A comprehensive transient heat transfer simulation of U-tube borehole heat exchanger considering porous media and subterranean water seepage

The present study involves a numerical simulation of a borehole heat exchanger (BHE) configuration that utilizes computational fluid dynamics (CFD) methodology. The BHE comprises a U-shaped pipe that facilitates the thermal exchange between water entering from one end and exiting from the opposite end. These heat exchangers are employed for both heating and cooling applications. This study concerns a system that employs water as its working fluid. The water enters a pipe at a higher temperature than that of the surrounding soil and subsequently exits with a lower temperature. The borehole wall temperature is examined in order to investigate the effects of inlet mass flow rate, backfill porosity, the presence of subterranean water, and its seepage velocity on the convection and conduction heat transfer, as well as on the system's performance. The results indicate that an increase in mass flow rate improves convection heat transfer. A porosity of 0.6 is deemed suitable under conditions of the absence of subterranean water, while a porosity of 0.2 is considered appropriate for backfill saturation and the presence of subterranean water. Also, an increase in subterranean water velocity seepage increases convection heat transfer, albeit at the expense of a decrease in system performance.