A Numerical Study of the Unsteady Interaction Effects on Diffuser Performance in a Centrifugal Compressor

Abstract

Interaction between rotating impeller and stationary diffuser in a centrifugal compressor is of practical importance in evaluating system performance. The present study aims at investigating how the interaction influences the unsteady diffuser performance and understanding the physical phenomena in the centrifugal compressor. A computational fluid dynamics (CFD) method has been applied to predict the flow field in the compressor, which has a conventional vaned diffuser (VD) and a low solidity vaned diffuser (LSVD). The radial gaps between impeller and diffuser and different flow coefficients are varied. The results obtained show that the major parameter that influences the unsteady variation of diffuser performance is due to the circumferential variation of the flow angle at the diffuser vane leading edge. The physical phenomena behind the pressure recovery variation are identified as the unsteady vortex shedding and the associated energy losses. The vortex core region as well as the shedding of vortices from the diffuser vane are triggered by the variation in the diffuser vane loading, which in turn is influenced by the circumferential variation of the impeller wake region. There is little unsteady variation of flow angle in the span-wise direction. This indicates that the steady state performance characteristics are related to the span-wise variation of flow angle, while the unsteady characteristics are contributed by the circumferential variation of flow angle. At design conditions, dominant frequency components of pressure fluctuation are all periodic and at near stall, these are aperiodic.

The effect of impeller–diffuser interactions on diffuser performance in a centrifugal compressor

“…Figure 14 shows the vortex core regions (the isosurface of the swirling strength) near the leading edge of the diffuser vane for the three cases. This vortex is also observed by Anish et al (2014). As the mass flow coefficient decreases, the area of the vortex core region increases -that is to say, the strength of the vortex is enhanced.…”

“…The time step was taken to be the time of an impeller blade passing 1/360 of a circle. This value is the optimum time step according to Anish and Sitaram (2009) and Anish et al (2014). The 'stage' method (ANSYS, 2011a) was adopted for analyzing the IDIs for steady simulations.…”

“…Reinhard (2003a, 2003b) and Robinson, Casey, Hutchinson, and Steed (2012) studied the impact of the size of the gap between the impeller and the vaned diffuser on the performance of the vaned diffuser; their findings indicate that a smaller gap results in a higher efficiency and a stronger IDI effect. Wilkosz, Zimmermann, Schwarz, Jeschke, and Smythe (2014) and Anish, Sitaram, and Kim (2014) investigated the impact of the IDI on the centrifugal compressors with both high and low Mach numbers, finding that the IDI strongly affects the aerodynamic performance of the diffuser and that it must be considered in the detailed design process. Anish et al (2014) noted that the main factor influencing the diffuser instability is the circumferential variation of the flow angle at the leading edge of the diffuser vane.…”

“…Wilkosz, Zimmermann, Schwarz, Jeschke, and Smythe (2014) and Anish, Sitaram, and Kim (2014) investigated the impact of the IDI on the centrifugal compressors with both high and low Mach numbers, finding that the IDI strongly affects the aerodynamic performance of the diffuser and that it must be considered in the detailed design process. Anish et al (2014) noted that the main factor influencing the diffuser instability is the circumferential variation of the flow angle at the leading edge of the diffuser vane. The aerodynamic loss in the diffuser is induced mainly by the generation and shedding of the vortexes which exist on the diffuser vane leading edge, on the suction side of the diffuser vane near the hub and on the diffuser vane trailing edge.…”

The Role of Impeller Outflow Conditions on the Performance of Vaned Diffusers

“…The increased loss may not be captured in the URANS simulation simply due to the limitations of RANS modeling, given that what is expected is an unsteady separation phenomenon. It is also possible that the effect of unsteadiness occurs at a different frequency than is allowed within a single passage, phase-lagged simulations; periodic unsteady phenomena uncorrelated to the frequency of the rotor blade passing was recently observed in work by Anish et al [25]. In summary, the hypothesis that the unsteadiness from the impeller has negligible influence on the diffuser aerodynamics is partially validated: across much of the operating range, around the best efficiency point, this is true.…”