Unsteady Behavior of Diffuser Stall in a Centrifugal Compressor With Vaned Diffuser

Abstract

In this study, the unsteady behavior of a diffuser rotating stall in a centrifugal compressor with a vaned diffuser was investigated through experiments and numerical analyses. From the casing static pressure measurements, it was determined that the diffuser stall propagated at 25% of impeller rotational speed in the vaneless space. The numerical results revealed the presence of a typical vortical structure on the diffuser’s leading edge. Under partial flow condition, a tornado-type vortex was generated on the diffuser’s leading edge. Furthermore, a longitudinal vortex at the shroud/suction surface corner (i.e., leading edge vortex (LEV)) was induced by the rolling-up flow on the diffuser suction surface. As the velocity was decreased, the development of the tornado-type vortex and LEV forms a substantial flow blockage within the diffuser passages. Furthermore, the diffuser stall cell was caused by the systematic vortical structure which consisted of the tornado-type vortex, LEV, and vortex in the throat area of diffuser passages. In addition to this, the developed LEV interacted with the next diffuser leading edge and formed the throat area blockage with the passage of time. Then, the tornado-type vortex and LEV developed by the throat area blockage and diffuser stall cell, which was caused by the systematic vortical structure, propagated to the succeeding diffuser vane. Therefore, the diffuser stall in the centrifugal compressor was caused by the evolution of the tornado-type vortex and LEV.

Evolution Process of Diffuser Stall in a Centrifugal Compressor With Vaned Diffuser

“…As Eck et al accurately pointed out, the rotating instability cells are induced by the small vortices propagating at the rotor leading edge [11]. We have also reported that the unsteady behavior of the diffuser stall is influenced by the unsteady vortices at the leading edge of the diffuser vane, during off-design operation [12]. Especially, we demonstrated that the diffuser stall cell is caused by the systematic vortical structure, which consisted of a tornado-type vortex, LEV, and a vortex in the throat area of the diffuser passages, as described in Fig.…”

“…The present simulations were conducted using an in-house computational fluid dynamics (CFD) code, which had been validated for various turbomachinery flows [12,18,19] [20], which was extended to the third order using the monotonic upwind scheme for conservative laws (MUSCL) interpolation [21]. The viscous flux was determined as a second-order central difference by using Gauss's theorem.…”

“…Both Coriolis and centrifugal forces were considered inertial force terms in the relative coordinate system. This DES code was able to transform appropriately the RANS-mode in regions near the wall, and the LES-mode inside the fluid volume [12].…”

“…1 Illustration of the Rotating Mechanism of the Diffuser Stall Cell. [12] can be recognized as a precursor of surge in the impeller or diffuser passage. Therefore, a better understanding of the physics of the rotating stalls leading to a surge is important in predicting of the surge and for the improvement of the stall margin.…”

Operating Range Extension of an Open Impeller Centrifugal Compressor Stage Utilising 3D Diffuser End Wall Contouring

“…A wide stable operating range of vaned diffusers is therefore important to meet these requirements. Fujisawa et al [4] and Fujisawa et al [5] studied the diffuser stall mechanism in an open impeller centrifugal compressor stage with a wedge diffuser. As initiation for diffuser instability a rotating diffuser stall was observed on shroud side.…”