PRESTALL BEHAVIOR OF SEVERAL HIGH-SPEED COMPRESSORS

High-speed compressor data immediately prior to rotating stall incepti on are analyzed and compared to stability theory. New techniques for t he detection of small-amplitude rotating waves in the presence of nois e are detailed, and experimental and signal processing pitfalls discus sed. In all nine compressors examined, rotating stall precedes surge. Prior to rotating stall inception, all the machines support small-ampl itude (< 1 percent of fully developed stall) waves traveling about the circumference. Traveling wave strength and structure are shown to be a strong function of corrected speed. At low speeds, a similar to 0.5 times shaft speed wave is present for hundreds of rotor revolutions pr ior to stall initiation. At 100 percent speed, a shaft speed rotating wave dominates, growing as stall initiation is approached (fully devel oped rotating stall occurs at about 1/2 of shaft speed). A new, two-di mensional, compressible hydrodynamic stability analysis is applied to the geometry of two of the compressors and gives results in agreement with data. The calculations show that, at low corrected speeds, these compressors behave predominantly as incompressible machines. The wave that first goes unstable is the 1/2 shaft frequency mode predicted by the incompressible Moore-Greitzer analysis and previously observed in low-speed compressors. Compressibility becomes important at high corre cted speeds and adds axial structure to the rotating waves. At 100 per cent corrected speed, one of these hitherto unrecognized compressible modes goes unstable first. The rotating frequency of this mode is cons tant and predicted to be approximately coincident with shaft speed at design. Thus, it is susceptible to excitation by geometric nonuniformi ties in the compressor. This new understanding of compressor dynamics is used to introduce the concept of traveling wave energy as a real ti me measure of compressor stability. Such a wave energy-based scheme is shown consistently to give an indication of low stability for signifi cant periods (100-200 rotor revolutions) before stall initiation, even at 100 percent corrected speed.