Investigation of Thermal Conductivity and Convective Heat Transfer Coefficient of Water-based ZnO Nanofluids

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Nanofluids are stable suspensions of nanoparticles in a conventional fluid. They have shown superior potential in heat transfer enhancement. In this research, ZnO/water nanofluids were prepared at various concentrations from 0.2 to 1.5vol%, and their thermal conductivity was measured. The results showed that the thermal conductivity of ZnO/water nanofluids depends on particle concentration and increases non-linearly with the volume fraction of nanoparticles. The effects of particles size and temperature on the thermal conductivity were also investigated at 1.5vol%. The results indicated that thermal conductivity enhanced with decreasing particles size and increasing with temperature. For nanofluids containing 10-15 nm and 45-50 nm particle sizes, the enhancements were 26.3 and 22.8% at 40oC, respectively. In this research, the convective heat transfer coefficient of ZnO/water nanofluids with the above particle sizes was also measured under laminar flow in a horizontal tube heat exchanger. It was observed that both nanofluids showed higher heat transfer coefficients compared to the base fluid at a constant concentration (1.5 vol%). For nanofluids with 10-15 nm and 45-50 nm particle sizes, the average heat transfer coefficient enhancement was 18.1 and 14.9% at Re=1115, respectively.