Heat transfer analysis of a vertical ground heat exchanger using numerical simulation and multiple regression model

Because of the importance of ground heat exchanger (GHE) design in a ground-source heat pump system, a three-dimensional, unsteady, and numerical model of a vertical GHE was developed using the finite-volume method. The construction and boundary conditions were described in a Cartesian coordinate system. To evaluate the effects of different characteristics such as thermal conductivity, volumetric heat capacity, inlet flow, inlet water temperature, soil porosity, Darcy⿿s velocity, and borehole depth on the heat flux of GHE,⿿>101 cases were studied. The numerical results were compared by varying the temperature of circulating water along the inlet and outlet pipes. Nine correlations that passed significance test were obtained by monadic regression and curve-fitting analysis using all the parameters. Further investigation was carried out to obtain the prediction equation of heat flux, and the fitted expression was derived by multiple stepwise regression analysis based on the orthogonal experimental design of five levels for nine parameters. A group of experimental tests were carried out in Nanjing, China to validate the prediction equation, and the results indicate that it can estimate the heat flux of GHE with the root mean square error of 15.23 W/m.