Dynamics of global instabilities in the vaneless diffuser: A numerical approach and its applications

The effect of inviscid main flow and backflow boundary layer at diffuser walls both contribute to the instability of vaneless diffuser. This study is carried out to investigate the instability induced by the inviscid main flow. The biglobal instability analysis adopted here is focused on the large time scale growth of perturbations imposed on a base flow. The diffuser considered has two parallel walls, and the undisturbed flow is assumed to be circumferentially uniform, isentropic, and to have no axial velocity. In order to access the given state is stable or not, a linearized Euler’s equations for compressible base flow is used with a small-amplitude perturbation assumption. The eigenvalue problem is established through the spectral collocation discretization. Compared with experimental data, the stability model proposed in this paper is verified. Based on this numerical approach, the influence of inflow condition and geometric parameters, especially the compressibility effect on the stability of diffuser are investigated. And based on the results from some experiment measurement, the reliability of the stability analysis is verified through comparison. It is showed that diffuser instability increases rapidly and the stall rotation speed decreases quickly with the increase of diffuser radius ratio. The largest critical inflow angle can be obtained when the wave number m is around 3–5 for the radius ratio between 1.5 and 2.2. The present biglobal stability method has the capability to predict the onset of rotating stall, especially for wide diffusers.