Characteristics of flow phase transition and heat transfer of N2O medium at the rock core scale of dry hot rock

The phase characteristics of the heat exchange medium play a crucial role in the thermal performance of the heat reservoir in Enhanced Geothermal Systems. A numerical model was developed at the core scale for a two-dimensional rough single fracture, incorporating the phase-change behavior of N 2 O. The thermal-hydraulic transport characteristics of liquid and supercritical N 2 O within a single fracture in hot dry rock were investigated under varying inlet velocity and temperature conditions. The results reveal the following key findings: (1) When N 2 O is in the supercritical state, vortex-like streamlines are more likely to form at the corners of the flow channel. Under high-velocity and low-temperature conditions, the temperature variation of N 2 O at the fracture edges exhibits a more pronounced sawtooth pattern. (2) At an inlet velocity of 0.01 m/s, the phase state of N 2 O varies at different locations within the fracture channel: liquid near the inlet and supercritical near the outlet. (3)Compared to the inlet temperature, the inlet velocity has a more pronounced effect on the heat extraction rate at the outlet. When the inlet velocity of N 2 O is 0.01 m/s and the temperature is 40°C, the maximum heat extraction rate reaches 830.89 W/m. These findings provide valuable insights into the thermal-hydraulic transport processes of heat exchange medium with phase-change characteristics in fractured reservoirs of hot dry rock.