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Abstract Activities by various authors on aerodynamics and control dynamics of rotating stall in axial compressor are first traced. Then, a process of stall cell evolution in a subsonic stage is discussed based on a 2-D CFD. A few numbers of vortices grow ahead of the rotor accumulating vorticity ejected from lightly stalled blades, and eventually organize a cell of circumferentially aligned huge vortices, which merge and recess repeatedly during the rotation. Such stall disturbance is intensified on trailing side of a circumferential inlet distortion and decays on the leading side. Considering these features, a new algorithm for stall warning is developed based on a correlation between pressure waveforms at each passing of a fixed blade. A remarkable change in the correlation level at near-stall provides a warning signal prior to the stall onset with sufficiently large time margin. This scheme is applied to achieve rotating stall prevention by actuating flaps installed on the hub.
The last issue is on characteristics of forward swept blade which has much increased throttle margin with decreased tip loss. A 3-D computation shows that a secondary vortex generated in suction surface mid span interacts to reduce the tip leakage vortex that initiates the stall.
For example, separation, with boundary layer fluid moving off the wall, occurs along the symmetry line on the end wall of a rectangular nozzle, where the streamwise pressure gradient is favorable (Greitzer et al. ). This type of three-dimensional separation, however, is associated with the confluence of boundary layer fluid due to cross-flow (Lighthill ) rather than by the inability of low stagnation pressure fluid to negotiate a pressure rise.
In addition, there is no stagnation of the separating fluid, and the primary effect is rather a change of direction as the fluid leaves the wall. As shown later, the stall indicator S correlates best with the diffusion parameter if the loading near the end wall is evaluated at a spanwise location of 10% chord.
This is within the end wall boundary layer thickness in multistage compressors but sufficiently away from the end wall surface to avoid interference with localized low pressure regions associated with spanwise turning of the cross-flow in the hub corner. To generalize different blade passage geometries, the calculations suggest nondimensionalizing the spanwise distance by chord. This paper presents a new criterion for estimating the onset of three-dimensional hub-corner stall in axial compressor rotors and shrouded stators. A simple first-of-a-kind description of hub-corner stall formation is developed which consists of (i) a stall indicator, which quantifies the extent of the separated region via the local blade loading and thus indicates whether hub-corner stall occurs, and (ii) a diffusion parameter, which defines the diffusion limit for unstalled operation. The stall indicator can be cast in terms of a Zweifel loading coefficient.
The diffusion parameter is based on preliminary design flow variables and geometry. Computational simulations and single and multistage compressor data are used to show the applicability of the criterion over a range of blade design parameters. The criterion also enables determination of specific flow control actions to mitigate hub-corner stall. As an illustration, a flow control blade, designed using the ideas developed, is seen to produce a substantial reduction in the flow nonuniformity associated with hub-corner stall.
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