Transient numerical simulation of the coaxial borehole heat exchanger with the different diameters ratio

In this paper, the thermal and hydrodynamic performance of the coaxial borehole heat exchanger (CBHE) with different diameters ratio, during heat injection into the ground are investigated. For this purpose, the energy and Reynolds Averaged Navier-Stokes (RANS) equations with SST k-omega turbulence model are numerically solved to simulate transient fluid flow and heat transfer. Numerical simulation results are validated with results of available experimental data. The reduction of heat transfer between the annular and central region and the increase of heat transfer between the annular region and ground is the main objective of this study to improve the thermal performance of the CBHE. The results show that the temperature difference between the inlet and outlet of the CBHE rises with the decrease of diameter ratio. In order to reduce costs, instead of using a CBHE with a high diameter ratio and more borehole depth, it is better to use a CBHE with a low diameter ratio and borehole depth. Also, the optimal diameter ratio is calculated for the lowest pumping power. Of course, due to the insignificance of the pumping power against the heat transfer at the different diameters ratio, the CBHE with a lower diameter ratio is recommended because it has better thermal performance and cooling.