Heat Transfer of an Impinging JET : Sensitivity Towards Inflow Conditions

The wall-heat transfer in the impingement region of a turbulent incompressible jet impinging onto a flat plate is assessed using direct numerical simulations.
For impinging jets, the mean wall-heat transfer distribution along the plate features a global maximum close to the jet axis and, for specific configurations, a secondary peak further downstream of the impingement region. While the appearance of the latter maximum has been the focus of many investigations, the occurrence of the former and its links to the inflow conditions of the jet have only been alluded to by existing studies. The present research considers two different inflow conditions: a fully developed turbulent pipe flow and a slightly convergent nozzle. The study shows that turbulent fluctuations, characterising the impingement region of the fully-developed turbulent inflow case, are significant in determining events of positive wall-heat transfer rates very close to the jet axis. On the contrary, similar events are not observed in the stagnation region of the convergent nozzle case, and the associated mean wall-heat transfer displays a markedly different distribution compared to the fully- developed turbulent inflow case.