Tip casing heat transfer measurements of a film-cooled turbine stage in a short duration facility

Abstract

An experimental study of the heat transfer to the tip casing of a fully scaled turbine stage was conducted. Pressure and surface temperature measurements were taken on the tip casing. Time-averaged heat flux data was computed and used to assess the influence of stage total pressure ratio, corrected speed and tip gap on casing heat flux.

The experimental work was conducted in the MIT Blowdown Turbine Facility using a highly loaded, film-cooled turbine stage. The facility is a short duration experimental structure capable of testing turbine stages under fully scaled conditions to produce useful test durations of 0.5 seconds. The turbine stage tested consisted of film-cooled turbine blades and nozzle guide vanes. During the course of this research, semi-infinite heat flux gauges were designed and fabricated by painting and baking thin platinum films onto machineable ceramic substrates. These gauges were used to experimentally measure surface temperatures on the tip casing with an estimated frequency response of 60 kilohertz. A tip casing insert was designed as an instrument holder to orient the heat flux gauges and highly sensitive pressure transducers on the tip casing surface to spatially resolve heat flux and static pressure.

A matrix of test conditions was devised to investigate the effects of various run conditions and tip gaps on tip casing flow. Both a 3.0% and a 1.5% tip-gap-to-span ratio were tested. Run conditions were established by varying the stage pressure ratio and the corrected speed. Results were compared within each test, between tests at the same tip gap, and between tests at different tip gaps. Time averaged data revealed the influence of rotor enthalpy extraction on casing heat flux. Pressure and corrected speed tests showed similar trends at both tip gaps. At design conditions, average endwall heat transfer was approximately 4% greater for the 1.5% tip gap.