Experimental investigation on the thermal performance of inserted helical tube three-fluid heat exchanger using graphene/water nanofluid

Heat exchangers are widely used for cooling and heating in every process industry. Here, a laboratory-scale experimental setup is used to investigate the thermal performance of inserted helical tube three-fluid heat exchanger using three different concentrations of graphene/water nanofluid (viz. 2, 4, and 6 volume percentage) as a working fluid. The influence of fluid flow in parallel configuration (viz. the flow of hot water, nanofluid, and air) is examined on the overall heat transfer coefficient, effectiveness and Nusselt number. The overall heat transfer coefficient increases with increasing all fluid flow rates. Significant improvements of the heat transfer coefficient between hot water and nanofluid and between nanofluid and air are found for the cases of nanofluids at the hot water flow or air flow rates, while a negligible effect of nanofluids between hot water and nanofluid is observed with increasing nanofluid fluid flow. Additionally, effectiveness decreases up to a minimum, after that it increases with increasing flow of hot water or nanofluid, while the effectiveness increases with increasing air flow and an improvement is found for the case of high concentration of nanofluid. A comparison between Nusselt numbers and correlations of a double-pipe helical tube heat exchanger shows a significant variation in results. Therefore, we proposed semiempirical correlations to estimate the Nusselt number and friction factor for the triple-fluid helical tube heat exchanger, and the correlations agree well with the experimental results. The semiempirical correlations would be useful for the flow of multiple fluids and several thermal communications in heat exchangers.