This paper reports an experimental and theoretical study of rotating stall in a vaneless diffuser which is coupled with a radial impeller. The experiments were conducted at 22 flow rates for two rotating speed: 1200rpm and 1800rpm. The measurements have consisted of: i/ unsteady pressure measurements delivered by two microphones flush mounted on the vaneless diffuser, ii/ 9 steady pressure taps mounted in one radial line on the diffuser to measure the pressure recovery in the vaneless diffuser. The stability of each stall mode was also studied by a 2D linear analysis; and the theoretical prediction was compared to experimental observations. The capabilities and limits of such an approach to predict the development of rotating stall have been evaluated. A non-dimensional analysis of the pressure losses at outlet was conducted to evaluate the effect of the instability development on the performance of the diffuser. It has shown that the arising of rotating stall has a positive effect on the diffuser performance.
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
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