Self-Acting Clearance Control for Turbine Blade Outer Air Seals

Abstract

Tip leakage losses constitute a major source of inefficiency in turbine rotor blades. Blade tip sealing has been a challenge due to the dynamic changes in clearance between the blade tips and the surrounding stationary casing. The variation is due to thermal and mechanical loads on the rotating and stationary structures. Improved sealing in both the high pressure compressor and the high pressure turbine can provide significant reductions in specific fuel consumption and compressor stall margin. Existing seal designs deteriorate. This increase in leakage reduces engine efficiency and thus, the engine burns more fuel to maintain the same power output. Turbine temperatures then rise toward material limits and decrease the overall life of the engine. A novel concept for controlling clearance between rotating blade tips and stationary outer air seal for gas turbine application is presented in this paper. The seal consists of segmented compliant sealing surfaces providing primary sealing, and a flexible sheet metal seal acting on the outer surface of the segments serving as a secondary seal. A commercial computational fluid dynamic (CFD) solver is used for the numerical flow analysis work to simulate the flow characteristics and spatially varying thermodynamic conditions in the flow passage between the blades tip and the primary sealing surface. The results showed that the seal surface geometry has a major impact on the flow field, and several design parameters were identified. A sudden flow contraction (step), as well as gradual converging and diverging seal surface influence differently the flow properties and consequently the seal surface pressures. Also, both pressure difference and clearance were found to be significant factors affecting the flow. The results presented in this study demonstrate the mechanisms which enable the seal to self-balance at a controlled clearance without contacting the blades.