This study is motivated by understanding the connections between the vortical structures in impinging jets and the wall heat transfer. The particular objectives of the study are: (1) examining how the stage of evolution of vortex pairing in the jet might influence the wall heat transfer; (2) establishing correlations between the vortex characteristics and the Nusselt number (Nu) distribution; (3) exploring the physics of the thermal boundary layer behavior and the associated near-wall flow that causes the enhancement and the deterioration in heat transfer during vortex-wall interaction; and finally (4) evaluating a newly published hypothesis of the mechanisms of the heat transfer enhancement and deterioration during this interaction. To address the first two objectives, CFD simulations are conducted of three simplified model problems involving the interaction of isolated axisymmetric vortex rings with a flat, constant-temperature, heated wall. The cases represent three scenarios of vortex-wall interaction: before (Case I), during (Case II) and after (Case III) pairing. The results show that when two vortices concurrently interact with the wall and undergo pairing (Case II), a significant instantaneous enhancement in Nu is attained in comparison to that associated with a single vortex interacting with the wall (Cases I and III). In all three cases, a deterioration in Nu is observed simultaneously with the enhancement (but at different radial locations) due to the formation of the secondary vortex (SV). However, the net effect of vortex-wall interaction on the heat transfer remains positive with Case II producing the highest heat transfer rate than the other cases.Two additional CFD cases are conducted to address the third objective. Both cases are the same as Case I except for one parameter. In the first of the additional cases, the thermal diffusivity is set to zero (=0) to understand the role of diffusion in heat transfer enhancement. Analysis of this case is complemented with a simple analytical model based on the unsteady 1D energy equation with wall-normal (axial) velocity perturbation. The results lead to the hypothesis that the axial velocity induced by the primary vortex (PV) toward the wall is the main factor for enhancement of the heat transfer on the downwash side of the vortex core by causing thinning of the thermal boundary layer (TBL). Thermal diffusion is found to limit this enhancement and cause the TBL to thicken when the downwash velocity weakens.In the second of the additional cases, the wall shear stress is set to zero (=0) to eliminate separation of the boundary layer, and hence evaluate the role of separation in deterioration of Nu. As in the case of Nu enhancement, the results show that the axial velocity is the leading factor driving the Nu deterioration. Surprisingly, eliminating separation leads to even smaller minimum Nu; found to be caused by closer approach of the PV toward the wall in the absence of the secondary vortex (due to separation elimination). Nevertheless, the overall effect of eliminating separation is positive since the closer proximity of the PV to the wall also causes significant Nu enhancement on the downwash side, producing a net positive Nu change.Finally, trajectories of selected fluid particles are tracked in a thermofluidic boundary-layer-resolved Lagrangian analysis in order to evaluate a recently published "surface renewal model" that explains the mechanisms of heat transfer due to vortex-wall interaction. The results show that while some elements of this hypothesis, regarding the heat transfer enhancement on the downwash side, are valid, the hypothesis is based on the wrong physics when it comes to the heat transfer deterioration on the upwash side.
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