The combination of variable speed control and adjustable\r\ndiffuser vanes offers an attractive design option for centrifugal\r\ncompressors applied in industrial applications where a wide\r\noperating range at high efficiency level and a favorable surge\r\nline is required. However, the knowledge about the impact on\r\ncompressor performance of a diffuser vane clearance between\r\nvane and diffuser wall which is mandatory since the diffuser\r\ngeometry adjustment has to take place during operation, is still\r\nnot satisfying.\r\nThis two-part paper summarizes results of investigations\r\nperformed at the Institute of Jet Propulsion and\r\nTurbomachinery at RWTH Aachen with an industrial-like\r\ncentrifugal compressor, featuring a design pressure ratio of 4\r\nand a design speed of 35200 rpm. Particular attention was\r\ndirected to the influence of the diffuser clearance on the\r\noperating behavior of the entire stage, the pressure recovery in\r\nthe diffuser and on the diffuser flow by a systematic variation\r\nof the parameters diffuser clearance height, diffuser vane angle,\r\nradial gap between impeller exit and diffuser inlet, and rotor\r\nspeed.\r\nCompressor map measurements provide a summary of the\r\noperating behavior related to diffuser geometry and impeller\r\nspeed, whereas detailed flow measurements with temperature\r\nand pressure probes allow a breakdown of the losses between\r\nimpeller and diffuser and contribute to a better understanding of\r\nrelevant flow phenomena. The results presented in Part I show\r\nthat an one-sided diffuser clearance does not necessarily has a\r\nnegative impact on the operation and loss behavior of the\r\ncentrifugal compressor, but instead may contribute to an\r\nincreased pressure ratio and improved efficiency.\r\nThe flow phenomena responsible for this detected\r\nperformance behavior are exposed in Part II [28], where the\r\nresults of detailed measurements with pressure probes at\r\ndiffuser exit and Particle Image Velocimetry (PIV)\r\nmeasurements conducted inside the diffuser channel, revealing\r\nthe complex and unsteady flow leaving the impeller and\r\npassing the diffuser channel, are discussed.\r\nThe experimental results are published as an open CFD\r\ntestcase “Radiver 2” [26], extending the experimental data base\r\nof the testcase "Radiver" published in 2003 by Ziegler [31].    «

The combination of variable speed control and adjustable\r\ndiffuser vanes offers an attractive design option for centrifugal\r\ncompressors applied in industrial applications where a wide\r\noperating range at high efficiency level and a favorable surge\r\nline is required. However, the knowledge about the impact on\r\ncompressor performance of a diffuser vane clearance between\r\nvane and diffuser wall which is mandatory since the diffuser\r\ngeometry adjustment has to take place during operat...    »