EXPERIMENTAL AND NUMERICAL EXAMINATIONS OF A TRANSONIC COMPRESSOR-STAGE WITH CASING TREATMENT

The flow in the blade tip vicinity of the transonic first stage of a multi-stage axial flow compressor with variable inlet guide vane (IGV) and casing treatment (CT) above the rotor is investigated experimentally and numerically with focus on the effects of the CT on flow structures and compressor performance. For the experimental part of this study, conventional performance instrumentation is used to estimate the operating condition of the compressor. Radial distributions of total temperature and total pressure are taken at the leading edges of the stators for comparison with simulations as well as for adjusting the operating conditions of the compressor. The velocity field in the rear part of the first-rotor is determined with Particle Image Velocimetry (PIV) at 90% and 96% radial height using two periscope light sheet probes. The employed PIV setup allows a spatial resolution of 0.7 mm x 0.7 mm and thus a similar resolution as the spatial discretization in the simulation. For the numerical part of the study, time-accurate simulations are conducted for the same operating conditions as during experiments. Additional simulations of the same configuration with smooth casing are conducted in order to estimate the effect of the CT on the flow. The examination of PIV measurements and corresponding simulations exposes complex vortical structures originating from the interaction of the rotor bow shock with the IGV trailing edge, CT, IGV wake and the tip leakage vortex. The associated induced velocities together with the general passage flow form a complex flow field with significantly altered blockage compared to a common flow field in the tip vicinity. Position and trajectory of the tip leakage vortex are deduced from interactions between tip leakage vortex and IGV wake / CT. The detailed comparison of the tip region of simulations with and without CT shows That the CT influences pressure rise and flow parameters in a wide radial range due. to a radial redistribution of the flow. Correspondingly, a rotor with CT can achieve an increased total pressure rise compared to a rotor with smooth casing, with only minor effects on the efficiency.