International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
Hydraulic and Thermal performance of
Microchannel
Shivani Desai#1, Dattatraya Subhedar*2
1, 2
Mechanical Engineering Department, CSPIT
Changa-388421, Gujarat, India.
Abstract— In the recent times, high power density trends and
temperature constraints integrated circuits have led to
conventional cooling techniques not being sufficient to meet the
thermal requirements. The ever increasing desire to overcome
this problem has led to worldwide interest in microchannel
cooling. Geometric configuration of microchannel plays a vital
role in heat transfer performance. Computational fluid dynamics
(CFD) provides a cost-effective and less time consuming output
for the analysis of microchannel. Hydraulic and thermal
behaviour of microchannel is studied. To optimize the heat
transfer rate in microchannel heat exchanger, simultaneous
effect of different parameters like size and shape of channel, fluid
properties, and flow property are considered. For the analysis of
effect of the different parameter on microchannel heat transfer
simulation is done on Fluent ANSYS 13. The results conclude
that the heat transfer through the channels can be increased by
decreasing the hydraulic diameter, increasing Reynolds number,
using conductive fluid. but at the penalty of increase in pressure
drop.
Keywords- computational fluid dynamics, microchannel, heat
transfer coefficient, pressure drop.
I. INTRODUCTION
Ever increasing energy demands, and concerns for space,
energy, and materials savings highlight the necessity for
miniaturized lightweight heat exchangers that can provide
high heat transfer. Compared with other conventional
counterparts, Microchannels have more beneficial features in
this respect. Microchannel in micro technology is a channel
with a hydraulic diameter below 1 mm.
Conventional Channels: Dh >3 mm
Minichannels: 3 mm ≥ Dh > 200 μm
Microchannels: 200 μm ≥ Dh > 10 μm
Transitional Channels: 10 μm ≥ Dh > 0.1 μm
Transitional Microchannels: 10 μm ≥ Dh > 1 μm
Transitional Nanochannels: 1 μm ≥ Dh > 0.1 μm
Molecular Nanochannels: 0.1 μm ≥ Dh [2].
Today’s electronic components are required to perform tasks
at a faster rate, and so high-powered integrated circuits have
been produced in order to meet this need. These high-speed
circuits are expected to generate heat fluxes that will cause the
circuit to exceed its allowable temperature. In order to solve
this problem, microchannel heat sinks were introduced in
1981 by Tuckerman and Pease [1].
As such, microchannels have significant potential for use as
heat exchanger components in typical thermal and energy
applications. These attributes render these heat sinks very
suitable for cooling devices such as high performance
microprocessors, laser diode arrays, radars, and high-energylaser mirrors.Microchannel reactor is a device in
which chemical reactions take place in a confinement with
typical lateral dimensions below 1 mm; the most typical form
of such confinement are microchannels [6]. Microchannels are
also used in this lab for drug delivery research.
In a channel the Nusselt number is constant for fully
developed flow for laminar flows. In the entrance region the
temperature and velocity profiles are developing and the
Nusselt number varies [4].
Reducing passage hydraulic diameters provides a larger
surface area per unit volume of heat exchangers for a given
mass flux of fluid flowing through the channel. In the fully
II Hydraulic performance of microchannel
developed laminar regime in microchannel, the local heat
transfer coefficient h varies with the channel size it is given by: The apparatus of pipe friction, shown in Fig. 1, is used for
k
experimental performance in which water taken from bottom
h  Nu
         Eq.1
tank is passed from aluminium pipe through pump to three nos.
Dh
Where k is the thermal conductivity of the fluid and Dh is the of U tube manometer used for pressure measurement. At
outlet water is collected in overhead tank.
hydraulic diameter of the channel.
The flow classification based on Knudsen number is used to
provide a classification scheme for microchannels,
minichannels and conventional channels. For this purpose, the
mean free paths of common gases such as oxygen, nitrogen
and hydrogen near one atmospheric pressure is considered [2].
Using this approach, Kandlikar and Grande (2002) proposed
the following classification scheme:
ISSN: 2231-5381
Table I Properties Of Water
Density
(kg/m3)
998.2
http://www.ijettjournal.org
Dynamic
viscosity
(kg/m s)
0.001003
Specific heat
(kJ/kg K)
Conductivity
(W/mK)
4.182
0.6
Page 1702
International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
This results the total pressure drop of 3413.39pa. By
comparing the both values of experiment and
simulationpercentage of error found is 1.37%.
Entrance effect
The Graetz number is defined as in Eq. (2) and is used as a
criterion for neglecting the entrance effects. Morini [5] stated
that the entrance effects on the average Nusselt number can be
neglected if the following inequality is satisfied: Gz < 10 The
Graetz number is defined as in Eq. 2 and is used as a criterion
for neglecting the entrance effects.
Gz 
For this case,
Fig. 1 Experimental setup
Where,
Pipe ID
= 17mm
Pipe OD
= 21mm
Length of section
= 1000mm
Tank volume
= 400 X 400 X 100=1.6e7mm3
Discharge
= Tank volume/time for filling tank
Density of mercury (Sm) = 13.6gm/mm3
Density of water (Sw)
= 1gm/mm3
Re . Pr d h ……………Eq. 2
L
Pr = 6.99
Reynolds number (Re) = 63.66
Graetz number ( Gz )= 7.56
Value of Graetz number is less than 10. Thus, the entrance
effects on the average Nusselt number can be neglected as
show in fig. 2.
D
(mm)
h1
(cm)
h2
(cm)
Hm
(cm)
Hf
(cm)
Time
(sec)
Reason for taking the entrance effect in account is the
channel’s effective length, especially when microchannel is
used for small are of heat transfer. If the flow is not fully
developed for the entire length for the given flow property ,
means of Graetz number is more than 10 there is no use of the
channel for hear t transfer. At that time we can vary the
diameter to get proper heat transfer.
17
27.5
29.90
2.403
30.28
31.3
Effect of diameter on pressure drop
Hf
= Hm (Sm-Sw)
Table II Experimental Results
Table IIII Calculations
Discharge
m3/sec
5.1118e-4
Velocity
m/sec
2.2521
Pressure drop
pa
3460.968
Effect of varing diameter on hydraudynmic performance of
microchannel is shown in fig. 3. The pressure drop increases
by reducing diameter for same reynolds number.
Simulation Results
With same input and same model condition of experimental
setup, having same boundary condition simulation is done
using ANSYS 13.0 for validation. Here in fig.2 velocity
contour is shown for the same simulation of experimental data.
Fig. 3 Pressure drop Vs Diameter at Re = 0.2
Poiseuille number for circular pipe is a constant value for
laminar flow, Po=f*Re is 16 as shown in fig. 4. There is no
change in it by changing the diameter of channel.
Fig. 2 velocity counter
ISSN: 2231-5381
http://www.ijettjournal.org
Page 1703
International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
Fig. 4 Poiseuille number Vs Diameter at Re = 0.2
Effect of flow property on pressure drop
Effect of varing reynolds number on hydraudynmic
performance of microchannel is shown in fig. 5. The pressure
drop increases by increasing reynolds number for same
diameter value.
Fig. 6 Experimental setup
Where,
Pipe ID
= 28mm
Pipe OD
= 32mm
Length of section = 400mm
Orifice diameter = 14mm
Table IIIV Properties of air at 315K
Dynamic
viscosity
(kg/m s)
19.2e-6
Density
(kg/m3)
1.1204
Fig. 5 Pressure drop Vs Reynolds number at D = 0.0002mm
Specific heat
(kJ/kg K)
Conductivity
(W/mK)
1007
0.027222
Table V shows the experimental values of temperature at
different point of the pipe section. Where, T2 to T5 are the
temperature of the thermocouple no. 2 to 5 from inlet to inner
surface of test pipe. Moreover, T1 andT6 is measured
temperature of air stream before and after test section.
Table V Experimental result
III Thermal performance of microchannel
Thermal performance is done with experimental behavior and
its simulation which is explained below.
Experimental setup
The apparatus shown in Fig. 7 consists of blower unit fitted
with the test pipe. The test section is surrounded by nichrome
bend heater. Four thermocouples are embedded on the test
section and two thermocouples are placed in the air-steam at
the entrance and exit of the test section to measure the air
temperature. Test pipe is connected to the delivery side of the
blower along with orifice to measure flow of air through the
pipe.it is to be noted that a part of total heat supplied is
utilized in heating the air. A temperature indicator with cold
junction compensation is provided to measure temperature of
pipe wall in the heat section at inlet and exit temperature, of
air flow through the section. Table I shows the properties of
dry air at 315 k.
ISSN: 2231-5381
T1(◦c)
T2(◦c)
T3(◦c)
T4(◦c)
T5(◦c)
T6(◦c)
42
58
62
63
58
46
Where, T2 to T5 are the temperature of the thermocouple no.
2 to 5 from inlet to inner surface of test pipe. Moreover, T1
andT6 is measured temperature of air stream before and after
test section.
Heat transfer rate (Qa) = m Cp (T6-T1) = 20W
Heat flux (Q)
= Qa / A
= 568.41 W/m2
Simulation Results
With same input and same model condition of experimental
setup, having same boundary condition simulation is done
using ANSYS 13.0 for validation. Here in fig.8 temperature
contour is shown for the same simulation of experimental data.
http://www.ijettjournal.org
Page 1704
International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
For this case,
Pr = 0.710
Reynolds number (Re) = 1500
Graetz number ( Gz )=0.532726
Value of Graetz number is less than 10.
Effect of diameter on heat transfer
For fully developed laminar flow the local heat transfer
coefficient is improved by changing its cross section shape, its
hydraulic diameter, by changing fluid etc.
Fig .9 shows the effect of reducing diameter on the heat
transfer along entire length of channel.
Fig. 7 Temperature contour
From the simulation, using ANSYS, the result are as shown in
Table VI.
Table VI Simulation result
T1(◦c)
T2(◦c)
T3(◦c)
T4(◦c)
T5(◦c)
T6(◦c)
42
56.59
59.13
60.32
61.93
46
CALCULATION
Average surface temperature (Ts) = (T2+T3+T4+T5) /4
Average fluid temperature (Tf) = (T1+T6) / 2
Average convective heat transfer (h) = Qa / ( AS (Ts- Tf) )
Fig. 9 Local Heat Transfer Coefficient through length Vs Diameter at,
Re = 1500
Where, As is surface area of channel
Effect of fluid on heat transfer
Results for experimental and simulation are compared as
shown in Table VII for the same modelling condition.
Conductivity of fluid also a parameter that affect the heat
transfer through channel. Here Fig. 10 shows the effects of
changing fluid on heat transfer through channel along length.
Higher the conductivity of fluid higher the heat transfer is in
the result.
Table VII Calculation
Results
Ts (◦c)
Tf (◦c)
H (w/ m2k)
Experimental
60.25
44
34.97
Simulation
59.49
44.013
36.72
Entrance effect
Fig. 10 Effect of changing fluid at Dh = 0.0002mm,Re = 1500
Effect of flow property on heat transfer
Change in flow property with same hydraulic diameter results
as shown in Fig.11. increase in reynolds number increases
heat transfer with penalty of increasr in pressure drop.
Fig. 8 Entrance temperature contour for D=0.2mm ,Re=1500
ISSN: 2231-5381
http://www.ijettjournal.org
Page 1705
International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
Fig. 11 Effect of chnging flow property at Dh = 0.0003mm
[2]
Satish G. Kandlikar, ―Microchannels And Minchannels– History,
Terminology, Classification and Current Research Needs‖ , First
International Conference on Microchannels and Minichannels April
24-25, 2003
[3]
Heat transfer and fluid Flow in minichannels and microchannels, Satish
G. Kandlikar, 2006 Elsevier Ltd.
[4]
P. Rosa, T.G. Karayiannis , M.W. Collins , ―Single-phase heat transfer
in microchannels: The importance o scaling effects‖ Applied Thermal
Engineering 29 (2009) 3447–3468
[5]
Satish G. Kandlikar , William J. Grande Evolution of ―Microchannel
Flow Passages . Thermohydraulic Performance and Fabrication
Technology‖, ASME International Mechanical Engineering Congress
& Exposition November 17-22, 2002
[6]
http://en.wikipedia.org/wiki/Microreactor
[7]
http://en.wikipedia.org/wiki/Graetz_number
Changing the cross section of the channel with same hydraulic
diameter results as shown in Fig.12. increases heat transfer
with penalty of increasr in pressure drop.
Fig. 12 Effect of chnging c/s of channel at Dh = 0.0002mm,Re = 1500
IV CONCLUSION
Heat transfer through the microchannel can be increased by
different ways like decreasing hydraulic diameter, using
conductive fluid, changing flow directions, increasing
Reynolds number, changing channel shape.
From the experiential and simulation results, for comparison
the outputs are almost same by the percentage of error by
6.2%.
Here Heat transfer through the channel is increasing by
increasing Reynolds number or by decreasing the hydraulic
diameter of the channel.
By using higher conductive fluid heat transfer is increased.
But in all matters heat transfer is increasing at the penalty of
higher pressure drop which can be reduced by decreasing the
length of the channel. It is the future scope of work to
optimize the length of the channel for same pressure drop and
high heat transfer.
REFERENCES
[1]
I. Hassan, P. Phutthavong, and M. Abdelgawad., ―Microchannel Heat
Sinks: An Overview of the State-of-the-Art‖, Microscale
Thermophysical Engineering, 8:183–205, 2004,pp183
ISSN: 2231-5381
http://www.ijettjournal.org
Page 1706