Role of large-scale coherent structures in impinging jet heat transfer.

A large-scale coherent structure responsible for the enhancement of stagnation-point heat transfer was successfully determined using a statistical analysis with conditional sampling in an impinging round jet. It was found that the optimal nozzle-to-plate spacing for maximal stagnation-point heat transfer is identical with the jet development length for a strong turbulent stream carrying large-scale energy-containing eddies. A physical model of heat transfer enhancement was experimentally derived from the viewpoint of turbulent surface-renewal induced by large-scale eddies impinging on the stagnation-point boundary layer. It was found that the effect of heat transfer enhancement is a function of the proposed surface-renewal parameter consisting of the characteristic turbulence intensity and frequency of the large-scale eddies.