This paper describes the role of tip leakage flow in creating the leading edge separation necessary for the onset of spike-type compressor rotating stall. A series of unsteady multipassage simulations, supported by experimental data, are used to define and illustrate the two competing mechanisms that cause the high incidence responsible for this separation: blockage from a casing-suction-surface corner separation and forward spillage of the tip leakage jet. The axial momentum flux in the tip leakage flow determines which mechanism dominates. At zero tip clearance, corner separation blockage dominates. As clearance is increased, the leakage flow reduces blockage, moving the stall flow coefficient to lower flow, i.e., giving a larger unstalled flow range. Increased clearance, however, means increased leakage jet momentum and contribution to leakage jet spillage. There is thus a clearance above which jet spillage dominates in creating incidence, so the stall flow coefficient increases and flow range decreases with clearance. As a consequence, there is a clearance for maximum flow range; for the two rotors in this study, the value was approximately 0.5% chord. The chordwise distribution of the leakage axial momentum is also important in determining stall onset. Shifting the distribution toward the trailing edge increases flow range for a leakage jet dominated geometry and reduces flow range for a corner separation dominated geometry. Guidelines are developed for flow range enhancement through control of tip leakage flow axial momentum magnitude and distribution. An example is given of how this might be achieved.

This paper investigates how the external geometry of a five-hole probe affects its accuracy and how internal geometry affects its settling time. An analytical model, which predicts settling time, is used to design an accurate, fast-settling, millimetre-scale probe. The paper has three components: First, results are presented from a series of area traverses performed with five-hole probes which range in head diameter from 0.99 to 2.67 mm. It is found that the smallest probe gives the greatest accuracy when traversing the shear layers in blade wakes. However, it takes 3.4 times longer to complete this traverse than compared to the largest probe. This is because traverses with small probes require more time to allow the pressure readings to settle between each traverse position. Second, an analytical model is developed which predicts settling time based upon the internal geometry of the probe. The approach adopted is capable of modeling any number of connected tubes with different lengths and diameters. It is validated against experimental measurements and is shown to give good agreement. This model can be used to ensure that probes are designed with acceptable settling times. Finally, the analytical model is used to design an optimised five hole probe. Use of the model highlights two important results which are required to reduce settling time: First, the length of the smallest diameter tubes, i.e. the ones in the probe head, should be minimised. Second, the volume of tubing downstream of the head should be minimised. Applying these principles to a new probe design cuts the total traverse time by 71%, whilemaintaining the highest value of accuracy.

In this paper, the influence of nonuniform bleed extraction on the stability of an axial flow compressor is quantified. Nonuniformity can be caused by several geometric factors (for example, plenum chamber size or number of off-take ducts), and a range of configurations is examined experimentally in a single stage compressor. It is shown that nonuniform bleed leads to a circumferential distribution of flow coefficient and swirl angle at inlet to the downstream stage. The resultant distribution of rotor incidence causes stall to occur at a higher flow coefficient than if the same total bleed rate had been extracted uniformly around the circumference. A connection is made between the analysis of nonuniform bleed extraction and the familiar DCθ criterion used to characterize inlet total pressure distortion. The loss of operating range caused by the nonuniform inlet flow correlates with the peak sector-averaged bleed nonuniformity for all the bleed configurations tested.

In this paper, the influence of non-uniform bleed extraction on the stability of an axial flow compressor is quantified. Non-uniformity can be caused by several geometric factors (for example, plenum chamber size or number of off-take ducts) and a range of configurations is examined experimentally in a single stage compressor. It is shown that non-uniform bleed leads to a circumferential distribution of flow coefficient and swirl angle at inlet to the downstream stage. The resultant distribution of rotor incidence causes stall to occur at a higher flow coefficient than if the same total bleed rate had been extracted uniformly around the circumference. The loss of operating range caused by the non-uniform inlet flow correlates with the peak sector-averaged bleed non-uniformity for all the bleed configurations tested. A connection is made between the analysis of non-uniform bleed extraction and the familiar “DCθ” criterion used to characterize inlet total pressure distortion.

The coupling between the bleed system and the flowfield of a downstream compressor stage is studied using two approaches. In the first approach, three-dimensional, full annulus, unsteady computations simulate the flow in a low-speed research compressor with nonuniform bleed extraction. Comparisons with experimental data show that the flow prediction in the main annulus is accurate to within 0.005 of flow coefficient and 0.5deg of flow angle. The computational fluid dynamics (CFD) is then used to provide a description of flow within the bleed system itself. In the second approach, a two-dimensional mean radius model, similar to that adopted by Hynes and Greitzer in the previous work on compressor stability, is used to simulate the response of the compressor to nonuniform bleed. This model is validated against experimental data for a single-stage compressor, and despite the inherent assumptions (two-dimensional flow and simplified compressor response), provides a satisfactory prediction of the flow for preliminary design purposes with orders of magnitude less computational cost than full 3D CFD. The model is then used to investigate the effect of different levels of bleed nonuniformity and of varying the axial distance between the bleed and the downstream stage. Reducing bleed nonuniformity and moving the stage away from the bleed slot are predicted to reduce the circumferential nonuniformity of the flow entering the stage.