Stability Enhancement of a Multistage Compressor by Air Injection

Cold and hot air injection upstream of the first rotor tip of a multistage compressor was tested experimentally. The compressor operating range was extended toward lower mass flow by more than 60% indicating a better throttling capability when air injection was activated. A strong dependency of the stability enhancement on the injected mass flow and injection velocity was found. Both increasing injection mass flow rate and increasing injection velocity led to a considerable extension of the throttling line. Comparable enhancements were achieved when reducing the number of nozzles and hence the injection mass flow. It was also found that injection of hot air, at temperatures comparable to air that bled off at a following stage, had no penalty on the stability enhancement. Investigation of the influence of air injection on radial work distribution showed that only small amounts of injected air were sufficient to lead to a significant radial work redistribution. This in turn changed the operating point of the first stage, leading to axial rematching and thus changed the whole operational behavior of the compressor.