Experimental research on stabilities, thermophysical properties and heat transfer enhancement of nanofluids in heat exchanger systems

Stable TiO2-water nanofluids are prepared by a two-step method, stabilities of nanofluids are investigated by precipitation method and transmittance method respectively, and thermal conductivities and viscosities are also measured. An experimental system for studying the heat transfer enhancement of nanofluids is established. and heat transfer and flow characteristics of TiO2-water nanofluids in heat exchanger systems with a triangular tube and circular tube are experimentally studied. The effects of nanoparticle mass fractions (omega = 0.1 wt %-0.5 wt %) and Reynolds numbers (Re = 800-10000) on the heat transfer and flow performances of nanofluids are analyzed. Fitting formulas for Nusselt number and resistance coefficient of nanofluids in a triangular tube are put forward based on the experimental data. The comprehensive performances of nanofluids in a triangular tube are investigated It is found that nanofluids in a triangular tube can significantly improve the heat transfer performance at the cost of a small increase in resistance coefficient compared with that in a circular tube, especially the resistance coefficients are almost the same between different nanopartide mass fractions at turbulent flow. It is also found that the comprehensive evaluation index eta decreases with Reynolds number at laminar flow but a critical maximum value appears at turbulent flow. (C) 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.