Jonathan Everitt, Benny Kuschel, Arian Roeber, Dr. Jürg Schiffmann
Advisor: Prof. Spakovszky
Although centrifugal compressors exhibit the same type of instabilities as axial compressors, rotating stall and surge are characterized by a much broader spectrum of unstable behavior. The wide variety of instability behavior, along with the inherently complicated flow in such a machine, are primary reasons that rotating stall and surge in centrifugal compressors are less well understood than similar phenomena in axial compressors. As a consequence, a general theory or a criterion for the onset of instability in centrifugal compressors does not exist.
Instead, correlations are used to describe the surge point for a certain class of centrifugal compressors and to estimate the stability limit based on a priori knowledge of blade row characteristics. The major limitation of these methods is that these characteristics are only available after experimental measurements and thus the method is not of predictive nature. This research project is different from past efforts in that the prediction is purely based on centrifugal compressor geometry and does not rely on correlations or a priori knowledge of compressor characteristics.
The approach is two-pronged. Previous research indicates that for certain classes of centrifugal compressors the inception of instability is in the diffuser; however the underlying fluid mechanics is not well understood. To gain insight, unsteady 3-D RANS calculations are being carried out on the isolated diffuser using an inlet flow field derived from full stage calculations. The inlet conditions are perturbed with a short wavelength total pressure disturbance. If the disturbance grows in time, the machine is deemed dynamically unstable, and the operating point is determined to be the surge point. The inlet conditions can be varied to determine what flow features need to be present in order to trigger instability.
The second prong to the approach borrows ideas from previous work on axial compressors and consists of 3-D steady RANS calculations to determine the body force distributions representing the effects of discrete blades on the flow field. The body forces are then coupled to a 3-D unsteady Euler solver. The compressor model is then forced with a short wavelength body force impulse in the vaneless space. The goal is to demonstrate that the method can accurately predict both the stall point and the type of stall inception pattern (spike or modal waves) in centrifugal compressors.
Advisor: Prof. Spakovszky
Although centrifugal compressors exhibit the same type of instabilities as axial compressors, rotating stall and surge are characterized by a much broader spectrum of unstable behavior. The wide variety of instability behavior, along with the inherently complicated flow in such a machine, are primary reasons that rotating stall and surge in centrifugal compressors are less well understood than similar phenomena in axial compressors. As a consequence, a general theory or a criterion for the onset of instability in centrifugal compressors does not exist.
Instead, correlations are used to describe the surge point for a certain class of centrifugal compressors and to estimate the stability limit based on a priori knowledge of blade row characteristics. The major limitation of these methods is that these characteristics are only available after experimental measurements and thus the method is not of predictive nature. This research project is different from past efforts in that the prediction is purely based on centrifugal compressor geometry and does not rely on correlations or a priori knowledge of compressor characteristics.
The approach is two-pronged. Previous research indicates that for certain classes of centrifugal compressors the inception of instability is in the diffuser; however the underlying fluid mechanics is not well understood. To gain insight, unsteady 3-D RANS calculations are being carried out on the isolated diffuser using an inlet flow field derived from full stage calculations. The inlet conditions are perturbed with a short wavelength total pressure disturbance. If the disturbance grows in time, the machine is deemed dynamically unstable, and the operating point is determined to be the surge point. The inlet conditions can be varied to determine what flow features need to be present in order to trigger instability.
The second prong to the approach borrows ideas from previous work on axial compressors and consists of 3-D steady RANS calculations to determine the body force distributions representing the effects of discrete blades on the flow field. The body forces are then coupled to a 3-D unsteady Euler solver. The compressor model is then forced with a short wavelength body force impulse in the vaneless space. The goal is to demonstrate that the method can accurately predict both the stall point and the type of stall inception pattern (spike or modal waves) in centrifugal compressors.
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Representation of bladed centrifugal compressor with axisymmetric body forces. |
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Unsteady pressure traces for pressure taps located at constant radius around vaneless space. Left: Response to forcing for an unstable operating point at 75% corrected design speed; two-lobed backward traveling modal pre-stall waves grow in amplitude. Right: Response to forcing for altered compressor at neutrally stable operating point at 70% corrected design speed; forward traveling, spike-like disturbance formed shortly after forcing. |
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