Investigation of heat transfer augmentation and friction factor in twisted tape-based earth-to-air-heat exchanger: a CFD-based analysis

Abstract

Earth air heat exchangers (EAHES) are an innovative way to heat and cool buildings by utilising the earth’s natural temperature. Utilizing twisted tape (TT) as an enhancement technique in the EAHE can significantly boost heat transfer passively. This study employs CFD simulations (Ansys Fluent 2022) with SST turbulence model for a fixed Reynolds number range (20,000–52,000). The Twist Ratio (y/w), representing the number of turns of TT inserts, is a critical roughness parameter. The ground temperature across whole soil domain is held constant at 26.7°C. Numerical results are validated against available data showing a close match, particularly at the outlet temperature. Maximum decrement in temperature at exit of EAHE for smaller twist ratio(y/w) is 19.53% as compare from conventional (plain pipe) EAHE for cooling mode. Nusselt number increment of the current design is 50.41% more than the previous design with TT inserts of Re = 51344, number of turns(y/w) = 15, respectively and for friction factor the maximum increment is 2.253 times without TT inserts EAHE at Re = 20573 for 15 turns, respectively. The maximum thermal performance factor improves by 1.19 times with TT inserts in EAHE, achieved at 15 turns and a Reynolds number of 51,344. Abbreviations: EAHE: earth to air heat exchanger; TT: twisted tape; LES: Large Eddy Simulation; RANS: Reynolds-Averaged Navier-Stokes.