COMBINED EFFECT OF FREE-STREAM TURBULENCE AND UNSTEADY WAKE ON HEAT-TRANSFER COEFFICIENTS FROM A GAS-TURBINE BLADE

The combined effect of free-stream turbulence and unsteady wakes on tu rbine blade surface heat transfer was studied. The experiments used a Jive-blade linear cascade in a low-speed wind facility. A turbulence g rid and spoked-wheel type wake generator produced the free-stream turb ulence and unsteady wakes. The mainstream Reynolds numbers based on th e cascade inlet mean velocity and blade chord length were 100,000, 200 ,000, and 300,000. Results show that the blade time-averaged heat tran sfer coefficient depends on the mean turbulence intensity, regardless of whether this mean turbulence intensity is from unsteady wake only, turbulence grid only, or a wake and grid combination. The higher mean turbulence promotes earlier boundary layer transition and causes much higher hear transfer coefficients on the suction surface. It also sign ificantly enhances the heat transfer coefficients on the pressure surf ace. The unsteady wake greatly affects blade heat transfer for low onc oming free-stream turbulence; however, the wake effect diminishes for high oncoming turbulence. The freestream turbulence also strongly affe cts blade heat transfer for a low wake passing frequency, but the onco ming turbulence effect diminishes for a high unsteady wake condition.