Experimental study on heat transfer and pressure drop characteristics utilizing three types of water based nanofluids in a helical coil under isothermal boundary condition

Background Nanofluids are an innovative attractive challenging field suitable for many heat transfer applications. The helical coil is preferred due to compactness and additional curvature flow disturbance. Method Heat transfer and pressure drop characteristics of Ag, ZnO and TiO2 water based nanofluids through helical coil tubes under isothermal boundary conditions are experimentally investigated with 0.25% nanoparticles volume concentration Ag/water, ZnO/water and TiO2/water nanofluids. Five coils are constructed with different coil pitches of 2, 4, 6, 8 and 10 cm. The flow rate of the nanofluid varies between 1 to 3.5 L/min with inlet temperature from 30 °C to 60 °C. Findings The results showed that increasing the coil pitch reduces Nusselt number and pressure drop. The nanofluids have a positive impact on the enhancement of heat transfer but with a pressure drop penalty. Average Nusselt number enhancement up to 11.8 %, 17.8 % and 28.7 % was achieved at Ag/water, ZnO/water and TiO2/water with average pressure drop increasing up to 25.4%, 23.5% and 28.5% respectively. The results showed that an increase in coil pitch decreases hydrothermal performance index (HTPI) and utilizing nanofluids increases HTPI for all types of nanofluids Ag/water, ZnO/water and TiO2/water. Novel correlations were developed to predict Nusselt number and pressure drop.