On the links between unsteady separation and Nusselt number deterioration during vortex-wall interaction

•Upwash-side heat transfer reduction in vortex-wall interaction is investigated.•The role of unsteady boundary layer separation in the reduction is examined.•Thermal boundary layer (TBL) thickening causes the reduction pre-separation.•Localized near-wall TBL thickening causes the reduction post-separation.•Blockage of the main vortex flow by the separated flow causes the near-wall effect.•Elimination of boundary layer separation causes further heat transfer reduction. A CFD study of an axisymmetric vortex ring interacting with a flat, constant-temperature, heated wall is conducted to study the physical nature of the mechanisms leading to deterioration in wall heat transfer due to the unsteady boundary layer separation resulting from vortex-wall interaction. The study utilizes a supplemental computation of a hypothetical flow configuration in which slip is allowed at the heated wall to eliminate separation of the boundary layer during the interaction. Comparison between the physical and hypothetical cases enables “isolation”, and hence understanding of the role of separation in the deterioration of Nusselt number (Nu). It is found that the nature of the near-wall upwash velocity leading to Nu deterioration is fundamentally altered by the presence of spearation. Without separation, the upwash is driven directly by the primary vortex ring. In contrast, with separation, the near-wall upwash is modified significantly by blockage from boundary layer separation and interaction with the secondary vortex ring. The modification is associated with the development of a near-wall “sublayer” that is responsible for the deterioration of Nu independent of the behavior of the overall thermal boundary layer thickness.