<b>Redefining Critical Angle of Inlet Distortion for Centrifugal Compressors</b>
Creators: name: Bond, Evan Henry
Publisher: name: Purdue University Graduate School
Year: 2025
Subjects: classificationCode: 401799
subject: Mechanical engineering not elsewhere classified
classificationCode: 400199
subject: Aerospace engineering not elsewhere classified
classificationCode: 401201
subject: Aerodynamics (excl. hypersonic aerodynamics)
classificationCode: 401204
subject: Computational methods in fluid flow, heat and mass transfer (incl. computational fluid dynamics)

description: With increasing demand for reduced carbon emissions and increased fuel costs, novel aircraft designs are being developed that reduce the wetted area of the aircraft leading to complex inlet installations for engine integrations. With this, an understanding of the effects of inlet distortion on the compression system is paramount. One key parameter that defines the response of the compression system to inlet distortions is that of the critical angle of distortion. This is the circumferential angle that a distortion must occupy before performance and stability of the compression system is changed. This effort investigates the mechanism by which the critical angle of distortion alters the performance and stability of a high-speed centrifugal compressor. With this, a more accurate estimate of the critical angle of distortion for compressors is developed that allows for characterization of this angle without the need for copious simulations and experimental test campaigns. This investigation was driven by computational fluid dynamic simulations that were utilized to determine the critical angle of inlet distortion. Once this was understood, inlet distortion screens were designed via use of porous inlet-only CFD models to generate similar distortion profiles to those used in the CFD campaign. Finally, these screens were tested and the distortion profiles of the screens investigated along with the performance and stability changes of the compressor due to increasing distortion extents.To determine the critical angle of distortion for the centrifugal compressor investigated, a computational fluid dynamics study of the compressor was conducted. In this effort, pure once-per-rev total pressure distortions were delivered to the compression system with the extent varied in terms of number of impeller main blade pitches. The effects of this on performance and machine static pressure rise characteristics was analyzed. These simulations were conducted using a full-annulus transient model to allow for distortion propagation through the passages to be as realistic as possible. The critical angle of distortion of the compressor was found to correspond to 4.5 pitches (or 95.3°) as at this point the compressor efficiency and total pressure ratio were exponentially deteriorated for any increase in distortion extent.With knowledge of the critical angle, an understanding of the mechanism by which this alters performance was presented in terms of reduced frequency. Advective, acoustic, and relative acoustic definitions of reduced frequency were analyzed to determine which correlated best with physical flow disturbances from the inlet distortion propagation through the impeller passage. Furthermore, rothalpy was investigated as a tool to track distortion through the passage as it is maintained along a streamline but contains information of the relative frame temporal pressure gradient due to disturbances in the absolute frame. Utilizing distortion tracking and reduced frequencies, the critical angle of inlet distortion was found to correlate with the acoustic reduced frequency. For acoustic reduced frequencies below unity, the compressor performance was degraded.With an understanding of the critical extent, inlet-only simulations were conducted to generate designs of total pressure screens to precipitate similar total pressure distortion profiles to the compressor for a design of experiment. These designs were evaluated experimentally using rotatable inlet rakes upstream of the compressor. A comparison between the experimental and CFD data for these distortion profiles showed discrepancies, which were investigated. The findings from this allowed an outline of best practices for future design work for generating total pressure distortion profiles using porous inlet-only models for design of experimental testing of inlet distortion related effects.Finally, the centrifugal compressor’s response to the designed inlet distortion screens was analyzed. The compressor was mapped from choke to surge at 80%, 90%, and 100% speed. These corresponded to subsonic, transonic, and supersonic inlet relative Mach numbers for the impeller. The compressor was found to be sensitive above the critical distortion extent with efficiency and stage total pressure ratio degraded. Surge margin was enhanced by use of the screens at 100% speed, but severely degraded at 80% and this was found to correlate with the work characteristic slope. The typical understanding of a more negative work characteristic slope being a more stable operating condition for the compressor was found to be untrue for the distortion screens tested. The compressor entered instability at the same value of work coefficient for all distortion conditions, which lead to a more positive slope of the work characteristic allowing for a wider operating range in terms of flow coefficient.