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The purpose of the investigation is to obtain the mechanism of affecting the performance and stability of an axial flow compressor with two kinds of multi distortion area. In the investigation, the compressor inlet distortion is made by the distortion generator, which is a column and is placed in the compressor rotor passage upstream. The five-passages unsteady numerical results show that the compressor performances for two kinds of multi distortion area are both lower than that for the clean inlet, and the compressor peak efficiencies for inlet distortions of 6×12 degrees and 6×24 degrees are about 3.5%, and 9.2% lower than that for the clean inlet, respectively. Further, two kinds of multi distortion area both reduce the compressor stability, and the stall margin improvements of −2.93% and −6.11% are provided by inlet distortions of 6×12 degrees and 6×24 degrees, respectively. The flow field analyses show that the flow conditions of the rotor tip inlet upstream deteriorate after the application of multi distortion area. The values of axial velocity and flow angle of incoming flows in some regions, which are near the shroud, become small due to the adverse effect made by the inlet distortion. So, the circulation capacity of the regions becomes low, and inlet blockages appear in some rotor tip passages. The bigger the distorted range is, the greater inlet blockages and corresponding flow losses are. When multi distortion area changes from 6×12 degrees to 6×24 degrees, there are obvious separations of the boundary layer near the blade suction surface in some blade tip passages, and the serious separations prevent the incoming flows from entering the blade tip passage. As a result, overflows occur at the leading edge of the blade tip. Furthermore, as the number of the distortion generator increases by a factor of one, the effects of inlet distortion on the amplitudes of the air relative velocity and static pressure in the rotor inlet upstream at the distortion generator passing frequency of 1076.5 Hz increase by a factor of about one.

Whole-passage simulations were conducted to simulate the stall inception process in a subsonic compressor rotor. The predicted results agreed well with available steady-state and time-resolving experimental data. The detailed flowfield analyses further demonstrated that the test rotor was of spike-type stall inception. The self-sustained unsteady cycle of tip separation vortex (TSV) was the underlying flow mechanism for the occurrence of the so-called ‘tip clearance spillage flow’ and ‘tip clearance backflow’. Furthermore, the correlation between the self-sustained cycles of TSV in the preceding and native passages was the flow mechanism for emergence of spike. For the test rotor, the distinctive flowfield feature during the development of spike was the involvement of ‘tip clearance back-spillage flow’.

Three-dimensional unsteady calculations were performed to explore the unsteady nature of tip clearance flow and its possible linking with the spike stall inception. It was found that the interaction of the broken-down leakage vortex with the tip clearance flow formed another distinctive vortex, denoted as the tip separation vortex. It formed below the rotor blade tip section and propagated diagonally inward. This vortex propagated across the rotor passage from the pressure side to the suction side with its vortex core filled with low-energy fluid. The spike emergence during stall inception included the breakdown of the tip leakage vortex and the formation and movement of the tip separation vortex.

By applying a concept similar to ‘domain scaling’ treatment often used in multi-stage turbomachinery flow field to the interface between the rotor blade passage and end-wall treatments, a series of computational studies were carried out to understand the physical mechanism responsible for improvement in stall margin of a modern subsonic axial-flow compressor rotor due to the axial-skewed slot casing treatment. Particular attention was given to examining the interaction between the rotor tip flow and the axial-skewed slot. In order to validate the multi-block model applied in the rotor blade end-wall region, the computational results for the modern subsonic compressor rotor both with and without axial-skewed slot casing treatment were correlated with available experimental test data. The computed results agree fairly well with the experimental data, and the computations are then used to describe the interaction between the rotor tip flow and axial-skewed slot. Detailed analyses of the flow visualization at the tip have exposed the different tip flow topologies between the cases with axial-skewed slot and with untreated smooth wall. It was found that the primary stall margin enhancement afforded by the axial-skewed slot casing treatment is a result of the tip flow manipulation. The repositioning of the tip vortex trajectory further towards the trailing edge of the blade passage and delaying the movement of incoming/tip clearance flow interface to the leading edge plane are the physical mechanisms responsible for extending the compressor stall margin.

On the basis of the test results of discrete axial and blade angle slot casing treatment, a new type of casing treatment was designed for a subsonic axial flow compressor rotor by optimizing various geometry parameters. To obtain a wide operating range and to minimize penalties in terms of isentropic efficiency, seven compressor configurations incorporating casing treatments of 0, 16.6, 33.3, 50, 66.6, 83.3, and 100 per cent rotor exposures were experimentally investigated. The results showed that significant improvements in stall margin are possible in all exposures and insignificant isentropic efficiency sacrifices are recorded in some exposures. Nearly 21.43 per cent stall margin improvement in terms of the corrected mass flow-rate was achieved with 33.3 per cent rotor blade tip axial chord exposure. The compressor built with 16.6 per cent rotor exposure was the best configuration in terms of maximum isentropic efficiency gain. The second issue of the paper was to offer a contribution to the understanding of the physical mechanism by which bend-skewed slot-casing treatment improves stall margin under subsonic conditions. By applying a concept similar to ‘Domain scaling’ approach (as often used in multistage turbomachinery flow-fields) to the interface between the rotor blade passage and end-wall treatments, a time-dependent three-dimensional numerical simulation was performed for the subsonic axial-flow compressor rotor with bend-skewed slot-casing treatment. The numerical results agreed well with the available experimental results. Detailed analyses of the coupled flow through bend-skewed slot-casing treatment and rotor blade passage under subsonic conditions led to some preliminary conclusions as to the flow physics involved in the stall margin improvements afforded by the use of bend-skewed slot-casing treatment.

This article investigates the flow field at the tip region of compressor rotor. In particular, the effect of stepped tip gaps on the performance and flow field of axial compressor was reviewed using experimental and computational methods. An axial compressor rotor with no inlet guide vanes was tested under subsonic conditions. A parametric study of clearance levels and step profiles was performed using eight different casing geometries. This study was aimed at comparing compressor performance in specified configurations. The experimental results showed that the inclusion of stepped tip gaps with the small clearance level gave increased pressure ratio, efficiency, and stall margin throughout the mass flow range at both speeds. However, when using medium and large clearance levels, the benefits of stepped tip gaps were not noticed for all rotor operating conditions when compared with the baseline case. Steady-state Navier-Stokes analyses were performed for cases involving small clearance level and stepped tip gap geometries. They highlighted the mechanisms associated with performance improvement. The numerical procedure correctly predicted the overall effects of stepped tip gaps. Detailed numerical simulation results showed that the interaction between the stepped groove flow and the blade passage flow could entrain the blockage produced by upstream tip leakage flow into the tip gap of adjacent blades of the compressor rotor. It is through this process that stepped tip gaps can help in dissipating blockage that was caused by upstream tip leakage flow. Thus, the path and extent of the blockage in the tip region are altered to increase the passage through-flow area, and so, the rotor performance can be improved.

In order to advance the understanding of the fundamental mechanisms of axial skewed slot casing treatment and their effects on the subsonic axial-flow compressor flow field, the coupled unsteady flow through a subsonic compressor rotor and the axial skewed slot was simulated with a state-of-the-art multiblock flow solver. The computational results were first compared with available measured data, that showed the numerical procedure calculates the overall effect of the axial skewed slot correctly. Then, the numerically obtained flow fields were interrogated to identify the physical mechanism responsible for improvement in stall margin of a modern subsonic axial-flow compressor rotor due to the discrete skewed slots. It was found that the axial skewed slot casing treatment can increase the stall margin of subsonic compressor by repositioning of the tip clearance flow trajectory further toward the trailing of the blade passage and retarding the movement of the incoming∕tip clearance flow interface toward the rotor leading edge plane.