The effects of blade passing on the heat transfer coefficient of the overtip casing in a transonic turbine stage

In a modern gas turbine engine, the outer casing (shroud) of the shroudless high-pressure turbine is exposed to a combination of high flow temperatures and heat transfer coefficients. The casing is consequently subjected to high levels of convective heat transfer, a situation that is complicated by flow unsteadiness caused by periodic blade-passing events. In order to arrive at an overtip casing design that has an acceptable service life, it is essential for manufacturers to have appropriate predictive methods and cooling system configurations. It is known that both the flow temperature and boundary layer conductance on the casing wall vary during the blade-passing cycle. The current article reports the measurement of spatially and temporally resolved heat transfer coefficient (h) on the overtip casing wall of a fully scaled transonic turbine stage experiment. The results indicate that h is a maximum when a blade tip is immediately above the point in question, while the lower values of h are observed when the point is exposed to the rotor passage flow. Time-resolved measurements of static pressure are used to reveal the unsteady aerodynamic situation adjacent to the overtip casing wall. The data obtained from this fully scaled transonic turbine stage experiment are compared to previously published heat transfer data obtained in low-Mach number cascade-style tests of similar high-pressure blade geometries.