Numerical study on a coaxial geothermal exchanger equipped with a new inner tube: Entropy generation, thermodynamic irreversibility analysis and exergy efficiency performance evaluation

Abstract

This paper focuses on the effect of inner tube fitted with CSF of different widths (H = 5–11 mm), thicknesses (W = 1.5–9 mm) and pitches (P = 50–200 mm) on the entropy generation, thermodynamic irreversibility and exergy efficiency performance of CGHE and gives the direction of improvement of CSF geometric parameters. A performance evaluation chart of exergy efficiency was also established to evaluate the exergy efficiency of each working condition. The results show that the assembly of CSF on the inner tube of the CGHE can effectively reduce the heat transfer entropy generation rate S˙gen,h, lowest 67.9% lower than that of the smooth inner tube, but the friction entropy generation rate S˙gen,f is significantly higher, highest being 10.33 times higher than that of the smooth inner tube. Increasing the width H and thickness and decreasing the pitch P both increase the entropy generation rate, but changes in width and thickness have a small effect on entropy, while changes in pitch P have a large effect on entropy, and their W˙loss follow the same pattern of variation as S˙gen. From Be and Ns it is clear that 99% is frictionally irreversible when Re > 30000, and frictional entropy generation is minimized at Re < 15000. When evaluated in terms of heat transfer improvement number NH, the direction of improvement of the CSF geometry parameters should be to increase the width H, thickness W and pitch P. In terms of exergy efficiency, the exergy efficiency for all cases results in greater than smooth inner tube coaxial GHE for the same pumping power and pressure drop conditions, but deterioration occurs for the same mass flow rate.