Measurement of Eccentricity Effects on Stability of a High-Speed Shrouded Centrifugal Compressor

Abstract

Eccentricity effects on stability has been experimentally measured with a high-speed shrouded centrifugal compressor in Korea Aerospace Research Institute. The research examines influences of eccentricity on the onset of rotating stall and surge, the types and the behavior of instabilities. For the experiments, a test section of a turbocharger test rig has been re-designed and manufactured

then, eccentricity can be adjusted by moving casings with highly accurate shims rather than moving a rotor shaft. In the test section, fast response pressure sensors (Kulite sensors) and gap sensors have been installed to measure unsteady flow and to monitor the relative center position between the rotor shaft and the casing. Totally thirty two Kulite sensors have been used and circumferentially located eight Kulite sensors have measured rotating stall. Those four sets of Kulite sensors have been located at impeller inlet, diffuser inlet, seal inlet, and seal gland to figure out where instability arises and to understand the behavior of instabilities—rotating stall and surge. Static pressures, total pressures, and total temperatures have been also measured to obtain steady performance—pressure rise coefficient. Experiments have been conducted at five levels of eccentricity. Baseline is 9.6% eccentricity and this case has been treated as concentric condition. The other four eccentricities are 19.6%, 26.2%, 37.8%, and 58.7%. All experiments have been operated at the design speed of 40,000 rpm. Experimental results present that: (1) the compressor directly goes into classic surge at the baseline case. (2) the compressor has two cells of rotating stall and stall cells rotate at the rate of 0.19 times rotor frequency at the baseline case. (3) overall pressure rise coefficient through the compressor has little influenced by eccentricity but pressure rise coefficient through the impeller has been degraded as eccentricity increases due to larger leakage along the secondary passage. (4) stalling flow coefficient is sensitive to eccentricity but the onset of surge is not sensitive to eccentricity

consequently, rotating stall precedes classic surge at larger flow coefficient as eccentricity increases.