The overall heat transfer coefficient (OHTC) of rock fractures is a fundamental parameter for characterizing the heat transfer behavior of rock fractures in hot dry rock (HDR) geothermal mining. Although a number of practical formulae for heat transfer coefficients have been developed in the literature, there is still no widely accepted analytical solution. This paper constructs highly accurate analytical solutions for the temperatures of the inner fracture wall and the fluid. Then they are employed to develop new definition-based formulae (formula A and its simplification formula B) of the OHTC, which are well validated by the experimental and numerical simulation results. An empirical correlation formula of heat transfer coefficient is proposed based on the definition-based formulae which can be directly used in the numerical simulations of heat transfer in rock fractures. A site-scale application example of numerical simulation also demonstrates the effectiveness of the empirical correlation formula.

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Keywords: Hot dry rock fractures, Temperature distribution, Convective heat transfer coefficient, Empirical correlation formula