Experimental and Numerical Investigation of Tip Clearance Effects in a High-Speed Centrifugal Compressor
thesisposted on 23.07.2020 by Matthew Francis Fuehne
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The objective of this research is to investigate the effects of tip clearance on the stage and component performance in a high-speed centrifugal compressor. The experimental data were compared against results from a numerical model to assess the ability of the numerical simulation to predict the effects of tip clearance. Experimental data were collected at Purdue University on the Single Stage Centrifugal Compressor (SSCC), a high-speed, high-pressure ratio test compressor sponsored by Honeywell Aerospace. Numerical simulations were completed using the ANSYS CFX software suite and part of the research computing clusters located at Purdue University.
Two tip clearances were tested, the nominal tip clearance and a tip clearance that is 66% larger than the nominal clearance, at speeds from 60% to 100% corrected speed. To compare data points with different tip clearances, various parameters were evaluated, and one was chosen. The value of TPR/inlet corrected mass flow rate best represented similar loading conditions, and thus similar incidences, for each tip clearance and was chosen as the best method for comparing similar data points taken with different clearances. Stage and component performance were focused on the sensitivity of each performance parameter to the changing of the tip clearance. The stage total pressure ratio and stage efficiency showed moderate sensitivity while the stage work factor showed much lower sensitivity. The impeller is more sensitive to changing tip clearances than the stage is, showing greater changes when comparing data from each tip clearance. The diffuser was on the same order of sensitivity as the impeller, with marginally higher sensitivities for some parameters. It was found that by the typical performance metrics, the diffuser performs worse at the nominal clearance than at the larger clearance. Upon further investigation though, the impeller is providing a higher static pressure and therefore, more diffusion, at the nominal clearance so the diffuser must perform less diffusion during nominal clearance operation.
To assess the validity of a prediction of the performance and sensitivity of the stage and components to the tip clearance, a numerical model was developed and validated. The numerical model was able to reasonably predict the stage performance with better comparisons of performance in the impeller and worse in the diffuser. The instrumentation in the experiment was replicated in the software to calculate performance the same way it is calculated experimentally so that the results would be comparable. While the performance of the stage and components was lacking in some areas, the trends predicted were similar to those calculated from the experimental data. As with the performance, the trends in the impeller matched very well between the experiment and the numerical simulation. The trends in stage and diffuser performance were predicted more accurately than the stage and diffuser performance maps and were able to capture the magnitude of the change in performance caused by changing the tip clearance.