Macroscale vortex impingement at a porous-fluid interface induces local heat-transfer enhancement

Abstract

Externally generated wakes can impinge on porous layers in several practical heat transfer applications. However, the transport of these macroscale vortices and their resulting influence on heat transfer in the porous layer remain unknown. In this paper, two-dimensional pore-resolved simulations are used to examine a square-bluff-body wake impinging on a porous layer composed of an in-line array of heated square obstacles at Re = 500-4000, Pr = 0.7 and 7.0, and porosities $ϕ$ = 0.75, 0.85, and 0.95. The incident macroscale vortices break down starting immediately at the porous-fluid interface. Following macroscale turbulent thermal transport in this entrance region, fluctuations farther downstream are aligned with the pore geometry and are generated at the microscale level by pore-throat shear, separation, and repeated obstacle wakes. Direct impact locally improves heat transfer at the porous-fluid interface by increasing the local Nusselt number relative to a non-impingement region. The interfacial heat-transfer enhancement varies with porosity, Reynolds number, and Prandtl number, with a peak enhancement of 18.2% observed for $ϕ$ = 0.85, Pr = 7.0, and Re = 1000. The wake-associated macroscale turbulent heat-flux contribution decays within a consistence entrance region extending approximately 3-4 unit cells into the porous layer across all investigated porosities, Reynolds numbers, and Prandtl numbers. For Re$\ge$1000, the persistent wake momentum deficit subsequently causes the heat-transfer contrast to become negative, resulting in lower heat transfer in the impingement region than in the non-impingement region.