Transient modeling of impinging heat transfer from an acoustically modulated turbulent air jet to a normally positioned flat surface

Abstract

The subject of this study is the numerical investigation of the impingement of an axisymmetric turbulent air jet on a flat surface, the influence of acoustic modulations on the coherent structures that form around the jet, and the effects on the heat transfer from the jet to the heated surface. The study showed how controlled acoustic perturbations influence the exit velocity profile from the nozzle and the formation of vortices in the boundary-layer of the jet. Since vortices are responsible for the redistribution of thermal energy transferred from the jet to the surface in impinging jet flow configurations, it is crucial to investigate whether their formation and evolution can be controlled. The results of the numerical simulations indicated very good agreement with experimentally measured velocity field. However, a problem arises in the prediction of the heat transfer because the standard k-ε model overestimates the values of the heat transfer coefficient in the stagnation zone because they were theoretically developed to use shear stresses for the generation of the turbulent kinetic energy, while in reality normal stresses are responsible for their generation in this flow situation. Due to the unstructured mesh used for the calculations, there are discrepancies in the results at larger edge distances. The complex flow at the impingement surface, the occurrence of secondary vortices and recirculation zones and their direct effect on the heat transfer cannot be fully captured by the mathematical model, even if the numerical errors are acceptable compared to experiments.