The combination of variable speed control and adjustable diffuser vanes offers an attractive design option for centrifugal compressors applied in industrial applications where a wide operating range at high efficiency level and a favorable surge line is required. However, the knowledge about the impact on compressor performance of a diffuser vane clearance between vane and diffuser wall which is mandatory since the diffuser geometry adjustment has to take place during operation, is still not satisfying.
The results of characteristic line and probe measurements presented in Part I [18] of this two-part paper showed that an one-sided diffuser clearance not necessarily need to lead to a negative impact on compressor operating behavior but is able to contribute to an increase in flow range, stall margin, pressure ratio and efficiency, as long as the diffuser passage is broad enough with respect to the clearance height.
In order to reveal the relevant flow phenomena, in Part II the results of detailed measurements of the pressure distribution at diffuser exit and Particle Image Velocimetry (PIV) measurements inside the diffuser channel performed at three clearance configurations and three diffuser angles at a fixed radial gap are discussed. It was found, that for defined diffuser configurations the clearance flow amplifies the diffuser throat vortex capable to reduce the loading of the highly loaded vane pressure side and to support a more homogenous diffuser flow. It turned out that the coaction of the geometry parameter diffuser vane angle and diffuser clearance height is of particular importance.
The experimental results are published as an open CFD testcase “Radiver 2” [17], extending the experimental data base of the testcase “Radiver” published in 2003 by Ziegler [24].
