Aerodynamic and heat transfer aspects of tip and casing treatments used for turbine tip leakage control

Axial flow turbine stages are usually designed with a gap between the tips of the rotating blades and a stationary outer casing. The presence of a strong pressure gradient across this gap drives flow from the pressure side of the blade to the suction side. This leakage flow creates a significant amount of energy loss of working fluid in the turbine stage. In a modern gas turbine engine the outer casing of the high-pressure turbine is also 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 aggravated by flow unsteadiness caused by periodic blade-passing events.;An experimental investigation of the aerodynamic and heat transfer effect of tip and casing treatments used in turbine tip leakage control was conducted in a large scale, low speed, rotating research turbine facility.;The effects of casing treatments were investigated by measuring the total pressure field at the exit of the rotor using a high frequency response total pressure probe. A smooth wall as a baseline case was also investigated. The test cases presented include results of casing treatments with varying dimensions for tip gap height of t/h=2.5%. The results of the rotor exit total pressure indicate that the casing treatment significantly reduced the leakage mass flow rate and the momentum deficit in the core of the tip vortex. The reductions obtained in the tip vortex size and strength influenced the tip-side passage vortex and other typical core flow characteristics in the passage. Casing treatments with the highest ridge height was the most effective in reducing the total pressure loss in the leakage flow of the test blades. This was observed at a radius near the core of the tip vortex. It appears that casing treatments with the highest ridge height is also the most effective from a global point of view, as shown by the passage averaged pressure coefficient obtained in the last 20% of the blade height.;The effect of the new blade tip concept, inclined squealer tip, on tip leakage flow with and without casing treatments is also investigated. The results of the rotor exit total pressure indicate that the inclined squealer tip arrangement has significant effects on both passage core flow and the interaction between the leakage vortex and the tip side passage vortex.;A steady-state method of measuring convective heat transfer coefficient on the casing of an axial flow turbine is also developed for the comparison of various casing surface and tip designs used for turbine performance improvements. The free-stream reference temperature, especially in the tip gap region of the casing varies monotonically from the rotor inlet to rotor exit due to work extraction in the stage. In a heat transfer problem of this nature, the definition of the free-stream temperature is not as straight forward as constant free-stream temperature type problems. The accurate determination of the convective heat transfer coefficient depends on the magnitude of the local freestream reference temperature varying in axial direction, from the rotor inlet to exit. The current investigation explains a strategy for the simultaneous determination of the steadystate heat transfer coefficient and free-stream reference temperature on the smooth casing of a single stage rotating turbine facility. The heat transfer approach is also applicable to casing surfaces that have surface treatments for tip leakage control. The overall uncertainty of the method developed is between 5% and 8% of the convective heat transfer coefficient.;The test cases presented show that the casing heat transfer is affected by the tip gap height. The heat transfer coefficient increases as the tip gap increases for both with and without casing treatments. It is also shown that the effect of ridge height on heat transfer coefficient is negligible for tip gap height of t/h=0.9%.