High-speed high-pressure ratio compressor surge is a transient breakdown in compression accompanied by an abrupt momentary reversal of gas flow. It commonly exists in dynamic type turbo compressors, particularly in the axial compressor of modern aero-engines. By Newton’s Laws of Motion, a force is needed to change the state of any motion. So what is the force that can cause such a dramatic motion as surge? What exactly triggers it, and how do we quantify the transient surge phenomenon? This paper attempts to answer these questions and discuss the transient dynamics of surge at its initial stage. It has generally been accepted that surge is precipitated by the onset of a rotating spike or stall, not only for low speed but for high-speed compressors too. The state of dynamic surge modeling today is best exemplified by the “Greitzer-Moore” model. However, it fails to incorporate the key elements of the transient nature of a surge inception: the extremely short time duration on millisecond scale and the shock wave presence observed experimentally. An indirect approach is taken in this paper to address the transient dynamics of stall and surge by using an analogy to the shock tube. The link is established based on observations that instant zero net through flow inside stalled cascade cell triggers stall/surge. The results from the analogy reveal that surge initiation simultaneously generates a pair of non-linear compression and expansion waves (CW & EW) and induced reverse fluid flow (IRFF). The dynamic forces for instant flow reversal are the pushing force of upstream propagating CW and the pulling force from downstream travelling EW. Surge Rules are deduced and then compared with experimental findings by previous researchers with good agreements. Moreover, the strength of the transient post-surge components, CW, EW and IRFF, can be estimated analytically or numerically by the shock tube theory from known pre-surge conditions and routes to surge.
Compressor surge is a complete breakdown in compression resulting in an abrupt momentary reversal of gas flow and the violent pressure fluctuation with relatively low frequency and high amplitude. It commonly exists in dynamic type turbo compressors, particularly axial compressor and jet engine, or turbo charger for reciprocating engines. It is generally accepted that surge is preceded by a rotating stall, a situation of a few stalled blades rotating around compressor annulus (cascade) with much higher frequency. In jet engine, violent surge event typically produces a frightening loud bang, lots of vibrations and could cause catastrophic structural failures if not timely managed. Naturally, as important matters as rotating stall and surge, there have been tremendous R/D efforts from academia, government and industry devoted to this area, especially since jet engines became the prime powerhouses for modern airplanes. Despite of all the efforts, there still seems to be a more urgent need to understand the physical characteristics of the transition from a rotating stall to surge that has mystified researchers due to its transient nature. Fundamental questions remain unanswered even today, such as: What exactly triggers the surge to take place from a rotating stall? What is the physical nature of a compressor system or a local incipient surge: is it a movement of wave or fluid particles or both? How to estimate the quantitative destructive forces of a severe surge, that is, the maximum possible surge strength? This paper attempts to answer these questions by applying the classical Shock Tube Theory to the transient process from rotating stall to surge. The Shock Tube analogy is established with the hypothesis (implied from experimental observations) that an instant zero through flow condition exists inside a stalled cascade cell or dynamic compressor that triggers surge. It is revealed that surge event consists of a pair of non-linear compression and expansion waves (CW & EW) that instantly reverse gas flow (IRFF) by the pushing force of upstream propagating CW and the pulling force from downstream travelling EW. The surge strength is shown to be proportional to the square root of the pressure ratio of the involved cascade or compressor. Surge Rules are deduced to predict the location of surge initiation, the minimum and maximum surge strengths, travelling directions and speed. Moreover, a pro-active control strategy called SEWI (Surge Early Warning Initiative) is proposed using the unique characteristics of CW-IRFF-EW formation of a cascade cell induced surge as precursors for subsequent warning and controls before the destructive compressor surge takes place.
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