Study of Heat Transfer Characteristics of Nanofluids in an Automotive Radiator

This paper presents an experimental study on heat transfer using nanofluid as coolants in engines. Previous studies shows that Al2O3 is found to be more effective in heat transfer due to its high conductive property which is found to increase with concentration. Particles having diameter in the range 10-3 to 10-6 m have low thermal conductivities and cause clogging in the flow section along with significant friction and are highly unstable in solution. Nanoparticles on the other hand are easily dispersed and cause minimal clogging or friction in the flow. In the present work, ethylene glycol-water solution is taken as a base fluid for nanoparticle dispersion. The ratio of water to ethylene glycol used is 80:20 and it has been noted out that heat conduction improved with increasing fraction of ethylene glycol. The experiments were conducted with flow rate of 4,5,6 and 7 L/min and the air flow rate inside the duct was kept constant at 4.9 m/s. The temperature of water in the reservoir is kept at 70°C. The nanoparticles used in this experiment are Cu and TiO2 having particle size less than 80nm. Result shows that there is an improvement of 24.5% in the overall heat transfer coefficient and there was also an increase of 13.9% in the heat transfer rate compared to the base fluid (80:20 Water: EG solution).


Introduction
In recent years weight reduction is the major focus on the automobile industry as it improves the fuel economy of the vehicle and its running cost. Engine cooling system is the one in which weight reduction can be achieved. Also, new technological improvement increases the thermal loads and hence require faster cooling. The customary methods of increasing the cooling rate (fins and micro channels) are already extended to their limits. Therefore, there is a critical need for new and innovative coolants that accomplish this high -performance cooling. Thermal conductivities of conventional engine coolants (lubricants and water) are low. Certain parameters of an automobile like engine performance, fuel efficiency, and emissions are heavily affected by convective heat transfer. The amount of heat produced during the combustion is important in enhancing the engine lifespan, oil cooling system and climate regulation. Developed thermal systems for modern vehicles need more compact heat exchangers, innovative heat transfer schemes and eco-friendly fluids with better heat transfer characteristics. The modern coolants with their enhanced thermal performance will cut down the overall size of heat 1234567890''"" exchanger (radiator) and may lessen the vehicle fuel consumption. To increase the heat transfer from heat exchanger and engine cooling system, several methods are adopted such as helically twisted tapes [1,2,3] nanotubes surface coatings [4] electric field and nanofluids in coolants [5] which in turn reduce the weight with the same given power.

Nanofluids
With the advancement of nanotechnology, nanofluids can be thought of as a coolant in heat exchangers [6, 7, and 8]. Nanofluids are liquid with suspensions of nanoparticles with at least one of their principal dimensions smaller than 100 nm. From previous investigations, nanofluids have been found to possess enhanced thermo physical properties such as thermal conductivity, thermal diffusivity and viscosity. Convective heat transfer coefficients of nanofluids are greater, compared to those of base fluids like ethylene glycol or water due to the increase in particle volume fraction, temperature and higher thermal conductivity of the nanoparticle that is dispersed in the fluid. The nanoparticle used include stable metals such as copper, gold and silver, metal oxides such as alumina, bismuth oxide, silica, titania and zirconia and allotropes of carbon such as diamond, single-walled and multi-walled carbon nanotubes and fullerenes [9][10][11][12]. Nanofluids are also used in micro channel cooling without any clogging and sedimentation problems.

Methods and Materials
Two methods are used for the preparation of nanofluids as documented in the literature [20]. In the present work, two-step process is adopted. In this process, nanoparticles are prepared and are dispersed in the base fluids with the help of mechanical agitation (stirring) or ultra-sonication method. The nanofluids are emulsified with or without the help of surfactants depending on the boundary properties of nanoparticles and the base fluids

Preparation of Nanofluids
The following steps are involved in preparation of Cu and TiO2 based hybrid nanofluids.
• Initially the base fluid having distilled water and ethylene glycol in the ratio 80:20 (water 80% and ethylene glycol 20% by volume) is taken and mechanically stirred for mixing of the two fluids.
• The second step involves measuring the quantity of nanoparticles containing Cu (~50 nm) in the ratio of 0.3% (wt by vol) and TiO2 (~20 nm)in the ratio of 0.4%( wt/vol) and adding it to the base fluid step by step while the base fluid is being stirred in the magnetic stirrer.
• Then the surfactant (CTAB) is added to the base fluid.
• The nanofluid is magnetically stirred for an hour to ensure proper dispersion of the nanoparticles in the base fluid.
• The next step involves conducting stability and sedimentation tests on the nanofluid prepared using KD2 PRO Thermal Analyzer (Decagon) The forced convective heat transfer experiment was conducted using three different fluids (1)distilled water, (2) base fluid and (3) nanofluids and their heat transfer characteristics are compared.

Experimental Setup
The schematic diagram of experimental setup is shown in figure 1. Initially the experiment was conducted using distilled water with the flow rate at 4,5,6 and 7 L/min. The air flow rate inside the duct was kept constant at 4.9 m/s throughout the experiment and was measured using pitot tube. The water in the reservoir tank (15 liters) was heated to 70ºC using electric heater. Once this temperature was achieved, the water is then circulated throughout the system while temperature was recorded at various points like water inlet, air inlet, water outlet, air outlet by using Data Accusation Card (DAC). The procedure was repeated for the base fluid (water and ethylene glycol) by taking 12 liters of distilled water and 3 liters of ethylene glycol in the reservoir tank. Similarly, for the nanofluids, Cu and TiO2 nanoparticles were taken in the ratio of 0.3% and 0.4% wt/vol concentration and they were dispersed in the base fluid and the procedure was repeated. The formulas used for the calculation of the results are given in appendix A.  Figure 2 shows the heat transfer rate with the flow rate for water, water : EG and nanofluid. It is noted that the heat transfer rate is greater for the nanofluid compared to other liquids. This type of higher heat transfer rate is attributed to the large surface area of the nanoparticle and higher thermal conductivity of nanofluids. From the fig., it is observed that there is an increase of 13.9% in heat transfer rate for nanofluids when compared to the base fluid. Eastman et. al [13] noted 40% increase in the 0.3% copper nanoparticle dispersed nanofluid compared to the ethylene glycol base fluid. Similarly Liu et. al [14] noted an increase of 23.8% in the thermal conductivity by adding 0.1% volume fraction of copper nanoparticle in water prepared by chemical reduction method. Yoo et. al [16] suggested that the surface to volume ratio is the dominant factor in enhancing the thermal conductivity rather than the thermal conductivity of the nanoparticle and the base fluid. Figure 3 shows the variation in Nusselt number with different coolant flow rate of various coolants. It is observed that the Nusselt number increases with the increase in the coolant flow rate, thereby increases the heat transfer. This may be due to the fact that the enhanced thermal conductivity of nanofluids increases the heat transfer performance [17]. Figure 4 shows the variation of the Nusselt number with Reynolds number. The experimental results exhibited good agreement with Dittus Boelter equation [18].The results show that the Nusselt number increases with the increase in the Reynolds number.
The overall heat transfer coefficient and Nusselt number increased with the flow rate. The overall heat transfer coefficient and Reynolds number is observed to increase with the flow rate which could be ascribed to the improved heat transfer properties of nanofluids, compared to that of the base fluid. From the figure 5, it is observed that there is an increase of 24.5% in the overall heat transfer rate for nanofluids when compared to the base fluid. This is due to the improved thermal conductivity of these particles, and the reduced thermal resistance offered by the flowing nanofluid on the wall surface of the inner tube. It is noted that, the mean free path available for particulate motion would have been augmented at lower volume concentrations of nanofluids. At higher volume proportions of nanofluids, due to the constricted diffusion of particles in the fluid medium, the rate of heat transfer is reduced [19].

Conclusion
The forced convective heat transfer was conducted in the test section using a car radiator with distilled water, water ethylene glycol solution and nanofluid respectively as coolants at the flow rates of 4, 5, 6 and 7 L/min and the following conclusions can be drawn from this: • The rate of heat transfer was found to increase by about 24.5% for the nanofluids when compared to base fluid (water: EG solution). • The results shows that the Nusselt number increases with the increase in the coolant flow rate following a trend similar to Reynolds number. • Finally, the experiment proves that nanofluids have high heat transfer rates to remove the excessive heat at high temperature and flow rates and have proved to be a better coolant than distilled water and water ethylene glycol solution. The heat transfer rate is calculated using Newton's law of cooling and the rate of heat transfer is given as The heat capacity rate is given by The minimum among the heat capacity rate is taken as Cmin and the other is taken as Cmax The heat capacity ratio which is a dimensionless quantity and is given by