70kW Automobile Radiator for Light Duty
Passenger Vehicle
By
S.W.A.S.T. SAMARATHUNGA
Department of Mechanical Engineering
University of Moratuwa
Sri Lanka
18th May 2022
ABSTRACT
Internal combustion engine serves as the prime mover of automobile even after the centuries of
it’s first use in the cause. However, regulating the temperature of the engine by effectively
removing the excess heat generated in the combustion process is vital for the safe and sustainable
operation of the system. Therefore, Radiator, a special heat exchanger which has been developed
overtime for the above requirement is delivering a critical service to the function of an
automobile. In this report, designing a radiator with a capacity of 70kW, for a light duty
passenger vehicle is analysed.
INTRODUCTION
Automobile, first being invented as a motored substitute to the horse carriages for a transport
medium, subjected into a tremendous evolution over the past century. In it’s journey of pushing
the performance boundaries further and further, the cooling of internal combustion engine which
powered the automotive also played a major role. Engine cooling is not a factor to be overlooked
as it mainly affects the engine as the too high temperatures causes metal weakening in
overheated engine parts, viscosity breakdown of the lubricating oils and accelerated wear of
engine parts due to the build up stress between those parts ultimately resulting in knocking,
cylinder deformation and piston deformation. Thus, the air-cooled heat exchangers specifically
‘Radiators’ are being employed in every automobile cooling system. Automobile cooling system
basically consist of radiator, water or coolant pump, electric cooling fan, pipe system, radiator
pressure cap, and thermostat. Further, heater core is also included in the system to direct the
engine heat into the passenger cabin for cabin heating purpose.
To identify radiator’s function of rejecting out the accumulated heat from coolant to the outer
environment, let’s investigate the overall function of automotive cooling system. The coolant is
being circulated through the coolant channels made in engine body around the combustion
cylinders where the major heat generation occurs. When the temperature of the coolant rises to a
certain level, thermostat opens the path for coolant to flow through radiator, with the help of
circulating pump, operates with the help of crank shaft motion. Control of this thermostat is
being undertaken by the car’s ECU in modern vehicles. When the coolant flows through the
radiator pipes, the heat is effectively dissipated to the air flowing through the radiator fins due to
either the motion of the automobile, or the radiator fan, which is an inward draft axial fan,
located right behind the radiator. The pressure regulating valve on the top of the radiator is to let
the excess coolant flow into the reservoir bottle in a case of increased pressure of coolant inside
the radiator. The cooled coolant is then flowing back into the engine block to complete the cycle.
While this process continues, if the passenger compartment requires to be heated, required
amount of coolant is to be flown through a secondary heat exchanger, where the draft flowing
through the exchanger routed into the passenger compartment. Further,
some oil cooling
arrangements are also available in the overall cooling process but those can be overlooked for the
Figure1:Automobile Cooling system with the Primary and secondary working fluid flows
sake of simplicity of the context.
Radiators were hand-built individually until around 1910, when the factory-built radiators were
started to appear in automobile. In around 1940, the passenger compartment heaters were
integrated into the vehicles and in 50th decade, automobiles equipped with Air conditioning
systems started to appear in the automobile market. Further, advancements made in internal
combustion engine technologies in latter decades such as super chargers and turbo chargers
highlighted the need of effective cooling of the engine walls thus, the evolution of radiators
occurred as a result. A peek at this evolution journey is beneficial in understanding the major
changes to the radiator throughout the history.
Radiator Material
From early days, radiators made of Copper or Brass (made of brass and lined with copper) was
used in automobile. Given the high thermal capacity of copper and Brass, an alloy of copper,
these devices were reliable but at the same time, were bulky and weighed a lot. Therefore,
despite the reliability, with the advancements of automobile technology and prioritizing of
lighter weight, radiator manufacturing material moved away from copper and it’s alloys to
Aluminium, a rather lighter and better option. Today, aluminium radiators are dominating the
high-performance car radiator market where both core of the radiator and the coolant tanks are
made of lighter aluminium. Further, there are plastic radiators in the market where the tanks are
made of plastic while the core of the radiator still being made of an alloy or metal. However,
these plastic radiators are not as efficient as the aluminium ones and further, the build of such
plastic arrangement as a radiator demands the need of entire replacement if a problem arises in
the device. In Al made radiators, this can be easily tackled by replacing the problematic part
only, eliminating the need of entire radiator replacement. However, what is being utilized most
in modern, general use cars is a hybrid plastic_ aluminium structure, which produce the
advantages of both designs while trying to eliminate the adverse effects of each.
Tube Arrangement
Radiator being a passive device, rejects out whatever heat that comes to it through coolant. The
coolant temperature which is exiting through the outlet of the radiator depends on the radiator’s
capacity of heat rejection. If the capacity is less, the outlet coolant line may be still in a higher
temperature than needed, causing inefficient cooling in engine walls. If the capacity is redundant,
the coolant will be cooled to a temperature lower than acceptable or unnecessary. In earlier
times, most of the radiators were made from copper fins and brass tubes where flat tubes and
corrugates louver fin design was used. Later during World War II, Russian tankers were using
circular tubes with flat fin aluminium radiators, and this design was later popularized in Europe
for smaller engines. But, even today, Flat pipe design with Corrugated fins is being utilized using
Aluminium as the manufacturing material. Further, the early day’s radiators were downflow
radiators where the coolant tubes were vertical where coolant flows from top to bottom. But
later, the cross-flow radiators gained the popularity with its added advantages of having same
tank and header design despite the length of the coolant tube channels, thus enabling the ability
to adapt the design for more frontal area with lesser effort and etc. However, since 90’s the
downflow radiators are again gaining the popularity due to characteristics like lower coolant
pressure drop and pumping power, and flexibility in attachments.
In addition to above information, the coolant used in radiators is also have evolved, where a 5050 mix of water and ethylene glycol is used along with some additives to give the liquid aspired
properties of lower freezing point, higher thermal capacity and while avoiding adverse properties
like casing rust.
To ensure proper heat transfer from the coolant to the air, the coolant flow inside the radiator
tubing has to be turbulent where the mixing of the coolant is ensured. To achieve this, structures
names vortex generators are inserted in the radiator tubing. Usually for round tubes rathe simple
structures like coiled wires or balls are being used while for flat tubes, special structures are
available.
METHODOLOGY
Materials and dimensions of the radiator design were chosen according to the current standards
and conventions. Since the aspired design is a coolant cross flow one, the length of the radiator
tubes was considered the main variable, while number of tube rows and columns were finalized
after iterations of calculations for the appropriate length of the radiator
After the selection of flow rates of coolant pump and flow fans, which were again done
iteratively while taking the help of available literature, thermal properties of both coolant and air
sides were obtained using property tables.
Since the required capacity of the radiator is 70kW, required UA value was calculated using the
NTU method.
Mass flow rates are calculated from volume flow rates: 𝑚̇𝑎𝑖𝑟 , 𝑚̇𝑐𝑜𝑜𝑙𝑎𝑛𝑡
With the specific heat capacities 𝑐𝑎𝑖𝑟 and 𝑐𝑐𝑜𝑜𝑙𝑎𝑛𝑡 ,
𝐶𝑎𝑖𝑟 = 𝑚̇𝑎𝑖𝑟 x 𝑐𝑎𝑖𝑟 and 𝐶𝑐𝑜𝑜𝑙𝑎𝑛𝑡 = 𝑚̇𝑐𝑜𝑜𝑙𝑎𝑛𝑡 x 𝑐𝑐𝑜𝑜𝑙𝑎𝑛𝑡 were calculated.
Using those,
𝐶𝑟 =
𝐶𝑚𝑖𝑛
𝑞𝑚𝑎𝑥 = 𝐶𝑚𝑖𝑛 (𝑇ℎ𝑜𝑡,𝑖𝑛 − 𝑇𝑐𝑜𝑙𝑑,𝑖𝑛 ) thus, effectiveness(ε) =
,
𝐶𝑚𝑎𝑥
𝑞𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑
𝑞𝑚𝑎𝑥
Using the standard equation for effectiveness for an cross flow heat exchanger in NTU method,
NTU value can be found.
ε = {1 − 𝑒
(
0.78 )
1
𝑁𝑇𝑈 0.22 )(𝑒 (−𝐶𝑟 𝑁𝑇𝑈
−1)
𝐶𝑟
}
But,
NTU =
𝑈𝐴
𝐶𝑚𝑖𝑛
,
Therefore, required UA for the radiator is found.
Heat transfer Ratios for both liquid(Coolant) side and the air side is then calculated.
Liquid Side:
From the flow rate, flow velocity Vcoolant was found and Re number for the velocity is found.
Nusselt number is then found accordingly to a internal flow of square shaped tube and hence the
hcoolant was found.
Air Side:
Air side heat transfer coefficient was also found similarly, but the length of the radiator remains
as a variable in the equations here, as it is associates with the flow area concerned to find the Vair.
Here also, the flow nature was considered by the value of Reynold’s number and the Nusselt
number equation was chosen accordingly.
Since the utilization of fins in a radiator is quite significant the overall surface efficiency must be
calculated for the external area. For that, fin efficiency was calculated priorly.
Fin efficiency (𝜂𝑓𝑖𝑛 ) =
m = √𝐾
ℎ𝑎𝑖𝑟 𝑃𝑓𝑖𝑛
tanh(𝑚𝐿)
𝑚𝐿
and
𝐴𝑙 𝐴𝑓𝑖𝑛,𝑓𝑎𝑐𝑒
L = height of the fin (Pfin: perimeter of the fin)
Here, a fair assumption was made that the fins have adiabatic fin tips as given the fin positioning
in a radiator, where the two consecutive tubes share the same fin tip, literally composing a
insulated fin tip condition. Further, even though the fins possess a sinusoidal shape in realty, a
straight rectangular shape has assumed for the sake of simplicity in calculating.
Overall surface efficiency:
𝑁
𝜂0 = 1 − 𝐴 𝑓𝑖𝑛
𝐴𝑓𝑖𝑛
𝑓𝑖𝑛+𝑏𝑎𝑠𝑒
(1 − 𝜂𝑓𝑖𝑛 )
Nfin = Number of Fins
Afin = Surface area of the fin face
Afin+base = (Nfin x Afin) + Abase
Abase = Aradiator – Afin face
Hence,
−1
1
1
)+ (
)]
𝑈𝐴 = [(
𝜂0 ℎ𝑎𝑖𝑟 𝐴𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙
ℎ𝑐𝑜𝑜𝑙𝑎𝑛𝑡 𝐴𝑖𝑛𝑡𝑒𝑟𝑛𝑎𝑙
Aexternal = Afin+base
Ainternal = Inner area of the tubes
Since the required UA value is known from the NTU method, the only variable remaining, the
length of the radiator tubes can be obtained.
However, to get a practically viable length, few iterations while changing the other parameters
had to be done.
CALCULATION
Given the capacity of the radiator(70kW), an all-Aluminium design was chosen adopting the
modern performance car radiators where the fins, tubing and the casing would be made of
Aluminium with cross coolant flow design.
1.8mm
19.6mm
Radiator tube specifications: Thickness - 0.18mm
10mm
6mm
Radiator Fin Specifications: Thickness - 0.1mm
All the specifications were considered adhering to the currently used industrial standards and
conventions, to ensure the validity of the designs.
Number of Tube lines = 240; Rows – 80, Columns - 3
Thus,
Radiator Height
-
630mm
Radiator Thickness
-
68.8mm
Generally available specifications:
Tube Area (As)
:
35.3 x 10-6 m2
Tube Hydraulic Diameter (Dh)
:
4As/P = 3.3 x 10-3 mm
Inlet Hose Diameter
:
31.75mm (1.25”)
Fan Diameter
:
0.3048m
Number of Fans Used
:
2
Coolant liquid Side:
Coolant flowing the in the radiator tube is considered as the 50% ethylene glycol which is the
generally used radiator coolant.
Flow rate of the pump
:
60 Litres per Minute (1x10-3 m3s-1)
[Obtained from similar literature]
Inlet Temp.
:
950C
Outlet Temp.
:
250C
⸫ Average Temperature
:
600C
For the obtained Tavg thermal properties of 50% ethylene glycol was obtained referring available
thermal property tables
For T = 600C,
Density(ρ)
:
1031.114kgm-3
Dynamic Viscosity
:
0.001198 kg/m.s
Specific Heat Capacity
:
3.386 kJ/kg.K
Thermal Conductivity
:
0.458 W/m.K
Prandtl Number
:
8.8596
Air Side:
Flow rate of the selected fans :
4000 (2000x2) CFM (1.89 m3/s)
[Obtained from data sheets]
:
350C
Density(ρ)
:
1.146kgm-3
Dynamic Viscosity
:
1.891 x 10--5 kg/m.s
Specific Heat Capacity
:
1.0067 kJ/kg.K
Thermal Conductivity
:
0.0267 W/m.K
Prandtl Number
:
0.7129
Ambient Air Temperature
For T = 350C,
Air Temperature for the properties were taken at a slightly higher value than the ambient
temperature to compensate the errors.
𝑚̇𝑎𝑖𝑟 = 1.146𝑘𝑔𝑚̇ −3 × 1.89𝑚3 𝑠 −1 = 2.17𝑘𝑔𝑠 −1
𝐶𝑎𝑖𝑟 = 𝑚̇𝑎𝑖𝑟 × 𝑐𝑎𝑖𝑟
= 2.17𝑘𝑔𝑠 −1 × 1.0067𝑘𝐽/𝑘𝑔𝐾
= 2.185𝑘𝐽/𝑠𝐾
̇ −3 × (1𝑒 − 3)𝑚3 𝑠 −1 = 1.03𝑘𝑔𝑠 −1
𝑚̇𝑐𝑜𝑜𝑙𝑎𝑛𝑡 = 1031.114𝑘𝑔𝑚
𝐶𝑐𝑜𝑜𝑙𝑎𝑛𝑡 = 𝑚̇𝑐𝑜𝑜𝑙𝑎𝑛𝑡 × 𝑐𝑐𝑜𝑜𝑙𝑎𝑛𝑡
= 1.03𝑘𝑔𝑠 −1 × 3.386𝑘𝐽/𝑘𝑔𝐾
= 3.49𝑘𝐽/𝑠𝐾
∴ 𝐶𝑚𝑖𝑛 = 2.19𝑘𝐽/𝑠𝐾
𝐶𝑟 = 0.63
𝑞𝑚𝑎𝑥 = 2.19(95 − 25) = 153.3𝑘𝑊
𝜀=
ε = {1 − 𝑒
(
70
= 0.46
153.3
0.78 )
1
𝑁𝑇𝑈 0.22 )(𝑒 (−𝐶𝑟𝑁𝑇𝑈
−1)
𝐶𝑟
1
0.46 = {1 − 𝑒 (0.63𝑁𝑇𝑈
}
0.22 )(𝑒 (−0.63𝑁𝑇𝑈0.78 ) −1)
}
𝑈𝐴
NTU = 0.79 = 2.19
∴ 𝑈𝐴𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 1.73𝑘𝑊
Heat Transfer Coefficients
Since Volume flow rate of coolant, 𝑄̇ = 1 x 10-3 m3/s, taking the effective cross-sectional area of
the all the pipes, the coolant flow velocity inside the radiator pipes is found.
𝑉𝑐𝑜𝑜𝑙𝑎𝑛𝑡
1 × 10−3
=
= 0.12𝑚𝑠 −1
35.3 × 10−6 × 240
𝑅𝑒 =
𝜌𝑉𝐷
𝜇
=
1021.114 ×0.12×3.3×10−3
0.001198
= 340.27
340.27 < 3000
Even though the Re number falls in the laminar range, with the presence of turbulators inside the
tubes, the turbulence is induced in real situation. But since turbulators has been omitted in the
design, flow is considered to be laminar.
Nussult number for a internal laminar flow through a rectangular tube with similar aspect ratio
lies at a constant value of 5.6
hcoolant =
𝑘 𝑁𝑢
𝐷
=
0.458 × 5.6
3.3 × 10−3
𝑉𝑎𝑖𝑟 =
= 777.2 W/m2K
𝑄𝑎𝑖𝑟
𝐴𝑟𝑎𝑑𝑖𝑎𝑡𝑜𝑟 − 𝐴𝑡𝑢𝑏𝑒𝑠
1.89𝑚3 𝑠 −1
=
(0.63 × 𝐿) − (((1.8 + 2 × 0.18) × 10−3 ) × 𝐿 × 80)
=
4.1
𝐿
Reynolds number is calculated assuming the flat tubes as flat plates
𝑅𝑒 =
𝜌𝑉𝐷
𝜇
=
4.1
×19.6×10−3
𝐿
1.891×10−5
1.146 ×
=
4870
𝐿
Assuming L < 1m, flow is considered to be laminar
Nusselt Number (Nu) =
ℎ 𝐷𝑢
𝑘
= 0.664 x Re0.5 x Pr0.33
= 0.664 x (4870/𝐿)0.8 0.7130.33 = 41.4/√𝐿
Therefore, convective heat transfer coefficient, hair =
𝑘 𝑁𝑢
𝐷
=
0.0267 × 41.4/√𝐿
19.6 × 10−3
= 55.8/√𝐿 W/m2K
To get the U for the finned surface, let’s consider the geometry of the radiator.
630mm
For the simplicity in calculation, fins are to be considered as flat straight plates with width of
19.6mm same as the tube widths and thickness of 0.01mm, rather than sinusoidal ones.
𝑚=
ℎ𝑎𝑖𝑟 × 𝑃
2ℎ𝑎𝑖𝑟
≈
𝐾𝐴𝑙 × 𝐴𝑓𝑖𝑛
𝐾𝐴𝑙 × 𝐻𝑓𝑖𝑛
𝑚=
70.35
4
√𝐿
KAl = 237W/m.k
4
Assuming √𝐿 = 1, 𝑚 = 70.35
𝜂𝑓𝑖𝑛 =
tanh(0.422)
= 0.945
0.422
Number of fins per unit length = 800 fins/m
Therefore, total number fins = 800 x L x (80 x 3) = 192,000 x L
Afin = 2 x 19.6mm x 6mm = 2.352x 10-4m2
AB = 2 Lradiator Wtube N – HfinWfinNfin =
Afin+base = NfinAfin + AB
= (49.45 x L) m2
NfinAfin = (45.12 x L) m2
𝜂0 = 1 − [
45.12
(1 − 0,945)]
49.45
𝜂0 = 0.95
For to obtain the UA value,
−1
1
1
𝑈𝐴 = [
+
]
𝜂0 ℎ𝑎𝑖𝑟 𝐴𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙 ℎ𝑐𝑜𝑜𝑙𝑎𝑛𝑡 𝐴𝑖𝑛𝑡𝑒𝑟𝑛𝑎𝑙
Aexternal = Afin base x Ntube = (49.45 x L) m2
Ainternal = (2Wtube +2Htube)LradiatorNtube = (10.27 x L) m2
1
−1
1
1730 = [
+
]
777.2 × 10.27𝐿
0.954 × 55.8/√𝐿 × 49.45𝐿
Lradiator = 0.95m
3D MODEL AND COMPUTATIONAL FLUID DYNAMICS ANALYSIS
A 3d model with obtained dimensions was developed using SOLIDWORKSTM and simulated in
using Ansys FluentTM.
However, due to extreme complexity in the geometry from the possession of very thin fins, the
computational power required for the entire model was not available. Further, the need of
simulating the entire model seemed bit redundant in the context of the requirement. Thus, a slice
of the radiator, comprising one tube with its fins on each side was modelled and used in
simulating software. The major drawback of this simplification is the missing out of effect of
parallelly placed tubes affecting each other’s air flow and air temperature. However, provided
the large air volume swept through the radiator fins as well as the relatively smaller radiator
thickness, the mentioned effect can assume to be negligible.
A major step to reduce the effect was designing the fluid domain, slightly omitting the tips of the
fins, such a way that it the prevents the convective heat absorption from the air domain to the
fins through the irrelevant fin tips.
Simplified model of one tube to be analyzed
Omitting of fin tips to avoid heat transfer error
Using the obtained CFD Results, the calculated results can be further analyzed. In this regard,
results of the simplified model can be multiplied accordingly.
REFERENCES
[1] Okhiria, Paul & Towoju, Olumide. (2021). Design and Analysis of an Automobile Radiator with a
14ºC Cooling Capacity. 4. 1147-1154.
[2] Ruchit Doshi, Shakshi Himmatramka, Janam Sanghavi, Jahnavi Patel, Vinit Katira.(2018).
Automobile Radiator Design and Validation. 2395-0072
[3] Y. Cengel, Heat and mass transfer. Boston, Mass.: McGraw-Hill, 2014.
[2]"Different Types of Car Radiators - Buy Radiator Online", Buy Radiator Online, 2022. [Online].
Available: https://www.buyradiatoronline.com/ourblog/different-types-of-car-radiator.
[3]"What Is Car Engine Coolant?", UTI Corporate, 2022. [Online]. Available:
https://www.uti.edu/blog/automotive/carcoolant#:~:text=All%20automotive%20coolants%20are%20glycol,propylene%20glycol%20is%20less
%20toxic.
[4] Ramesh J. Ladumor, Prof. V. Y Gajjar, Prof. K.K.Araniya. “A Review Paper on Analysis of
Automobile Radiator”
[5] Hardikkumar B. Patel, Mr. Deepu Dinesan.(2014) “Optimization and Performance Analysis of An
Automobile Radiator Using CFD - A Review”
[6]C. Oliet, A. Oliva, J. Castro and C. Pérez-Segarra, "Parametric studies on automotive
radiators", Applied Thermal Engineering, vol. 27, no. 11-12, pp. 2033-2043, 2007. Available:
10.1016/j.applthermaleng.2006.12.006.
[7] Shah, R. K. (2003). “Advances in Automotive Heat Exchanger Technology. SAE Transactions”, 112, 631–
641. http://www.jstor.org/stable/4474543
[8]"Tubes with inserted turbulators", Hydro.com, 2022. [Online]. Available:
https://www.hydro.com/en/aluminium/products/welded-tubes/tubes-with-inserted-turbulators/.
[9]K. Allen, "Turbulators in Heat Exchangers: Types and Purposes | The Super
Blog", Superradiatorcoils.com, 2022. [Online]. Available:
https://www.superradiatorcoils.com/blog/heat-exchanger-turbulators-types-andpurposes#:~:text=Ball%20turbulators%20are%20true%20to,increasing%20velocity%20by%20reducin
g%20volume.
[10]2022. [Online]. Available:
https://www.yonaka.com/Acura_Integra_1994_to_2001_Race_Radiator_w_Fans_p/ymar027.htm.
[11]S. Ahmed, M. Ozkaymak, A. Sözen, T. Menlik and A. Fahed, "Improving car radiator performance
by using TiO2-water nanofluid", 2022.