Understanding working fluids flow and heat transfer in Enhanced Geothermal System (EGS) is critically important for assessing production capabilities and heat extraction rates from hot dry rock (HDR) effectively. Water is a conventional working fluid of EGS; therefore, it is necessary to study water–rock coupling heat transfer within HDR fractures. A single fracture is a basic element of complex fracture systems of EGS, so there has been many researches investigating water flow and heat transfer through a single fracture within a cylindrical HDR specimen for preliminary study, and heat transfer coefficient (HTC) is often adopted to evaluate heat transfer performance between water and rock, but there is a lack of knowledge about the inner surface temperature ( T i ), which is a key parameter to calculate HTCs, due to limitations of current experimental methods and analytical solutions. Numerical modeling can be an alternative way to help us have a more comprehensive understanding of the distributions of T i , but the 2D numerical models only reveal the distribution of T i along the flow direction without its distribution perpendicular to the flow direction, namely, the distribution along radius. Therefore, it is essential to conduct the corresponding 3D numerical modeling to obtain the radial distribution of T i . In this paper, a 3D numerical model accounting for surface morphology is proposed to simulate water flow and heat transfer through a single fracture within a cylindrical HDR specimen and verified by experimental data. The results reveal that the radial distribution of T i can be described by a quadratic function mostly, and neither the surface morphology nor the phase state has influence on the tendency of T i . Finally, four relatively stable formulas to calculate HTCs so far are compared and discussed.

中文翻译：

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单个裂缝内水-岩耦合传热的 3D 数值模拟

了解增强型地热系统 (EGS) 中的工作流体流动和传热对于有效评估干热岩 (HDR) 的生产能力和热提取率至关重要。水是EGS的常规工作流体；因此，有必要研究 HDR 裂缝内的水岩耦合传热。单个裂缝是EGS复杂裂缝系统的基本要素，因此对圆柱形HDR试件内单个裂缝的水流和传热进行了初步研究，常采用传热系数（HTC）来表示。评估水和岩石之间的传热性能，但缺乏关于内表面温度 (T i ) 的知识，这是计算 HTC 的关键参数，由于当前实验方法和分析解决方案的局限性。数值建模可以作为一种替代方法来帮助我们更全面地了解 T i 的分布，但二维数值模型仅揭示了 T i 沿流向的分布，而没有其垂直于流向的分布，即沿半径分布。因此，必须进行相应的 3D 数值建模以获得 T i 的径向分布。在本文中，提出了一种考虑表面形态的 3D 数值模型，以模拟通过圆柱形 HDR 试样内单个裂缝的水流和传热，并通过实验数据进行验证。结果表明，T i 的径向分布主要可以用二次函数来描述，并且无论是表面形貌还是相态都对T i 的趋势没有影响。最后，比较和讨论了迄今为止计算 HTC 的四个相对稳定的公式。