Film cooling on a convex surface: influence of external pressure gradient and Mach number on film cooling performance

The film cooling performance on a convex surface subjected to zero and favo urable pressure gradient free-stream flow was investigated. Adiabatic film cooling effectiveness values were obtained for five different injection geo metries, three with cylindrical holes and two with shaped holes. Heat trans fer coefficients were derived for selected injection configurations. CO2 wa s used as coolant to simulate density ratios between coolant and free-strea m close to gas turbine engine conditions. The film cooling effectiveness re sults indicate a strong dependency on the free-stream Mach number level. Re sults obtained at the higher free-stream Mach number show for cylindrical h oles generally and for shaped holes at moderate blowing rates significant h igher film cooling effectiveness values compared to the lower free-stream M ach number data. Free-stream acceleration generally reduced adiabatic film cooling effectiveness relative to constant free-stream flow conditions. The different free-stream conditions investigated indicate no significant effe cts on the corresponding heat transfer increase due to film injection. The determined heat flux ratios or film cooling performance indicated that cool ant injection with shaped film cooling holes is much more efficient than wi th cylindrical holes especially at higher blowing rates. Heat flux penaltie s can occur at high blowing rates when using cylindrical holes.