International Journal of Heat and Mass Transfer, Vol.137, 740-750, 2019
Heat transfer and fluid flow characteristics of nanofluids with various filling ratios under synchronous vibration
In the combustion engine working process, significant heat energy is transferred to the combustion chamber, and particularly to the piston. Therefore, the piston must be efficiently cooled with increased thermal loading. When the piston power density exceeds 0.3 kW/cm(2), it must be cooled by a cooling chamber. To assess nanofluid flow and heat-transfer characteristics in the cooling chamber, visualization experiments and numerical simulations were performed using water-based Al2O3 and SiO2 at 2% concentration by volume in a straight circular pipe under synchronous vibration with the piston. The filling ratio, vibration frequency, and nanoparticle shape all have important impacts on cooling. Experimental data verified numerical simulation results that the optimum heat transfer filling ratio = 53.4%, under which the convective heat-transfer coefficient of nanofluids reached the maximum 1914.4 W/(m(2) K). The heat-transfer enhancement coefficient was proportional to the vibrational frequency. Within the experimental limits and with high filling ratio, the convective heat-transfer coefficient increased by 9.5% when the vibrational frequency increased from 2.55 Hz to 3.65 Hz. Nanoparticle shape also had a major impact on heat transfer. Heat transfer with rod-shaped Al2O3 nanoparticles was significantly better than with spherical SiO2 nanoparticles. Under the condition of 3.65 Hz, compared with water, the enhancement amplitude of spherical nanoparticles with the same volume concentration was 8.0%, but that of the rod-like nanoparticles could be increased by 13.4%. The fluids mixed best for a rotor rotation angle between 180 degrees C and 270 degrees C, and the degree of nanofluid chaos was better than that of purified water in a reciprocating vibration cycle, as verified by experiments and numerical simulations. (C) 2019 Elsevier Ltd. All rights reserved.