Numerical study on the heat transfer characteristics between supercritical carbon dioxide and granite fracture wall

A two-dimensional numerical model was developed to investigate the flow and heat transfer characteristics of supercritical carbon dioxide (scCO2). The numerical results of flow and heat transfer characteristics of scCO2 through straight granite fracture were found to be in good agreement with the experimental results. Moreover, the heat transfer characteristics between scCO2 and rough granite fractures were also investigated by using the proposed model. The confining temperature outside the specimen was set at 200 °C and the pressure of scCO2 was 8 MPa. Four different case studies with the flow rates of 0.75, 0.51, 0.35, and 0.13 kg h−1 were, respectively, designed. The overall and the local heat transfer coefficients (OHTC, LHTC) were used to characterize the heat transfer properties of these cases. Furthermore, the effects of flow rate, fracture roughness, and phase state of CO2 on the heat transfer characteristics of CO2 were investigated. The results indicated that increased roughness or flow rate could improve the heat transfer performance of scCO2. Moreover, on the same area of the fracture, the temperature of the fracture wall was found to be always higher compared to that of scCO2; much similar to the water behavior and the OHTC of scCO2 increased with the increase in the injection flow rate. Surface morphology of the fracture significantly influenced the LHTC distribution of scCO2, and LHTC roughly exhibited a negative correlation with the local waviness of fracture. This was pronounced at the sunken positions of the fracture surface, which exhibited significantly larger LHTCs compared to the prominent positions. The heat transfer characteristics of CO2 were found to be closely related to its phase state. CO2 in the denser phase showed better heat transfer performance.