Dynamic compressors are susceptible to aerodynamic instabilities while operating at low mass flow rates. These instabilities, rotating stall and surge, are detrimental to engine life and operational safety, and are thus undesirable. In order to prevent stability problems, a passive technique, involving fuel flow scheduling, is currently employed on gas turbines. The passive nature of this technique necessitates conservative stability margins, compromising performance and/or efficiency. In the past, model based active control has been proposed to enable reduction of margin requirements. However, available compressor stability models do not predict the different stall inception patterns, making model based control techniques practically infeasible. This research presents active stability management as a viable alternative. In particular, a limit detection and avoidance approach has been used to maintain the system free of instabilities. Simulations show significant improvements in the dynamic response of a gas turbine engine with this approach. A novel technique has been developed to enable real-time detection of stability limits in axial compressors. It employs a correlation measure to quantify the chaos in the rotor tip region. Analysis of data from four axial compressors shows that the value of the correlation measure decreases as compressor loading is increased. Moreover, sharp drops in this measure have been found to be relevant for stability limit detection. The significance of these drops can be captured by tracking events generated by the downward crossing of a selected threshold level. It has been observed that the average number of events increases as the stability limit is approached in all the compressors studied. These events appear to be randomly distributed in time. A stochastic model for the time between consecutive events has been developed and incorporated in an engine simulation. The simulation has been used to highlight the importance of the threshold level tosuccessful stability management. The compressor stability management concepts have also been experimentally demonstrated on a laboratory axial compressor rig. The fundamental nature of correlation measure has opened avenues for its application besides limit detection. The applications presented include stage load matching in a multi-stage compressor and monitoring the aerodynamic health of rotor blades.

This thesis deals with a procedure for determining the complete processing of aerodynamic flow instabilities (rotating stall and surge) in a centrifugal compressor. At small flows the performance of a compressor system is limited by the surge line, which is caused by flow instabilities. Numerical solution is obtained using the method of transfer matrix. This system is simulated through several models with local resistances that represent the dissipation of pressure energy. Pulses are excitated in these models by the pressure jump placed before the centrifugal compressor. From the frequency-amplitude characteristics for the selected range of frequencies and flow the impedance characteristic of the compressor system is determined. We are looking for problematic frequencies in this characteristic that cause flow instabilities in the compressor system.

<p dir="ltr">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.</p><p dir="ltr">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.</p><p dir="ltr">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.</p><p dir="ltr">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.</p><p dir="ltr">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.</p>