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A systematic investigation of the effects on compressor performance of recessing (trenching) the case over the rotor tips was conducted in a four-stage research compressor. Variations in the slope and depth of the trench, rotor tip penetration into the trench, distances of start and termination of the trench relative to the blade edges, and rotor-tip clearances were evaluated. The effects of each geometric parameter on compressor efficiency and stall margin were determined. A new sloped trench configuration was developed and found to be superior to any other trench configuration tested, giving in one case a 0.5 point improvement in efficiency without adversely affecting stall margin.
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Wisler et al. (1989) studied this question.
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