An Experimental Study of Surge Control in a Helicopter Gas

An experimental study was performed to identify the surge inception dynamics and to demonstrate an active surge control on a 600 HP class Allison 250-C30 gas turbine engine. Throttling orifices inserted between the compressor and the combustor were used to move the steady state operating line toward the surge line. The compressor was then adjustably throttled into surge using water injection downstream of the throttling orifices. The actuation for forced response tests and feedback surge control was one-dimensional high pressure air injection through the inducer bleed slots near the impeller face. The air injection was modulated with a high speed valve which has a bandwidth of 310 Hz. The baseline of interest was 95% design speed with the steady mean air injection. System identification at the baseline conditions identified a surge mode at 30 Hz and an acoustic mode at 100 Hz. High speed data showed that during surge inception a period of 100 Hz oscillation is followed by a few cycles of 30 Hz oscillation. The surge mode oscillation appears as a short burst which lasts only for 1 or 2 cycles prior to surge. A resonance analysis with the estimated engine parameters showed that a distributed model, which accounts for the compressibility in the duct of the compression system, can predict both resonance frequencies. Various linear control laws were designed and tested. However, none of them eliminated the strong 100 Hz acoustic resonance from the downstream of the compressor. No stable operating range extension was achieved. A new actuator for the downstream of the compressor must be developed for further study of the surge inception mechanism and active surge control. Thesis Supervisor: Alan H. Epstein Title: R.C. MacLaurin Professor of Aeronautics and Astronautics Thesis Supervisor: James D. Paduano Title: C.R.Soderberg Associate Professor of Aeronautics and Astronautics