This study investigates the multiscale roughness effects on hydrodynamic heat transfer (or heat transfer associated with water flow) in a single fracture, based on direct numerical simulations of fluid flow and heat transfer in fracture-matrix systems with or without secondary roughness considered. The simulation results show that the primary roughness controls the low velocity regions and the overall flow direction in the fracture, while the secondary roughness enhances the complexity of the flow near the local sharp cornered asperities and promotes the generation of eddies. The changes of flow behavior could in turn profoundly affect the water temperature distribution and local heat transfer coefficient in the fracture-matrix system. In addition, two competing mechanisms impacting the overall heat transfer coefficient (h) due to the multiscale roughness are revealed. On one hand, the increase in flow velocity induced by primary and secondary roughness increases h. On the other hand, the eddy zone induced by the secondary roughness can trap the high-temperature fluid and inhibit the heat transfer from the fracture surface to the ambient fluid, which could decrease h. The macroscopic effect of surface roughness on h is the result of the joint action of these two competing influence mechanisms.

本研究基于对考虑或不考虑二次粗糙度的裂缝-基质系统中的流体流动和传热的直接数值模拟，研究了多尺度粗糙度对单个裂缝中流体动力传热（或与水流相关的传热）的影响。模拟结果表明，初级粗糙度控制着裂缝中的低速区域和整体流动方向，而次级粗糙度增强了局部尖角粗糙附近流动的复杂性，促进了涡流的产生。流动行为的变化反过来又会深刻影响裂缝-基质系统中的水温分布和局部传热系数。此外，两种相互竞争的机制会影响整体传热系数 ( h) 由于多尺度粗糙度被揭示。一方面，初级和次级粗糙度引起的流速增加增加了h。另一方面，二次粗糙度引起的涡流区可以捕获高温流体并抑制从裂缝表面到周围流体的热传递，这可能会降低h。表面粗糙度对h的宏观影响是这两种相互竞争的影响机制共同作用的结果。