Experimental and numerical evaluation on influential factors for thermal performance of coaxial-type ground heat exchanger

Thermal behaviour of Ground Heat Exchanger (GHEX) is affected by environmental factors, operating conditions, and the configuration of heat exchange pipes. Moreover, heat transfer in GHEXs is governed by the convection phenomenon among the circulating fluid, the wall of heat exchange pipe, the borehole grout and the ground formation. The convection is variable with flow conditions within the heat exchange pipe (i.e., laminar or turbulent flow), which is a significant variable in the thermal performance of GHEXs. Especially, in case of coaxial-type GHEXs, the occurrence of turbulent flow with sufficient heat exchange area is considerably critical due to the geometric characteristics of the concentric tube-in-tube configuration. In this paper, the coaxial-type GHEX was constructed in a test bed, and the in-situ thermal performance test (TPT) was carried out to estimate the thermal performance of the coaxial-type GHEX. In addition, a Computational Fluid Dynamic (CFD) model was developed by calibrating with the field test results. In order to simulate turbulent flow inside the pipe, the K-epsilon model was applied. Then, parametric studies were conducted with consideration of various configurations (i.e., depth, inner and outer diameters, and flow rate) to figure out the optimal configuration for coaxial-type GHEXs. As the flow rate and depth increased, the heat exchange amount of coaxial-type GHEX increased. In particular, as the hydraulic diameter increased, the heat exchange amount increased, which means that a reduction in the fluid velocity through the annular space is more dominant in determining the heat exchange amount.