Abrupt Variations in Deep Surge Frequencies in High-Pressure-Ratio Multi-Stage Axial Flow Compressors

Abstract

High-pressure-ratio multi-stage axial flow compressors tend to show complicated behaviors of frequencies in deep surges, including discontinuous changes. The phenomena for a nine-stage compressor were investigated by one-dimensional numerical simulations. The phenomena were found to occur somewhere in the range of roughly (90 – 100)% of the design speed, where incomplete surge recoveries, i.e., premature surge recoveries accompanied by immediate re-stallings, tend to occur, resulting in longer surge periods and lower surge frequencies. The incomplete surge recoveries are found to occur by premature unstalling owing to re-ingestions of the reversed hot air produced by the preceding surge, which is too hot for the compressor to work normally. The re-ingestion temperature, and the duration time of the hot re-ingestion relative to the normal suction appear to affect significantly the recovery process. It tends to occur essentially in the range of (90 -100)% of the design speed, where the compressor working conditions are changeable sensitively to small changes in the working conditions. The situations are found to be governed mainly by the following parameters; the representative tip Mach number, the reduced resonance frequency related with the suction flowpath, and the relative location of the compressor in the whole flowpath. Sufficiently below and above the sensitive speed range, the situations keep their respective stable and continuous surge behaviors. In the higher speed range, the deep surges having lowered frequencies at higher speeds become the normal state. The phenomena depend strongly on the design conditions, i.e., speed and pressure ratio and number of stages. The above conclusions suggest the natures of the discontinuous frequency behaviors in high-pressure-ratio multi-stage compressors, although they are qualitative in that they are strongly influenced by the present particular design conditions and the accuracies of the employed analysis.