Casing Modification for Increasing the Surge Margin of a Centrifugal Compressor in an Automotive Turbine Engine

The significance of broadening the flow range of the compressor in an automotive gas turbine engine by lowering its surge-limited flow is cited. Following a review of flow instabilities observed in the centrifugal compressor, data are presented from a particular machine. Review of these data suggest that surge is associated with the conditions inducing rotating stall, and that the rotating stall occurs initially in the diffuser. Employing a simplified two-dimensional analytical model of the flow at the rotor-diffuser interface, it is found that flow relief in the third dimension should delay the onset of surge. Incorporating such relief in the shroud casing is shown experimentally to reduce surge flow.