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The performance and efficiency of a centrifugal compressor are usually affected by the highly complex 3-dimensional flow structures which develop in the flow field of the compressor. Several experiments and research using numerical analysis have been reported, however, there are still many unknown physical phenomena that need to be studied, in order to optimize the design and improve the efficiency of turbomachines, especially those installed on hydrogen-powered fuel cell electric vehicles (FCEVs). In this study, the 3-dimensional vortex structures were analyzed using the critical-point theory and the probabilistic definitions, for an air supply device mounted on the commercial hydrogen FCEVs. The behavior of the complex 3-dimensional vortex structures at the design flow rate and low flow rate were elucidated. A tip leakage vortex was observed to develop at the leading edge of the main blade at all flow rates, which caused interference to the splitter blade. At 60% of the design flow rate, a vortex breakdown occurred at the tip leakage vortex near the leading edge of the main blade, and a reverse flow at 50% chord length of the main blade's suction surface. The boundary layer which developed at the leading edge of the main blade's suction surface at all flow rates led to the creation of a hub separation vortex by interfering with the boundary layer developed at the hub surface as a result of the centrifugal force. In addition, the boundary layer developed at the hub and shroud surface created a horseshoe vortex as it moved downstream and interfered with the leading edge of the main blade and splitter blade. It was confirmed that the behavior of the tip leakage, hub separation, and horseshoe vortex structures determined the aerodynamic performance of the centrifugal compressor. The average pressure difference improved by 1.47% of the entire flow rate after optimizing the compressor design. [ABSTRACT FROM AUTHOR]

Diffusers are one of the most important factors determining the centrifugal compressor performance. The present work is aimed at providing a detailed understanding of the underlying flow and loss mechanisms in three different diffusers in a compact centrifugal compressor stage. Experimental and computational studies were conducted for various diffuser configurations, e.g. two pipe diffusers and one wedge diffuser, while keeping the throat in all the three geometries. It was found that both the pipe diffuser configurations had better aerodynamic performance than the original wedge diffuser. Furthermore, the pipe diffuser with a fishtail arrangement exhibited greater performance improvement, but had more distortion outflow than the wedge diffuser and the radial pipe diffuser because of the strong jet and wake structure caused by the fishtail turn. Nevertheless, the fishtail configuration has a smaller discharge swirl angle, which would have a positive impact on the performance of the combustor. As a result, the fishtail pipe diffuser configuration was recommended in compact centrifugal compressors. [ABSTRACT FROM AUTHOR]

An appropriate diffuser following an impeller is critical to realize a high efficiency and a wide operating range for a centrifugal compressor stage. The purpose of this study is to investigate the effect of different types of vaned diffusers on a large mass flow coefficient centrifugal compressor stage performance under different operating conditions and to reveal the loss mechanisms in the vaned diffusers. Five vaned diffusers are studied. Flow fields and vortices in a conventional diffuser and a rib diffuser are first examined. Then, vortices and flow fields in three different tandem diffusers are analyzed in detail. For the three tandem diffusers, only circumferential position of first row vanes relative to the second row vanes is different. Results show that the stage with the conventional diffuser possesses the shortest operating range. The rib diffuser has less loss due to the weaker tip leakage vortex, while its static pressure recovery coefficient is lower, and the loss in the following return part is higher. Comparison results between the three tandem diffusers imply that when the trailing edge of the first row vane is close to the pressure surface of the second row vane, the stage with the tandem diffuser has a better performance. This is ascribed to the interaction of the tip leakage vortex and suction surface vortex, which decreases the total loss, especially reduces the loss induced by the suction surface vortex. When the trailing edge of the first row vanes is close to the suction surface of the second row vane, the loss is increased since the leading edge vortex has a large strength and surrounds the suction surface vortex. Therefore, the reasonable interaction of vortices in a tandem diffuser can bring a high performance of the centrifugal compressor. [ABSTRACT FROM AUTHOR]