Transient heat transfer processes in a single rock fracture at high flow rates

Abstract Understanding the process of heat transfer in fractures and the influence of fracture surface morphology is crucial for optimized heat mining. In this work, we investigated transient heat transfer behavior at high flow rates, ranging from 80 to 350 mm/s, at various inflow temperatures. A split sandstone core was installed in an experimental setup that allowed the fluid to be circulated, cooled and reheated. Water was circulated through the fracture and a temperature step was induced with the help of a heat exchanger upstream to the core. Thermocouples before and after the core enabled the measurement and evaluation of transient cooling and heating curves. The thermal breakthrough curves reveal the capacity of the rock to maintain maximum outflow temperature for a substantial amount of time. Thermal and mass dispersion coefficients are comparable but analytical solutions for heat transport can only be used with strong limitations. Heat transfer coefficients were determined using the stationary state. The evaluation of the surface roughness identifies its substantial influence on the heat transfer coefficient. Our experiments show that flow velocities beyond 200 mm/s lead to cold outflow temperatures, causing undesired cooling and prolonged heat transfer processes in the reservoir. These effects should not be underestimated in terms of the efficiency of geothermal energy generation.