Experimental Study of Surge Control in the Allied Signal LTS-101 Helicopter Turboshaft Engine

摘要

Surge dynamics and surge control techniques are experimentally examined on an Allied Signal LTS-101 helicopter engine. Diffuser throat injection is used for actuation. A mean level of 95 psig air injection modifies the system dynamics. Steady state injection creates a relatively large, surge- free region of near zero characteristic slope at the speedline peak, which is conducive for control. Low frequency unsteadiness is observed prior to surge for the open loop baseline case. With mean air injection, the system has a 27 Hz surge mode, and a series of higher frequency acoustic resonances, the first of which occurs at 68 Hz. These modes are one dimensional, with no rotating stall development. Surge is stabilized in the LTS-101 using active feedback control. The control strategy is to sense upstream static pressure patterns at multiple axial locations and feed back the measured signals to a high speed valve in the air injection flow path. At 95% N1 corrected, an H-infinity compensator achieves a 0.98% reduction in stalling mass flow. This equates to 10.7% increase in the overall operating range of the engine. The H-infinity compensator achieves range extension by damping the surge mode. The scheme successfully avoids excitation of the acoustic resonances. Less complex feedback schemes are unable to achieve compressor stalling mass flow reduction. This research effort incorporates a rigorous, systematic robust control redesign process. A linear, acoustic model is derived for the transfer function relation between the high speed valve command and the static pressure signal at the inlet. The model predicts a surge mode at 27 Hz and various acoustic frequencies. Based on this model, the 68 Hz mode is attributed to acoustic resonances in the system, which includes the LTS- 101 and inlet ducting.