A novel three-dimensional numerical model to simulate heat transfer inside a double U-tube borehole with two independent circuits

Abstract

In this paper, a three-dimensional numerical model is developed to investigate the transient heat transfer inside a double U-tube borehole with two independent circuits. The thermal resistance and capacity approach is applied to develop this model. Moreover, a new thermal network is proposed based on experimental data for a double U-tube borehole with two independent circuits. In this model, the axial heat conduction and thermal capacities of the grout and fluid carriers are taken into account. Besides, based on the experimental data, the grout inside the borehole for each layer is divided into five zones. The accuracy of the proposed model is investigated with the available experimental data and a fully three-dimensional unsteady CFD model. The suggested model is examined in two modes of long- and short-time simulations. The temperature and mass flow rate of inlet fluids are constant for long-time simulations. Moreover, two modes of variable fluid inlet temperature and variable fluid mass flow rates are considered for the short-time simulation to examine the accuracy of the proposed model against sudden changes of temperature and mass flow rate of the inlet fluids. Also, the proposed model is compared against the existing analytical and numerical models to investigate the heat transfer inside a double U-tube borehole with two independent circuits. Based on the results, it is found that the accuracy of the developed model is higher than the existing models.