Qiushi Li

Experimental investigations on instability evolution in a transonic compressor at different rotor speeds

Experimental investigations are conducted to study the instability evolution in a transonic axial flow compressor at four specific rotor speeds covering both subsonic and transonic operating conditions. Two routes of evolution to final instability are observed in the test compressor: at low rotor speeds, a disturbance in the rotor tip region occurs and then leads to rotating stall, while at high rotor speeds, a low-frequency disturbance in the hub region leads the compressor into instability. Different from stall and surge, this new type of compressor instability at high rotor speed is initiated through the development of a low-frequency axisymmetric disturbance at the hub, and we name it “partial surge”. The frequency of this low-frequency disturbance is approximately the Helmholtz frequency of the system and remains constant during instability inception. Finally, a possible mechanism for the occurrence of different instability evolutions and the formation of partial surge are also discussed.

Effects of Radial Loading Distribution on Partial Surge Initiated Instability in a Transonic Axial Flow Compressor

Partial surge is a new type of instability inception in transonic axial flow compressors and occurs in the form of axisymmetric low-frequency disturbances localized in the hub region. In this study, the evolution of instability in a transonic axial flow compressor at different rotor speeds shows that at 65% of the design rotor speed, partial surge does not occur; at 78% of the design rotor speed and higher, partial surge occurs and leads to compressor instability. The result of RANS simulation of the same compressor then suggests that the level of blade loading in the hub region could be highly correlated to the occurrence of partial surge. Next, the radial distribution of blade loading near the stall point is varied by introducing inlet distortion — alternately mounting specially designed screens at the inlet of the compressor. The experimental results suggest that high hub loading near the stall point would lead to partial surge initiated instability. The general effects of radial loading distribution on the type of stall inceptions are also discussed.Copyright

Experimental Study of Compressor Instability Inception in a Transonic Axial Flow Compressor

An experimental investigation is conducted to study the details of instability inception in a transonic axial flow compressor. The experimental results indicate that the compressor instability is initiated through the development of a low-frequency axisymmetric disturbance. The frequency of this disturbance is approximately the Helmholtz frequency of the test facility. The low-frequency disturbance can be detected over 3000 rotor revolutions before the compressor becomes unstable. Further experimental investigations illustrate that this low-frequency axisymmetric disturbance is initiated at the hub region of the compressor. This new kind of instability inception is termed “partial surge.” Examination of the design parameters of the compressor indicates that a high diffusion factor in the rotor root region might be the cause of the partial surge type instability inception.Copyright

Effects of inlet radial distortion on the type of stall precursor in low-speed axial compressor:

Experimental investigations of the effect of inlet blade loading on the rotating stall inception process are carried out on a single-stage low-speed axial compressor. Temporal pressure signals from the six high response pressure transducers are used for the analysis. Pressure variations at the hub are especially recorded during the stall inception process. Inlet blade loading is altered by installing metallic meshed distortion screens at the rotor upstream. Three sets of experiments are performed for the comparison of results, i.e. uniform inlet flow, tip, and hub distortions, respectively. Regardless of the type of distortion introduced to the inflow, the compressor undergoes a performance drop, which is more severe in the hub distortion case. Under the uniform inlet flow condition, stall inception is caused by the modal type disturbance while the stall precursor switched to spike type due to the highly loaded blade tip. In the presence of high blade loading at the hub, spike disappeared and the compressor once again witnessed a modal type disturbance. Hub pressure fluctuations are observed throughout the process when the stall is caused by a modal wave while no disturbance is noticed at the hub in spike type stall inception. It is believed that the hub flow separation contributes to the modal type of stall inception phenomenon. Results are also supported by the recently developed signal processing techniques for the stall inception study.

Experimental investigations on the frequency of partial surge

Partial surge is characterized by axisymmetric low-frequency disturbance localized in the hub region and with the potential risk of causing full compressor instability in a transonic axial flow compressor. To find out what determines the low frequency of partial surge, most typical vibro-acoustic frequencies in the test compressor system are first examined, and the observed frequency of partial surge is found to be the closest to the Helmholtz frequency estimated with Greitzer’s duct-compressor-plenum model. Experiments are then conducted to observe the frequency of partial surge during the evolution of instability in the same compressor system with varied lengths of compressor inlet duct and inlet duct in front of the settling chamber and at the same rotor speed. The result also suggests that the dominant frequency of partial surge can be determined by the Helmholtz frequency of the system. When the length of compressor inlet duct is increased beyond a critical range, a different type of system oscillation is also observed in the form of a disturbance at higher frequency. The occurrence of this disturbance is discussed in this article with an extended theory of system oscillation.

Catastrophe-Theory-Based Modeling of Airfoil-Stall Boundary at Low Reynolds Numbers

Airfoil stall at low Reynolds numbers is a complex nonlinear dynamic phenomenon, which is characterized by catastrophe and hysteresis. It is difficult but important to mathematically describe the s...