Quantitative evaluation of the effects of transient heat tranfer on axial flow compressor stability

The purpose of this investigation is to study the effects of transient heat transfer on the stability of axial flow compressors. During speed transients the state of the compressor can be considered non-adiabatic. This is due to heat transfer between the compressor metal parts and the air flow. For instance, during a deceleration, heat is transferred from the hot metal to the cooler air. This phenomenon is detrimental to the stability margin of the compressor. The density changes of the air and boundary layer reactions during heat transfer reduce the margin of stability. When a deceleration is immediately followed by an acceleration the surge margin reduction becomes even worse. The response of a compressor during an acceleration can reduce the surge margin even further. Since the surge margin has already been diminished due to heat transfer, the possibility of the compressor becoming unstable is greater.

First, this investigation thoroughly researched the topic of the effects of heat transfer on compressor stability. Then using engine test data the thermal energy storage capacity for a typical engine and the heat transfer rate during a speed transient were quantified. The transient investigated was a "bodie" transient, which is a standard engine stability test for gas turbine engines. The "bodie" transient is a deceleration followed by an acceleration. Instabilities usually occur during the acceleration part of the transient. Only pre-stall conditions were analyzed, i.e., only the deceleration was investigated. The heat transfer rate was quantified for four sets of back to back data points that were run at similar operating conditions. One of the tests encountered a stall; the other did not. Through an analysis procedure, it was determined that the stalled test had a greater heat transfer rate than the unstalled test. This demonstrates the link between the occurrence of compressor instability and heat transfer during speed transients.