Patrick B. Lawless

An Experimental Study of Vane Clocking Effects on Embedded Compressor Stage Performance

Previous research has shown that vane clocking, the circumferential indexing of adjacent vane rows with similar vane counts, can be an effective means to increase stage performance, reduce discrete frequency noise, and/or reduce the unsteady blade forces that can lead to high cycle fatigue. The objective of this research was to experimentally investigate the effects of vane clocking in an embedded compressor stage, focusing on stage performance. Experiments were performed in the intermediate-speed Purdue three-stage compressor, which consists of an IGV followed by three stages. The IGV, Stator 1, and Stator 2 vane rows have identical vane counts, and the effects of vane clocking were studied on Stage 2. Much effort went into refining performance measurements to enable the detection of small changes in stage efficiency associated with vane clocking. At design loading, the change in stage efficiency between the maximum and minimum efficiency clocking configurations was 0.27 points. The maximum efficiency clocking configuration positioned the Stator 1 wake at the Stator 2 leading edge. This condition produced a shallower and thinner Stator 2 wake compared with the clocking configuration that located the wake in the middle of the Stator 2 passage. At high loading, the change in Stage 2 efficiency associated with vane clocking effects increased to 1.07 points; however, the maximum efficiency clocking configuration was the case where the Stator 1 wake passed through the middle of the downstream vane passage. Thus, impingement of the upstream vane wake on the downstream vane leading edge resulted in the best performance at design point but provided the lowest efficiency at an off-design condition.

Active Control of Rotating Stall in a Low-Speed Centrifugal Compressor

This paper addresses rotating stall control in centrifugal compressors, presenting the results of an experiment directed at the suppression of stall in the Purdue Low-Speed Centrifugal Research Compressor. The control technique uses a phase-controll ed inlet distortion generated by an array of 12 airinjection ports located in the compressor inlet endwall. The active control of first-mode (one-cell) rotating stall was successfully demonstrated. In the cases investigated, the stall control system provided only slight increases in stall margin. However, the control system demonstrated an ability to restore system stability to the compressor when exhibiting a developed stall condition.

Rotating Stall Acoustic Signature in a Low Speed Centrifugal Compressor: Part 1 — Vaneless Diffuser

An experimental study is performed to identify spatially coherent pressure waves which would serve as precursors to the development of an instability in the Purdue Low Speed Centrifugal Research Compressor when configured with a vaned diffuser. To achieve this, sensitive electret microphones were uniformly distributed around the circumference in the inlet and diffuser sections of the compressor. Fourier analysis of simultaneously sampled data from these microphone arrays was employed to identify the development of dominant spatial modes in the pressure field in the compressor. Three different diffuser geometries were investigated, resulting in three different instability pathologies. The rotating stall patterns observed in the compressor demonstrated propagation rates near impeller speed, and from one to four cells. These instabilities would classically be described as impeller stall, although the conditions appeared to arise simultaneously in the diffuser and impeller and were typically of similar magnitude in both locations. The excitation of the pressure waves, as indicated by spatial Fourier analysis, occurred five to fifteen impeller revolutions before small changes were evident in the raw microphone signals, and fifteen to twenty-five revolutions before the stall condition could be considered fully developed.Copyright

High-Speed Centrifugal Compressor Surge Initiation Characterization

An experimental study is performed to characterize the behavior of a high-speed centrifugal compressor as it approaches instability. To achieve this, data at the inlet and exit of the centrifugal compressor are analyzed. Three inducer bleed conditions are examined. Data analysis indicates that the disturbance was a 9-lobed stall pattern occurring in or near the diffuser and suggests that the phenomena is different than that typically referred to as impeller stall. The component pressure characteristics show a reduction in diffuser performance corresponding to the rise in the spatial mode magnitude, with minimal effect on the impeller. It is suggested that the rotating stall condition that was observed in this compressor may play a similar role to that observed when rotating stall initiates surge in multistage axial compressor.

Particle Image Velocity Investigation of a High Speed Centrifugal Compressor Diffuser: Spanwise and Loading Variations

An efficient diffuser is essential to a modern compressor stage due to its significance in stage performance, durability, and operability. To address the need for data that describe the complex, unsteady flow field in a vaned diffuser, particle image velocity is utilized to characterize the spanwise and circumferential variations in the flow features in the vaned diffuser passage of a transonic centrifugal compressor. The spanwise variation in the diffuser flow field is further investigated by the comparison of three different operating conditions representative of low, nominal, and high loading. These data demonstrate that not only the diffuser flow field is highly dependent on the operation conditions, e.g., hub-to-shroud variation increases with loading, but also the circumferential periodicity, created by the highly three dimensional impeller discharge flow, generates a larger unsteadiness toward the hub region of the vaned diffuser.

Investigation of seal purge flow effects on the hub flow field in a turbine stage using particle image velocimetry

In high temperature turbines, air from disk cavities is forced through the vane-rotor seal to prevents hot gas ingress into these cavities. This purge air plays a role in the effectiveness of platform film cooling as well as interacting with the formation of the horseshoe vortex on the downstream rotor. The objective of this work is to investigate and characterize the aerodynamic signature of this interaction thus comprising a first step towards deeper understanding of the effect on platform heat transfer and the formation of the horseshoe vortex. Toward this end an investigation is performed on the first stage of the Purdue Research Turbine using Particle Image Velocimetry (PFV). The flow field is interrogated in the near hub region of the intra-stage space downstream of the first vane row. Two dimensional (radial and axial) velocity data from a measurement spaced between vane wakes are acquired. The data acquired are phase-locked to rotor position, thus characterizing the effect of the rotor potential field on the emergent seal flow behavior. Results are presented for three seal flow rates, ranging over values typical for actual gas turbine engine rotor seals.

Characterization of abrupt rotating stall initiation in an axial flow compressor

The detection of rotating stall precursors has become a focus of current research because of the desirability of implementing a control scheme well before a compressor enters a stall condition. To identify spatially coherent pressure waves that would serve as precursors to the development of an instability in a low-speed, axial-flow compressor, the Purdue Axial Flow Research Compressor was instrumented with sensitive electret microphones uniformly distributed around the compressor circumference. Fourier analysis of simultaneously sampled data from these microphone arrays was employed to identify the development of dominant spatial modes in the pressure field in the compressor. The transition to stall was observed to be an abrupt process, with the eruption of a stall cell on rotor blades corresponding to a strong rise in the spatial mode magnitude. However, a weak, circumferentially distorted pressure wave was detected that began adjusting to the ultimate phase propagation velocity of the finite stall pattern from within 5-26 revolutions prior to a significant rise in the mode magnitude and the indication of a stall cell on the rotor blade. Comparison of the first and second harmonics of this signal indicate that this disturbance is represented in the spatial domain by an impulsive-type waveform, and thus likely represents a small, propagating flow separation.

Particle Image Velocimetry Characterization of High-Speed Centrifugal Compressor Impeller-Diffuser Interaction

High-density instantaneous and rotor phase-locked velocity field measurements are made in the centrifugal compressor diffuser vaneless space by means of the particle image velocimetry technique. The technique is challenged in this application by a small length scale and large velocities yielding very small flow time scales and demanding the use of extremely small flow tracking particles. The experiments reveal a detailed picture of the rotor phase-lockedaverageimpellerexitanddiffuserinlet flowfieldasisusefulforunsteady flowmodeling.Strongpassage velocity gradients exist across the vaneless space up to the diffuser vanes, showing that the phase-locked averaged impeller exit flow presents a highly unsteady non-mixed-out velocity field for the downstream vane row. This unsteady vane intake is thus characterized by a strong fluctuation in incidence angle with the passing blade event. Measured time variation in the velocity field indicates a significant time-varying potential field due to incidence fluctuations. Nomenclature Rduct = inlet duct radius r = radius � = radius ratio (diffuser leading-edge radius/impeller tip radius) � = 1 standard deviation

Vane Clocking in a Three-Stage Compressor: Frequency Domain Data Analysis

Blade row interactions affect compressor performance and durability. As design systems expand to account for these interactions, a better understanding of the underlying physics is necessary. In this paper, results from a vane clocking experiment in a three-stage compressor are discussed. Efforts are focused on the second stage, specifically the change in stator 2 wake profiles with respect to the placement of the stator 1 wake. The two clocking conditions presented position the wake from stator 1 at the leading edge of stator 2 and in the middle of the stator 2 passage. The time-accurate data are Fourier decomposed to determine the relative magnitudes of the frequencies in the spectrum. With data acquired at 50 circumferential locations spanning one vane passage for each clocking configuration, an enormous amount of data is collected, and a useful method for synthesizing this information is presented. Results show that, by placing the stator 1 wake at the leading edge of stator 2, the stator 2 boundary-layer response to the large incidence variations associated with the rotor 2 wakes is dampened, resulting in a thinner and more shallow stator 2 wake.

Instability Pathology of a High Speed Centrifugal Compressor

An experimental investigation is performed to characterize a high-speed centrifugal compressor as it approaches instability and during subsequent surge cycles. To achieve this, data are analyzed from the inlet, the diffuser and the exit of the compressor. Analysis of the data indicates the presence of two rotating stall modes prior to and during surge cycles. A nine cell mode pattern is shown to erupt prior to the initial surge cycle. The second rotating stall mode is a single cell mode that coincides with the initiation of the surge cycles. Both stall modes are shown to be located at or near the diffuser.Copyright