Core-scale investigation of convective heat transfer of supercritical pressure CO2 in hot rock fracture with various inclinations

Abstract

Abstract Extracting heat from hot dry rock by injecting working fluids is recognized as an important way of geothermal utilization. In recent years, using CO2 as the working fluid has been paid more attentions since its advantage of saving water, appropriate properties, as well as the potential capacity of large-scale CO2 geological storage. Our previous research has found that the effect of thermal properties variation of CO2 on heat transfer performance in horizontal fracture can’t be ignored when fluid temperature is in the range of Tp,c (pseudo-critical temperature) to 1.2 Tp,c. However, the combine effect of thermal properties variation with different fracture inclinations on heat transfer performance is not clear. In this paper, core-scale experiments and numerical simulations were conducted to investigate laminar convective heat transfer of supercritical pressure CO2 in a vertical smooth parallel-plate fracture and a rough fracture with various inclinations. The convective heat transfer characteristics in smooth horizontal and vertical fractures were compared in terms of velocity distribution, heat transfer coefficient and Nusselt number at different working conditions. It was found that in the vertical fracture, CO2 flow near the pseudo-critical point (Tf,in/Tp,c\xa0=\xa01.1) appeared heat transfer deterioration at low Rein number, followed by recovering to a comparable value with the Nu number in horizontal fracture. While the impact of buoyancy force on the heat transfer away from the pseudo-critical point (Tf,in/Tp,c\xa0=\xa02.9) was comparatively small in this study, especially when Gr/Re2 was smaller than 0.1. The heat transfer performance in a rough fracture was lower than that in the smooth fracture under the same conditions and the effect of the fracture inclination on the heat transfer performance was not significant. A correlation of Nusselt number in a rough rock fracture was proposed with extra considerations of the thermal property variation and fracture inclination. This work would provide a deeper understanding of the influence of fracture inclinations on heat transfer performance in geothermal development.