Conjugate heat-transfer and integrated performance assessment of supercritical CO2 flow in self-affine rough geothermal fractures

Supercritical CO 2 (sCO 2 ) is a promising working fluid for enhanced geothermal systems, but fracture-scale heat extraction depends on the coupled effects of surface roughness, flow conditions, and strongly variable thermophysical properties. A two-dimensional conjugate heat-transfer model was established for an independent smooth fracture and five self-affine rough fractures, accounting for the dependence of CO 2 thermophysical properties on temperature and pressure. After validation against published transient experiments, calculations were conducted for 96 combinations of fracture roughness and operating conditions. Roughness produced a non-monotonic thermo-hydraulic response, as moderate roughness enhanced heat transfer through acceleration, recirculation, and boundary-layer renewal, whereas stronger roughness caused rapidly increasing nonlinear pressure losses. The performance evaluation criterion (PEC) characterizes the balance between heat-transfer enhancement and flow-resistance penalty. PEC variations were influenced mainly by the roughness profile and inlet mass flow, whereas rock temperature played a smaller role. At matched characteristic Reynolds numbers, high PEC values remained concentrated at H = 0.6 − 0.7 , and increasing Reynolds number improved this balance without shifting the preferred range. These findings provide direct fracture-scale insights for sCO 2 -based geothermal systems, guiding the selection of fracture morphology and operating conditions to balance heat extraction and flow-resistance penalties.