Effect of Nano-fluids on Heat Transfer Enhancement in Automotive Cooling Circuits

Unceasing technological development has compelled the automobile industry to manufacture engines with higher efficiencies. To meet the weight requirements of a vehicle, a radiator with optimum size and design is required. Fins are generally used in radiators to increase the rate of heat transfer. Moreover, conventional approach of improving the heat transfer by using micro-channels and fins have some limitations. Additionally, Ethylene Glycol and water have very low thermal conductivities, therefore there is a dire need of introducing new fluids to increase the heat transfer in automobile cooling circuits. Recent researches have shown that Nano-fluids perform better than conventional fluids in heat transfer applications. Leong et al. [33] studied the heat transfer performance of Copper and Ethylene Glycol Nano-fluid and found a better rate of heat transfer than the base fluid alone. The Reynolds number for air and coolant was kept 5000 and 6000 respectively and a 3.8% increase in the rate of heat transfer was narrated for 2% volume fraction. The study also suggested a decrease in the dimensions of frontal surface. Hussein et al. [34] carried out a study on the heat transfer performance of SiO2 and TiO2 Nano-fluid with water. Volume concentration, volume flow rate and inlet temperature of Nano-fluid was kept 1-2%, 2-8 LPM, 60°-80°C respectively and flow was kept laminar. The results indicated that Nusselt number increased slightly with the volume concentration and inlet temperature of the Nano-particles and showed a significant rise with the increase in volume flow rate. 

Subhedar [35] carried out an experimental study to observe the heat transfer performance of water monoethylene glycol (MEG) and Nano-particles of Al2O3 with a volumetric ratio of 50-50%. MEG is usually used as a coolant in radiators. The flow was kept laminar and an automobile cooling system was replicated in the experimental system. The Nano-fluid preparation consisted of two-step method. Ultrasound technique was used to disperse the Nano-particles (20nm diameter) of Al2O3 in a mixture of MEG. The experimental study showed an increase in the heat transfer relative to the base fluid. With the increase in volume concentration from 0-8%, an increase in the overall heat transfer coefficient was noticed. The increase of the inlet temperature from 65 °C to 85 °C. Lastly, the study also established that when the volume concentration of Nano-particles of Al2O3 is 2%, it permits a reduction of 36.9% in the heat exchanger surface. Bozorgan et al. [36] performed numerical analysis on the radiator of a diesel engine. He circulated the Nano-fluid of CuO-water through the flat tubes. The study concluded that an increase of 23.8% and 10% compared to base fluid was attained in pumping power and overall heat transfer coefficient respectively when the speed of the vehicle was 70km/h and the volume concentration of Nano-fluid was 2%. 

Hwa Ming Nieh [7] carried out an experimental study in which he used Nano-coolant (NC), Titania (TiO2) and Alumina Al2O3 to increase the heat transfer of an air-cooled radiator. The study concluded that Nano-fluids perform better than Ethylene Glycol in the terms of heat dissipation and efficiency. Moreover, experimental data revealed that Nano-fluid of TiO2 is efficient than the Nano-fluid of Al2O3. Gulhane and Chincholkar [37] performed experimental analysis on the radiator of a car to determine the heat transfer rate. The Nano-fluid of Al2O3 with water was taken in lower concentration in this study. The result revealed that with the increase in inlet temperature of the coolant, coolant flow rate and particle concentration, heat transfer coefficient of the coolant also rises. They noticed a maximum increase of 45.87% in the heat transfer coefficient. Ray and Das [39] used three different types of Nano-particles i.e. silicon dioxide, copper oxide and aluminum oxide in a car radiator. The base fluid for all three Nano-particles was a mixture of water and Ethylene Glycol in a ratio of 40:60. It was observed that a mixture with lower concentrations i.e. 1% performed better than the mixture that had higher concentration of Nano-particles. A reduction of 35.3% in the pumping power and 7.3% in the heat transfer surface was achieved for 1% volume concentration of Al2O3 Nano-fluid. 

In the case of CuO Nano-fluid, pumping power reduction was comparatively lesser i.e. 33.1% and heat transfer surface reduction i.e.7.2% was almost similar to Al2O3 Nano-fluid. SiO Nano-fluid performed lowest but still a reduction of 26.2% and 5.2% was attained in the pumping power and heat transfer surface respectively. Ali et al. [40] studied the heat transfer performance of ZnO Nano-particles at different volume concentrations i.e. 1%, 2%, 3%, and 8%. The flow rate and Reynolds number was kept in the range of 7-11 LPM and 17,500-27,600 respectively. All the concentrations of Nano-fluid showed a better heat transfer performance compared to the base fluid. The maximum increase of 46% was achieved for 2% volume concentration of the Nano-particles. When the concentration was increased up to 3%, a decrease in the heat transfer was noticed. In addition to that change in inlet temperature from 45°C to 55°C, an increase of 4% was observed in heat transfer velocity. Moghaieb et al. [41] performed the experimental analysis to study the convective heat transfer of a car engine for water and Al2O3 Nano-fluid. He used different diameters [21-37 nm] of Nano-particles in his study. He found that with the increase in flow velocity, the heat transfer coefficient increase whereas it decreases with the increase in the temperature. For a volumetric concentration of 1%, 78.76 % increase in the heat transfer was attained as compared to the traditional fluid. 

Elbadawy [42] performed a numerical analysis to study the flow characteristics and thermal performance of water based Nano-fluid of Al2O3 and CuO. He observed that Nano-fluids can increase the rate of heat transfer and obtained a major volume reduction in the radiator. He found that pumping power is depends upon the concentration of Nano-particles and Reynolds number of the flow. Moreover, 38% and 45% increase in the rate of heat transfer was obtained for CuO and Al2O3 Nano-fluid respectively. Optimum value of volume concentration of both Nano-fluid was found to be 4.5%. At this value, a reasonable increase and decrease in the heat transfer and pumping power was achieved respectively. Harsh et al. [43] carried out an experimental analysis on the heat transfer performance of Nano-fluids in an engine. A solution of Ethylene Glycol (EG) and water (80:20) was used as a base fluid for the dispersion of Al2O3 Nano-particle. Three different values of flow rate were chosen to be 4, 6 and 7 LPM whereas the speed of air flow inside the duct was 4.9 m/sec. The study concluded that with the use of Nano-particle, 24.5% and 13.9% increase in the overall heat transfer coefficient and rate of heat transfer can be obtained. In this research, the diameter of Nano-particles of Al2O3 was 45 nm and its volumetric concentrations was kept between 2% and 6%. The analysis was carried out on three different operating ranges of car (High, Medium and Low) and study was focused on determining the rate of heat transfer, pumping power and outlet temperature of radiator. The obtained results were then compared with the results obtained using base fluid. 

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