A Numerical Analysis for the Effect of the Borehole Characteristics and Carrier Fluid Flow on The Thermal Performance of Ground Heat Exchangers
The ground heat exchanger represents a principal part of the ground-coupled heat pump and plays a major role in its thermal performance. The present study outlines the effect of the borehole depth and the carrier thermal fluid mass flow rate on the overall performance of vertical single and parallel flow in cross-orientation (PFCD) double U-tube heat exchangers. Five borehole depths in the range of (25-55) m were examined at a water flow velocity range of (0.1-0.5) m/s. The simulation process was accomplished by a 3-dimensional model built by the COMSOL Multiphysics 5.4 software at a steady-state condition. The double U-tube heat exchanger was simulated with full loading of water flow in which each of the U-tubes handles the same flow rate as that of the single U-tube one. Water temperature monitoring with depth showed that the temperature drop between inlet and exit ports of heat exchangers increases with the reduction of flow velocity and depth increase. The heat transfer rate of the double U-tube configuration at the borehole depth of (50) m fell in the range of (1.8-2) times the load of the (25) m depth for the examined range of flow velocity. The double U-tube showed an increase in the heat transfer rate of (17-27) % as compared to that of the single U-tube depending on the borehole depth. The carrier fluid temperature drop across the heat exchanger has revealed a dependence on the borehole depth and flow rate. The thermal resistance of the examined configurations showed a minor dependence on the borehole depth.
Keywords - 3-Dimensional Numerical Analysis, Borehole Depth, Double U-tube, Ground Heat Exchanger