Impeller–Diffuser Interaction in a Centrifugal Compressor

Abstract

A study has been conducted, using an unsteady three-dimensional Reynolds-averaged Navier-Stokes simulation, to define the effect of impeller-diffuser interaction on the performance of a centrifugal compressor stage. The principal finding from the study was that the most influential aspect of this unsteady interaction was the effect on impeller tip leakage flow. In particular, the unsteadiness due to the upstream potential effect of the diffuser vanes led to larger viscous losses associated with the impeller tip leakage flow. The consequent changes at the impeller exit with increasing interaction were identified as reduced slip, reduced blockage, and increased loss. The first two were beneficial to pressure rise, while the third was detrimental. The magnitudes of the effects were examined using different impeller-diffuser spacings and it was shown that there was an optimal radial gap size for maximum impeller pressure rise. The physical mechanism was also determined: When the diffuser was placed closer to the impeller than the optimum, increased loss overcame the benefits of reduced slip and blockage. The findings provide a rigorous explanation for experimental observations made on centrifugal compressors. The success of a simple flow model in capturing the pressure rise trend indicated that although the changes in loss, blockage, and slip were due largely to unsteadiness, the consequent impacts on performance were mainly one-dimensional. The influence of flow unsteadiness on diffuser performance was found to be less important than the upstream effect, by a factor of seven in terms of stage pressure rise in the present geometry. It is thus concluded that the beneficial effects of impeller-diffuser interaction on overall stage performance come mainly from the reduced blockage and reduced slip associated with the unsteady tip leakage flow in the impeller.Downloaded From: http://turbomachinery.asmedigitalcollection.asme.org/ on 05/29/2015 Terms of Use: http://asme.org/terms Table 4 Change of stage total-to-total pressure ratio and other performance parameters due to a reduction of radial gap size r 2 ЈÕr 2 from 1.092 to 1.054 Journal of Turbomachinery OCTOBER 2000, Vol. 122 Õ 785 Downloaded From: http://turbomachinery.asmedigitalcollection.asme.org/ on 05/29/2015 Terms of Use: http://asme.org/terms

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

“…14), which results in high values of absolute angle. 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.…”

Effect of high negative incidence on the performance of a centrifugal compressor stage with conventional vaned diffusers

“…Shum et al [10] found that the alignment of the diffuser vanes with a spatially averaged and time-averaged impeller exit flow was the main parameter affecting the diffuser performance. Shum et al also conclude that there is an optimum radial gap between the impeller exit and diffuser leading edge, because at some decreasing radius, increasing losses overtake the benefits of a smaller blockage achieved with a smaller radial gap.…”

Experimental and numerical analyses to enhance the performance of a microturbine diffuser

“…This aspect has been documented by several researchers (see, among others [3,4]). Owing to this complexity, some authors have underlined the weak points of a simple approach that attempts to describe the flow entering the diffuser without the direct effect of impeller interaction [5][6][7]. On the other hand, with the help of both experimental and numerical simulations, other researchers have indicated that the circumferential non-uniformities are less important than those occurring from hub to shroud in the neighborhood of the best efficiency working point [3,8].…”