Numerical investigation of a centrifugal compressor with various diffuser geometries for fuel cell applications

To meet the air supply requirements for automotive fuel cell applications, the centrifugal compressor supplying the fuel cell requires a wide operating range. This paper describes a numerical study of a centrifugal compressor with a variable-vaned diffuser geometry that delivers a wide operating range. The geometry variability is achieved adjusting the angle of the diffuser vanes about their own axes. In real applications, this would require gaps between the diffuser vanes and end walls; the negative influence of these gaps on the flow and compressor efficiency is shown by a numerical investigation of diffusers with different vane angles with and without gaps that reduce the efficiency by up to 2.7 percent. Furthermore, the compressor performance is compared numerically with various fixed-vaned diffuser geometries, which are designed to shift the compressor map towards lower mass flows and accordingly increase the surge margin by up to 45 percent. These measures cause an increase in efficiency at high speeds and a reduction at low and medium speeds.