Effect of a Sinusoidal Leading Edge Undulation on the Rotor Tip Gap Flow in a Highly Loaded Axial Compressor

In this paper we present a numerical investigation of the interaction between a sinusoidal chord length undulation at the leading edge and the rotor tip gap flow. A “2.5-stage” research compressor representative of the front stages of the high-pressure compressor of a modern turbofan engine serves as a test case. First, a linear cascade with a moving endwall was used to investigate the fundamental flow mechanics. RANS simulations were conducted to carry out a parametric study on the design variables amplitude, wavelength, and phase of the sine function at the blade tip. Favorable designs were found to reduce the flow blockage caused by the tip leakage vortex, with the most promising configuration increasing the mass flow in the tip region in the order of 20%. We attribute the reduction in flow blockage to two effects: Streamwise vortices induced by the undulated leading edge interact with the tip leakage vortex, mitigating the vorticity of the tip leakage vortex. In addition, the pressure difference between suction side and pressure side is reduced at the blade tip resulting in a smaller tip gap flow. As a result, the trajectory and the strength of the tip leakage vortex are changed. The design principles derived from the cascade study were then applied to the first blade row of the compressor. Again, a mass flow increase in the rotor tip region was observed, confirming the findings of the cascade study. The final design enhances the surge margin of the compressor in the order of 20% at part-load while maintaining efficiency at the design point. In summary, we elucidate the passive flow control mechanism of a sinusoidal leading edge undulation at a rotor tip. We demonstrate by numerical analysis that applying this undulation to one rotor of a “2.5-stage” high-pressure compressor improves stability at part-load without compromising efficiency at the design point.