Tip clearance flows in axial compressors: stall inception and stability enhancement
The flow in the rotor tip clearance gap in an axial compressor influences the loss levels and the stall onset point. The role of tip clearance flows in the stall inception process is still debated, though. The aim of this thesis is to improve the fundamental understanding of the tip clearance flow field and its role in the stall inception process. The knowledge gained is used to develop a new casing treatment for an aero-engine core compressor. Detailed static pressure, total pressure, and velocity measurements were made in the rotor tip clearance gap at design and near stall flow conditions. The resulting overtip flow maps showed the flow field in more detail than previously possible. Flow variations around the annulus and among blade passages were analyzed and found to increase as stall is approached. This increase was not uniform around the annulus or among passages; instead, some blade passages exhibited unique behaviour compared to all the other passages. The flow variations in the unique passages were linked to small physical irregularities in compressor blading. Detailed overtip measurements were also made at stall inception to investigate the formation of the spike disturbance. These measurements, the first of their kind, showed that the disturbance which initiates the formation of the spike destabilizes the boundary between the reversed flow and incoming flow. After destabilization, reversed flow increasingly dominates the passage until the neighbouring passage is affected. The spike was found to originate most often from those regions in the compressor where the flow variations are highest and was not, as suggested by some research, associated with the forward movement of the tip leakage vortex. A new casing treatment for axial compressors was proposed and tested. The casing treatment extracts air from over the rotor blade tips and re-injects it upstream of the rotor blade leading edges into the tip region through discrete re-circulation loops. The overtip location of air extraction is unique and enables self-regulation of the amount of flow re-circulated: a minimum amount of air is re-circulated at compressor design conditions (thus minimizing any loss of efficiency) and a maximum amount of air is re-circulated near the stability limit (thus maximizing stall margin). Modest stall margin improvements (2%) were achieved without any loss of compressor efficiency at design conditions.