e-Prime - Advances in Electrical Engineering, Electronics and Energy 1 (2021) 100009
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e-Prime - Advances in Electrical Engineering, Electronics and
Energy
journal homepage: www.elsevier.com/locate/prime
A review of the state-of-the-art in electronic cooling
Zhihao Zhang, Xuehui Wang, Yuying Yan∗
Fluids & Thermal Engineering Research Group, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK
a r t i c l e
Keywords:
Thermal management
Review
Electronics cooling
Heat transfer
i n f o
a b s t r a c t
The cooling or thermal management issues are facing critical challenges with the continuous miniaturization
and rapid increase of heat flux of electronic devices. Significant efforts have been made to develop high-efficient
cooling and flexible thermal management solutions and corresponding design tools. This article reviews the latest
progress and the state-of-the-art in electronic cooling, which could help inspire future research. The commonly
used methods in electronic cooling, classified into direct and indirect cooling, are reviewed and discussed in
detail. Direct cooling consists of air cooling, spray and jet impingement cooling, immersion cooling, and droplet
electrowetting. As for indirect cooling, the most popular and hot topics of using microchannel, heat pipe, vapour
chamber, thermoelectric, and PCM are overviewed. The effectiveness of the thermal management methods for
different-level requirements of electronic cooling and the ways how heat transfer capability can be improved are
also introduced in detail. Meanwhile, the pros and cons of these thermal management methods are discussed based
on their inherent heat transfer performances/characteristics, optimisation methods, and relevant applications. In
addition, the current challenges of electronic cooling and thermal management technologies are explored, along
with the outlook of possible future advances.
1. Introduction
Since Werner Jacobi introduced the early concept of the integrated
circuit in 1949 [1], the integrated circuit has developed rapidly in half a
century. An integrated circuit is usually a small chip made of semiconductor material silicon, manufactured by even the several-nanometreprocess and could accommodate millions to billions of microstructures
such as transistors, resistors, and capacitors. Nowadays, the integrated
circuit has been used in almost all electronic devices, and modern life
has also been integrated with various electronic products deeply, as
shown in Fig. 1. These applications have significantly improved efficiency and quality of modern peoples’ work, production, and life.
Then, against the fast development of the integrated circuit industry,
the famous “Moore’s Law” was proposed in the 1960s [2]. Although the
“Moore’s Law” is an empirical relation in production essentially rather
than natural law, its accuracy has been proven for decades and the law
has been widely used to guide research and development goals in the
semiconductor industry. Based on the “Moore’s law”, the number of
transistors that can be accommodated on the integrated circuit will keep
increasing, as shown in Fig. 2(a). Simultaneously, with increasing demand of market development, current electronic devices move towards
miniaturisation, thinness, and lightness. However, more transistors and
a smaller device size mean more power and greater heat flux density.
Up to now, the heat flux density of advanced server equipment chips
∗
can reach the order of 1 MW/m2 , and the heat flux density of phased
array radar and other equipment can even reach 5 −10 MW/m2 [3]. As
a result, these scenarios pose great challenges to thermal management
technologies in order to maintain the operations within safety ranges.
As shown in Fig. 2(b), the problems of temperature is identified as the
cause of approximately 55% of electronic equipment failures, according
to a study conducted by the US Air Force Avionics Integrity Program [4].
In addition, Black’s equation [5] indicates that the increase of temperature would accelerate the failure process of electronic devices. Therefore, dealing with temperature issues of electronic devices has become
increasingly important nowadays.
As mentioned above, heat generation is increasing along with the
continuous improvement of the integrated circuit manufacturing process. Therefore, the integrated circuit industry needs efficient thermal
management technologies, which are expected to keep improving the
performance and reliability of electronic devices. Even though traditional air cooling can address the heat dissipation issues by optimizing
the heat sink design for some general electronic devices [7], more other
cooling schemes for advanced high-performance electronic devices are
critically needed. Therefore, as shown in Fig. 3, a series of electronic
cooling methods has been proposed and studied in depth. Usually, these
thermal management concepts are divided into active and passive cooling methods [8]. The main difference between them is that the passive
cooling system is generally based on natural convection, while the active cooling system needs to be supplied by the external energy, usu-
Corresponding author.
E-mail address: yuying.yan@nottingham.ac.uk (Y. Yan).
https://doi.org/10.1016/j.prime.2021.100009
Received 5 August 2021; Accepted 6 October 2021
2772-6711/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
Z. Zhang, X. Wang and Y. Yan
e-Prime - Advances in Electrical Engineering, Electronics and Energy 1 (2021) 100009
Fig. 1. Typical application areas of integrated circuits.
Fig. 2. (a) The development trend of chip maximum power consumption, heat flux density, and approximate transistor counts in the past 20 years [6]; (b) distribution
of failure causes of electronic equipment [4].
Fig. 3. Classification of commonly used thermal management methods.
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e-Prime - Advances in Electrical Engineering, Electronics and Energy 1 (2021) 100009
ally electric energy, to enhance the heat dissipation capability of the
heat sink. Naturally, the active cooling system generally has better heat
transfer ability, so most cooling methods are developing based on active
cooling and are widely used in the thermal management of high-power
electronic devices. Hence, in this review article, the active cooling systems are introduced in detail owing to their excellent heat transfer performance and wide ranges of applications. Furthermore, as shown in
Fig. 3, thermal management technologies are also divided into coolings
of direct or indirect contact with the cooling fluid, based on whether the
cooling medium contact with the targets.
For direct contact cooling, liquid cooling has attracted more attention due to its better heat dissipation than conventional air cooling.
amongst them, spray cooling has been studied extensively [9,10]. The
spray cooling method atomizes droplets through high-pressure pumps
and nozzles, and covers the entire heating surface (insulating surface)
of an electronic device. Spray cooling has the advantage of high heat
transfer capability, excellent temperature uniformity, and sizeable cooling area. The jet impingement cooling is similar to spray cooling but
does not need droplet atomization. The study on jet impingement heat
transfer enhancement focuses on optimising jet parameters, liquid characteristics, and heating surface structures [11]. Besides, the immersion
cooling method also offers excellent cooling performance and could be
applied to data centres [12] and servers [13]. Simultaneously, droplet
electrowetting is employed by researchers to control hot spots of electronic devices. Due to the controlling of droplet motion can be achieved
by adjusting the surface wettability, the design of surface structures or
mophologies could significantly influence the method of thermal management. As for the methods of indirect coolingthey refer to the cooling medium dissipating heat through heat sinks, and have also been
studied extensively and applied widely. The pipeline heat exchanger
is the most common component for the external heat sink. Unlike direct contact cooling, the indirect contact cooling needs to consider the
contact thermal resistance of external heat sinks. Therefore, thermal interface materials (TIMs) are proposed and plays an essential role in the
thermal management of electronic devices [14]. The microchannel heat
sink gains much attention amongst indirect cooling methods due to its
high heat transfer capability and compact size. Therefore, microchannel
cooling has been widely used in the thermal management of small and
high-power electronic devices. The balance of flow resistance reduction
and heat transfer enhancement is the main objective of microchannel
heat sink optimization. Much research work has been done to explore
the effect of microchannel structure parameters [15], liquid characteristics [16,17], and phase change process [18,19] on flow and heat transfer performance. Besides, the heat pipe and vapour chamber as conduct heat devices are also commonly used in the thermal management
of electronic devices. Usually, the heat pipe and vapour chamber have
similar working principles like wick structures and two-phase flow and
heat transfer process. Both of them would go through the liquid evaporates in the hot end, and the vapour condenses in the cold end, then
the liquid is back to the hot end through the wick structure with the
help of capillary force. The heat transfer characteristics in the heat pipe
and vapour chamber are typically one-dimensional heat transport and
two-dimensional heat spreading [20]. Nowadays, the miniaturization
and high efficiency of the heat pipe and vapour chamber are a research
trend to be suitable for smaller-sized electronic devices like laptops or
smartphones. Many studies focus on this topic and they will be discussed
in detail in this review article. In addition, the thermoelectric (TE) cooler
also is one of the feasible choices for the thermal management of electronic devices. It has the advantages of noise-free, no pollution, and
long operating life, so many studies focus on optimising TE material
to improve the cooler working performance [21]. Moreover, the phase
change material (PCM) cooling method is also a potential thermal management technology. It is based on the principle of latent heat storage,
which maintains the temperature constant with the high energy storage density [22]. For electronic devices with pulsed heat flux density,
the PCM-based heat sink can effectively absorb the heat during pulse
operation and return to the device for devices during low-temperature
operation, so electronic devices’ working temperature can be relatively
stable. So far, many studies have been conducted to optimise the structures of PCM-based heat sink [23], improve PCM characteristics [24],
and consider combination with other kinds of thermal management devices [25].
Overall, the background of electronic devices cooling is discussed
and overviewed. The present work comprehensively reviews the stateof-the-art methods of thermal management for electronic devices,
mainly focusing on active cooling. Considering whether the cooling
medium directly contacts with electronic devices as a criterion, the
rest of the review is divided into two parts:Section 2 introduces direct contact cooling, such as air cooling in Section 2.1, spray cooling in Section 2.2, jet impingement cooling in Section 2.3, immersion cooling in Section 2.4, and droplet electrowetting in Section 2.5.
Then, Section 3 is concerned with indirect contact cooling, and TIMs
are introduced because of their significant effect on cooling performance. iMcrochannel, heat pipe, and vapour chamber are discussed in
Sections 3.1, 3.2 and 3.3, respectively. Besides, the TE cooler is introduced in Section 3.4 and Section 3.5 discusses the method of PCM-based
thermal management. Finally, the conclusion and outlook of the thermal management methods for electronic componts and devices are presented. This review article is expected to provide a useful reference for
the research in electronic devices cooling.
2. Direct contact with cooling fluid
2.1. Air cooling
The easiest way to achieve electronic thermal management is to use
natural or free convection cooling, which is an economical and convenient method. Based on this, Meng et al. [26] studied the effect of mounting angle on the natural convection heat transfer. The results show that
the straight-fin heat sink has the best and worst cooling performance
at the mounting angle of 90° and 15°, respectively. But as shown in
Fig. 4, the radiation and free convection are just recommended for heat
dissipation lower than 1550 W/m2 heat flux density [27]. Therefore,
for high-power electronic devices, forced air or liquid cooling is more
reliable. According to thermal management methods, externally forced
cooling can be divided into two scenarios. First, the cooling fluid (air or
liquid) directly acts on the surface of the electronic device. Second, the
heat is dissipated by an intermediate heat sink to expand the heat exchange area and further enhance the heat transfer process. Nowadays,
the study focus on natural cooling for electronic devices are quite limited
[28]. It may be due to the natural cooling method is a mature technology
for low-power electronics, such as TV or VCR [29]. For some thermal
management areas, such as data centres, forced liquid cooling (with or
without phase change) is critical. Therefore, the thermal management
methods with the liquid as the cooling medium will be comprehensively
reviewed and discussed in this paper.
2.2. Spray cooling
Spray cooling is one of the most effective thermal management methods for high-power electronic devices due to its high heat flux dissipation ability and sizeable cooling area [9]. It uses a nozzle to break down
the cooling fluid into numerous tiny droplets by high pressure. Then the
droplets impact directly on the heated surface to achieve the effect of
enhancing heat transfer. As shown in Fig. 5, the practical spray cooling process can be divided into typical three stages at different wall
temperatures. The first stage is single-phase regime, in which wall temperature maintains in a lower range and almost increases linearly, and
no phase change occurs in the cooling fluid essentially. Then, the spray
cooling would be transitted to two-phase regime with an increase of surface temperature, and the slope of the curve increases significantly. The
bubble nucleation process needs a great deal of energy to overcome the
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Fig. 4. The range of heat flux density applies to conventional cooling methods [6,27].
Fig. 5. Typical spray cooling curve (93 ml/min,
Tsat = 57 °C, ΔTsub = 28 °C) [30].
energy barrier that enhances heat absorption. The liquid film agitation
caused by droplets impact also plays a role in improving heat transfer
performance. Finally, when the surface temperature increases to a specific critical value, the heat flux will stop to increase. Many factors and
parameters can affect the spray cooling. According to the spatial configuration of the spray cooling system, these can be categorised into nozzle
parameters, cooling fluid characteristics, and heating surface properties.
These parameters are coupled with each other over the influence of the
spray cooling system.
For spray cooling, the placement and structure of the nozzle have
a critical influence on heat transfer performance, and some parameters
like inlet temperature, pressure or subcooling degree also make the impacts [31]. Therefore, many researchers have carried out studies on it.
Liu et al. [32] revealed that with the increase of pressure difference of
nozzle injection, the atomized droplets would be smaller, similar results have also been obtained by other researchers [33], and it was
reported that higher spray pressure would help enhance heat transfer
performance [34]. In addition, inlet temperature of the nozzle is also
proved to be able to significantly affect the heat transfer performance
of flash spray cooling [35]. Liu et al. [36] revealed that the cooling capability would increase with spray frequency of cooling fluid under the
same temperature difference between the cooling fluid and the heating
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Fig. 6. the process of spray cooling and droplets velocity photographed by a high-speed camera and PIV when the nozzle pore size is (a-b) 1.2 mm; (c-d) 1.5 mm;
(e-f) 2.0 mm [37].
surface. They also found that the pulsed spray cooling could increase
specific heat flux compared with continuous spray cooling. Bao et al.
[37] used Particle Image Velocimetry (PIV) system to observe the velocity distribution of atomized droplets with different nozzle pore sizes, as
shown in Fig. 6. They found that the heat transfer performance increases
with nozzle pore size, making the atomized droplets velocity lower but
more evenly distributed; it also increases with surface coverage areas.
In the meanwhile, it is also found that the number of atomized
droplets increases with nozzle height [38], and the average diameter
of the atomized droples decreases with the hight [39]. Besides, many
studies show that the relation between the cooling effect and spray
height is not linear. An optimum value is existed to achieve the best
heat transfer coefficient [40], and the optimum spray height decreases
with the flow rate [41]. Lin et al. [42] noticed that the spray height
determines the spray droplet characteristics and coverage area, and the
interaction of these parameters further affects the cooling effect. They
also revealed that the suitable nozzle orifice diameter needs to ensure
the spray to have enough mass flow and proper flow velocity at the same
time. Meanwhile, the tilt angle of the nozzle could also affect the heat
transfer performance [43]. Gao et al. [41] presented that the influence
of nozzle inclination depends on whether the change of nozzle tilt angle
makes the flow flux more significant or not.
In the practical thermal management process, the multi-nozzles
spray cooling method is commonly used. So the arrangement of multinozzle also has a significant influence on the spray cooling process, such
as the mean heat flux increases with horizontal spacing and nozzles offset distance [44]. Jiang et al. [38] combined the cold plate and linearly
arranged multi-nozzles to propose a novel compact spray cooling module, making maximum temperature difference of heat sources less than
6.5 °C when the heat flux was 304.70 W/cm2 . Xie et al. [45] suggested
that compared with a large number of nozzles, the small number nozzles with high flow mass will produce a more extensive heat flux. Hou
et al. [46] carried out a study of numerical simulation and noted that
the heating surface heat flux increased with nozzle numbers. Further-
more, there is a critical number when considering the performance and
economic optimisationof a spray cooling system.
As a medium for absorbing heat, the liquid thermal properties can
have a significant effect on spray cooling performance. Therefore, researchers have carried out studies on various feasible cooling fluids,
such as binary liquid (alcoholic liquid) [47], surfactants solution [48],
and nanofluid [49]. A binary solution is a commonly used method to
change the characteristics of the liquid. Liu et al. [50,51] noted that
the mixed ethanol and pure liquid water could effectively improve heat
transfer coefficient and reduce power consumption. However, Pati et al.
[52] suggested that adding methanol to water could reduce heat flux
due to the relatively high contact angle. So different kinds of organic
compounds could have a different effect on the cooling performance.
In addition, the study shows that an optimal value of surfactants added
could be obtained to maximize the heat transfer effect on binary liquid
spray cooling [53]. The surfactants could make the liquid surface tension reduce within a specific concentration range, so the droplets could
wet the heating surface better and improve the heat transfer ability [54].
It has also been proved that nanofluid can enhance heat transfer coefficient for spray cooling [55].
Moreover, the phase change process of liquid in spray cooling also
poses an excellent application prospective in electronic thermal management. Many studies focus on low boiling point cooling fluid, such
as R134a [55]. It is found that the performance of flash spray cooling of this kind of working fluid improved with the inlet temperature [35] or sub-cooling degree [39]. In addition, the Leidenfrost phenomenon is also a matter of concern, which caused the thin vapour film
between the heating surface and liquid droplets, and would worse the
heat transferperformance. Thus, lots of studies have focused on how to
suppress this phenomenon. Recently, Limbeek et al. [56] conducted a
study on the influence of ambient conditions on the Leidenfrost temperature and revealed the droplet changing regimes under different substrate temperatures, as shown in Fig. 7. They also reveal that the Leidenfrost temperature would increase with ambient pressure. Liang et al.
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Fig. 7. The snapshots of ethanol droplets under a serious substrate temperature and the colour circle region mean the different regimes [56].
Fig. 8. The SEM images of the (a) pyramid fin micro-structured surface; (b) square fin micro-structured surface; (c) nano-porous surface [63].
[10] summarized the mathematical models about the Leidenfrost temperature in their review paper. The detailed introduction to the Leidenfrost phenomenon is not the focus of this review article. More research
progress can refer to the review paper published by Zhong et al. [57] and
Talari et al. [58].
The physical characteristics of the heating surface also have a significant effect on thermal management performance. Many researchers
have reported that surface roughness could help improve spray cooling
performance [39,59]. Chen et al. [60] carried out numerical simulations
and pointed that compared with liquid temperature, spray pressure, nozzle height, and wall temperature have the most apparent effect on heat
transfer. Recently, Zhao et al. [61] have found that the effect of surface
orientation (such as upward, downward, and vertical) on spray cooling is not significant. Wang et al. [62] found that the micro-structured
hydrophobic surfaces could effectively improvethe rate of spray cooling
and bubble removal. Zhou et al. [63] prepared the macro-structured and
the nano-porous flat surface, as shown in Fig. 8. They found that the fins
macro-structured surface could effectively improve heat transfer performance compared with a smooth flat surface; and this may be because of
the increase of the contact area, nucleation sites, and wettability at the
same time. Likewise, Xu et al. [64] indicated that types of micro- and
nano-structured surfaces could enhance the heat transfer performance
of the spray cooling effectively. Xu et al. [65] and Ekkad et al. [66] reviewed and discussed the effects of surface properties and structures on
spray cooling performance.
In summary, spray cooling is proven to be an effective solution of
thermal management for high-power electronic devices. Structural parameters of nozzles and physical properties of working fluid affect the
spray cooling performance. Meanwhile, spray cooling has its shortcomings, such as high-pressure operating conditions and clogging of the nozzle during practical operation. Understanding the heat transfer mechanisms of spray or film nucleate boiling is still lacking [67]. In some particular fields, such as aerospace, the influence of gravity on the spray
cooling performance also lacks understanding [68]. Therefore, there is
still a great deal of research worth of studying to further improve the
efficiency and reliability of spray cooling.
2.3. Jet impingement cooling
Jet impingement cooling is one of effective cooling methods with
relatively low thermal resistance; and has widely been employed as r
thermal management solutions for power electronics devices and industry. A schematic diagram of distinction of air-jet impingement regions is
shown in Fig. 9. The jet impingement cooling process is similar to spray
cooling but does not need droplet atomization. Nevertheless, the fluid
flow and heat transfer mechanism is still very complex. Many studies
have been carried out to understand jet impingement cooling mechanism and improve the heat transfer performance. Like spray cooling, research on the application of jet impingement cooling and heat transfer
enhancement is also carried out from the nozzle configurations, liquid
properties and surface structures.
Nozzle parameters and arrangements have a significant effect on the
jet impingement cooling performance. Tepe et al. [70] revealed that the
extension of the jet hole could enhance the heat transfer process and alleviate the negative effects of crossflow and eliminate misalignment of impinging areas. Through numerical simulation, Sabato et al. [71] studied
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Fig. 9. Regions division of the jet impingement cooling [69].
the influence of jet numbers, nozzle diameter and arrangement on the
cooling performance. They found that with the increase of jet numbers,
the marginal heat transfer could be improved, but the pumping power
was also rising, and the heat transfer was enhanced with the reduction of nozzle diameter. Wu et al. [72] also found that the heat transfer
coefficient of jet impingement cooling decreased with nozzle diameter
increasing. Wen et al. [73] pointed that the nozzle geometry and arrangement defined the jet momentums under serious cross-sections and
whole flow field complexity, respectively. For jet impingement cooling,
the best cooling effect is concentrated in the central impinging area but
rapidly declining when moving away from this area. Hence, multiple jets
arrangement is often using to address this issue [74]. Besides, they also
revealed that multi-jet cooling could be more effective than single-jet
cooling by the experimental research, as shown in Fig. 10 [75].
In addition, the performance jet impingement cooling is normally
largely relevant to working fluid or liquid thermal proiperties. Wu et al.
[76] used paraffin to fabricate particulate form of nano PCM and add
them into water as the cooling medium. The results showed that the
heat transfer coefficient could be improved by 50% compared with water for jet impingement cooling. Rehman et al. [77] completed a simulation of using PCM slurry and nanofluid as cooling medium. The results
showed that the addition of PCM and nanoparticles can enhance heat
transfer capability and decrease ethe stagnation point temperature, but
result in an increase of pressure drop. They also reported that the PCM
slurry could not only improve heat transfer performance but also reduce pressure drop compared with nanofluid under the same jet velocity
and particle loading. Selimefendigil et al. [78] compared the influence
of four nanoparticle shapes (spherical, blade, brick, and cylindrical) on
the jet impingement cooling performance. The results showed that cylindrical nanoparticles show the best heat transfer performance in terms
of Nusselt number. Ekiciler et al. [79] also pointed that the shape of
nanoparticles could affect jet impingement cooling. Modak et al. [80] indicated that jet impingement cooling Nusselt number increases with the
nanoparticle concentration and Reynolds number. Meanwhile, the sed-
iment of nanoparticles on the surface could enhance wettability. For
more information about nanofluid jet impingement cooling, it can be referred to the review article recently published by Mohammadpour et al.
[81]. For jet impingement boiling, there are many methods reported,
which could improve heat flux, such as modification of heat transfer surface [82,83], improvement of fluid properties [84], optimization of nozzle structures [85], and other methods [86,87]. The readers could also
refer to the review articles published by Devahdhanush et al. [88] and
Fan et al. [89], which introduce various ways for improving the critical
heat flux of jet impingement boiling and the factors that could influence
jet impingement boiling in details.
Furthermore, the effect of electronic devices’ surface material and
properties on jet impingement cooling have been studied by a few researchers recently. Wei et al. [90] found that compared with cooling
substrate or base plate, direct cooling for the backside of electronic device was more effective. Selimefendigil et al. [78] pointed that corrugated surfaces had a minor effect on the heat transfer enhancement than
flat surface at low Reynolds number. However, the Nu would increase
about 6.99–8.87% athigh Reynolds number. Wang et al. [75] studied on
finned heat sinks and reported that the copper foam could positively affect jet impingement cooling performance. Wiriyasart et al. [91] applied
jet impingement to cool heating surfaces with rectangular, circular, and
conical micro pin-fins, respectively; and their results showed that the
circular micro-pin-fins could offer better heat transfer enhancement.
In short, like spray cooling, jet impingement cooling also could be
an up-and-coming option for thermal management of high-power electronics, owing to its effective heat transfer performance, lower thermal resistance, and simple system. However, jet impingement cooling
faces some issues that need to be deal with. One apparent shortcoming of single-phase jet impingement cooling is that a significant surface
temperature fluctuation due to heat transfer coefficient away from the
stagnation zone drops drastically. The flow obstruction between closely
spaced multiple jets would also impact the cooling performance under
some specific conditions.
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Fig. 10. (a) Experimental devices; (b) cross-section of the chip-level jet impingement cooler; (c) schematic diagram of the thermal test chip with temperature sensors
and heater cells; (d and e) temperature distribution map of the single-jet and multi-jet (4 × 4) impingement cooling based on the experimental measure results [75].
Table 1
The commonly used coolants in immersion cooling.
Fluids
Thermal conductivity (Unit: W/m•K)
Cooling mechanism
Boiling point ( °C)
Ref.
Ethanol
Water-ethylene glycol (1:1)
Minera oil
Novec 649
Novec 7000
FC-3283
FC-43
FC-72
0.167 [103]
0.37
0.13
0.059
0.075
0.066
0.065
0.057
Two phases
Single/Two phase
Single-phase
Two phases
Two phases
Single-phase
Single-phase
Two phases
78
107.3
–
49
34
128
178
56
[100]
[94]
[93]
[97]
[104]
[105]
[105]
[106]
2.4. Immersion cooling
proved the critical heat flux and heat transfer coefficient by 30% and
455%, respectively. Besides, other methods, such as Taguchi Method
optimization [98], microporous Cu coating [99], Baffle layout [100],
and applying nanofluid [101] are also studied and reported. It has been
proved that all these methods could help improve heat transfer performance of immersion cooling performance. In addition, the safety and
stability of immersion cooling have also attracted muchattention from
both academic society and industry. Ramdas et al. [102] found that immersion cooling would not damage the PCB. Ali et al. [99] carred out
numerical simulations and reported that the Von-Mises stress of chips
cooled by immersed dielectric liquid was always within safety limits in
their simulations.
Immersion cooling can efficiently cool electronic devices but have
some issues that need to be handled. For example, one study has suggested that the fluff floating in the coolant could be adsorbed on the heat
exchange surface, which could suppress the cooling efficiency [107].
The balance between electric insulation and thermal conductivity of the
coolant is also an important research topic for immersion cooling. Meanwhile, further research is still needed to address the ageing of electronic
devices with various coolants.
Immersion cooling is one of the effective thermal management methods. It uses dielectric fluid or coolant with good thermal conductivity
and very poor electric conductivity properties, immersing the electronic
devices to achieve cooling. Immersion cooling has been widely used
to cool like data centres [12] and servers [13]. The commonly used
coolants are listed in Table 1. Wang et al. [92] proposed a hybrid cooling
method to combine spray cooling and immersion cooling. They found
that the hybrid cooling method improved the heat flux by up to 65.6%
under certain conditions than traditional spray cooling. Similarly, Patil
et al. [93] also pointed that the immersion cooling of flowing dielectric fluid could improve the cooling performance by about 46% compared with natural convection. As mentioned above, many studies use
dielectric fluid as cooling fluids but not water due to its poor insulation
properties. However, as shown in Fig. 11(a–e), Birbarah et al. [94] by
coating Parylene C on the printed circuit board (PCB), made immersion cooling with water and water-ethylene glycol liquid be possible.
They said that the water always has a better heat transfer coefficient
for both natural convection and nucleate pool boiling than dielectric
fluid.
Much research has been conducted and shown that immersion cooling is also an effective thermal management solution for power electronic devices [95,96].Methods to enhance the heat transfer of immersion cooling has also been reported. Hsu et al. [97] fabricated three
kinds of surface, namely Si nanowire (SiNW), Si micropillar (SiMP),
and two-tier Si nanowires combining with Si micropillar (SiNW/MP) to
help enhance the heat transfer pf immersion cooling. They found that
compared with the usual plain SiO2 surface, the SiNW/MP surface im-
2.5. Droplet electrowetting
Electrowetting, which is one of the effective methods to control wetting phenomena of droplets, has been widely studied in scientific and
industrial communities [108]. Electrocapillary, the electrowetting phenomenon cornerstone, was first proposed in 1875 by Gabriel Lippmann
[109]. The equation used to describe the relation between electric field
effect and droplet contact angle is called the Young–Lippmann equation,
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Fig. 11. (a and b) Schematic diagram of immersion cooling of PCB; (c–e) under the different heat flux conditions the generation of bubbles taken by high-speed
camera [94].
which could be expressed as [110]:
𝑐 𝑜𝑠𝜃 = 𝑐 𝑜𝑠𝜃𝑒𝑞 + 𝐶 𝑉 2 ∕2𝛾
The hot spot temperature decreased from 172 °C to below 100 °C in
475 s. For more thermal management studies on droplet electrowetting,
the review paper recently by Yan et al. [119].
In terms of how to improve droplet electrowetting heat transfer, a
few research works can be identified. Cheng et al. [112] reported their
work of coating the copper screen to improve the effect of cooling, in
which droplet splitting and merging to make thin-film existence on the
target region to control thin-film evaporation on the hydrophilic surface. Bindiganavale et al. [123] suggested out that the effect of EWOD
phase change cooling could be improved by using super hydrophilic
nanoporous coating, promoting the emergence of thin-film and then
enhancing heat transfer. Sur et al. [122] proposed an alternating current electrowetting (ACEW) method to improve boiling heat transfer. In
addition, the efforts of using CNT (Carbon nanotubes) to EWOD were
also made in the studies; and the researchers found that it could help
reduce the resistance of droplet friction effectively [124]. Papathanasiou et al. [125] conducted a review about the effect of surface design
on electrowetting. It was shown that other cooling liquids would also
improve the heat transfer performance. The liquid alloy droplet could
be a choice for the electrowetting cooling, and the heat transfer rate
could be increased by two orders of magnitude compared with water
[126]. Chakraborty et al. [127] rereportd the application of nanofluid
to improve the dynamic changes ofdroplet contact angle, contact radius, and amplitude; and suggested thatthe morphology changes of the
droplet could increase the disturbance inside the droplet then improve
heat transfer.
Even though the droplet electrowetting cooling method is a novel
and valuable method, it still faces many challenges for further improving
the cooling capability and application range. For example, the velocity
of droplet transport could be improved to lift the limitation n cooling
capability of high power electronic devices [114,128].
(1)
where 𝜃𝑒𝑞 is the equilibrium contact angle when the external voltage
is zero, and C is the capacitance per unit area, V is voltage for direct
2 (effective voltage)
current electrowetting, and V2 should change to 𝑉𝑒𝑓
𝑓
for alternating current electrowetting.
Nowadays, the droplet electrowetting phenomenon has seen more
application in the thermal management of electronic devices. One of
the representatives is EWOD (Electrowetting on Dielectric) technology
[111], which has the advantages like quick response [112], hot-spot
cooling [113], and low energy consumption (no pressure drop) [114].
As shown in Fig. 12, the main heat transfer enhancement methods of
EWOD are divided into changing wettability, controlling fluid movement and enhancing phase change heat transfer. Xu et al. [115] studied
the process mechanism of the droplet from stationary to motion from
the perspectives of EWOD characteristics and polarization theory. They
indicated that there are three stages to make the motion of the droplet,
which is the change of the contact angle and surface tension, the deformation of the droplet, and the generation of large enough hydrostatic
pressure difference. Bahadur et al. [116] reported that the voltage for
electrowetting could change the wetting state of the droplet (such as
Cassie-Baxter state or Wenzel state); and in their study, a simplified prediction model of droplet movement based on the energy minimization
principle was developed. Through combining the effect of electrowetting and thermocapillary, Pamula et al. [117] recorded the phenomenon
that the tiny droplets automatically move to the hot spots; and they suggested that this is owing to the temperature difference induced surface
tension changes. Park et al. [118] studied two-phase droplet electrowetting cooling on a single plate EWOD configuration, which could improve
the heat transfer effectively compared with the traditional single phase.
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Fig. 12. (a) Schematic diagram of electrowetting principle [120]; (b) the influence of electrowetting on the wetting state (left: Cassie state, right: Wenzel state)
[116]; (c) based on the electrowetting to move droplets to hot spots [121]; (d) improve the boiling heat transfer by electrowetting [122].
Fig. 13. (a) Schematic diagram of contact thermal resistance; (b) the use of TIM.
3. Indirect contact with cooling fluid
3.1. Microchannel cooling
In the previous part of the article, thermal management methods
about direct contact of the cooling fluid have been reviewed. This part
will focus on indirect contact cooling, which means the cooling medium
dissipates the heat combined with the external heat sink structure. It is
worth mentioning that the air gap between the electronic device and
heat sink surfaces, as shown in Fig. 13(a), significantly impacts the
heat removal process. Therefore, thermal interface materials (TIMs), as
shown in Fig. 13(b), are an effective medium that fill the gap between
the surfaces and provide better heat transfer [129]. Nowadays, many
solutions have been proposed for improving the thermal conductivity
of TIMs, such as adding nanoparticles [130], using carbon nanotube arrays (CNTs) [131,132], continuous metal phase [133]. There are various
TIMs has been developed, and they can be divided into three categories
[134], namely: filler-based (grease, adhesives, etc.), metal/solder (alloys, metallic foils, etc.), and carbon-based (graphene, graphite, etc.).
As the progress of TIMs research is beyond the scope of this work,
more detailed information can be referred to the articles mentioned
above.
In 1981, the microchannel heat sink method was proposed by Tuckerman and Pease for the first time [135]. Mehendale et al. [136] and
Kandlikar et al. [137] defined microchannel of hydraulic diameter
ranged from 1 to 100 𝜇m and 10 to 200 𝜇m, respectively. The microchannel heat sink has a strong heat dissipation capability that reaches
more than 1000 W/cm2 [135] and better temperature uniformity after
optimization [138]; and a typical microchannel heat sink applied for
thermal management of chipis shown in Fig. 14(a–d). Thus, as shown
in Fig. 14(e), the study about microchannel cooling gradually draw
more attention to researchers, and the factors that could affect the microchannel cooling capability have been extensively studied. In this section, the studies about microchannel cooling are divided into two aspects: microchannel structure and cooling fluid properties; and hope
these could give a general introduction to the research of microchannel
cooling.
About microchannel cooling research, many studies focus on the design of microchannel structures because of the significant effects on flow
and heat transfer process. In this review, the optimization of microchan-
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transfer is shown in [159]. A general positive effect on the heat transfer
under single-phase microchannel flow is reported in [146]. Recently,
Kang et al. [147] proposed a method for optimising microchannel surface roughness by simulated annealing; which could ensure the optimal
heat transfer performance under the specific pressure drop condition.
Jones et al. [148] also reported their results of that the surface roughness could affect the heat transfer coefficients and pressure drop of the
microchannel flow boiling under appropriate conditions. Moreover, the
concept of bionic has been applied to microchannel surface design. Tang
et al. [149] fabricated a bionic surface based on dragon louse wing, and
achieved drag reduction by 35.4% compared with smooth surface. So
far, the concept of surface wettability has been widely studied and applied to heat and mass transfer processes [150]; its effect on microchannel cooling has also been studied extensively. On bionic approaches,
hydrophobic and superhydrophobic surfaces have been to microchannels to reduce wall share stress and pressure drop effectively [151,152].
Rahbarshahlan et al. [153] numerically simulated the effect of a hybrid
surface of hydrophobic and hydrophilic, the results showed that the horizontal pattern has better heat transfer performance than the longitudinal pattern. In addition, the surfaces of hydrophobic and heterogeneous
wettability could help enhance boiling heat transfer [154,155], and the
horizontal pattern could enhance flow boiling and reduce pressure drop
of microchannels [156].
As for the microchannel structures, many researchers have reported
that the significant effect of channel shape on cooling performance.
Firstly, inlet shape of microchannel can affect heat transfer and hydraulic performance has been proved [157,158]. Gunnasegaran et al.
[159] reported that the microchannel of rectangular cross-section offered the best cooling performance compared the channels with trapezoidal and triangular sections. Also, some studies suggested that the
circular cross-section shape posed better heat transfer capability than
the square and rectangular ones [160,161]. Therefore, it is necessary to
conduct in-depth and comprehensive studies to understand the effect of
microchannel inlet with different cross-section on heat transfer and hydraulic performance. In addition, double-layer microchannel can offer
more effective cooling performance [162]. In terms of the design of microchannels, more uniform cooling capability and temperature distribution, like the manifold layout, often plays an essential role in controlling
flow distribution, as shown in Fig. 15. The study on the manifold design
can better guide the layout of flow channel and realize the balance of
flow resistance reduction and heat transfer enhancement. For more information of manifold design and study, review articles [163,164] could
be referred.
In general, there are many methods for changing and optimising the
shapes or confugrations of microchannels. For example, compared with
Fig. 14. Microchannel heat sink example (a) microchannel structure details;
(b) microchannel flow visualization; (c) back-side of the heat sink; (d) IR temperature map [139]; (e) The number of published articles that the article title,
abstract, keywords contain “microchannel” (blue line) or “microchannel” and
“cooling” (red bar) from 2011 to 2020 based on the database of Scopus®. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
nel structures is mainly classified based on the changes of structural parameters of microchannels (such as material type, surface treatment, and
channel configuration, etc.) and combined with other enhanced heat
transfer structures (such as pin-fin structure, porous medium, and vortex
generator, etc.). Firstly, it is accepted that the materials of microchannel heat sinks significantly influence cooling performance. Koşar et al.
[140] analysed the heat sink material like Si, Cu, Steel, Glass, Quartz,
and Polyimide through numerical simulations. They found that Nu decreases with lower thermal conductivity material. Muhammad et al.
[141] compared different materials such as copper, aluminium, tungsten, and silicon, and found that higher thermal conductivity materials offer better cooling performance; and the conclusions are consistent
with [142,143]. Sarowar et al. [144] have recently studied microchannel heat sinks based on ultra-high temperature ceramics (UHTCs) materials HfB2 , TiB2 , and ZrB2 , and concluded that the UHTCs materials
canbe effectively applied to microchannel heat sinks and the HfB2 presentes better cooling performance. More information about microchannel fabrication and material selections the review paper published by
Prakash et al. [145] may be referred.
As a mode of forced convection heat transfer process, the channel
surface properties of microchannel cooling considerably affect the performance and quality of thermal management. How will the surface
roughness caused by machining process affect the fluid flow and heat
Fig. 15. Different types of manifold layout (a) dividing; (b) combining; (c) parallel; (d) reverse; (e) normal flow [163].
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Fig. 16. Commonly used microstructures in the microchannel heat transfer enhancement (a) dimple [171]; (b) pin-fin [172]; (c) rib [173]; (d) groove [174]; (e)
cavities [175]; (f) vortex generator [176]; (g) interrupted wall [177]; (h) metal foam [178].
conventional straight channels, wavy channels can provide better heat
transfer capability [165,166]. Some studies focus on the design of microchannel structures to improve heat transfer performance. The common feature of these designs or optimizations is to increase the disturbance of the boundary layers. It was proved that the perturbation of
the thermal boundary layer could enhance the fluid flow heat transfer [167]. Thus, as shown in Fig. 16, many microstructures have been
applied to microchannel optimizations, these include dimple or protrusions surface, pin-fin structure, rib, groove, cavities, vortex generator,
interrupted wall, and metal foam, etc. Other review articles, such as
[168–170] about design of microchannel structures and optimisation of
microchannel heat sinks should also be viewed.
As mentioned above, the design of channel structures has played an
essential role in enhancing the process of flow and heat transfer in microchannels. On this basis, the other factors of convective heat transfer
process in the channel, such as working fluid thermal proiperties and
phase change process, also significantly influence the cooling performance. In addition to water (or deionized water), the most commonly
used working liquid in microchannel cooling, various other types of
fluid, such as refriegerants and nonafluids, have been applied to microchannel heat transfer. Probably, nanofluid is the one that has drawn
much attention from academic or industrial societies, as its higher thermal conductivity [196,197],so it has been widely implemented to improve microchannel heat transfer performance [179]. In fact, the base
fluids still largely affect microchannel cooling performance of nanofluids. Recently, Li et al. [180] applied carbon-acetone nanofluid into microchannel cooling, and found that the carbon-acetone nanofluid increased heat transfer coefficient by 73%, as carbon nanoparticle enhanced the thermal properties of acetone. Lyu et al. [181] compared
the water and kerosene as base liquid, and reported that the kerosene
did not contribute to the enhancement of flow and heat transfer performance. Shamsuddin et al. [182] reported their experimental results and
suggested that adding surfactants could reducee microchannel pressure
drop under certain conditions without negatively impacting heat transfer capability. In short, the above studies show that the effect of the
base liquid of nanofluid is still worth studying in-depth. Up to now, the
nanofluid still faces some issues that need to be solved seriously, like
clogging, fouling, and corrosion of microchannel [169]. For more information of nanofluids applied to microchannel cooling, review articles
[17,183–188] may be referred.
As one type of coolants, liquid metal has some unique thermal properties compared with conventional cooling fluid. For example, the thermal conductivity of water is about 0.6 W/m•K. but the liquid metal
Ga68 In20 Sn12 could reach 39 W/m•K [189] and has broader temperature
adaptability. Therefore, liquid metal is suitable for thermal management
of high heat flux electronic devices. It was used in heat pipe for space
power cooling decades ago [190], and has also been used in chip cooling
first in 2002 by Liu and Zhou [191]. So far, the use of liquid metal in
the microchannel have been widely studied. Sarafraz et al. [192] found
that even though the pressure drop increased after adding Indium into
Gallium, the cooling performance was improved due to the increase of
gallium-indium eutectic mixture thermal conductivity. Sarowar et al.
[193] also reported their results bycomparing with other gallium alloys
like EGaInSn, EGaIn and GaSn, and found that the GaIn have the best
cooling performance for microchannel applications. Muhammad et al.
[141,194] indicated that thermal conductivity and specific heat of liquid
metal are key factors affecting the cooling performance of microchannel
heat sink. They also compared various Ga alloys like EGaInSn, EGaIn,
GaSn and GaIn, and found that the EGaInSn coolant introduced the highest microchannel pump power. Therefore, although, liquid metal could
offer higher heat transfer capability than water and nanofluid, it often
has highest pump power consumption. Indeed, liquid metal has its problems. It may cause corrosion to the microchannel heat sink and affect
the safety of electronic equipment, so careful further research is still
needed for material selections and flow design.
As mentioned above, the method of single-phase microchannel cooling has successfully targeted the thermal management of electronic devices. However, the capability of single-phase flow microchannel cooling is not sufficient for electronic devices with higher heat flux. Therefore, making the coolant undergoing a phase change process such as
flow boiling can effectively improve the capability of microchannel cooling [195]. Due to the latent heat of boiling can dissipate the heat flux
and keep wall temperature uniform under low flow rate [196],. several
methods have been developed to enhance the capability of heat transfer by the mode of microchannel flow boiling; and these can be divided
into microchannel structure optimization and working fluid modification. Recently, Oudah et al. [197] proposed a microchannel inlet restrictors structure, which could improve the critical heat flux (CHF) of
the microchannel heat sink due to the inhibition of instability, but it
also caused the increase of pressure drop significantly. This was also
noticed by Ren et al. [198], who proposed a method for fabricating parallel microgroove in the microchannel, as shown in Fig. 17. Thus, the
flow boiling instabilities could be controlled effectively and both the
CHF and heat transfer coefficient (HTC) could be improved by up to
155% and 72%, respectively, without affecting pressure drop. Hedau
et al. [199] found that the cooling capability declined with the decrease
of channel number, which means the increase of channel width, lead
to the bubble confinement effect and increased liquid film thickness.
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Fig. 17. (a–c) Schematic diagram of the microgroove etching in microchannel; (d and e) effect of microgroove on the microchannel flow boiling [198].
Hou et al. [200,201] proposed a thermal test vehicle fabricated in the
PCB grooves and could significantly improve the heat dissipation capacity of the heat sink. Based on their experimental results, Gedupudi
et al. [202] proposed a simple 1-D model that could evaluate the effect
of channel dimensions or thermofluid conditions on the upper bound
of pressure fluctuations amplitude and flow reversals. Vontas et al.
[203] applied volume of fluid (VOF) method to simulate the effect of
microchannel surface wettability on the flow boiling characteristics. The
results showed that wettability presented a crucial impact on HTC, and
the liquid film evaporation and contact line evaporation played the dominant role in the hydrophilic and hydrophobic surfaces, respectively. Lin
et al. [156] also found that heterogeneous wetting surface could help improve the HTC by 39.55%. The effect of microstructure on microchannel flow boiling also attracted the attention of researchers. Deng et al.
[223] proposed an open-ring pin fin for microchannel and found that it
could improve heat transfer performance and suppress the instabilities
of flow boiling when it is inline arranged. Tiwari et al. [204] reported
that the wavy wall microchannel would have better heat transfer performance than the straight wall and suppress two-phase flow instabilities.
In the meantime, the external field could also affect the boiling heat
transfer, Zu et al. [205] pointed out that the electric field applied to
the boiling bubble can promote the vortices motion, enhancing the wall
heat transfer process. In addition, the effect of nanofluid on flow boiling
also be conducted. Soleimani et al. [206] simulated the nanofluid twophase flow boiling in the microchannel, and found that the addition of
nanoparticlescould not significantly improve heat transfer performance.
Zhang et al. [207] experimentally studied the graphene oxide nanofluid
flow boiling in microchannel and found that graphene oxide leads to the
generation of nonporous deposition, which may block active nucleation
sites and deteriorate heat transfer performance. Therefore, the significance of using nanofluids in microchannel flow boiling needs further
study and discussion. For more information about microchannel flow
boiling, heat transfer enhancement and inhibition of instability, floowing review articles [18,19,208–213] should be helpful.
Microchannel cooling has been proven to be a very effective thermal
management method for high-power electronic devices. There is a great
deal of research on microchannel cooling every year. Up to now, the optimal design of microchannel has been studied in-depth, but fabricated
microchannel is still plagued by machining accuracy, manufacturing
costs and process simplicity. The fabrication methods of microchannels
also need improvement. The critical parameters of microchannel heat
sinks, such as material, size, flow channel layout, and microstructure
placement, also the general method of optimizing design are all need to
be explored. Although nanofluids applied to microchannel could help
enhance cooling ability, after a certain period of working, the deposition
of nanoparticles is still a thorny problem that needs to be solved, leading to clogging inmicrochannels, making it ineffective and even damaging electronic devices. Flow boiling is certainly an effective way for
enhancing heat transfer, but more feasible methods for reducing flow
resistance and suppressing two-phase instability are worthy of further
exploration.
3.2. Heat pipe
As an efficient heat exchange device, heat pipes have attracted wide
range of attentions from heat and mass transfer society since it was
patented by Gaugler in 1944 [214]. The basic structures and working
principles of the heat pipe are as shown in Fig. 18(a). Heat pipes have
been widely used in the thermal management of high-power electronic
devices due to its simple structure, high reliability, and efficient heat
transfer capability [215]. Attributable to the continuous improvement
of the manufacturing process, the ultra-thin heat pipe (UTHP) can also
be fabricated and used in the electronic device, as shown in Fig. 18(b–c).
A number of methods have been developed and employed for improving the heat transfer performance of heat pipes. From the perspective of structural optimization, the wicking structure has been studied
dramatically due to its essential role in flow and heat transfer inside the
heat pipe. Shioga et al. [218] proposed a new manufacturing method
that could erase the gap between the evaporator container inner wall
and the wick and suppress the evaporator container vapour leakage.
Zhou et al. [219,220] proposed a series of spiral woven mesh (SWM)
wick, which could realize the balance of increasing heat transfer capability and reducing structural weight for the UTHP. They also proposed
a kind of composite wick structure composed of copper foam and mesh,
as shown in Fig. 19(a–d), which had the favourable characteristics of
better heat transfer performance and mechanical strength at the same
time [221]. Chen et al. [222] also proposed a kind of surface functional
wicks which could provide better capillary performance to guarantee the
excellent heat dissipation capability of the UTHP. Wang et al. [223] inspired from the principle of mimicking biology and biomimetics to propose a novel hierarchical wick structure for the heat pipe and proposed a
mathematical model to describe the capillary pressure and capillary rise
rate. Li et al. [224] developed a kind of process that fabricated the superhydrophilic copper mesh wick for the flat plate heat pipe. Comparing
with the pure copper sheet or pyrolytic graphite flake, this kind of heat
pipe increased the effective thermal conductivity by 80 and 36 times,
respectively. In addition, the inclination angle is an essential parameter
that affects the heat pipe heat transfer performance and many studies
considered that. Zhang et al. [225] pointed that the extra wicking structure enhanced the heat transfer performance under smaller inclination
angles (30° and 60°) compared with the heat pipe without a wick. But
under the vertical condition, the smooth heat pipe presented lower thermal resistance and temperature difference. Yao et al. [226] also found
that the inclination angle significantly affected the cooling capability
of the micro heat pipe. The effective thermal conductivity decreased
from 5820 to 295 W/m•K when the inclination angle change from 90° to
−75° Further, Li et al. [227] also noticed that the sintered porous wicking structure ensured adequate capillary pressure and protection com13
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Fig. 18. (a) Schematic diagram of the heat pipe working process [216]; (b and c) a kind of UTHP applied on the laptop thermal management [217].
Fig. 19. SEM photograph of (a) copper foam; (b) mesh partial; (c) enlarged view of UTHP; (d) cross-section view of UTHP [221].
pared with no sintering. Hence, the porous wicking structure can counteract the influence of centrifugal accelerations to a certain extent and
make the heat pipe maintain better heat load. Besides, there are other
studies contribute to optimize the structure of the heat pipe. Tharayil
et al. [228] used nanoparticles to coat the evaporator which can enhance the heat transfer due to the boiling process was stimulated. Ling
et al. [229] based on the constructal law designed a novel leaf-shaped
oscillating heat pipes which posed a certain degree of heat transfer enhancement effect.
In addition, the working fluid of the heat pipe also influences
the thermal performance. Sardarabadi et al. [230] revealed that the
nanofluid combined with the sodium functional group could improve
thermal performance. Xu et al. [231] indicated that the mixture of DI
water and HFE-7100 could improve the heat transfer limits of heat pipes.
Ramkumar et al. [232] also pointed that acetone could improve the
heat transfer coefficient by 79.81% and reduce the thermal resistance
by 10.52% compared with methanol as a working fluid. Meanwhile, the
filling ratio of the working fluid in the heat pipe also has a significant
effect on the cooling capability [233,234], and Zhou et al. [221] pointed
that higher or lower filling ratios may cause the deterioration of the heat
transfer effect owing to the increase of thermal resistances. Furthermore,
the analysis of working conditions of the heat pipe is also of great significance to the application of heat pipe. Werner et al. [235] pointed
out that the thermal resistance can be used as an indicator to determine
the heat pipe boiling limit occurrence. Wang et al. [236] based artificial
neural network (ANN) method proposed a model which could predict
the thermal resistance of the pulsating heat pipe under various working
conditions directly. A more detailed introduction of the heat pipe can
refer to review articles published in recent years [237–239].
The heat pipe is an effective thermal management method, and its
advantages are beyond doubt. However, the heat pipe also faces some
issues that need to be handled. First, to improve capillary and heat transfer performance, the wicking structure needs to find more optimized
design methods and manufacturing processes, like hydrophilic surface
treatment and multiscale fabrication. In addition, the UTHP has a wide
range of application prospects under the trend of flattening and flexible electronic devices. Therefore, the design of UTHP needs to continuously improve its structural stability and keep the balance between
heat transfer enhancement and portability. Thechallenges for material
selection and structural design, as well as the universal design guide
are all worthy of being considered. As both shell material selection and
the combination quality of wick and shell also significantly affect the
working ability and stability of heat pipes, the optimization of material
and packaging technology should be full of opportunities. Moreover,
the artificial intelligence technology has also been applied to heat pipes
design, and the optimization of its learning and calculation methods is
worth of further study.
3.3. Vapour chamber
Vapour chamber, as a heat dissipation structure, has been used for
thermal management of electronic devices. The vapour chamber has advantages of favourable temperature uniformity, suitable for devices with
high heat flux, large surface area, and high reliability. Its basic structures and working principle are shown in Fig. 20. The vapour chamber
is usually an enclosed space, and the bottom side of the steam chamber
is in contact with the heat source. The working fluid is heated to evaporate and diffuse to the upper-side condensing surface, and then the
steam is re-condensed on the upper side. Finally, the working liquid is
absorbed by the wick material again and distributed to the heat source
through capillary force. By repeating this process cyclically, the heat
dissipation process of the electronic device is realized. Although both
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Fig. 20. Schematic diagram of the vapour chamber [240].
the vapour chamber and heat pipe are based on the two-phase flow and
heat transfer of working liquid, the difference between them can normally be identified from their applications.s. The heat pipe is used for
unidirectional or one-dimensional heat transfer from the heat source to
the cold source. But the vapour chamber is more used for making the
two-dimensional heat spreading to the external heat sink. Materials usually being selected for the fabrication of vapour chamber include copper,
which has excellent thermal conductivity, aluminium with lightweight
and lower cost, and silicon, which can mitigate the coefficient of thermal expansion mismatch between the semiconductor die and the heat
sink.
Like the heat pipe, the wick structure of the vapour chamber also
plays a significant role in the internal fluid heat and mass transfer process, affecting the overall cooling performance. Peng and Liu et al.
[241,242] based on the biomimetic method, proposed a leaf-vein-like
fractal network for the wick of the vapour chamber. The results showed
that this kind of leaf-vein system could make the thermal resistance
smaller and have a favourable ability of temperature uniformity at the
same time. And also, there are studies showing that fabricating the
tree-shape and leaf venation groove on the evaporator surface could
bring a similar heat transfer enhancement effect [243,244]. Chen et al.
[245] combined the micropillar and copper mesh wick structure to propose a novel ultra-thin vapour chamber (UTVC), as shown in Fig. 21(a–
c). The results have shown that this kind of UTVC can have 30 times
stronger thermal conductivity than pure copper, and the change of maximum heat transfer performance would be less than 11% under reverse
gravity. Li et al. [246] experimentally studied the effect of copper powder and foam wick on the thermal performance of the vapour chamber, which can be seen in Fig. 21(d–h). They pointed out that the copper foam improved temperature uniformity, but the copper powder was
more effective in reducing thermal resistance. Velardo et al. [247] proposed a kind of hybrid wick structures, screen mesh and sintered powder combined wicks, which are better than the copper heat sink significantly. The porous sintering wick sheet has also been proved to improve the heat transfer performance [248]. Meanwhile, the thickness of
wicks can affect the temperature rise of the vapour chamber. It’s best
to reduce their thickness as much as possible and avoid affecting the
strength [249]. Wang et al. [250] revealed that the appropriate configuration of the vapour chamber evaporator and condenser wick porosity should make the maximum pressure drop in the wick close to but
not exceed the maximum capillary pressure, then the better cooling effect could be obtained. They also pointed that make the wick porosity of the evaporator a bit larger than the condenser can promote the
circulating flow of liquid. Patankar et al. [251] conducted notable research on the condenser-side wick structure for improving temperature
uniformity. The biporous wick structure they proposed could make the
peak-to-mean temperature of the vapour chamber surface decrease up
to 37%. The mesh wick structure also can relieve the fluctuation of wall
temperature [244].
Besides, other structural parameters of the vapour chamber also
could affect the cooling capability. As shown in Fig. 22(a–d), Chen
et al. [252,253] designed to combine the vapour chamber with the direct bonded copper (DBC) through the Sn–Pb material. They found that
the total thermal resistance could reduce by 41.6% when used vapour
chamber module compared with the copper plate. The thermal stress
decreased by 20%, and the minimum failure cycle lifetime could extend
by 9%. Velardo et al. [254] based on the experimental and numerical
studies, pointed that the effective thermal conductivity of the vapour
changed from 1900 to 2400 W/m•K when the heat source size change
from 20 × 20 mm to 35 × 35 mm, respectively. It means that the heat
source size can influence the thermal performance of the vapour chamber owing to the discrepancy of the phase change and transfer process.
Huang et al. [255] revealed that the synergy of ensuring the structural
support effect and reducing vapour pressure drop could be achieved
when the spacing and diameter of the support columns decrease. They
also pointed that for the UTVC, the spaced structure was better compared with the layered structure. Meanwhile, the effect of surface wettability on the vapour chamber evaporator and condenser was also be
studied. Yang et al. [256] combined wettability patterned surface with
the nanomesh wick structure of the evaporator for the UTVC, and the
result in their study showed that this kind of hybrid structure improved
the effective thermal conductivity by 210.7% and provided better temperature distribution. Koukoravas et al. [257] designed a kind of hybrid
wettability surface to replace the metal wick of the condenser for enhancing condensation performance. The results showed that the smallest thermal resistance could be obtained when 65% of the condenser
surface was superhydrophilic. In addition, the method of vapour compression refrigeration cooling has also been a good choice for thermal
management owing to its good thermodynamic efficiencies. The miniaturization of the vapour compression refrigeration and explore the optimizing parameters to meet the needs of electronic devices has attracted
the attention of researchers nowadays. Yee et al. [258] proposed design
guidelines for mesoscale cooling of the vapour compression refrigeration, and their assembly can achieve a 65% increase in cooling capacity.
Poachaiyapoom et al. [259] pointed out that both the heating surface
temperature and coefficient of performance would decrease with the increase of compressor speed. For more information about the compact
vapour compression refrigeration systems could refer to the review paper by Barbosa Jr et al. [260].
It is difficult to fully simulate the working process of the vapour
chamber, owing to its internal complex physical processes like heat
transfer with phase change and multiphase mass flow. However, many
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Fig. 21. (a) Cross-section view of UTVC with micropillar and copper mesh wick; (b and c) SEM image of liquid and vapour channel of UTVC [245]; (d–g) SEM image
of copper powder with size of 265 ± 85, 143 ± 37, 90 ± 15, and 66 ± 9 𝜇m, respectively; (h) SEM image of the copper foam [246].
Fig. 22. (a) Schematic diagram of the integrated circuit packaging; (b) vapour chamber working process with IGBT heated; (c) IGBT prototype integrated with
vapour chamber; (d) the temperature distribution in IGBT under the condition of chip power loss and diode power loss [253,261].
studies still focus on proposing the simplified heat and mass transfer
model of vapour chamber due to the great convenience of numerical
simulation. Liu et al. [262] proposed a reduced-order model to describe
the influence of heat flux and liquid charge on the vapour chamber thermal performance, which guides the mini type vapour chamber design.
Patankar et al. [263–265] developed the transient thermal performance
operation model of the vapour chamber. The model was targeted to investigate transient thermal performance and optimization of the vapour
chamber. It also can be used to select the structural parameters or even
the working liquid of the vapour chamber under certain specific conditions. Naturally, the working liquid characteristics also have an important impact on the thermal performance of the vapour chamber. When
the working liquid changed from the DI water to the ethanol, the heat
transfer and temperature uniformity could be better [242]. Kim et al.
[266] found that using the nanofluid as the working fluid in the UTVC
made the thermal resistance reduce by 56% compared with the DI-water
and obtained better temperature uniformity. As for the filling ratio of
working liquid, many studies have shown that the thermal resistance
first decreased and then increased with the filling rate, so an optimal
filling ratio value exists for the vapour chamber[244,267,268].
In short, the vapour chamber has been successfully applied in the
thermal management of electronic devices. However, some issues also
limit the better application of vapour chambers. For the vapour chamber combined with the integrated circuit, the packaging process and
the precision of the filling ratio is difficult to control, which is essential
for the reliability and working performance of the vapour chamber. Besides, the start-up issue during the use of the vapour chamber also needs
to be handled carefully [247]. Simultaneously, like the heat pipe, the
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Fig. 23. Schematic diagram of the TE module based on (a) Seebeck effect; (b) Peltier effect [272].
optimization of wicking structure and UTVC design to provide higher
capillary and heat transfer capability would still need to identify more
methods to meet the challenge of miniaturized electronics.
power collected by TEGs is enough for TECs and can ensure the maximum temperature of the surface hotspot would not exceed the threshold.
Similarly, Lin et al. [276] proposed a novel designed self-driven TEGTEC system, which could improve the cooling capability and maximum
drop of temperature by 75% and 76.8%, respectively. In addition, other
kinds of ways to apply the TE module on electronic thermal management
are also constantly being proposed. Lee et al. [277] display a dynamic
thermal management program based on TECs, controlled adaptively according to online information. After applying this system to the mobile
device, the results showed that the processor speed loss would reduce
from 19.2% to 1.8%. Mathew et al. [278] realize the integration of thermoelectric cooling on the dynamic random access memories (DRAMs),
which can control the temperature below 85 °C. Li et al. [279] compared
the high-power LEDs working with and without the TE cooling module,
as shown in Fig. 24(a–b), and the results have shown that the TE cooling
module could make the junction temperature of the LEDs reduce by 17
°C. Cai et al. [280] also pointed that it is necessary to formulate the cooperative operation strategy of thermoelectric active cooling (TAC) and
thermoelectric self-cooling (TSC) to realize maximum energy efficiency
and minimize entropy production. Zhang et al. [281,282] proposed a
solution for optimizing and evaluating the TE cooling system. It is a
concise and simple method to explore the relationship between the junction temperature or cooling power with the electrical current and realize
thermal enhancement. Barrubeeah et al. [283] proposed an analytical
model, which could guide optimization to improve the cooling capacity
by 70%. Moreover, to achieve effective thermal management for flexible
electronic devices is a big challenge;and the poor flexibility is the main
bottleneck for the TE cooling method applying to wearable devices. Recently, Hou et al. [284] studied on Bi0.5 Sb1.5 Te3 /epoxy thick films, and
proposed a flexible device of TE cooling, as shown in Fig. 24(c). They
pointed out that this TE module could improve the stable temperature
difference by 24% under the same applied current compared with other
groups, which shows a way to improve the cooling performance of the
flexible TE module. Kattan et al. [285] based on a thin-film TE module, proposed a kind of on-demand cooling solution for the mobile chip.
They pointed that this solution could reduce the average temperature
by 10 °C, and the energy collected by the TE module can supplement
89% of the cooling cost.
Simultaneously, the TE module could combine with other kinds of
heat sink like pin-fin, heat pipe, vapour chamber, and microchannel to
improve thermal management performance. Siddique et al. [286] pro-
3.4. Thermoelectric cooling
The thermoelectric (TE) materials could use temperature difference
(heat transfer) to realize the conversion of thermal energy and electrical
energy was found by Thomas Johann Seebeck in 1921 [269], which is
usually called the Seebeck effect, as shown in Fig. 23(a). After that, a
TE module can transfer heat based on the Peltier effects discovered by
Jean Charles Athanase Peltier in 1834 [270]. The Peltier effect means a
temperature difference is produced through the electrical current flowing between two electrical connection points, as shown in Fig. 23(b).
Then, the flat plate at one side of the thermoelectric device will absorb
heat (cold side), and the other side will generate heat (hot side). The
TE module cooling has the advantages of no pollutant emission, noisefree, favourable reliability, heat transfer performance not affected by
gravity. Therefore, the TE module also is widely used in the thermal
management of electronic devices [271].
Many methods have been carried out for improving the heat transfer
or cooling capability of the TE module. Firstly, the TE module material
significantly affects thermal performance. A study pointed that only relying on the existing known TE materials cannot compete with passive
cooling. Hence, the new material needs to be discovered [273]. Mao
et al. [274] pointed that it is essential to find the material with higher
ZT, which means the thermoelectric materials figure of merit and the
equation is shown in Eq. (2), where 𝜌 is the electrical resistivity, S is the
Seebeck coefficient, T is the temperature, and k is the thermal conductivity. They also concluded that the materials of n-type Mg3 Bi2−x Sbx ,
n-type Ag2 Se and p-type CsBi4 Te6 promise research directions to improve the thermal performance of the TE module [274].
𝑍𝑇 =
𝑆2𝑇
𝜌𝑘
(2)
As mentioned above, the hotspot issue is crucial in the thermal management of electronic devices. sabre et al. [275] based on the “sustainable self-cooling framework”, which mean installed the thermoelectric
generators (TEGs) on the cold area of the integrated circuit and the
thermoelectric cooler (TECs) would be fabricated on the hotspot region
to achieve the effective cooling. The results showed that the electronic
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Fig. 24. Schematic diagram of the high-power LEDs (a) without TE module; (b) combined with the TE module [279]; (c) a prototype flexible TE cooling device
based on Bi0.5 Sb1.5 Te3 /epoxy thick film [284].
posed to combine the TE module with the liquid cooling system. The
results showed that this kind of system could reduce the temperature
of the hot area by 4 °C and had a lower cost to improve economic efficiency. Belarbi et al. [287] found that the cooling performance for
CPU could improve by 15% after the air-jet impingement cooling combined with thermoelectric cooling. Sun et al. [288] proposed a coupled
TE module with the gravity-assisted heat pipe for electronic equipment
cooling. The experimental results showed that compared with typical
air-cooling, combined with heat pipe could reduce the electricity consumption and enhance the cooling capability by 39.3% and 64.8%, respectively. Lin et al. [289] designed the TE module combined with a
microchannel heat sink filled with TiO2 nanofluid, which could cool the
LED substrate below 53.1 °C under the 65 °C ambient temperature and
show feasibility in a high-temperature environment. Huang and Li et al.
[290–292] proposed a novel kind of concentric cylindrical TEG system
different from the typical square TEG system and combined it with the
heat pipe. The experimental and numerical results show that this kind
of combination could improve the heat transfer in the radial direction
and working performance of TEG, which can enhance the power output
of the TEG owing to the generation of electric power strongly relays on
the heat transfer improvement.
In all, TE cooling as one of the effective cooling methods is regarded
as a promising thermal management technology for electronic devices.
At present, improving the performance of TE materials, such as finding the new kind of material inexpensive and with higher ZT, is still
the main challenge. Besides, improving the efficiency of the TE cooling
system also relies on the optimal design of the cooperative application
of TE modules, which is worth research based on the different kinds of
electronic devices.
mal performance. Yang et al. [295] proposed a new kind of PCM, the
low melting point metal (LMPM), which provide high thermal conductivity and latent heat characteristics. Krishna et al. [296] added Al2 O3
nanoparticles to the Tricosane. The results showed that this kind of
nano-enhanced PCM could improve the thermal conductivity by 32%.
Through the melting impregnation method, Feng et al. [305] fabricated
a novel polyethylene glycol (PEG)/mesoporous carbon FDU-15 composites PCM, which improved the thermal conductivity by 60% when compared with PEG. Farzanehnia et al. [297] thoroughly added multiwall
carbon nanotubes (MWCNTs) to the Paraffin wax obtained nano-PCM,
which could decrease cooling time by 6% compared with PCM without MWCNTs and reduce the peak temperature of the integrated circuit.
Arshad et al. [298–303] also proposed adding a series of nanoparticles
and composite materials into the PCM to enhance thermal conductivity, melting time, heat storage capability, and heat transfer rate density.
Materials commonly used to improve the thermal performance of PCM
are shown in Fig. 25.
As mentioned above, PCM is usually combined with various kinds of
heat sinks to maximize its advantages, and the pin-fin heat sink is the
most versatile amongst them. The commonly seen pin-fin and its derivative structures combined with PCM are shown in Fig. 26(a–h). Ghanbarpour et al. [305] thorough numerical simulation that found the number
and height of pin-fins had a more noticeable effect on the cooling capability than thickness. Desai et al. [306] designed six kinds of pin-fin
structures for the PCM-based heat sinks, including rectangular, triangular, circular, and their pyramid-derived structures. The numerical results
showed that the increase of pin-fin number (from 9 to 100) reduced the
substrate temperature, and the pyramid structure didn’t provide better heat transfer t. They also pointed that the triangular geometry had
the best cooling capability under the same mass distribution. Bondareva
et al. [307] revealed that the increase of pin-fin width would induce
circulation flows between pin-fins, and the heat transfer rate would decrease. They also pointed that the total melting time of PCM decreased
with the elongation of the pin-fin. However, the study pointed out that
blindly raising the length of the pin-fin might reduce the heat energy
storage capability of PCM-based heat sink, so the parameters of the pinfin need to be studied and configured carefully [308]. Kalbasi et al.
[309] also revealed that the pin-fin structure could bring both positive
and negative effects on the PCM-based heat sink at the same time. The
positive effect is improving the thermal conductivity of the heat sink, but
the negative effect is the phase change enthalpy would also be sacrificed.
Therefore, the safe operation time of PCM-based heat sink is dependant
on the balance of positive and negative effects, which is consistent with
the opinion proposed by Mosavi et al. [310]. In addition to the typical
pin-fin structure, other kinds of enhanced heat transfer structures similar to pin-fin have also been proposed and optimized for PCM thermal
management. Righetti et al. [311] proposed a 3D periodic pyramidal cell
constructed by aluminium ligaments to combine with PCM. The experimental results showed that the ligaments with a greater number and less
thickness (more pyramidal cells) provided lower junction temperature
and better temperature uniformity. Based on the topological optimiza-
3.5. PCM-based heat sink
Phase change material (PCM) is a series of substances that can absorb
or release heat through a phase change process (usually a solid-liquid
phase change process) within a specific temperature range. As early as
1901, John A Kyle applied for a patent that proposed using palmitic acid
and stearic acid to achieve heat preservation for protecting tin sheets
during manufacturing [293]. Nowadays, the PCM has been widely used
in many fields [22,294], and it also has advantages in electronic thermal management. For example, for the electronic device with periodic
changes in heating power, PCM can absorb the heat when the power consumption is at peaks and release the heat when the power consumption
is at troughs, thereby ensuring the smooth operation of the electronic
equipment. Usually, the PCM would not be directly used for the integrated circuit or packaging but combine with other kinds of heat sink
like pin-fin, making the best use of PCM characteristics and realising
better thermal management performance.
As a medium for heat absorption, storage and release, the characteristic of the PCM have a significant effect on the cooling capability.
Therefore, lots of studies focus on the improvement of the PCM ther-
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Fig. 25. PCM ESEM image of (a) LMPM EBiInSn/silicone composites [304]; (b) TiO2/RT-35HC composites [302]; (c) Al2O3/RT-35HC composites [299]; (d) CuO/RT35HC composites [299]; (e) graphene nanoplatelets (GNPs)/RT-28HC composites [298]; (f) multiwall carbon nanotubes (MWCNTs)/RT-28HC composites [298]; (g)
graphene oxide (GO)/RT-35HC composites [303]; (f) reduced graphene oxide (rGO)/RH-35HC composites [303].
Fig. 26. (a–c) Circular, triangular, and rectangular pin-fins [313]; (d) pyramidal cell structure [311]; (e) tree-shape structure [312]; (f) star pin-fins [314]; (g) plate
pin-fin [305]; (h) honeycomb structure [315].
tion, Xie et al. [312] proposed a kind of tree shape structure, which has
advantages in generating more convection cells, inducing more vigorous
natural convection during PCM melting and improving the heat transfer
performance.
Metal foam has been proven to enhance heat transfer and has been
widely combined with PCM to improve cooling capability. Kothari et al.
[316] pointed out that the PCM combined with metal foam provided
a preferable cooling capability to pure PCM. Similarly, Yang et al.
[317] combined the PCM with the commonly used Cu foam, as shown
in Fig. 27(a–c). They indicated that the PCM-based heat sink decreased
the junction temperature by 10–20 °C compared to the Copper entity
heat sink. Recently, Qureshi et al. [318,319] proposed four kinds of
metallic foam units and numerical simulates its periodic structure as
shown in Fig. 27(d–j). The results indicated that compared with the tra-
ditional metallic foam unit Kelvin unit, under the same porosity level
and isothermal condition, the Gyroid, IWP, and Primitive unit could reduce the melting time by 31%, 40.3%, and 35.3%, respectively. So, these
three kinds of units could improve the average heat transfer coefficient
and cooling performance, and the IWP unit showed optimal temperature
uniformity.
In addition, the PCM also could combine with other cooling methods
such as the heat pipe, vapour chamber and TE module, which could get
a favourable heat dissipation effect [305]. Krishna et al. [296] found
that the heat pipe evaporator temperature could decrease by 25.75%
after being combined with PCM and save 53% of fan power. Behi et al.
[320] also noticed that the PCM could contribute 86.7% to the required
cooling performance for the PCM-heat pipe thermal management system
in their study. Simultaneously, the structure optimization of PCM-heat
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Fig. 27. SEM image of (a) metallic foam with low volume fraction; (b) metallic foam with high volume fraction; (c) Cu metallic foam filled with PCM [317]. Unit
structure of (d–g) kelvin, gyroid, IWP, and primitive; (h) periodic foam structure; (i and j) liquid fraction at 30 and 90 s [318,319].
pipe heat sinks also be developed. Qu et al. [321] combined the paraffin
wax PCM with the multi-layers 2D and 3D oscillating heat pipes (OHPs).
The experimental results showed that the total time needed for melting
the PCM of the multi-layers 3D-OHPs was longer than the 2D-OHPs, so
the multi-layers 3D-OHPs was considered to have the better cooling capability. Both multi-layers 2D and 3D-OHPs showed better performance
for the solidification process compared with PCM used only. Besides,
the microchannel heat sink can also associate with the PCM, which effectively improves cooling capability. Ho et al. [322] combined nanoencapsulated phase change material (NEPCM) with water as a kind of
suspension applied into the microchannel. Their experimental results
showed that this kind of PCM-microchannel heat sink could enhance
the heat transfer performance by 70%. Meanwhile, they also found its
performance index decreased under the high Re due to increased viscosity and the reduction of sensible heat. Yan et al. [323] designed to put
the PCM outside the microchannel heat sink and showed that the PCM
layer on the top of the microchannel heat sink did not affect the thermal
resistance.
Finally, although PCM has been widely used in the thermal management of electronic devices, it still faces some issues that need to be
studied in-depth. The use of nanoparticles could improve the thermal
management performance, but the effect of concentration on heat transfer characteristics like latent heat is still not fully clear. More accurate
numerical models and analytical methods are also necessary for detailed
evaluatingPCM working process and performance under different conditions. With the increasing application of PCM to various fields, not only
more chemical or physical means are needed for improving the thermal
performance of PCM, but also the eco-friendly of PCM should be drawn
more attention.
impingement, and immersion coolings has a low thermal resistance owing to the cooling fluid can interact directly with the heating surface of
the electronic devices. The droplet electrowetting can enhance the cooling ability of hot spots because its capable of controlling the directional
movement of the liquid droplet to the target area. As for the indirect
cooling methods, microchannel cooling demonstrates efficient heat dissipation ability and provide better temperature uniformity by two-phase
heat transfer. Its compact and lightweight structure also makes it possible to use various electronic devices and even other fields. The heat pipe
and vapour chamber as kinds of heat spreaders have an excellent ability
to transport heat to the external heat sink, which provides more ideas
for the architecture of the thermal management systems of the electronic
device. Moreover, TE cooling has the advantages of noise-free and no
pollutant emission; thus, making it a highly competitive thermal management solution in several specific scenarios. Finally, due to the unique
properties of heat-absorbing, storing and releasing, PCM can be applied
together with other types of heat sinks and heat spreaders.
Based on the conducted study, to inspire the future development
of advanced thermal management technologies, the following could be
considered:
1) the improvement of structure designs will still play an essential role
in the thermal management of electronic devices. By improving the
core structure of thermal management system, such as the nozzle in
spray cooling, the system cooling capability can be improved effectively. Also, electronic devices are developing towards miniaturization, portability and flexibility nowadays, bringing more challenges
to thermal management solutions. Therefore, it is necessary to continue studying and designing novel thermal management structures
to adapt to the development of electronic devices.
2) the thermal conductivity of materials used for heat sink, heat spreaders, and TIM is critical tothermal management. It is necessary to
find suitable alternative materials or processing methods to improve
thermal conductivity. More breakthroughs are neccesary to find the
materials with higher ZT for TE cooling. In addition, the effects of
thermal stress and corrosion on device materials also need to be carefully considered.
3) whether it is direct cooling or indirect cooling, the surface properties,
such as wettability and roughness, significantly affect the flow and
heat transfer of the cooling fluid. Therefore, for different cooling
methods, clarifying the effect of different surface properties on their
cooling capability is of great significance to the design of thermal
management systems.
4. Conclusion and outlook
In this paper, the state-of-the-art and the latest progress of commonly
used methodologies and technologies applied for thermal management
of electronic devices have been critically reviewed. This covers the major methods of direct cooling and indirect cooling to provide a comprehensive overview of the available thermal management technologies
and possibly inspiring new methods. Their heat transfer performance,
optimization, and application are particularly highlighted. It can be concluded that with the increase of heat flux of electronic devices, compared
with air cooling, liquid cooling is more adaptable to the requirements
of efficient thermal management. Direct contact cooling like spray, jet
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4) the nanofluid with higher thermal conductivity has been used in
electronic cooling. However, it still faces the difficulties, such as
easy formation of deposits and blockages in channels and nozzles
after long-term operation; and sometimes it may lead to a damage
ofthermal management systems and even the electronic devices.
5) the phase change process enhance heat transfer process effectively
for liquid cooling, and it is usually used for the thermal management
of the high-power electronic device. It may also cause instabilities
like the pressure drop oscillations in the microchannel flow boiling,
which still need to find some effective methods to control or eliminate. The complete numerical simulation of two-phase flow process
is still a complex and challenging task, and more accurate and efficient numerical methods still need to be further explored.
6) the thermal management system usually consists of many components, and the optimization and new ways to reduce the overall thermal resistance and energy consumption are still required. To design
a thermal management system also needs to consider the issue of
eco-friendly and recyclable.
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgment
The authors acknowledge the financial support for this work from
European H2020-MSCA-RISE (778104) of ThermaSMART project, the
UK APC & Innovate UK-Funded Project 113167, and the doctoral degree
scholarship of China Scholarship Council (CSC) .
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Mr. Zhihao Zhang received B.S. and M.S. degrees from China
University of Petroleum in 2017 and 2020, respectively. He is
currently a PhD with Fluids & Thermal Engineering Research
Group in the Faculty of Engineering, University of Nottingham, UK. His research interests include thermal management
of electronic equipment and the heat and mass transfer mechanism of droplet evaporation processes.
Dr. Wang received his doctoral degree at Zhejiang University
in June 2017. He was the winner of the Second prize of Black
Sea international competition in 2018. He joined the University of Nottingham in 2018, where he works as a research
fellow, dedicating to developing advanced thermal management spray cooling technologies for high-speed electric motors. His main research interests are refrigeration, thermodynamics, heat transfer and thermal management technologies.
He has involved several projects funded by the national natural science foundation of China (NSFC), Innovate UK as key
members. He has contributed to more than 50 published papers in reputational international journals and conferences,
and two of these papers were indexed by ESI (1%). He also
was authorized 11 patents. He served as the secession chair
in UKHTC 2019 in Nottingham, and with editorships of Energy Engineering, Frontier in Energy Research journals, and
Journal of Robotics and mechanical Engineering.
Dr Yuying Yan, who obtained PhD at City, University of
London in 1996, is a professor of thermofluids engineering
in faculty of engineering at University of Nottingham, UK.
With more than 30-year experience, his research covers widely
range area of flow and heat transfer including heat transfer
enhancement, phase changes, surface wetting, and nature inspired solutions for energy efficiency, as well as energy storage and thermal management for power electronics and energy systems. He has supervised 36 PhD students, 10 postdoctoral fellows, and published over 400 refereed research papers. He is a senior editor for e-Prime: electrical, electronic &
energy; associate editor for Case Studies in Thermal Engineering, and honorary editorial board member for International
Journal of Heat and Mass Transfer, Nature group journal: Scientific Report, and Journal of Bionic Engineering, Automotive
Innovation, Thermal Science & Engineering Progress, and Energy Storage & Saving.
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