Hydrothermal analysis for a slinky-type horizontal borehole heat exchanger equipped with turbulator considering various ground depths

Abstract

This work critically evaluates hydrothermal behavior inside the slinky-type horizontal borehole heat exchanger with turbulator. The employed turbulator is the spiral twisted tape. The simulations are performed using the finite volume approach in the laminar flow area (Re = 250–400) using a commercial CFD program. Four ground depths (600, 1000, 1400, and 1800 mm) are selected for the proposed heat exchangers, and the outcomes are compared with and without the proposed turbulator. The obtained numerical results demonstrate that in all of the proposed situations, the average Nussel number and heat transfer coefficient grow as the Reynolds number increases. For a given Reynolds number, increasing the depth of the proposed borehole heat exchanger results in a greater heat transfer coefficient. Also, the turbulator-equipped model's outlet temperature values are higher than those of the non-turbulator-equipped model due to the presence of spiral twisted tape. More swirl flows result from this, which speeds up the rate at which heat is transferred from the fluid inside the proposed heat exchanger to the nearby soil. Moreover, the pressure drop increases as the Reynolds number augments, and this tendency is the same for both cases with and without turbulators. Furthermore, the model with 1000 mm depth at Re = 350 has the best thermal performance, and the model with 1400 mm depth is in the second level at Re = 350. The model with 1800 mm depth is attributed to having the lowest thermal performance in all analyzed Reynolds numbers because of the model's more considerable pressure drop. The model with a depth of 1800 mm has the lowest thermal performance at the lowest value of the researched Reynolds number (Re = 250). The models, including 600, 1000, and 1400 mm depths, have thermal performances of approximately 23.53, 11.76, and 8.82 % higher, respectively, than the reference model (1800 mm depth). According to the lowest Reynolds number trends, the model with a depth of 1800 mm has the lowest thermal performance at Re = 400, the maximum value of the analyzed Reynolds number in this case. Compared to the reference model (1800 mm depth), the thermal performance of the other models—including those with 600, 1400, and 1000 mm depths—is roughly 6.67, 11.11, and 26.67 % greater, respectively.

Keywords

• Renewable energy
• Geothermal energy
• Borehole heat exchanger
• Turbulator
• Heat transfer enhancement