Experimental investigation and theoretical model of heat transfer of saturated soil around coaxial ground coupled heat exchanger

This study is performed to investigate the thermal performance of saturated soil around coaxial ground coupled heat exchanger (GCHE). Based on artificial glass micro-balls as porous medium, heat-transfer experiments are conducted. Then a theoretical model with Darcy’s natural convection is developed and some numerical solutions are obtained using Keller-Shooting method. There is a better coincidence between theoretical and experimental results, and theoretical model greatly reduces the limitations of experimental setup and reveals the heat-transfer performance within a wider range. The results indicate that heat transfer mainly happens near the outer wall of coaxial GCHE and inclines to stabilization at far-field. The inlet temperature, initial temperature of porous medium and the flow rate are major factors affecting heat transfer. Compared with operation mode II, the heat-transfer rate in operation mode I are higher for the same flow rate and initial temperature of porous medium. There is a linear relationship between dimensionless temperature gradient along the outer wall of GCHE and the dimensionless height.