Cavitation inception and the associated flow structure in the tip region of a ducted propeller are investigated experimentally at varying advance ratios (J) using high-speed imaging and stereoscopic particle image velocimetry (SPIV) measurements in a refractive index-matched facility. At design and higher J values, inception occurs in axially aligned secondary vortices, located between the blade suction side and the tip leakage vortex (TLV), circumferentially after the trailing edge. With decreasing J, the inception shifts first to the TLV, and then along its core towards the leading edge. High-resolution SPIV data follow the evolution of TLV, tip leakage flow, near wake and several secondary vortices. Time-resolved SPIV at 30 kHz enables calculation of all three mean vorticity components, hence capturing axial vortices, and identifies the origin of flow structures. At high J values, inception occurs when quasi-axial vortices are stretched by the circumferential TLV and co-rotating secondary vortices located in the shear layer connecting the TLV to the suction side blade tip. With decreasing J, inception shifts to the TLV and towards the leading edge owing to earlier rollup and higher vortex strength, along with earlier breakup, evidenced by high core turbulence and a decrease in peak vorticity despite an increase in circulation.

Effects of air injection on a centrifugal compressor performance are studied numerically and results are presented in this paper. Flow field is simulated based on solution of the Reynolds-Averaged Navier-Stokes equations utilizing the well-known k-ε turbulence modeling. To find optimum arrangement of the air injectors, including their numbers, positions and setting angles and also their mass flow rates, 10 different cases were proposed in this investigation. Results of velocity and pressure fields and streamlines patterns on various blade-to-blade and streamwise planes provided necessary data for analyses and discussions. These results revealed that a proper air tip injection can weaken the tip leakage flow strength and alleviate the blockages to the main stream near the casing. Hence, this technique can be used to extend the stable operating range of the compressor and delay the probable stall commencement. In addition, compressor can produce higher pressure ratios in cases of proper injections in comparison to no-injection case. Optimum injection configuration improved the stall margin of the proposed compressor by nearly 15.8%. Numerical results for no-injection case were compared with those of the authors own experimental works carried out on a proper test rig, which showed close agreement.

This paper deals with tip leakage flow structure in subsonic axial compressor rotor blades row under different operating conditions. Analyses are based on flow simulation utilizing computational fluid dynamic technique. Three different circumstances at near stall condition are considered in this respect. Tip leakage flow frequency spectrum was studied through surveying instantaneous static pressure signals imposed on blades surfaces. Results at the highest flow rate, close to the stall condition, showed that the tip vortex flow fluctuates with a frequency close to the blade passing frequency. In addition, pressure signals remained unchanged with time. Moreover, equal pressure fluctuations at different passages guaranteed no peripheral disturbances. Tip leakage flow frequency decreased with reduction of the mass flow rate and its structure was changing with time. Spillage of the tip leakage flow from the blade leading edge occurred without any backflow in the trailing edge region. Consequently, various flow structures were observed within every passage between two adjacent blades. Further decrease in the mass flow rate provided conditions where the spilled flow ahead of the blade leading edge together with trailing edge backflow caused spike stall to occur. This latter phenomenon was accompanied by lower frequencies and higher amplitudes of the pressure signals. Further revolution of the rotor blade row caused the spike stall to eventuate to larger stall cells, which may be led to fully developed rotating stall.