The flow and heat transfer characteristics of supercritical mixed-phase CO2 and N2 in a 3D self-affine rough fracture

Abstract

Utilizing a three-dimensional self-affine function, we constructed the fracture morphology and established a numerical model considering the thermal-hydraulic coupling (TH) mechanism to simulate the flow and heat transfer processes of CO2 and N2 in a single fracture under supercritical mixed-phase conditions in dry hot rock. The fractal dimension of the fracture was set to 2.5, and the Joint Roughness Coefficient (JRC) roughness coefficient was 13.71. The model accurately simulated the flow and heat transfer processes of supercritical mixed-phase CO2 and N2 fluids within the fracture under the conditions of 50 MPa underground pressure, 200 °C rock exterior temperature, initial temperature of 32 °C for the heat transfer fluid, and inlet velocities ranging from 0.005 to 0.03 m/s. The results revealed that the output thermal power curves for nine different ratios of CO2 and N2 could be classified into three categories.Among the four conventional thermodynamic parameters, specific heat capacity and density significantly impacted output thermal power. The convective heat transfer process between the fluid and the rock wall resulted in temperature fluctuations, primarily influenced by the sharp edges of the fracture wall. As the inlet velocity increased, the outlet-inlet temperature difference monotonically decreased. The fluid with a composition of 45 % N₂ and 55 % CO₂ exhibited the smallest reduction in temperature, with a value of 50.52 °C. Meanwhile, the output thermal power monotonically increased, with the fluid composition of 5 % N2 and 95 % CO2 experiencing the most significant increase, reaching 122.22 W.