Tianyu Pan

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

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.

Using forward end-sweep to reduce transonic cantilevered stator losses to improve compressor performance

ABSTRACT The complicated flows in a transonic cantilevered stator would cause serious aerodynamic losses and reducing these losses would contribute much to improving the overall aerodynamic performance of a compressor. The paper conducts an overview study of the potential aerodynamic losses in a transonic cantilevered stator and presents a systematic numerical study for the effects of forward end sweep on reducing aerodynamic losses. A 2-D stator blade is used to throw light upon the physical mechanism of the forward end sweep on reducing the losses. At the casing region, forward end sweep has led to a redistribution of the blade load along the chordwise direction, higher in the front part and lower in the rear part. This result leads to a lower pressure gradient from the 30% chord location to the trailing edge and can be beneficial for the reduction of the corner separation. At the hub region, on one aspect, forward end sweep reduces both the peak Mach number and the size of the high-Ma region of the blade suction surface, thus decreasing the shock loss near the hub. On the other aspect, it also reduces the peak load and the strength of the leakage vortex, therefore alleviating the flow blockages. Simultaneously, the forward end sweep is applied to a commercial and high-performance multistage compressor to further validate its feasibility. When the technique is applied to the transonic first four stator rows, an improvement of adiabatic efficiency is achieved by ∼ 0.85% and the compressor stable operating range is slightly increased. The results suggest that the forward end-swept transonic cantilevered stators have a significant potential for reducing the stator losses. They would provide guidelines to advance the cantilevered stator design and further improve the compressor aerodynamic performance. Abbreviations: h, stagnation enthalpy; , stagnation pressure; p, static pressure; U, blade rotational speed at tip; V, velocity; , non-dimensional wall distance; , total pressure loss coefficient; , flow coefficient (Vx/U); , stage load coefficient (Δh/U2); , adiabatic efficiency; , total pressure ratio; in, inlet; out, outlet; LE/TE, leading/trailing edge; PS/SS, pressure/suction surface

A region-segmentation combining surrogate model based on L-indicator and N-fold cross-validation technique

ABSTRACT For complex engineering problems, for which the mathematical models may be linear, low-order nonlinear or even high-order nonlinear, surrogate models which have high adaptability and accuracy are required. This article develops a method for constructing a region-segmentation combining surrogate model. It is based on the idea that in the entire experimental domain, different local regions may present different characteristics (linearity, low-order nonlinearity and high-order nonlinearity), and the entire domain should be divided into several subregions to be approximated by different surrogates so as to achieve high prediction accuracy in local regions. The preferred models in each subregion then constitute a weight-average combining surrogate model. The investigations reveal that the new model not only is more adaptive to analytically unknown functions, but also gives more accurate predictions. The method has been applied to three benchmark problems and a practical engineering problem, and the results maintain validity.