EFFECT OF UNSTEADY WAKE ON DETAILED HEAT-TRANSFER COEFFICIENT AND FILM EFFECTIVENESS DISTRIBUTIONS FOR A GAS-TURBINE BLADE

Unsteady wake effects on derailed heat transfer coefficient and film c ooling effectiveness distributions from a gas turbine blade with film cooling are obtained using a transient liquid crystal technique. Tests were performed on a five-blade linear cascade at a axial chord Reynol ds number of 5.3 x 10(5) at cascade exit. Upstream unsteady wakes are simulated using a spoke-wheel type,wake generator: The test blade has three rows of film holes oil the leading edge and two ro,vs each on th e pressure and suction surfaces. Air and CO2 were used as coolants to simulate different coolant-to-mainstream density ratio effect. Coolant blowing ratio for air injection is varied from 0.8 to 1.2 and is vari ed from 0.4 to 1.2 for CO2. Results show that Nusselt numbers for a fi lm-cooled blade are much higher compared to a blade without film injec tion. Particularly, film injection causes earlier boundary layer trans ition on the suction surface. Unsteady wakes slightly enhance Nusselt numbers but significantly reduce film cooling effectiveness on a film- cooled blade col,compared with a film-cooled blade without wakes. Nuss elt numbers increase slightly but film cooling effectiveness increases sig,significantly with an increase in blowing ratio for CO2 injection . Higher density coolant (CO2) provides higher effectiveness at higher blowing ratios (M = 1.2) M;whereas lower density coolant (Air) provid es higher effectiveness at lower blowing ratios (M = 0.8).