Optimized design on the thermohydraulic performance of the helical coil heat exchanger

The helical coil heat exchanger (HCHX) is one of the most popular heat transfer devices in heat pump water heaters. In this study, the Computational Fluid Dynamics (CFD) software, ANSYS FLUENT, was employed to predict the thermohydraulic performance of an HCHX, including the overall heat transfer coefficient and Fanning friction factor. Using different sizes of the HCHX, comparing the CFD results with experimental data or correlations available in the literature revealed that both results on thermohydraulic performance agreed well. The objective of this study was to choose the Taguchi method coupled with grey relational analysis (GRA) to optimize the HCHX design with the improvement of thermohydraulic performance. Among the selected three factors in the optimization process, it was found that the coil pitch and coil diameter were the two most important factors in influencing the thermohydraulic performance of HCHXs, while the outer diameter of the helical tube had little impact. Consequently, the optimal design of the HCHX increased the overall heat transfer coefficient by 131% and decreased the friction fanning factor by 6%, resulting in the increase of heat transfer rate by 47% and the reduction of pumping work by 72%. Moreover, the predicted optimal thermohydraulic performance was verified by the predictive formula of the Taguchi method. © 2022 The Author(s)