A New Simplified Model of Post Stall Transients inAxial Compression Systems

Based on the theory developed by Moors and Greitzer, a new simplifying approximation, which takesinto account the influence of higher harmonics of rotating waves, is proposed in this paper to get a simplified model of post stall transients in axial compression systems. This approximation leads to a set ofthree simultaneous nonlinear first order partial differential equations. The further investigation of poststall behavior for different response modes of instabilities (rotating stall and/or surge), recoverability,prestall period during stall inception, and the effect of compression system parameters on them canbe carried out by this model and has been discussed in detail in the present paper. It has been foundthat stall inception exhibits a large prestall period in the region with small slope of compressor characteristic, and in this region, final throttle setting, compressor characteristic and time-lag parametershave a strong influence on the period. The inertia parameters of blade rows have a strong influenceon the recoverability of compression systems and the blockage of stall cell at recovery point. Somequalitative comparisons with available experimental results and experience are made, and it shows thatthe proposed model is very simple and reliable.

Numerical Simulation of Flow Instabilities in HighSpeed Multistage Compressors

In the present paper, a nonlinear multi 'actuator disk' model is proposed to analyze the dynamicbehavior of flow instabilities, including rotating stall and surge, in high speed multistage axial compressors. The model describes the duct flow fields using two dimensional, compressible and unsteadyEuler equations, and accounts for the influences of downstream plenum and throttle in the system aswell. It replaces each blade row of multistage compressors with a disk. For numerical calculations,the time marching procedure, using MacCormack two steps scheme, is used. The main purpose of thispaper is to predict the mechanism of two dimensional short wavelength rotating stall inception and theinteraction between blade rows in high speed multistage compressors. It has been demonstrated thatthe model has the ability to predict those phenomena, and the results show that some system parameters have a strong effect on the stall features as well. Results for a five stage high speed compressorare analyzed in detail, and comparison with the experimental data demonstrates that the model andcalculating results are reliable.

Numerical Simulation of Active Suppression of Rotating Stallin Axial Compression Systems

In the present paper, a theoretical model is proposed to analyze the transient behavior of suppression of rotating stall in axial compression systems through the use of an additional disturbance. The governing equations of the model are a set of simultaneous nonlinear first order partial differential equations,and for numerical calculations, a simple explicit time marching method can be used. The influence of system parameters on the suppression effectiveness and the interaction between rotating stall and surge have been discussed initially. The analysis of the influence of system parameters presents that both the B parameter and axisymmetric compressor characteristic have significant effect on the stabilization effectiveness of a control strategy. The effectiveness decreases as the value of B and the number of stages or stage loading of the compressor increase. It has been found that the onset flow rate of rotating stall and surge in a compression system may be different, and there is a strong interaction between these two kinds of instabilities. The onset flow rate of pure one dimensional surge depends on the value of B and axisymmetric compressor characteristic, besides the slope of the compressor characteristic.In some cases, when rotating stall which is the natural mode of instability in a compression system is suppressed, one dimensional surge can occur. It often limits the effectiveness of a control strategy to suppress rotating stall. But when surge is initiated by rotating stall, it is also possible to inhibit the occurrence of surge by suppressing rotating stall in a compression system.