PERFORMANCE INFLUENCE OF LEADING-EDGE TUBERCLES ON A HIGH-SPEED AXIAL COMPRESSOR

Abstract

Gas turbine engines are one of the crucial innovations of the modern world and developments have progressed from their introduction almost a century ago. Historically the compressor has been the target of engineering improvements, including efforts to expand performance and operability. Engine performance is governed in part by achievable compressor pressure ratio, which is limited by compressor stall. To mitigate this, many active and passive flow control devices have been tested over the decades, most of which have never had a practical application. But a bio-inspired passive flow control device known as a tubercle has shown promising results when applied to aircraft wings and other lifting surfaces, including 2-D compressor profiles. In this work, a three-phase programme involving tubercle optimization, 2-D and 3-D experiments have been undertaken to better understand performance improvements in various airfoil operation regimes. Self-Organizing Maps (SOM), an artificial neural network technique, were used to compare tubercle geometric and performance parameters. Two new geometric parameters: Reynolds number based on hydraulic diameter and tubercle aspect ratio, and one new performance parameter: post-stall operability area are introduced in this work to characterize the 3-D flow associated with tubercles. SOM results indicated that higher maximum coefficient of lift and stall angle can be achieved with lower tubercle amplitude and wavelength. Low-speed wind tunnel tests were conducted to expand the data set and findings indicated that optimized geometry is around amplitude of 3% chord and wavelength of 11% chord. A compressor cascade was designed based on the Rolls-Royce A-250 first stage rotor blade mean conditions. Total pressure measurements, flow visualization and pressure sensitive paint were used to assess tubercle and baseline performance. Against a range of tubercled test articles, the 3% chord amplitude and 11% chord wavelength geometry was ultimately selected for 3-D testing in the Rolls-Royce A-250 axial compressor at operational static conditions. Compressor blades were modified using Electrical Discharge Machining and the compressor with both modified rotor and stator blades showed performance improvements at 80% speed when tested with bleed valve partially open. But these trends were not sustained at higher speeds. The A-250 stall is characterized by stall cell formation at the first stage, eventually developing into compressor stall. Of significance, when tubercles were installed, the compressor successfully eliminated stall cell formation until stall initiated in the final stage, defying the predictions of the Greitzer model. Despite the overall reduced compressor map performance, tubercles successfully increased pressure ratio at 80% even with open bleed valve. Tubercles can potentially improve the overall performance if optimized to individual compressor stages and with improved manufacturing methods.