Nicole L. Key

Comparison of Turbine Tip Leakage Flow for Flat Tip and Squealer Tip Geometries at High-Speed Conditions

The tip leakage flow characteristics for flat and squealer turbine tip geometries are studied in the von Karman Institute Isentropic Light Piston Compression Tube facility, CT-2, at different Reynolds and Mach number conditions for a fixed value of the tip gap in a nonrotating, linear cascade arrangement. To the best knowledge of the authors, these are among the very few high-speed tip flow data for the flat tip and squealer tip geometries. Oil flow visualizations and static pressure measurements on the blade tip, blade surface, and corresponding endwall provide insight to the structure of the two different tip flows. Aerodynamic losses are measured for the different tip arrangements, also. The squealer tip provides a significant decrease in velocity through the tip gap with respect to the flat tip blade. For the flat tip, an increase in Reynolds number causes an increase in tip velocity levels, but the squealer tip is relatively insensitive to changes in Reynolds number.

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.

Sensitivity of Multistage Compressor Performance to Inlet Boundary Conditions

Proper boundary conditions are required to accurately compute any flowfield. Although compressor designs are accomplished in large part with computational fluid dynamics tools, confidence in these tools exists because of code validation using high-quality experimental datasets. It has been widely recognized that inlet distortion has an impact on compressor performance. However, the impact of small distortion in inlet total pressure and total temperature profiles, as found in a research facility, on compressor performance predictions has not been well documented. In this paper, the inlet flow conditions for a multistage research compressor are shown. Compressor performance calculations using the measured inlet conditions are compared with results obtained with different inlet conditions to determine the sensitivity of the calculated multistage compressor performance to these conditions. Effects of radial profiles, endwall boundary-layer thickness, and circumferential nonuniformity are considered.

Considerations for Measuring Compressor Aerodynamic Excitations Including Rotor Wakes and Tip Leakage Flows

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.

Tailoring Inlet Flow to Enable High Accuracy Compressor Performance Measurements

Abstract To accomplish the research goals of capturing the effects of blade row interactions on compressor performance, small changes in performance must be measurable. This also requires axi-symmetric flow so that measuring one passage accurately captures the phenomena occurring in all passages. Thus, uniform inlet flow is a necessity. The original front-driven compressor had non-uniform temperature at the inlet. Additional challenges in controlling shaft speed to within tight tolerances were associated with the use of a viscous fluid coupling. Thus, a new electric motor, with variable frequency drive speed control was implemented. To address the issues with the inlet flow, the compressor is now driven from the rear resulting in improved inlet flow uniformity. This paper presents the design choices of the new layout in addition to the preliminary performance data of the compressor and an uncertainty analysis.

The Effects of Tip Leakage Flow on the Performance of Multistage Compressors Used in Small Core Engine Applications

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.

Experimental Investigation of Factors Influencing Operating Rotor Tip Clearance in Multistage Compressors

An analysis of compressor rotor tip clearance measurements using capacitance probe instrumentation is discussed for a three-stage axial compressor. Thermal variations and centrifugal effects related to rotational speed changes affect clearance heights relative to the assembled configuration. These two primary contributions to measured changes are discussed both independently and in combination. Emphasis is given to tip clearance changes due to changing loading condition and at several compressor operating speeds. Measurements show a tip clearance change approaching 0.1 mm (0.2% rotor span) when comparing a near-choke operating condition to a near-stall operating condition for the third stage. Additional consideration is given to environmental contributions such as ambient temperature, for which changes in tip clearance height on the order of 0.05 mm (0.1% rotor span) were noted for temperature variations of 15°C. Experimental compressor operating clearances are presented for several temperatures, operating speeds, and loading conditions, and comparisons are drawn between these measured variations and predicted changes under the same conditions.

Experimental investigation of a forced response condition in a multistage compressor

It is nearly impossible to remove all resonant crossings from the operating range of a multistage compressor; thus, designers rely on models to predict the resonant response amplitude to determine if the crossing is acceptable. An accurate model can reduce unexpected forced-response problems encountered during engine testing. Although work has been done to improve such models, the confidence in these models comes from validation with quality test data including both the aerodynamic forcing function and the response of the blade row. Recent trends in turbomachinery, including higher blade loadings and use of integrally bladed rotors, lead to increased potential for forced-response issues. The Campbell diagram crossing for the excitation associated with the upstream and downstream vane rows on the embedded rotor first torsion vibratory mode is measured. The rotor response will be characterized with tip-timing measurements and the aerodynamic forcing functions measured. Unsteady casing pressures over the vib...

Compressor Vane Clocking Effects on Embedded Rotor Performance

VANE clocking is the circumferential indexing of adjacent vane rows with similar vane counts. The clocking configuration can be arranged so that either all downstream vanes are positioned in the wakes shed from the upstream vanes, or the upstream vane wakes pass through every passage of the downstream vane row without any interaction with the vane surface. Researchers have shown that vane clocking can impact discrete frequency noise, the unsteady forces acting on the embedded rotor, and stage efficiency. Stage efficiency, in general, can be affected by changes to stator loss, rotor loss, or work done on the flow by the rotor. Researchers tend to focus on how the downstream stator loss is affected by vane clocking, but the role, if any, the rotor flowfield plays in vane clocking effects on stage efficiency has not been fully investigated. Many clocking studies have been performed in turbine environments, but fewer compressor clocking studies have provided conclusive results because the more common low-speed compressor research facilities generate a small pressure rise per stage, leaving the even smaller changes associated with vane clocking indiscernible. One of the few vane clocking studies that report a measurable and repeatable vane clocking effect on stage efficiency has been performed in the intermediate-speed Purdue University three-stage axial compressor research facility [1,2]. With similar vane counts for the IGV, Stator 1, and Stator 2, the effects of vane clocking were isolated to the embedded stage by clocking Stator 1 with respect to Stator 2. The objective of this Note is to investigate how vane clocking affects rotor loss and work done on the flow. This is accomplished with both steady total temperature measurements and unsteady tangential flow angle measurements. Two loading conditions were investigated: design point loading and a high loading condition. Vane clocking effects were more than three times stronger at high loading than at design loading. In addition, the larger potential field associated with the high loading condition would make any vane clocking effects on rotor performance, if present, more obvious. Three spanwise locations of the flowfield were interrogated: 30, 50, and 70%span. Thesewere chosen because at design loading, vane clocking effects were strongest at 70% span. At high loading, the hub variations were out-of-phase with the tip variations because of the significant skew in the Stator 1 wakes by the time they convected to the Stator 2 leading edge plane. Therefore, the clocking trends at 30% span were opposite of the trends at 70% span, and thus, interrogating these spanwise locations would allow for this type of trend to be identified in the rotor flowfield data, if present. The experiments were performed in a compressor featuring geometry representative of the rear stages of a high-pressure compressor, with engine representativeMach numbers and Reynolds numbers [3]. The two clocking configurations in this Note, CL3 and CL6, represent offsets of 32 and 83%vane passage (vp), respectively. These include the configuration leading to wake impingement on the downstream vane row, and the other, half a passage out of phase with the first, allows the wakes to pass through downstream passages. For these studies, the relative position of the IGV and Stator 1 was held constant, as was the relative position of Stator 2 and Stator 3. All results are acquired at 100% corrected design speed, 5000 rpm. To determine the effects of vane clocking on stage 2 total temperature ratio, measurements were acquired at the exit of Stator 1 and Stator 2 with seven-element Kiel head total temperature rakes. The thermocouple sensor and thermocouple-grade extension wire was designated as “special limits of error,” and for type T thermocouples, the manufacturer specified uncertainty was 0.5 °C. In an effort to reduce this value, a temperature bath was used to verify that the uncertainty for this group of wires was actually only 0.1 °C for the temperature range of interest. A miniature cross-film sensor was used to acquire instantaneous flow angle and velocity information in the axial-tangential plane. The uncertainty in the tangential flow angle measurements acquired with the cross-film sensors are 1 deg . All time-accurate data were acquired at a sampling frequency of 120 kHz (Rotor 2 blade pass frequency is 2.75 kHz at design speed), with 200 revolutions of data used to characterize an ensemble-averaged (EA) revolution.

Unsteady Vane Boundary Layer Response to Rotor–Rotor Interactions in a Multistage Compressor

Experiments have been performed in a three-stage axial compressor to investigate unsteady stator boundary-layer transition in an embedded stage environment. Quasi-wall shear stress data have been acquired on the suction surface of the second stage stator, in addition to unsteady flow angle measurements at the stator inlet. With two rotor blade rows upstream of the instrumented stator, blade row interactions representative of a multistage environment are present. The different blade counts for the upstream rotors provide an unsteady flowfield at the stator inlet featuring a beating pattern. There is no portion of fully laminar boundary-layer flow on this vane. The vane boundary layer is transitional near the leading edge, and transition to a fully turbulent boundary layer is complete by 45% suction side length. The presence of the rotor 1 wakes, when arriving in between the rotor 2 wakes, increases the normalized quasi-wall shear stress in the leading-edge region by 4% and decreases the amount of laminar t...