The influence of foreign gas injection and slot geometry on film cooling effectiveness

Abstract

Measurements of foreign gas concentration are presented for a wide range of velocity and density ratios. The mainstream boundary layer thickness and slot lip geometry were varied in order to investigate the importance of these parameters through the range of velocity and density ratios covered. The value of mass concentration at the wall increased continuously with mass flux injected, though the improvement was relatively small for velocity ratios greater than unity. For a given mass flux of foreign gas the wall value of moss-concentration (and hence by analogy the adiabatic wall film cooling effectiveness based on enthalpy) was slightly greater for the lightest gas injected. The corresponding wall value of mole-fraction (analogous to effectiveness based on adiabatic wall temperature) was considerably greater for the lightest gas. Increasing the slot lip thickness lowered the film cooling effectiveness and its influence increased as the coolant density was reduced. The effect of increasing the mainstream boundary layer thickness was much smaller though still significant when a light gas coolant was used. The present results are compared with other experimental data and with the predictions of the boundary layer model of film cooling flows.