Experimental and analytical study of the overall heat transfer coefficient of water flowing through a single fracture in a granite core

An experimental and analytical study was conducted to improve the understanding of the heat transfer characteristics between flowing water and granite fracture wall. A systematic experimental study of water flow and heat transfer through a single fracture in a well-characterized granite specimen was performed under confining pressures of 0, 3 and 6 MPa and confining temperatures of 70, 80, 90 and 100 °C. Based on the test data, four existing formulas of the overall heat transfer coefficient (OHTC) were evaluated. Compared to other formulas that may predict abnormal OHTC values (extremely large or negative values), Formula D and the newly proposed Formula E were suitable indicators of the OHTC under our experimental conditions. The proposed method is recommended because of its rigorous theoretical basis, concise form and effective prediction results. Based on the proposed formula E, we found that OHTCs were positively correlated with flow rates. However, wider fracture apertures resulted in smaller OHTCs, suggesting that wider apertures decrease the heat transfer efficiency. Additionally, most of the slopes of the OHTC-aperture curves were similar. Finally, we investigated the conditions for which OHTC formulas produced abnormal values.