Linear stability analysis of rotating stall in a wide vaneless diffuser

Abstract

A two-dimensional linear stability analysis of rotating stall in a wide radial vaneless diffuser is performed to determine the onset and the characteristics of rotating stall. In addition, a dedicated experiment test rig was used to study the abilities and the limits of the theoretical model. The characteristics of rotating stall (number of modes, propagation velocity, critical angle at which the instability may appear) are calculated by this method and compared to experimental results. Besides, the growth rate of each stall mode has been calculated to determine the dominant stall mode. The ability and limits of this approach is discussed. KEYWORDS LINEAR STABILITY ANALYSIS, ROTATING STALL, RADIAL WIDE VANELESS DIFFUSER NOMENCLATURE V Stable velocity ω Angular velocity R Diffuser radius ratio σ Growth rate of rotating stall h Width of diffuser ρ Density P Pressure SUBSCRIPTS n Number of cells 2 Impeller outlet Q Flow rate 3 Diffuser inlet Г Circulation 4 Diffuser outlet i Imaginary unit θ Tangential direction t Time r Radial direction u Perturbation velocity rs Rotating stall p Pertubation pressure B Basic solution r Radius c Critical condition θ Angle d Design condition z Axial direction SUPERSCRIPTS GREEK LETTERS-Dimensional variables α Flow angle ~ Perturbation variables ζ Vorticity

Instabilities identification based on a new centrifugal 3D impeller outflow model

“…More detailed descriptions of the experimental measurements, data acquisition, and adopted pump operating points can be found in previous papers [15,19,[22][23][24][25] that operate the same apparatus.…”

Effect of inlet leakage flow on the instability in a radial vaneless diffuser

“…Their work was extended, considering the diffuser geometry variations 6 . These unforced rotating instabilities have been theoretically investigated using two different approaches: a 3D wall boundary layer stability analysis 7 and a two-dimensional inviscid approach only assuming a core flow region in the vaneless diffuser hub-to-shroud sections 8,9 in case of wide diffuser ratios, i.e. for diffusers whose ratio between the height (b 2 ) and the outlet-to-inlet diffuser radius difference (R 4 − R 3 ) is Γ = (R 4 − R 3 )/b 2 O(1).…”