A Study on Impeller-Diffuser Interaction: Part II — Detailed Flow Analysis

Abstract

The interaction between impeller and diffuser is considered to have strong influence on the flow in highly loaded centrifugal compressors. However, the knowledge about this influence is still not satisfying. This two-part paper presents an experimental investigation of the effect of impeller-diffuser interaction on the unsteady and the time averaged flow configuration in impeller and diffuser and the performance of these components. The flat wedge vaned diffuser of the investigated stage allows an independent adjustment of diffuser vane angle and radial gap between impeller exit and diffuser vane inlet. Attention is mainly directed to the radial gap, as it determines the intensity of the impeller-diffuser interaction. In part I it was shown that smaller radial gaps improve diffuser pressure recovery, whereas impeller efficiency is hardly affected. Part II focuses on the reasons for these effects. Measurements with a laser-2-focus velocimeter in the highly unsteady flow field between the impeller exit region and diffuser throat were performed at three different diffuser geometries allowing a detailed flow analysis. Especially the unsteady results show that for a smaller radial gap more impeller wake fluid is conveyed towards the highly loaded diffuser vane pressure side reducing its loading and leading to a better diffusion in the diffuser channel. Concerning the impeller flow, it was found that a smaller radial gap is leading to a noticeable reduction of the wake region at impeller exit. The experimental results are intended to be published as an open CFD testcase under the name “Radiver”.

Effects of Radial Gap Ratio between Impeller and Vaned Diffuser on Performance of Centrifugal Compressors

“…Therefore, it can be realized that this discrepancy in work input value for different radial gap ratios is because of the recirculation that is a factor of work input generation in the impeller. As a result, the recirculation in impeller increases by reducing the radial gap ratio [18]. However, the impeller performance characteristics at Mu = 0.69 are not affected by changing the radial gap ratio.…”

“…The load on the rear part of the pressure surface reduces in a larger radial gap size. Therefore, the diffuser can have superior diffusion [18]. However, according to Figure 20, as the impeller inlet tip-speed Mach number is less than the unity value (M u = 0.69), the flow does not accelerate substantially.…”

“…The load on the rear part of the pressure surface reduces in a larger radial gap size. Therefore, the diffuser can have superior diffusion [18]. Diffuser pressure recovery (CP) is one of the most common factors for evaluating the diffuser performance [37], which is the capability of the diffuser to diffuse.…”

Flow Characteristics of a Pipe Diffuser for Centrifugal Compressors

“…This vortex also occurs in other radial bladed diffusers applied in centrifugal compressors. Ziegler et al (2003) studied the unsteadiness of this vortex in a wedge-type diffuser, and Stahlecker et al (1998) detected the vortex in a vaned diffuser by experimental methods. The vortex near the back wall can be seen clearly in plane02, implying that it is fully generated before entering the diffuser throat.…”

Effects of a Nonuniform Tip Clearance Profile on the Performance and Flow Field in a Centrifugal Compressor

“…The tip clearance height is 0.7 mm for the stationary impeller, and the vaneless diffuser has a constant Table 1, and a schematic diagram of the meridional plane is shown in Figure 1, where the sections 2 M and 2M indicate the measurement positions. A complete description of the geometrical configurations and operating conditions can be found in Ziegler et al [15][16][17].…”