Rotor tip clearance height and the associated tip leakage flow have a significant effect on the performance and stability of compressors. Existing studies considering tip clearance effects on stability have been primarily limited to low-speed compressors, and many of these evaluated single-stage machines, which may not adequately represent stall trends for engine-scale compressors. Furthermore, test campaigns for engine-scale compressors cannot provide instrumentation accessibility required for detailed stage performance and stall investigations. Using results collected from a three-stage intermediate-speed axial compressor with appreciable density rise, this study addresses these needs. In this paper, three rotor tip clearances are tested, ranging from 1.5 to 4% span (1 to 3% chord). Previous studies have primarily shown a transition from short-length-scale spikes to long-length-scale modes as the clearance is increased, whereas the present study shows the opposite: a transition from modes to spikes with increased tip clearance. As a result, these data emphasize that a definitive trend does not exist between the stall inception mechanism and increasing tip clearance. Instead, the clearance effects alter stage matching with speed and change the stall inception mechanism. These results also elicit future research by preliminarily suggesting that stall inception mechanisms may be predictable from steady performance measurements collected in the stalling stage.

Large rotor tip clearances and the associated tip leakage flows are known to have a significant effect on overall compressor performance. However, detailed experimental data reflecting these effects for a multistage compressor are limited in the open literature. As design trends lead to increased overall compressor pressure ratio for thermal efficiency benefits and increased bypass ratios for propulsive benefits, the rear stages of the high-pressure compressor will become physically small. Because rotor tip clearances cannot scale exactly with blade size due to the margin needed for thermal growth considerations, relatively large tip clearances will be a reality for these rear stages. Experimental data have been collected from a three-stage axial compressor to assess performance with three-tip clearance heights representative of current and future small core machines. Trends of overall pressure rise, stall margin, and efficiency are evaluated using clearance derivatives, and the summarized data presented here begin to narrow the margin of tip clearance sensitivities outlined by previous studies in an effort to inform future compressor designs. Furthermore, interstage measurements show stage matching changes and highlight specific differences in the performance of rotor 1 and stator 2 compared to other blade rows in the machine.

Stall is a type of flow instability in compressors that sets the low-flow limit for compressor operation. During the past few decades, efforts to develop a reliable stall warning system have had limited success. This paper focuses on the small nonlinear disturbances prior to deep surge and introduces a new approach to identify these disturbances using nonlinear feature extraction algorithms including phase-reconstruction of time-series signals and evaluation of a parameter called approximate entropy. To the best of our knowledge, this is the first time approximate entropy has been used for stall warning, and thus, its definition and utility are presented in detail. The technique is applied to stall data sets from two different compressors: a high-speed centrifugal compressor that unexpectedly entered rotating stall during a speed transient and a multistage axial compressor with both modal- and spike-type stall inception. In both cases, nonlinear disturbances appear, in terms of spikes in approximate entropy, prior to surge. The presence of these presurge spikes indicates the potential of using the approximate entropy parameter for small disturbance detection and stall warning. The details of the nonlinear feature extraction algorithm, including guidelines for its application as well as results from applying the algorithm to rig-level data, are presented.

The unsteady flow field generated by the rotor provides unsteady aerodynamic excitations to the downstream stator, which can result in vibrations such as forced response. In this paper, measurements of the rotor wake and rotor tip leakage flow from an embedded rotor in a multistage axial compressor are presented. A unique feature of this work is the pitchwise traverse of the flow field used to highlight the changes in the rotor exit flow field with respect to the position of the surrounding vane rows. Results acquired at midspan focus on characterizing an average rotor wake, including the effects on the frequency spectrum, from a forced response perspective. While many analyses use an average rotor wake to characterize the aerodynamic forcing function to the downstream stator, this study explores the factors that influence changes in the rotor wake shape and the resulting impact on the spectrum. Additionally, this paper investigates the flow near the endwall where the tip leakage vortex is an important contributor to the aerodynamic excitations for the downstream vane. For the first time, experimental data are presented at the rotor exit, which show the modulation in size and radial penetration of the tip leakage vortex as the rotor passes through the upstream vane wake. As computational models become more advanced, the ability to incorporate these aerodynamic excitation effects should be considered to provide better predictions for vane vibratory response.

The effects of humid air on the performance of a multistage research compressor and new methods of humidity accounting to ensure appropriate representation of performance parameters are investigated in this paper. Turbomachinery textbooks present methods of correcting speed and mass flow rate using perfect gas assumptions, but these methods can reduce the ability to achieve repeatable compressor performance when using unconditioned air in a climate where absolute humidity may vary. Instead, a new method is introduced, which models humid air as a real gas and circumvents the need for assumptions in the correction process. In the area of compressor research, the ability to measure small changes in performance parameters and ensure repeatable results is essential. Errors of more than 0.5% can result from using perfect gas assumptions to calculate corrected speed, which can lead to misrepresented performance parameters beyond the uncertainty of the measurements. Multiplicative correction factors based on analytical data are also introduced as an alternate method of applying the new real-gas method, and these correction factors are compared to those derived by previous authors applying ideal gas methods for humidity accounting. This is the first time in open literature that experimental results for a component of a gas turbine engine are presented comparing a humid air correction method with traditional correction methods.

The inception and evolution of rotating stall in a high-speed centrifugal compressor are characterized during speed transients. Experiments were performed in the single stage centrifugal compressor (SSCC) facility at Purdue University and include speed transients from subidle to full speed at different throttle settings while collecting transient performance data. Results show a substantial difference in the compressor transient performance for accelerations versus decelerations. This difference is associated with the heat transfer between the flow and the hardware. The heat transfer from the hardware to the flow during the decelerations locates the compressor operating condition closer to the surge line and results in a significant reduction in surge margin during decelerations. Additionally, data were acquired from fast-response pressure transducers along the impeller shroud, in the vaneless space, and along the diffuser passages. Two different patterns of flow instabilities, including mild surge and short-length-scale rotating stall, are observed during the decelerations. The instability starts with a small pressure perturbation at the impeller leading edge (LE) and quickly develops into a single-lobe rotating stall burst. The stall cell propagates in the direction opposite of impeller rotation at approximately one-third of the rotor speed. The rotating stall bursts are observed in both the impeller and diffuser, with the largest magnitudes near the diffuser throat. Furthermore, the flow instability develops into a continuous high frequency stall and remains in the fully developed stall condition.