Natalie R. Smith

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.

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...

Vane Clocking Effects on Stator Loss for Different Compressor Loading Conditions

Vane clocking is the relative circumferential indexing of adjacent vane rows with similar vane counts, and it affects discrete frequency noise, forced response, and stage performance in axial compressors. The objective of this paper is to investigate vane clocking effects on stator 2 loss for many loading conditions in a three-stage axial compressor. Although previous studies have focused on two or three operating conditions, this is the first to investigate vane clocking effects over many loading conditions. Detailed circumferential traverses of stagnation pressure were acquired for six clocking configurations at six different loading conditions: a low-loading condition, two conditions near design point, two high-loading conditions, and a near-stall condition. Stator 2 wake widths and depths were similar for different clocking configurations at low loadings, but the high-loading cases exhibited larger changes associated with vane clocking, especially near the stator hub. This resulted in different radial...

The Development of a Multistage Axial Compressor Research Facility

Recent renovations to the Purdue 3-stage axial compressor research facility have led to improved inlet flow to the facility and expanded driveline capabilities. The original layout included a front-driven compressor, which led to challenges with uniformity of inlet air temperature and velocity profiles. The installation of a new electric motor, with variable frequency drive speed control provided the opportunity to address this issue and thus, the compressor is now driven from the rear allowing for improved inlet flow. The research compressor at the heart of the redesign, is representative of the rear stages of Rolls-Royce high pressure compressors, with engine representative Mach numbers and Reynolds numbers. The design choices of the new layout are presented in addition to the preliminary performance data of the compressor and an uncertainty analysis.

Vane Clocking Effects on Stator Suction Side Boundary Layers in a Multistage Compressor

The stator inlet flow field in a multistage compressor varies in the pitchwise direction due to upstream vane wakes and how those wakes interact with the upstream rotor tip leakage flows. If successive vane rows have the same count, then vane clocking can be used to position the downstream vane in the optimum circumferential position for minimum vane loss. This paper explores vane clocking effects on the suction side vane boundary layer development by measuring the quasi-wall shear stress on the downstream vane at three spanwise locations. Comparisons between the boundary layer transition on Stator 1 and Stator 2 are made to emphasize the impact of rotor-rotor interactions which are not present for Stator 1 and yet contribute significantly to transition on Stator 2. Vane clocking can move the boundary layer transition in the path between the wakes by up to 24% of the suction side length at midspan by altering the influence of the Rotor 1 wakes in the 3/rev modulation from rotor-rotor interactions. The boundary layer near the vane hub and tip experiences earlier transition and separation due to interactions with the secondary flows along the shrouded endwalls. Flow visualization and Stator 2 wakes support the shear stress results.

Reconciling Compressor Performance Differences for Varying Ambient Inlet Conditions

Careful experimental measurements can capture small changes in compressor total pressure ratio (TPR), which arise with subtle changes in an experiments configuration. Research facilities that use unconditioned atmospheric air must account for changes in ambient compressor inlet conditions to establish repeatable performance maps. A unique dataset from a three-stage axial compressor has been acquired over the duration of 12 months in the Midwest U.S., where ambient conditions change significantly. The trends show a difference in compressor TPR measured on a cold day versus a warm day despite correcting inlet conditions to sea level standard day. To reconcile these differences, this paper explores correcting the compressor exit thermodynamic state, Reynolds number effects, and variations in rotor tip clearance (TC) as a result of differences in thermal growth.

Effects of Tip Clearance on Stall Inception in a Multistage Compressor

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...

Blade-Row Interaction Effects on Unsteady Stator Loading in an Embedded Compressor Stage

Blade-row interactions drive unsteady blade forces in compressors. This paper presents a perspective on understanding how the pitchwise variations in the flow at the exit of the rotor affect the surface pressures on the downstream vane. The rotor wakes and tip leakage flows are the primary unsteady flow features that drive unsteady lift on the vane. However, those rotor flow features are affected by their interaction with the wakes from upstream vanes. Thus, the interaction with stator 1 and rotor 2 must be understood to adequately characterize the interaction between rotor 2 and stator 2. This paper uses vane clocking, or the circumferential shift in successive vane rows of similar counts, to illuminate these blade-row interaction effects on the downstream vane surface pressure distribution. To accomplish this, experiments are performed in a three-stage axial compressor where high-frequency response pressure transducers are flush-mounted in the stator 2 pressure and suction surfaces are flush-mounted at ...