New publication on compressor exit velocity triangles

11 May 2022


A diagram which gives a particularly useful insight into the relevance of centrifugal compressor impeller design parameters was originally published in German in a Brown Boveri Company internal note in 1941.

The diagram has been extended and used extensively in several forms in our recent book published in 2021 by Cambridge University Press.


However, at the ASME Turbo Expo, taking place in Rotterdam, the Netherlands, 13th-17th June, 2022, Professor Mick Casey will present a technical paper which uses the diagram in other ways, not included in the book.



The full reference is:


Casey, M. and Robinson, C. (2022) Some properties of the exit velocity triangle of a radial compressor impeller, GT2022-79368, ASME Turbo Expo 2022, Rotterdam, Netherlands 13-17th June, 2022.


Important aspects made clear by discussing various forms of 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 the reaction 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 in this diagram at off-design conditions provides a new approach to estimate the mean slip factor from a measured or calculated compressor performance map.
  • Different impeller design styles can be categorized in this diagram to 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 important aid to compressor stability.


Radial Flow Turbocompressors is available from Cambridge University Press with a discount or via Google Books

For those already owning a copy, a list of corrigenda can be downloaded from PCA's website by following this link.