A CFD modelling for optimizing geometry parameters for improved performance using clean energy geothermal ground-to-air tunnel heat exchangers

In this numerical study, different geometric parameters have been optimized for improved performance using a clean energy geothermal Ground to Air Tunnel Heat Exchanger (GATHE). A commercial computational fluid dynamic code ANSYS Fluent was used, and the code has been validated with the available experimental and analytical results. Three soil samples were used to analyze the thermal properties, each with a thermal conductivity of 0.65, 1.25 and 3.50 Wm- 1K-1. The critical soil thickness was found to be 10 times the pipe diameter and the lowest outlet temperature of 300.1 K was achieved for a 60 m pipe with a soil thermal conductivity of 3.50 Wm- 1K-1. To further enhance the performance of the system, multiple fin arrays were introduced into the ductwork at 0.5 m, 1.0 m and 2.0 m pitches. For a 10 m long pipe with fins installed every 0.5 m, an outlet temperature of 306.7 K was observed. With 4 fins for temperature variation, performance increased by 19.4 % compared to the case without fins. When the fin spacing is 1 m, the temperature drop increases by 5.6 %, and when the fin spacing is 2 m, the temperature drop increases to 7 %, with an outlet temperature of 306.1 K. Finally, a novel set of multiple blocks in diverging and converging patterns was introduced to restrict airflow and further improve heat dissipation. Compared to a single-fin block, the outlet temperature of the 4fin block was reduced by 14.52 % due to the increased contact area, thus improving the thermal performance of the heat exchanger system.