SOME PROPERTIES OF THE EXIT VELOCITY TRIANGLE OF A RADIAL COMPRESSOR IMPELLER

Abstract A diagram that demonstrates important aspects of the exit velocity triangle of a radial impeller is introduced and explained. Although this diagram is based on the published work of Mehldahl in 1941, it is not widely known in the radial compressor community. It provides a formidable coherence to the understanding and selection of the exit velocity triangle for impeller designs across many different applications. Important aspects of the design made clear in the diagram are as follows: The degree of reaction is determined primarily by the work coefficient, and decreases as the work coefficient increases, but for typical designs it remains nearly constant at off-design conditions.The inlet and exit velocity triangles can be shown in the same diagram and this visualizes the deceleration of the relative velocity along the casing streamline and of the meridional velocity across an impeller, and shows the acceleration of the relative flow on the hub streamline.The work coefficient is the primary determinant of the level of diffusion in the impeller.The slope of the impeller work versus flow characteristic in this diagram provides a new approach to estimate the mean slip factor from a measured compressor performance map.Different impeller design styles can be categorized in this diagram and show that different impellers are needed for use with vaned and vaneless diffusers.The absolute velocity at the diffuser inlet of a backswept impeller increases with a decrease in flow rate along the operating line, which is an aid to compressor stability.