A simple criterion for three-dimensional flow separation in axial compressors

Abstract

Most modem blade designs in axial-flow compressors diffuse the flow efficiently over 20% to 80% of blade span and it is the endwall regions that set the limits in compressor performance. This thesis addresses the estimation, control and mitigation of three-dimensional separation near the hub corner in axial- flow compressors. A simple method to estimate the onset of hub comer separation in compressor blade passages has been developed. A parameter is defined to quantify the combined effect of adverse pressure gradient and secondary flow which are the two main mechanisms contributing to the formation of three-dimensional flow separation. There is a critical value of the parameter at which the onset of three-dimensional flow separation occurs. Data from existing research and production compressors show the generality of the separation criterion. The new parameter captures the alleviating effect of boundary layer skew on three-dimensional flow separation. Using this concept, a flow control scheme has been developed to mitigate hub comer separation by injecting spanwise momentum from the blade suction surface. A proof of concept flow control experiment demonstrates a reduction in stagnation pressure loss coefficient of 8% with an injection flow of 0.8% of the cascade mass flow.