Abstract
The heat transfer coefficient (HTC) is an important parameter describing the heat transfer characteristics between rock fracture walls and flowing fluid. This study summarizes and evaluates the existing typical overall heat transfer coefficient (OHTC) formulas (A - D) and proposes four new OHTC formulas (E - H). As a novelty different from the existing analytical methods, the new method solves the temperature distribution of a cylindrical rock sample in the form of integral, and thus obtains the expression of fracture surface temperature. Various experimental data were employed to evaluate each formula. The adaptability and stability of OHTC formulas are closely related to the flow rate. Based on the commonness of each formula, the key parameter affecting OHTC value is presented. Fluid flow and heat transfer in a single fracture is numerically modeled. Comparison of outlet water temperature obtained by numerical method and experimental measurement shows that the accuracy of formulas D, G and H is higher. For comparison with the parallel-plate model, the fractures with heterogeneous aperture are constructed, and heterogeneous HTC is adopted. The results show that the water temperature contours in fractures modeled with homogeneous HTC are more tortuous, and the temperature contours modeled with heterogeneous HTC are smoother. As the flow rate increases, the difference of outlet temperature between fractures with homogeneous and heterogeneous apertures increases.