Thermo-hydraulic performance of the U-tube borehole heat exchanger with a novel oval cross-section: Numerical approach

In this paper, a novel U-tube Borehole Heat Exchanger (BHX) with an oval cross-section is proposed for improving the performance of Ground Source Heat Pump systems (GSHP). To study the thermo-hydraulic performance of Borehole Heat Exchanger (BHX) with an oval U-tube: A three-dimensional, symmetric and unsteady state multi-physics Computational Fluid Dynamic (CFD) simulations are carried out by ANSYS FLUENT environment. The conjugate heat transfer and fluid flow are simulated. Borehole thermal resistance, fluid inlet and outlet temperatures, fluid temperature profile along pipe length and pressure drop inside the tube are calculated and analyzed. The oval shape dimensions are adapted, then a comparison between the circle and the oval cross sections is carried out; four different cases are investigated. As a result, the CFD results show a good agreement with their experimental pairs via Thermal Response Test (TRT) test which was done during September 2017 by our laboratory members. Besides, the results show that the oval shape enhances the heat transfer process and achieves the lowest borehole thermal resistance of 0.125 m K/W, also, it decreases the thermal resistance by 18.47% compared with the circular one. Besides, the oval shape shows a pressure drop of 32% less than the circular tube. Consequently, the oval shape could enhance the coefficient of performance (COP) of GSHP, decrease the borehole length, decrease the operating pressure and cut down initial and operating costs. Also, it fit easily inside tight borehole, so it is preferable in high-densely urban areas.