Laser imaging investigation of transient heat transfer processes in turbulent nitrogen jets impinging on a heated wall

This paper is focused on simultaneous two-dimensional temperature and velocity field measurements to study heat transfer processes of ambient temperature nitrogen jets impinging on a heated wall. Instantaneous temperature fields were measured by laser-induced fluorescence (LIF) of toluene seeded as a tracer in the nitrogen carrier flow. High repetition rate particle image velocimetry (PIV) data were recorded simultaneously to investigate interactions between velocity and temperature fields. Different heat transfer phenomena were observed by parametric variation of Reynolds number, turbulence intensity and surface temperatures. Whereas laminar flows at low Reynolds numbers exhibited a smooth thermal boundary layer, highly dynamic flares were observed ejecting hot gas from the near wall region into the free stream at increased Reynolds numbers. Rotating vortex structures were identified as the leading source of flare generation. Additionally, turbulent heat flux was quantified and shown to significantly contribute to increased heat transfer rates.

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