Unsteady Analysis of a Pulsating Alternate Flow Pattern in a Radial Vaned Diffuser

Abstract

In centrifugal compressors, Mild Surge (MS) leads to unstable operation. Previous experimental work on a centrifugal compressor designed and built by Safran Helicopter Engines (SafranHE) showed that MS corresponds to the pulsation of an alternate stall pattern at the Helmholtz frequency of the test rig on two channels in the radial diffuser. The present contribution experimentally investigates the impact of the Inlet Guide Vane (IGV) stagger angle on this alternate flow and numerically studies the topology of this pulsating alternate flow. The experimental investigation is performed with unsteady pressure sensors, and shows that the IGV stagger angle only impacts the pulsation frequency of the alternate flow pattern. This change is explained by the dependence of the Helmholtz frequency on the compressor inlet section. The topological analysis of the average flow field, computed from wall-resolved Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations, demonstrates that the saddle point (major critical point) in the corner hub/suction side of the stalled blade migrates upstream while staying in the corner if the mass flow rate decreases. One main blade over two is stalled on both sides because the flow originating from this corner separation circumvents the trailing edge and migrates upstream along the pressure side. In the simulation, the pulsation of the alternate stall is coupled with the reflection of acoustic waves on the inlet and outlet planes, regarded as an environmental effect.

Numerical investigation of a transonic centrifugal compressor at high rotational speed

“…Pressure and temperature are measured with 130 steady sensors to derive performance. Unsteady pressure measurements are also carried out using 52 case-mounted Kulite transducers to investigate aerodynamic phenomena, see (Moënne-Loccoz et al, 2020;Poujol et al, 2022) for details. Regarding some specifics, the overall stage total-to-static pressure ratio is around 4, and the Mach number at the exit of the impeller is about 1.2 at the rotational speed of interest derived from the peripheral velocity.…”

“…Experimental observations of the alternate flow pattern are based on an alternate rate derived by projecting all sensors in a single or two passages. Spectral analysis of pressure fluctuations reveals the lock-in phenomenon which may occur between the alternate pattern and the Helmholtz frequency of the test facility (Poujol et al, 2022). When the compressor is operated in mild surge, this pattern phase-locks with the associated planar pressure wave.…”

“…In papers (Moënne-Loccoz et al, 2020;Poujol et al, 2022), experimental analysis at part-speed conditions based on pressure transducers reveals the establishment of an alternate flow pattern in the radial diffuser at low massflow rates. To identify this phenomenon, the pressure sensors distributed over the circumference at the diffuser leading edge are used to derive the experimental alternate rate A detailed given by equation 3 with p 1p (p 2p ) the pressure projected in 1 channel (2 channels), θ the circumferential coordinate and θ RD the pitch of the radial diffuser.…”