Some Properties of the Exit Velocity Triangle of a Radial Compressor Impeller

A diagram is described which demonstrates important aspects of the exit velocity triangle of a radial compressor impeller with no inlet swirl. Although this diagram is based on work dating back to 1941, it is not widely known in the radial compressor community. It aids the selection of the exit velocity triangle for impellers and gives significant insight into impeller-diffuser matching. It can be used to analyze performance maps from both experimental and computational fluid dynamics (CFD) studies. Important aspects made clear in the diagram are as follows: The degree of reaction and the de Haller number of the impeller are both determined primarily by the work coefficient, and both decrease as the work coefficient increases; the degree of reaction is also affected by the exit flow coefficient, and for typical backswept impeller designs, it remains nearly constant at off-design flow conditions; the inlet and exit velocity triangles can be shown in the same diagram to visualize the deceleration of the relative velocity along the casing streamline and of the meridional velocity across an impeller, together with the acceleration of the relative flow on the hub streamline; the slope of the impeller gas path work coefficient versus exit flow coefficient at off-design conditions can be added to the diagram and this provides a new approach to estimate the mean slip factor from a measured or calculated compressor performance map; the diagram can be used as a useful template to compare different impeller design styles and to explain why different impellers are needed for use with vaned and vaneless diffusers; and the absolute velocity at the diffuser inlet of a backswept impeller increases with a decrease in flowrate along the operating line, which is an important aid to compressor stability.