Numerical simulations of rotating stall in axial flow compressors

Gas turbine compressor performance may encounter deterioration during service for various reasons such as damage by debris from the casing or foreign objects impacting on the blades, typically near the rotor's tip. Moreover, mal-schedule of Variable Stator Vanes (VSVs) during start-up may also result in performance deterioration and reduction in the surge margin. Ability to assess the effect of compressor deterioration using Computational Fluid Dynamics (CFD) is important at both design stage and in service. Compressor blade damage breaks the cyclic symmetry and the VSVs mal-schecule creates mis-match between stages together with geometric variations, thus computations are desirable to be performed using full annulus assemblies. Furthermore, downstream boundary conditions are also unknown during rotating stall or surge and simulations become difficult. This research presents unsteady time-accurate CFD analyses of compressor performance with tip curl blade damage in a single stage axial flow compressor and VSVs mal-schedule in a 3.5 stage axial flow compressor. Computations were per- formed near stall boundary to predict rotating stall characteristics. The primary objectives are to characterise the overall compressor performance and analyse the detailed flow behaviour. Computations for the nominal blade configurations were also performed for comparison purposes for both compressors. All unsteady simulations were performed at part speeds with a variable nozzle downstream representing an experimental throttle. For the blade damage study, two different degrees of damage for one blade and multiple damaged blades were investigated and compared with the results from the undamaged case. For the VSVs mal-schedule study, the first two stators were assumed to be variable and were used to create mal-schedule vane settings for the investigation. The effects of blade damage and VSVs mal-schedule on the aerodynamics performance and rotating stall characteristics for both compressor assemblies were investigated respectively and discussed in detail.