Effect of the Unsteadiness on the Diffuser Flow in a Transonic Centrifugal Compressor Stage

Abstract

The study is focused on the analysis of the flow structure within the vaned diffuser of a transonic high-pressure centrifugal compressor stage. The analyzed time-dependent flow field comes from unsteady computations of the stage using a 3D Navier-Stokes code with a phase-lagged technique, at an operating point close to the design point. A good comparison with available experimental data allowed the use of CFD for investigating the details of the flow in order to assess the effect of the unsteadiness in the diffuser flow development. Applying various data processing techniques, it is shown that the unsteadiness is due to the jet and wake flow structure emerging from the radial impeller and to the pressure waves brought about by the interaction between the vane bow shock wave and the impeller blade. The interaction between the pressure waves and the vane pressure side boundary layer leads to a pulsating behavior of separated bubbles within the diffuser. The pressure waves are similar in shape and strength whatever the blade height. The observed change in the flow field from hub to tip is due to migration of the low momentum fluid contained in the wake toward the pressure side/hub corner.

Active Flow Control in a Radial Vaned Diffuser for Surge Margin Improvement: A Multislot Suction Strategy

“…This result confirms the high influence of the unsteadiness on the flow in the diffuser entry zone, especially in centrifugal compressors with small radial gap between the impeller and the vaned diffuser [33]. The scrolling of the impeller blades in front of the diffuser vanes generates intense high pressure waves, in the same manner as already observed in another transonic Safran Helicopter Engines centrifugal compressor [34]. These high pressure waves are reinforced when crossing the diffuser throat.…”

Experimental and numerical investigation of the flow field in a high-pressure centrifugal compressor impeller near surge

“…Figure 15 shows the difference between the absolute flow angle calculated near surge and the absolute flow angle calculated at peak According to Shum et al [5] who stated that the diffuser performance is mainly influenced by the incidence of the time-averaged flow, it is thought from the results given in Figs 14 and 15 that the strong instabilities inception could be initiated near the hub of the vaned diffuser entry zone. Moreover, it has been shown [14] that, at peak efficiency, the wake material migrated in the vaned diffuser passage towards the hub, leading to an accumulation of low momentum fluid near the hub. Therefore, an additional feeding in low momentum fluid near the hub as described in Fig.…”

Unsteady pressure measurements in a high-speed centrifugal compressor

“…The first part concerns the pressure transducers whose main features and requirements may be found in the review of Ainsworth et al [4]. Not only the time resolution but also its resistance to high temperatures has to be checked.…”

“…The Turbomachinery Team of the Fluid Mechanics Acoustic Laboratory (LMFA) at Ecole Centrale de Lyon (ECL) has two high-speed compressor test rigs dedicated to studies of the flow in a 3½-stage axial compressor named CREATE designed and built by Snecma, and in a transonic centrifugal compressor stage TM designed and built by Turbomeca. Several works [1] [2][3] [4] have been published on detailed measurements acquired in these machines using pressure and temperature probes and a laser anemometer. Ensemble averages of the data acquired with the laser anemometry technique lead to an interesting description of the flow if only the phenomena phased with the rotation speed of the machine are under investigation.…”