The off-design performance prediction of centrifugal compressors

Molenkamp, D.J. (2016) The off-design performance prediction of centrifugal compressors.

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Abstract:In this report, a centrifugal compressor performance prediction model is presented, that was made for Aeronamic. Aeronamic is a company that designs and produces subsystems for the aerospace industry. Centrifugalcompressors are often used in these systems. The current design strategy involves a time consuming process of trial and error, where a compressor is optimized for multiple operating points that must be reached. A MATLAB model is requested, that can evaluate the off-design performance of a given compressor design. This can then be used in a computational optimization algorithm to automate the design for multiple operating points. Based on a given geometry, upstream conditions and operating conditions, the prediction tool calculates the flow and thermodynamic conditions through the compressor. The underlying theory is the mean streamline analysis, which is a proven method for preliminary performance evaluation of turbomachinery. In this theory, averaged velocities are considered, that are calculated at intermediate “stations” through the compressor. These stations are located at the inlet and exit of every component, for example the impeller inlet and exit. Velocity triangles are employed to find the relevant velocities and flow angles. First, the impeller is analyzed, based on the assumption of ideal, isentropic behaviour. Subsequently, empirical loss models are added to account for losses in the flow. These consist of expressions for all separate loss components. Two entire sets of loss models are implemented. Validation is done against a commercial package, that is able to predict off-design performance. Furthermore, some test data is employed. The best set of loss models is chosen and turned out to give a very accurate prediction of the pressure ratios, based on four test compressor (impeller) geometries, using air as the working medium. A vaneless diffuser was added but not thoroughly validated yet. Further work also includes adding a vaned diffuser and volute. An outline of prediction methods for the operating limits of a compressor is made. Some of these are implemented in the code. Choke can be quite reasonably predicted. Finding the surge limit turns out to be a lot harder in a preliminary design stage. An empirical relation for the surge limit from the literature is implemented. The off-design prediction model makes use of a real gas model, so it can be used with refrigerants as well. This is proven to work when the correct efficiency is specified. However, the set of loss models underestimates the losses for these mediums and still needs some extra improvement. The MATLAB model is structured in such a way, that it can be readily implemented in Aeronamic’s in-house optimizer so it can be used for optimization studies.
Item Type:Internship Report (Master)
Aeronamic B.V., the Netherlands
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:centrifugal compressors, performance, off-design
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