Study of Vaneless Diffuser Rotating Stall Based on Two-Dimensional Inviscid Flow Analysis

Abstract

Rotating stalls in vaneless diffusers are studied from the viewpoint that they are basically two-dimensional inviscid flow instability under the boundary conditions of vanishing velocity disturbance at the diffuser inlet and of vanishing pressure disturbance at the diffuser outlet. The linear analysis in the present report shows that the critical flow angle and the propagation velocity are functions of only the diffuser radius ratio. It is shown that the present analysis can reproduce most of the general characteristics observed in experiments: critical flow angle, propagation velocity, velocity, and pressure disturbance fields. It is shown that the vanishing velocity disturbance at the diffuser inlet is caused by the nature of impellers as a “resistance” and an “inertial resistance,” which is generally strong enough to suppress the velocity disturbance at the diffuser inlet. This explains the general experimental observations that vaneless diffuser rotating stalls are not largely affected by the impeller.

Analysis of Vaneless Diffuser Stall Instability in a Centrifugal Compressor

“…In other words, the flow mode can be described as a rotating instability consisting of two counter-rotating flow cells relative to each other. This qualitative description is in agreement with results of other research groups, e.g., [5,9]. The same feature can be attributed also to the lower flow angle (second and third columns) but with a larger number of rotating cells distributed evenly around the circumferential.…”

“…The rotating stall feature was illustrated as two high static pressure cells at low velocity with reversing flow and two lower pressure cells at higher outgoing velocity. This was later confirmed in the 2D linear stability analysis by [5]. These cells were reported to be rotating at sub-synchronous rotational speed.…”

“…For the actual evolution of the dynamic character and thus prediction of the number of rotating stall cells and their propagation speed around the annulus, different research groups have considered a time-evolving calculation assuming 2D-unsteady, inviscid and incompressible flow, see e.g., [5][6][7]. The effect of the blades on the flow is fed at the diffuser inlet boundary and a Dirichlet condition with static pressure is considered on the outlet surface.…”

Experimental Study and Theoretical Analysis of the Rotating Stall in a Vaneless Diffuser of Radial Flow Pump

“…The analysis is based on the assumptions of a 2D, non-viscous flow with axisymmetric boundary conditions at inlet. The detail of the methodology used to predict the flow stability is available in Tsujimoto [8]. The results of this analysis are compared to the present experimental work to identify if such a methodology is able to predict the unstable modes developing.…”

“…(1) Theoretical analysis, which includes three dimensional analysis (Jansen [3], Senoo et al [4,5], Frigne [6], Dou et al [7]) and two dimensional analysis (Tsujimoto [8], Moore [9] and Abdelhamid et al [10]). For two dimensional analysis, it was suggested the 2D core flow instability might be responsible for rotating stall.…”