Session: 40-04 Centrifugal Compressors 2
Paper Number: 79368
79368 - Some Properties of the Exit Velocity Triangle of a Radial Compressor Impeller
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 by 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.
Presenting Author: Michael Casey PCA Engineers Limited
Presenting Author Biography: Michael Casey studied Engineering Science in Oxford University, and completed his D.Phil. thesis on "Cavitation inception on hydrofoils" there in 1974. <br/><br/>He held postdoctoral positions in Durham and Cambridge Universities and then worked as an engineer and manager for nearly 30 years in various international companies (WS Atkins, Sulzer Turbo, Rolls Royce, and Sulzer Innotec), working on turbomachinery design methods, CFD and experimental methods. <br/><br/>From October 2003 to March 2011 he was Professor of Thermal Turbomachinery in Stuttgart University, Germany. In retirement, he still acts as a consultant for PCA Engineers Limited in the UK. He is a Fellow of the Institution of Mechanical Engineers and of The American Society of Mechanical Engineers. Together with his co-author he has recently published a book entitled Radial Flow Turbocompressors with Cambridge University Press.
Authors:
Michael Casey PCA Engineers LimitedChris Robinson PCA Engineers Limited
Some Properties of the Exit Velocity Triangle of a Radial Compressor Impeller
Paper Type
Technical Paper Publication