Experimental study on convective heat transfer of supercritical carbon dioxide in granite fractures

Carbon dioxide (CO 2 ), owing to its superior thermodynamic properties, is a promising geothermal working fluid. The development of CO 2 -enhanced geothermal systems (CO 2 -EGS) requires an accurate characterization of its convective heat transfer, particularly in the supercritical state (SC-CO 2 ), within rock fractures. To this end, an experimental apparatus was developed, and the tests were conducted on granite fractures across temperature (50 °C-180 °C) and flow rate (9-21 mL/min) ranges. The results indicate that both the equivalent hydraulic aperture and permeability of the fracture decrease with increasing temperature, with the reduction rate diminishing at higher temperatures. The temperature field evolution exhibits three distinct phases: rapid decrease, slow decrease, and thermal equilibrium, with the final equilibrium temperature decreasing significantly at higher flow rates. Furthermore, a novel formula for the convective heat transfer coefficient (HTC) is proposed, which explicitly incorporates actual SC-CO 2 enthalpy obtained from the NIST REFPROP database based on the Span-Wagner equation of state. This formula eliminates the non-physical negative HTC values generated by conventional methods, thereby providing a physically consistent description of heat transfer. The proposed correlation reveals that the HTC increases with flow rate, driven primarily by a linear proportionality to the mass flow rate. Conversely, the HTC decreases with rising temperature, with the rate of reduction diminishing at higher temperatures. This behavior is governed by the temperature-dependent decline in both the isobaric heat capacity and the enthalpy difference of SC-CO 2 . Consequently, this work establishes a more accurate and physically consistent framework for quantifying the HTC of SC-CO 2 in fractured rock.