Review of unsteady aerodynamic problems and control strategies for the blade tip flow of axial compressors

When axial compressors operate under high-loading conditions, inherent unsteady flow phenomena emerge in the tip region as a consequence of the tip leakage flow. These aerodynamic phenomena are collectively known as tip flow unsteadiness. It has been proven that tip flow unsteadiness not only serves as an excitation source of both non-synchronous vibration and tip noise but is also the cause of short-length-scale stall inception. These structural and aerodynamic problems have become common issues in highly loaded axial compressors, which require a breakthrough in the study of tip flow unsteadiness. This article begins with a review of past research on unsteady tip flow phenomena, examining them from the perspectives of self-excited unsteadiness and rotating instability (RI). Detailed discussions are presented on the relationships between RI and tip clearance noise, non-synchronous vibrations, and stall inceptions. This is followed by explanations for the origin of the tip flow unsteadiness. Six theories proposed in existing literature are classified, including vortex shedding, tip leakage vortex breakdown, the interaction between tip leakage flow and adjacent flow, rotating instability vortex, tip secondary vortex, and shear layer instability. There have been only limited investigations of control techniques aimed at suppressing tip flow unsteadiness. These methods are classified according to their control mechanisms and the corresponding control effects are presented. Additionally, recommendations for future advancements in these fields are presented.