Numerical study on the heat transfer performance evaluation, flow characteristics, exergy efficiency, and entropy generation analysis of a novel coaxial geothermal heat exchanger

In order to improve the geothermal heating capacity of buildings, this paper proposes a coaxial geothermal heat exchanger (CGHE) with an inner tube assembled with continuous spiral fins (CSF). The thermal performance and flow characteristics of a CGHE with inner tubes assembled with different widths of CSF were numerically investigated at different rotational directions and rotational speeds (rpm = -30, -60, -90, -120, 30, 60, 90 and 120). The heat transfer mechanism in different rotational directions and rotational speeds was revealed and the heat transfer performance ,exergy efficiency and entropy generation were analyzed. The results of the study showed that the assembly of different width CSF on the inner tube of the CGHE exhibited completely different thermal performance and flow characteristics in the case of clockwise and counterclockwise rotation. Nu decreases in clockwise rotation and increases in counterclockwise rotation, but with small changes. The f of each H is always greater than the static f when rotating clockwise, and each width f increases as the clockwise rotation speed increases. And the f of each H in counterclockwise rotation is opposite to that in clockwise rotation. The PEC of each H decreases as the clockwise rotation speed increases and increases as the counterclockwise rotation speed increases, with a maximum PEC = 1.813 for H = 11 mm and rpm = 120. And counterclockwise rotation can improve the exergy efficiency of CGHE. The entropy generation rate is significantly higher than that of smooth tubes when Re exceeds 12,639.