Numerical simulation and experimental verification on heat transfer process of a vertical U-bend ground heat exchanger

In order to simulate the heat transfer process of vertical U-bend GHE(ground heat exchanger), a two-region vertical U-tube analytical solution model is presented and developed firstly by dividing the heat transfer region of vertical U-bend buried coil into two parts at the boundary of borehole wall, and the two regions are coupled by the borehole wall temperature. Both steady and transient heat transfer method are used to analyze the heat transfer characteristics inside and outside borehole respectively. The transient borehole wall temperature is calculated for the soil region outside borehole by use of the variable heat flux cylindrical source model. As for the region inside borehole, considering the variation of fluid temperature along the length and the heat interference between two adjacent legs of U-tube, a steady-state heat transfer analytical solution model is developed based on element energy conservation. The validation of the model developed is done by experimental data and the results indicate that the calculated fluid exit temperatures of the GHE by the model are agreed well with the test data.