Water and ethylene glycol as conventional coolants have been widely used in an automotive car radiator for many years. These heat transfer fluids offer low thermal conductivity. With the advancement of nanotechnology, new generation of heat transfer fluids called "nanofluids" have been developed and researchers found that these fluids offer higher thermal conductivity compared to that of conventional coolants. This study focused on the application of ethylene glycol based copper nanofluids in an automotive cooling system. Relevant input data, nanofluid properties and empirical correlations were obtained from literatures to investigate the heat transfer enhancement of an automotive car radiator operated with nanofluid based coolants. It was observed that, overall heat transfer coefficient and heat transfer rate were increased with the usage of nanofluids in engine cooling system compared to those of ethylene glycol (i.e. basefluid). It was later identified that about 3.8% of heat transfer enhancement could be achieved with addition of 2% copper particles with basefluid at Reynold number of 6000 and 5000 for air and coolant, respectively. In addition, reduction of air frontal area was estimated. It has been found that heat transfer rate increased with the increase of volume fraction of copper particles in a basefluid.
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Keywords: Nanofluids; Automotive radiator; Heat transfer enhancement.
A Total heat transfer area, m2
Af/A Fin area on one side/total area
C Heat capacity rate, W/K
Cp Specific heat J/kgK
C* Cp, min/Cp, max
Dh Hydraulic diameter, m
DP Pressure drop, Pa
EG Ethylene glycol
f Fanning friction factor, dimensionless
G Mass velocity, kg/m2s
H Total water flow length, m
h Heat transfer coefficient, W/mK
j Colburn factor, dimensionless
k Thermal conductivity, W/mK
L Fin length for heat conduction from primary to midpoint between plates for symmetric heating, m
NTU Number of heat transfer units
Nu Nusselt number
P Pumping power
Pr Prandtl number
Re Reynolds number
t Fin thickness, m
T Temperature, K
U Overall heat transfer coefficient W/m2K
W Mass flow rate
Ïƒ Minimum free flow area/frontal area
Fin efficiency of plate fin
Total surface temperature effectiveness
m Dynamic viscosity, Ns/m2
Volumetric flow rate, m3/s
Ï Density, kg/m3
Ïˆ Volume fraction of particles
e Heat exchanger effectiveness
a Total one side of heat transfer area/ total volume
fr frontal area
f fluid (basefluid)
Continuous technological development in automotive industries has increased the demand for high efficiency engines. A high efficiency engine is not only based on its performance but also for better fuel economy and less emission. Reducing a vehicle weight by optimizing design and size of a radiator is a necessity for making the world green. However, traditional approach of increasing the cooling rate by using fins and micro channel have already reached to their limit . In addition, heat transfer fluids, such as water and ethylene glycol exhibit very low thermal conductivity. As a result there is a need for new and innovative heat transfer fluids for improving heat transfer rate in an automotive car radiator.
Nanofluids seem to be potential replacement of conventional coolants in engine cooling system. Recently there have been considerable research findings highlighting superior heat transfer performances of nanofluids. Yu et al.,  reported that about 15-40% of heat transfer enhancement can be achieved by using various types of nanofluids. With these superior characteristics, the size and weight of an automotive car radiator can be reduced without affecting its heat transfer performance. This translates into a better aerodynamic feature for design of an automotive car frontal area. Coefficient of drag can be minimized and fuel consumption efficiency can be improved.
Therefore, this study attempts to investigate the heat transfer characteristics of an automotive car radiator using ethylene glycol based copper nanofluids as coolant. Thermal performances of an automotive car radiator operated with nanofluids are compared with a radiator using conventional coolants. The effect of volume fraction of the copper nanoparticles with basefluids on the thermal performance and potential size reduction of a radiator were also carried out.
2. Nanofluids in enhancing thermal conductivity
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Eastman et al.  reported that thermal conductivity of ethylene glycol nanofluids containing 0.3% volume fraction of copper particles can be enhanced up to 40% compared to ethylene glycol basefluid. Hwang et al.  found that thermal conductivity of the nanofluids depends on the volume fraction of particles, thermal conductivity of basefluid and particles. Lee et al.  studied thermal conductivity of low volume concentration of alumina (Al2O3) aqueous nanofluids produced by two-step method. Authors conclude that, the thermal conductivity of aqueous nanofluids increases linearly with the addition of particles. Thermal conductivity of zinc dioxide -ethylene glycol (ZnO-EG) based nanofluids was investigated by Yu et al. and they observed about 26.5% enhancement of thermal conductivity when 5% volume fraction of zinc dioxide nanoparticles added in ethylene glycol. Present study concluded that size of nanoparticles and viscosity of the nanofluids played a vital role in determining the nanofluids thermal conductivity enhancement ratio.
Mintsa et al.  investigated the effect of temperature, particle size and volume fraction on thermal conductivity of copper oxide and alumina water based nanofluids. Authors suggested that thermal characteristics can be enhanced with volume fraction, temperature and particle size. Authors found that the smaller the particle size, the greater the effective thermal conductivity of the nanofluids at the same volume fraction. Contact surface area of particles and fluid and Brownian motion can be increased when smaller particles are used. This consequently increased thermal conductivity of nanofluids.
3. Nanofluids in enhancing forced convective heat transfer and relevant pressure drop
Namburu et al.  numerically analyzed turbulent flow and heat transfer to three types of nanofluids namely copper oxide (CuO), Alumina (Al2O3) and Silicon dioxide (SiO2) in ethylene glycol and water, flowing through a circular tube under constant heat flux. Results revealed that nanofluids containing smaller diameter of nanoparticles offer higher viscosity and Nusselt number. Nusselt numbers are also increased at higher particles volume fraction. It is observed that at a constant heat flux (50 W/cm2) with a constant Reynolds number (20000), heat transfer coefficient for 6% CuO nanofluid has increased 1.35 times over the base fluid. At the same particles volume fraction, CuO nanofluids provide higher heat transfer coefficient compared to other types of nanofluids.
Ding et al.  found that convective heat transfer coefficient of nanofluids has the highest value at the entrance length of a tube, starts decreasing with axial distances and eventually reaches to a constant value in a fully developed region. At a given flow and particle concentration, aqueous-based carbon nanofluids offer highest improvement. Zeinali, et al.  experimentally investigated convective heat transfer to laminar flow of Alumina-water (Al2O3/water) nanofluids inside a circular tube with constant wall temperature under different concentrations of nanoparticles. Results clearly indicated that heat transfer coefficient of nanofluid increases with nanoparticle concentration and greater than basefluid distilled water at a constant Peclet number. Authors found that experimental results were much higher than predicted results. Yu et al.  conducted heat transfer experiments of nanofluids containing 170-nm silicon carbide particles at 3.7% volume concentration. The results showed that heat transfer coefficients of nanofluids are 50-60% greater than those of basefluids at a constant Reynold number.
Kim et al.  investigated effect of nanofluids on the performances of convective heat transfer coefficient of a circular straight tube having laminar and turbulent flow with constant heat flux. Authors found that convective heat transfer coefficient with alumina nanofluids improved by 15% and 20% for laminar and turbulent flow respectively. However, no improvement in convection heat transfer coefficient was noticed for amorphous carbonic nanofluids in turbulent flow. This showed that thermal boundary layer plays a dominant role in laminar flow while thermal conductivity plated a dominant role in turbulent flow.
Ku et al. conducted an experimental study on pressure drop of nanofluids containing carbon nanotubes in a horizontal tube. Authors had reported that the pressure drops of nanofluids became almost the same as that of distilled water (basefluid). The viscosity of nanofluids decreases with the shear rate and thus, the difference of pressure drop between nanofluids and distilled water at elevated flow rate also decreases.
Duangthongsuk and Wongwises conducted an experimental study on the heat transfer performance and pressure drop of TiO2-water nanofluids, flowing in a turbulent flow regime. They reported that the pressure drop of nanofluids was slightly higher than the base fluid and increased with the increasing volume concentrations.
4. Input data and operating characteristics
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The characteristics of the radiator considered in this study are shown in Tables 1 and 2. Other important thermal properties of nanofluids are calculated from empirical correlations shown in Section 5. Thermo-physical properties of ethylene glycol are presented in Table 3 and the thermal conductivity of nanofluids used in the analysis is obtained from Eastman et al.  as shown in Figure 1.
5. Mathematical formulation of ethylene glycol based copper nanofluids in an automotive car radiator
Mathematical correlations shown in Section 5.1 are taken from references [5, 15-17]. In this paper comparison was made between the radiator's heat transfer rateÂ operated usingÂ ethylene glycol coolant and nanofluids. It highlighted not only the influence of nanofluids but also volume fraction of copper nanoparticles to the heat transfer rate of a radiator. Described equations are being incorporated to aid the comparison.Thermal performances of a radiator can be calculated using Equations (1)-(26), data shown in Tables 1-3 and Figure 1. Calculations were done on air and coolant sides.
5.1 Air side calculation
(a) Air heat capacity rate, Ca can be expressed as
(b) Heat transfer coefficient, ha can be expressed as
(c) Fin efficiency of plate fin, hf can be expressed as
(d) Total surface temperature effectiveness, can be expressed as
5.2 Nanofluid Side Calculation
(a) Heat transfer Coefficient can be expressed as
(b) Heat capacity rate, Cnf can be expressed as
(c) Heat Exchanger effectiveness for cross flow unmixed fluid, e can be expressed as
(d) Overall heat transfer coefficient, based on air side can be expressed as
Neglecting wall resistance and fouling factor
(e) Pressure drop can be expressed as
(f) Pumping power can be expressed as
(g) Total heat transfer rate can be expressed as
6. Results and discussions
6.1 Influence of volume fraction of copper particles to thermal performance of an automotive car radiator
In this section, analysis of thermal performances of an automotive car radiator at constant Reynolds number of air (4000) and coolant (5000) have been done. With the increase of volume fraction of copper nanoparticles, the dynamic viscosity of nanofluids has been increased. Dynamic viscosity of nanofluids in this study is calculated from the correlation by Lee et al.  as shown in Equation 12. This parameter influences mass flow rate of nanofluids in an automotive car radiator. From Equation 11, it is found that the mass velocity of nanofluids has increased with the increase in volume fraction of copper nanoparticles due to higher dynamic viscosity of the nanofluids. It can also be explained from equation below.
In this equation is varied and other parameters are kept constant. This value is then substituted into Equation 13 to calculate the coolant mass flow rate which can be manipulated as shown in equation below.
However, volumetric flow rate of nanofluids is decreased with decrease of volume fraction of copper nanoparticles as calculated using Equation 14 and presented in Figure 2. In this study, it is found that Prandtl number of nanofluids based coolant decreases exponentially with volume fraction of copper nanoparticles, mainly due to higher thermal conductivity of nanofluids. Nanofluids exhibit higher thermal conductivity due to increase in Brownian motion, formation of nanolayer etc. Prandtl number of nanofluids is calculated using Equation 15 and the is an influential parameter for Prandtl number. It may be stated that higher value of this property will lead to lower value of Prandtl number. These relationships are shown in Figure 3. Figure 3 also showed lower coolant Nusselt number with the addition of copper particles, based on Equation 10. In this equation the coolant Reynolds number is kept constant while the coolant Prandtl number is varied. This study also found that ethylene glycol based copper nanofluids demonstrated higher overall heat transfer coefficient for air side as calculated using Equation 22. The relationship is shown in Figure 4 where overall heat transfer coefficient for air side is increased with copper nanoparticles. For instance, an overall heat transfer coefficient, 164 W/m2K can be achieved for 2% Cu +EG nanofluid compared to 142 W/m2K for basefluid. This indicates lower air side area is required to achieve 142 W/m2K overall heat transfer coefficient if 2% Cu+ EG nanofluid is used. Hence, estimated reduction of air side area up to 15.31% at 2% Cu+ EG particles was achieved. This study also found that heat transfer rate is increased exponentially as the volume fraction of copper particles are increased as shown in Figure 5. This improvement is calculated using Equation 26. It can be deduced that effectiveness of the radiator is increased with the application of nanofluids. However the effectiveness value does not increase substantially although the improvement of overall heat transfer coefficient is significant.
6.2 Influence of air Reynolds number on thermal performance of a radiator
The effect of air Reynolds number on the thermal performance of a radiator is discussed in this section. Coolant volumetric and mass flow rates, Nusselt and Prandtl numbers did not experience any change since the coolant Reynolds number was fixed at 5000. Only air Reynolds number and fraction of copper nanoparticles were varied in this section. With the increase of air Reynolds numbers the air heat transfer coefficient was increased. This will influence the thermal performance of the radiator system. Substituting higher value of air heat transfer coefficient into Equation 22, overall heat transfer coefficient based on air side of the radiator is obtained and shown in Figure 6. This property is found proportional with air Reynolds number. Nanofluids with higher copper volume fraction generates higher overall heat transfer coefficient than a basefluid. Same scenario happened for heat transfer rate where it is proportional to air Reynolds number as shown in Figure 7. About 3.8% of heat transfer improvement can be achieved with addition of 2% copper particles at 6000 and 5000 Reynolds number for air and coolant respectively. Based on the overall heat transfer coefficient and heat transfer rate improvement, percentage reduction of air frontal area can be estimated, at these mentioned Reynolds number. Percentage improvement of heat transfer rate is increased with air Reynolds number.
6.3 Influence of coolant Reynolds number on thermal performance of a radiator
This section presents the effect of coolant Reynolds number on the thermal performance of a radiator at a fixed air Reynolds number (4000). As the coolant Reynolds numbers are increased, the mass and volumetric flow rates are also increased. Overall heat transfer coefficient based on air side is increased with coolant Reynolds number as shown in Figure 8. The value of this property for nanofluids is higher than that of a basefluid. Therefore, heat transfer area reduction for the same value of overall heat transfer coefficient can be achieved by using nanofluids. Heat transfer enhancement was also observed with coolant Reynolds number. For instance, with the addition of 2% copper particles, 1.4% improvement of heat transfer rate has been achieved at 4000 and 7000 Reynolds number for air and coolant respectively. It is also observed that the improvement percentage is decreased with decrease of coolant Reynolds number. Figure 9 shows heat transfer rate of a radiator using nanofluid is higher than that of a radiator using ethylene glycol.
6.4 Comparison of coolant pressure drop
This section analyses the coolant pressure drop and pumping power at the radiator at a fixed coolant flow rate (0.2 m3/s) and an air Reynolds number (4000) with varying copper nanoparticles volume fraction. Initially, the coolant volumetric flow rate was converted to mass flow rate by using Equation 14. This equation is manipulated as shown below to calculate the coolant mass flow rate.
Coolant mass flow rate is found to be increased with the addition of copper nanoparticles due to its higher density. Then, coolant mass velocity is calculated using Equation 13. This value is substituted into Equation 11 to get the coolant Reynolds number. The pressure drop has been calculated by using fanning friction factor based on Equation 24, which then substituted into Equation 23.
It was observed that the coolant pressure drop increased with the addition of copper nanoparticles. The result reveals that 110.97 kPa of pressure drop obtained by adding 2% copper particles compared to 98.93 kPa for a basefluid alone. Due to this extra pressure drop a higher coolant pumping power is needed. The pumping power is calculated based on the Equation 25. Calculated results indicate that about 12.13% increase in pumping power is observed at 2% addition of copper nanofluids compared to a basefluid alone. These trends are shown in Figures 10 and 11. Increase in density increases pressure drop of flowing liquids. Adding particles in base fluid increases density of the liquid and augments pressure drop at a low percentage in the present region of study. Similar results were reported by Ko et al.  and Duangthongsuk and Wongwises .
Following conclusions can be drawn from this study:
(a) A better heat transfer performance was obtained for a radiator operated with ethylene glycol based nanofluids compared to coolant ethylene glycol.
(b) Heat transfer rate is increased with increase in volume concentration of nanoparticles (ranging from 0% to 2%). About 3.8% heat transfer enhancement was achieved with addition of 2% copper particles at 6000 and 5000 Reynolds number for air and coolant respectively.
(c) Thermal performance of a radiator using nanofluid or ethylene glycol coolant is increased with air and coolant Reynolds number.
(d) Estimated 18.7% reduction of air frontal area is achieved by adding 2% copper nanoparticles at Reynolds number of 6000 and 5000 for air and coolant respectively.
(e) 12.13% estimated augmentation in pumping power is achieved by using nanofluid of 2% copper particles at 0.2 m3/s coolant volumetric flow rate.