Experimental and computational studies were conducted to study the role of the tip leakage flow in axial compressor stall and the relationship between the tip clearance flow field and surge margin extension from circumferential groove casing treatment.
The CFD results were used to identify the existence of an interface between the approach flow and the tip leakage flow. The experiments used a surface streaking visualization method to identify the time-averaged location of this interface as a line of zero axial shear stress at the casing. The axial position of this line moved upstream with decreasing flow coefficient in both the experiments and computations. The line was consistently located at the rotor leading edge plane at the stalling flow coefficient, regardless of inflow boundary condition. These results were successfully modeled using a control volume approach that balanced the reverse axial momentum flux of the tip leakage flow with the momentum flux of the approach fluid. Non-uniform tip clearance measurements demonstrated that movement of the interface upstream of the rotor leading edge plane leads to the generation of short length scale rotating disturbances. Therefore, stall was interpreted as a critical point in the momentum flux balance of the approach flow and the reverse axial momentum flux of the tip leakage flow.
Experimental measurements of surge margin extension from seven CGCT configurations with a fixed groove geometry demonstrated that the contribution of individual grooves in a multi-groove casing to surge margin extension is an (a) additive and (b) linear function of the smooth wall tip clearance axial momentum flux at the location of a each groove. Extending the axial momentum model to include the influence of a CGCT showed that circumferential grooves reduce the tip leakage flow axial momentum through radial transport. The equivalent force due to a circumferential groove was demonstrated to be related to the smooth wall tip clearance axial momentum flux through a coefficient of drag that had a log-linear dependence on groove aspect ratio.