Convective heat transfer performance of nanofluids in concentric tube heat exchanger

Abstract

This research studied the convective heat transfer performance of nanofluids in a concentric tube heat exchanger. Nanofluids is a new technology for heat transfer in cooling systems and suspension stability of nanofluids is a major issue since it will affect the performance of nanofluids in heat exchanger. The addition of surfactant will increase the dispersion of nanoparticles in the base fluids and increase its stability. The objectives of this research were to formulate the stable nanofluids with addition of surfactant and to investigate the stability, properties, and heat transfer performance of the nanofluids in concentric tube heat exchanger. Aluminium oxide (Al2O3) and titanium dioxide (TiO2) nanoparticles were added to the base fluid, ethylene glycol (EG), with addition of polyvinylpyrrolidone (PVP) as surfactant by two-step method. First, the optimum stability ratio of nanoparticles to surfactant was determined by preparing 5 samples of 0.50 vol% nanoparticles with addition of different concentration of surfactant. Thus, each sample had different ratio and was observed for a few weeks by visual observation. Then, 4 samples of nanofluids (0.25, 0.50, 0.75 and 1.00 vol %) were prepared based on the optimum stability ratio. The properties and heat characteristics of nanofluids were analyzed at different concentration (0.25, 0.50, 0.75 and 1.00 vol%) of nanofluids and at 4 different temperatures which were, 30, 45, 60 and 75 °C. From the observation, the Al2O3 nanofluids was stable with the addition of 0.1 vol% surfactant, while TiO2 nanofluids was stable with 0.4 vol% surfactant. The thermal conductivity of nanofluids was higher than the base fluids and increased as the temperature and concentration increased. Nevertheless, thermal conductivity for Al2O3 nanofluids was slightly higher compared to TiO2 nanofluids. As known, viscosity of nanofluids increases with increasing nanoparticles concentration and reducing nanofluids temperature. Viscosity of these nanofluids was the highest at 1.00 vol% and at 30 °C temperature for both. The heat transfer performance was enhanced by using nanofluids as compared to base fluids alone. Heat transfer coefficient was increased by increasing nanoparticles concentration and temperature. All nanofluids gave the highest heat transfer enhancement at 1.00 vol% and 75 °C. However, heat transfer enhancement by Al2O3 nanofluids was higher compared to TiO2 nanofluids due to the high thermal conductivity of Al2O3 nanofluids. Thus, the research proved that nanofluids can be used to replace the traditional working fluids as coolant or thermal fluids.