An experimental study of flat plate film cooling with cylindrical holes by using PIV, HWA and PSP techniques

An experimental study was conducted to investigate both the aerodynamic and the thermal behavior of film cooling jets exiting from three cylindrical holes on a flat plate. Measurements have been carried out in a low-speed wind tunnel, at low inlet turbulence intensity level, with blowing ratio (BR) varied in the range between 0.5 and 1.5. A high-resolution Particle Image Velocimetry (PIV) system was used to conduct detailed flow field measurements in the streamwise plane, while the Hot Wire Anemometry (HWA) technique provided measurements of velocity components in the vertical-lateral planes at two locations downstream of the holes. Concerning the thermal measurements, the binary pressure-sensitive paint (PSP) technique has been employed to map the adiabatic film cooling effectiveness (η) on the surface of interest, at density ratio of DR = 1 and 1.5, by using N2 and CO2 as coolant stream, respectively. 2-D flow field measurements in the streamwise plane have shown that the cooling jet stays attached to the flat plate surface at BR < 1. Conversely, jets lift-off at BR > 1. Thermal measurements at DR = 1 confirmed this behavior: at BR = 0.5, the highest levels of film cooling effectiveness are achieved immediately downstream of the hole whereas BR values of 1 and 1.5 have a detrimental impact on the thermal coverage. With a focus on the case at BR = 1, another set of experiments was conducted with CO2 as coolant to investigate the influence of DR on cooling efficiency, either at matched BR or momentum flux ratio (I). The latter was found to be the more appropriate parameter to scale cooling effectiveness results when comparing low and high DR cases.