The influence of borehole lengths on a numerical model of a double-tube vertical ground heat exchanger

Abstract

Abstract GHEs are underground pipes that transmit heat between the earth and pipes. ANSYS Fluent was used to numerically investigate the thermal and hydraulic performance of a vertical twin tube ground-linked heat exchanger in a pile foundation in cooling mode. The CFD analysis uses three borehole length models—M1-10 m, M2-15 m, and M3-20 m—with flow rates from 1-100 lit/min and Reynolds numbers from 162 to 1625. The numerical results show that the average heat transfer rate of the Model M1 was 13% higher than that of the Model M2, 32% higher than that of the Model M3, and 95% higher than that of the Model M2. The pressure drop was minimal for 1 to 5 lit/min (laminar flow condition) but very high for 10 to 100 lit/min. The bore cavity length increased the pressure drop and decreased the average output temperature. These two-tube vertical GHEs with optimized length and selection of pipe materials can improve natural convection heat transfer by contacting ground waters.