Abstract: The development and utilization of shallow geothermal energy is related to the successful achievement of the “doublecarbon” target. The study on seepage and heat transfer characteristics of the rock fracture in the target reservoir is critical to the effectiveness of the groundwater source heat pump system. However, most of the previous studies on seepage and heat transfer in fractures considers a single factor such as rough surface, flow velocity, confining pressure, temperature for the evaluation of its heat transfer efficiency. The understanding of the evolution mechanism of seepage and heat transfer and its contribution degree by the change of shallow geothermal reservoir (confining pressure and rock temperature) at multiple flow rates have not been systematically investigated. Therefore, based on a self-developed seepage and heat transfer system of fractured rocks, this paper carried out the seepage and heat transfer test under multiple flow rates for sandstone with a single fracture. Subsequently, the evolution characteristics of single fracture seepage and heat transfer under multiple flow rates were analyzed. Finally, the influenced mechanism and contribution degree of sensitive parameters (confining pressure and rock temperature) in shallow geothermal on seepage and heat transfer were discussed. The test results show that: (1) The trends of outlet water temperature and convective heat transfer coefficient with the increase of water flow rate have obvious transition stages. (2) The increase of confining pressure leads to the fracture closing and the thinning of flow boundary layer, while the increase of temperature gradient causes a higher outlet water temperature and convective heat transfer coefficient due to the increase of heat flux density. (3) The contribution of rock temperature is significantly higher than that of confining pressure. Especially when the temperature increases from 60 ℃ to 70 ℃ , the convective heat transfer coefficient increases dramatically.

Key words: shallow geothermal, sandstone, a single fracture, multiple flow rates, convective heat transfer coefficient