Surge dynamics and unsteady flow phenomena in centrifugal compressors

Abstract

Detailed time resolved measurements of centrifugal compressor surge has been obtained on an automotive turbocharger for two very different compression systems one with a large downstream volume and one with a much smaller downstream volume. These measurements show impeller stall at the inducer tips to be a key phenomena in initiating surge. The inducer tip stall, which is dominant over other types of stall in the compressor, is observed to be non-rotating and asymmetric due to the presence of an asymmetric downstream volute. The most severe stalling of the impeller occurs at a circumferential position nearest the volute tongue position and is due to a circumferential flow distortion set up by the . volute. The vaneless diffuser is seen to be destabilizing but does not initiate surge by abrupt stalling. Rotating stall was found to be unimportant in surge initiation. New evidence is presented concerning the dynamic behavior of the compressor characteristics in surge operation.

Instantaneous compressor characteristics in surge when operating in a large volume(large B-parameter) system are found to be flatter than the time averaged ones for a small volume(small B-parameter) stabilized system. A physical mechanism accounting for the difference between the two measured characteristics is the slow development time and differing circumferential extent of the inducer stall present. The flatness of the large B characteristic contrasts with the characteristic of an axial. compressor operating in surge and leads to slow growth of the surge massflow instability. A dynamic model has been developed which includes effects of speed variations, compressibility, and time lags. The inclusion of speed variations changes the time domain behavior of the compression system substantially from the results obtained with constant speed.

A precursor period of mild surge whose length depends on the amount of throttling is shown to be present before deep surge and is due to the speed variations. Both speed variations and time lags in compressor behavior are shown to introduce a stabilizing effect on compressor behavior in mild surge. The results of this model agree qualitatively as well as quantitatively with the measured experimental system dynamic behavior. The dynamic behavior observed has some properties of importance for the new field of active control of surge instabilities in centrifugal compressors. The flatness of the instantaneous compressor characteristic, the existence of a mild surge precursor period, and slow growth of the surge instability are favorable conditions for a relatively simple active control strategy to stabilize the compression system and eliminate surge. Also the dynamic model developed may be useful for exploring alternative active control strategies.