An experimental study of density ratio effects on the film cooling injection from discrete holes by using PIV and PSP techniques

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An experimental study was conducted to investigate the performance of film cooling injection from a row of circular holes spaced laterally across a flat test plate. While a high-resolution Particle Image Velocimetry (PIV) system was used to conduct detailed flow field measurements to quantify the dynamic mixing process between the coolant jet stream and the mainstream flows, Pressure Sensitive Paint (PSP) technique was used to map the corresponding adiabatic film cooling effectiveness on the surface of interest based on a mass-flux analog to traditional temperature-based cooling effectiveness measurements. The cooling effectiveness data of the present study were compared quantitatively against those derived directly from the temperature-based measurements under the same or comparable test conditions in order to validate the reliability of the PSP technique. The effects of the coolant-to-mainstream density ratio (DR) on the film cooling effectiveness were investigated by performing experiments at fixed blowing ratios by using either Nitrogen (DR=0.97) or CO2 (DR=1.53) as the coolant streams for the PSP measurements. An accompanying analysis of scaling quantities, such as the coolant-to-mainstream momentum flux ratio, I, and bulk coolant-to-mainstream velocity ratio, VR, in addition to the most-commonly used blowing ratio (i.e., the coolant-to-mainstream mass flux ratio), M, was also conducted to illuminate the extent to which the flow scenario can be described using purely kinematic or dynamic means. It was found that the scaling quantities that give more weight to density ratio (i.e., blowing ratio, M, and then momentum ratio, I) have more success to collapse measurement data from varying density ratio of the coolant flows for the cases with relatively low coolant flow rates, while the coolant-to-mainstream bulk velocity ratio, VR, may be used with some success to scale the film cooling effectiveness for the cases with higher coolant flow rates.