No abstract

This paper presents a stability model that can make a rapid prediction of the rotating stall inception in turbomachinery and provide the spatial distribution of the corresponding instability mode. In addition, this model can take the three-dimensional geometry of blades and complex flow details in the compressor into consideration, and the solution of the development process of small perturbations can be converted to a nonlinear eigenvalue problem. We propose a solution method by converting the nonlinear eigenvalue problem into a generalized one; then, it can be solved by the Arnoldi algorithm. The proposed method can shorten the elapsed time from hundreds of hours to a few minutes, as compared with the methods adopted in previous works, substantially reducing the computational cost. Furthermore, the spatial distribution of eigenvectors can be obtained to investigate the characteristics of the perturbation mode, which can be applied as a foundation to set the inlet/outlet boundary conditions and select the eigenvalue representing the rotating stall inception. In the cases of a transonic isolated rotor and a subsonic one-stage compressor, the results are in accordance with those measured in experiments, verifying the accuracy and effectiveness of the stability model. Therefore, the model can be applied to evaluate the flow stability in the design stage of compressors with low computational cost.

There has been little research on the system stability of the flow past a blade when the domain is finite in the transverse direction. In this study, we first explore the influence of the domain size and periodic boundary conditions in the transverse direction on the stability of the flow past a single blade in a finite domain. As the transverse width decreases, the base flow and instability characteristics change. Furthermore, the stability of the flow past a cascade including n blades is analyzed. There exist n perturbation modes corresponding to different temporal growth rates and frequencies with various staggered spatial distributions of the dominant region, embodied as the inter-blade phase angle in the frequency domain. The critical value and leading mode are related to the blade number. Therefore, when analyzing the stability of the flow past a cascade, it is important to extend the domain to the real blade number. The influence of the geometric cascade parameters on system stability is also studied. Finally, when the circumferential periodic flow is broken in a cascade including n blades, there exists only one eigenvalue near the stability boundary. The system stability deteriorates, and the critical Reynolds value drops sharply, even when several blades are restaggered to have a smaller angle of attack. From the distribution of the leading mode, the initial location of unsteadiness is associated with the region of maximum deficit in the velocity profile of the wake flow.

Inlet swirl distortion is generally considered as a type of velocity distortion, and inlet guide vanes (IGVs) are widely used in the multi-stage compressor of aero-engines to eliminate the tangential velocity of the swirl flow. However, few studies have explored whether there still exists some negative influence of inlet swirl distortion on the compressor, even after the installation of IGVs. Therefore, in this study, the influence of various types of inlet swirl distortions on a multi-stage compressor with the installation of IGVs is investigated. A swirl distortion generator installed in the inlet duct was designed to produce various types of swirl flow patterns. When the distortion intensity increased to some degree, there still existed a decrease in the compressive capability and an obvious additional efficiency loss. The inlet twin swirl distortion was accompanied by total pressure distortion, so even with the installation of IGVs, there was still a significantly negative influence on the performance of the multi-stage compressor, especially the stall margin. Subsequently, to improve the stall margin under inlet swirl distortion, the stall precursor-suppressed (SPS) casing treatment was installed in the first stage of the multi-stage compressor. It could enhance the stall margin of the compressor with no obvious change in the characteristic curves and no additional efficiency loss under various types of inlet swirl distortions, and its mechanism was verified by capturing the dynamic pressure characteristics.