Multiscale Roughness Influence On Hydrodynamic Heat Transfer In A Single Fracture

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.