Behaviors of Surge Frequencies in Multi-stage Axial Flow Compressors over a Wide Range of Speeds

The fundamental behaviors of deep surge frequencies in axial-flow compressors have come to be described as a rather simple framework for low pressure-ratio compressors, in terms of two essential reduced frequencies concerning resonance frequencies and surge frequencies, on the basis of numerical-experimental results. For high-pressure-ratio multi-stage compressors, however, the frequency behaviors over a wide range of compressor speeds tend to give drastic deviations from the simple framework, including abrupt changes and apparent irregularities. The cause was investigated here by numerical simulations on a nine-stage axial flow compressor. Generally, deep surge frequencies of high-pressure ratio compressors tend to lower gradually, from low speeds toward higher ones. At somewhat below the design speed, the frequencies tend to decrease precipitously, settling to some very low levels of values. The discontinuous drops in the frequencies are shown to result from occurrences of short-term incomplete surge recoveries in the stalling phase of surge. The phenomenon tends to elongate the duration of the stalled phase and the whole surge duration, resulting in the lowered surge frequency. The phenomenon is caused by the flow reversal of high-temperature air in the stalling phase of surge and the re-suction of the hot air in the unstalling phase. The degree of the peak temperature and the intensity of the interferences between the hot-air re-suction phenomenon and the intrinsic surge appear to affect significantly the additional short-term premature stalling of the compressor. It could be strongly related with the high average temperature due to the hot air re-suction, and the duration time of the suction. The complicated behaviors thus become qualitatively explainable, though only by examinations on numerical-simulation results.