Experimental Determination of Transition Reynolds Number for Unsteady Film Cooling

In order to investigate the unsteady effect on transition in film cooling, an 11-m long Ludwieg Tube, consisting of a test section placed between the high pressure and low pressure sections of a shock tube, has been constructed. With this device, a controlled unsteady, low subsonic flow lasting for a period of several milliseconds is obtained. The transition Reynolds Number is determined from the output of thin film heat flux transducers having a response time of a fraction of a microsecond. The results indicate that, in the case of flow without gas injection into the boundary layer, the transition Reynolds Number is one order of magnitude smaller than the critical Reynolds Number for steady wedge flow with the same pressure gradient. With injection, the transition Reynolds Number is small near the injection slot; far downstream, it increases asymptotically to the value for flow without injection.