An Experimental Investigation of the Performance ...

Proceedings of ASME Turbo Expo 2015: Turbine Technical Conference and Exposition GT2015 June 15 – 19, 2015, Montréal, Canada

GT2015-42325 An Experimental Investigation of the Performance Impact of Swirl on a Turbine Exhaust Diffuser/Collector for a Series of Diffuser Strut Geometries

Song Xue, Stephen Guillot, Wing F. Ng, Jon Fleming Techsburg, Inc Christiansburg, VA, USA [email protected], [email protected], [email protected], [email protected] K. Todd Lowe Virginia Tech Blacksburg, VA, USA [email protected] Nihar Samal, Ulrich E. Stang Solar Turbines San Diego, CA, USA [email protected], [email protected]

effects of the profiled struts. In this case the decreased swirl reduces the flow asymmetry responsible for the reduction in pressure recovery attributed to the formation of a localized reverse-flow vortex near the bottom of the collector. This research indicates that strut setting angle and, to a lesser extent, strut shape can be optimized to provide peak engine performance over a wide range of operation.

ABSTRACT A comprehensive experimental investigation was initiated to evaluate the aerodynamic performance of a gas turbine exhaust diffuser/collector for various strut geometries over a range of inlet angle. The test was conducted on a 1/12th scale rig developed for rapid and accurate evaluation of multiple test configurations. The facility was designed to run continuously at an inlet Mach number of 0.40 and an inlet hydraulic diameter-based Reynolds number of 3.4×105. Multi-hole pneumatic pressure probes and surface oil flow visualization were deployed to ascertain the effects of inlet flow angle and strut geometry. Initial baseline diffuser-only tests with struts omitted showed a weakly increasing trend in pressure recovery with increasing swirl, peaking at 14° before rapidly dropping. Tests on profiled struts showed a similar trend with reduced recovery across the range of swirl and increased recovery drop beyond the peak.

NOMENCLATURE Alphabet Cp pressure recovery coefficient [-] ∆Cp change in recovery (Cpi-Cpref) Cpref recovery for diffuser-collector model at 0° Dev_Cp recovery deviation from the mass average value h diffuser passage height [%] Ps static pressure [Pa] Pt total pressure [Pa] ReDH Reynolds Number, basis: inlet hydraulic diameter U velocity [m/s] Greek δ error/uncertainty Subscript 1,2 inlet plane, exit plane x,y,z axial, radial, lateral direction Operator ሺ ሻ indicates mass averaging of the quantity ( )

Subsequent tests for a full diffuser/collector configuration with profiled struts revealed a rising trend at lower swirl when compared to diffuser-only results, albeit with a reduction in recovery. When tested without struts, the addition of the collector to the diffuser not only reduced the pressure recovery at all angles but also resulted in a shift of the overall characteristic to a peak recovery at a lower value of swirl. The increased operation range associated with the implementation of struts in the full configuration is attributed to the de-swirling

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Copyright © 2015 by ASME and Solar Turbines Incorporated

INTRODUCTION

(a)

The performance of an industrial gas turbine engine is strongly influenced by the pressure recovery afforded by a properly designed exhaust configuration. Certain applications call for an exhaust collector that is designed to divert the turbine flow radially to the corresponding site interface. Pressure recovery is maximized through the upstream diffuser and collector in order to reduce back pressure at the exit of the engine, subsequently facilitating an increased expansion ratio of the turbine [1].

Plane 1 (Inlet DAQ)

Industrial gas turbines often operate at part-load conditions for extended periods of time. As a result, the turbine exhaust is subjected to a wide range of inlet conditions affecting the performance of the engine. To improve the efficiency it is critical to understand the diffuser-collector system performance characteristics at different inlet flow swirl angles. Numerous studies have documented the effects of inlet angle on the performance of annular diffusers including Lohmann et al. [2], Kumar and Kumar [3], Vassiliev et al. [4], and Pietrasch and Seume [5].

Back Wall

Research by Goudkov et al. [6] focused on assessing the effects of the flow passing through the tip gap of the blade row upstream of the diffuser. Their work indicated that the resulting jet-type leakage helps to suppress any boundary layer separation on the shroud. The inlet boundary conditions provided for testing documented in this paper are derived from an actual engine configured with an upstream shrouded blade. Since such airfoils are characterized by a significant reduction in tip leakage the absence of any inlet tip jet-flow does not compromise the validity of the results provided in this paper.

(b)

The present paper details the findings of a series of experiments conducted to study the effects of inlet angle on the performance of a diffuser-collector system with different diffuser strut definitions. The performance characteristics of diffusers with no struts, airfoil profiled struts, and cylindrical struts are compared at nominal inlet angles of 0°, 7°, 14°, 21°, and 35°.

Support bracket for diffuser-only configuration

Figure 1: Section view of the 1/12th scale facility: (a) diffuser -collector model (profiled struts); (b) diffuser only model (no struts).

from the large exit area of the pipe flow conditioner accelerates smoothly into the diffuser-collector test section. The turbulence grid (red) is deployed to obtain the desired turbulence level (Tu = 4%) at the inlet of the diffuser. It consists of a 1.59 mm thick porous metal disc placed upstream of the swirl vanes at the exit of the contraction flange [8]. Swirl vanes (black) were placed directly downstream of the turbulence grid to set the desired angle distribution at the inlet of the diffuser. A boundary layer contraction section (green) was positioned at the inlet of the diffuser (yellow).

TEST FACILITIES In the previous study by Guillot et al. [7], a 1/12th scale facility was deployed to qualitatively assess the flow features of a typical exhaust collector, considering quantitative differences due to Reynolds number effects. In the full-scale production model, the diffuser-collector system is optimized for the exhaust conditions of the turbine. Therefore, in order to create similarity between the full scale operation and the wind tunnel research, several flow-conditioning devices were deployed upstream of the 1/12th scale model as shown in Figure 1. By adding a turbulence grid and a boundary layer contraction section, the inlet flow was adjusted to replicate the velocity/turbulence profile leaving the last blade row of the turbine.

INSTRUMENTATION, DATA ACQUISITION, ANALYSIS A customized LabVIEW program is used to acquire, calculate, and record time-averaged temperature and pressure values at a user defined rate and length of time in the 1/12th-scale diffuser-collector facility. The program was set to record a total of 3,200 samples taken within a 4 second time span. Furthermore, this program provided the capability to control a single-axis stepper motor used to traverse a 3-hole pressure

In the 1/12th scale test facility, a Panther WA 3032-3300D positive displacement blower is employed to provide steady flow at a Mach number of 0.4. The primary purpose of the contraction (blue) is to ensure that the uniformly distributed air

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probe to various radial depths at the diffuser inlet. It also facilitates control of a two-axis stepper motor used to traverse a 5-hole probe at the collector exit.

A two-dimensional traverse system was used to index a five-hole probe in a grid across the exit section of the exhaust collector. This traverse pattern for each data set was automated using LabVIEW to control two stepper motors. (Figure 3).

Diffuser Inlet Instrumentation Six static pressure taps located directly upstream of the strut ring (inlet to the diffuser) were incorporated on both the casing and hub to provide an inlet circumferential static pressure distribution profile. These provided details of diffuser inlet non-uniformity due to back pressure created by the collector. Radial profiles at the diffuser inlet plane were acquired through traverses of a United Sensor 3/16’’ WAC-187 three-hole probe. This wedge-shaped, 2-D directional probe provided inlet angle, total pressure, and static pressure by using three triangularly-spaced pressure ports. The probe was developed to eliminate sensitivity to pitch angles up to 30° [9]. Six equally-spaced probe access holes were located at the inlet plane. Note that the polar coordinates specified throughout the paper are referenced at 0° to the 12 o’clock location (top) as shown in Figure 2. At each circumferential location the probe was traversed in 10% increments at five different radial depths inside the annulus ranging from 30% to 70% of the entire case-to-hub radial span.

Figure 3: Isometric view of the exit traverse plane (collector back wall and hub removed).

An Aeroprobe 3D five-hole probe was deployed to acquire time-averaged pressure data at four second intervals for a total of 800 points within the collector. The probe provided an accurate resolution of velocity vectors as high as 60° from the probe axis [13]. Flow Visualization Technique Surface oil visualization tests were performed to provide details of the flow field in non-accessible sections of the diffuser-collector. A thin layer of 500 cs silicone fluid mixed with a yellow AX-16-N fluorescent pigment was painted on surfaces of interest before the blower run. A test duration of 10 minutes facilitated dispersion of the oil-pigment mixture to adequately reveal the flow structure on the diffuser and collector surfaces. The resulting images were recorded with the aid of a high resolution SLR camera in a dark light environment. Performance Indices Pressure recovery is a standard term used to relate the actual static pressure rise to the maximum theoretical (ideal) pressure rise, thus equating to a number less than one. This parameter is derived from mass averaged total and static pressures per equation 1:

(1)

The present paper focuses on the comparison between different configurations and test conditions. Therefore, pressure recovery data is provided relative to the value obtained for the diffuser-only model, without struts for axial inlet flow conditions (ΔCp = Cp- Cpref).

Figure 2: (a) Upstream view of the circumferential radial traverse plane; (b) Pressure tap locations on hub

Collector Exit Instrumentation

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calculations revealed that this was being driven by the flow at the bottom, pinched side of the collector.

Primary Uncertainties All individual instrument mean-square uncertainties were summed via the propagation of errors equation to define the instrumentation accuracy of each performance variable found in Table 1. Overall uncertainty is determined by measurement errors associated with both instrumentation and testing. More details of the uncertainty analysis are provided by Boehm [14]. Table 1: Table of Uncertainties of Performance Variables

Figure 5: Measured inlet static pressure coefficient circumferential distribution (radial averaged), diffuser-collector, 7° inlet swirl.

GEOMETRY OF DIFFUSER STRUTS

Keil probe surveys were completed to accurately capture the inlet flow conditions. Table 2 shows radially-averaged flow angles for the five swirl vane designs operating under three strut configurations. The results vary only slightly from the design targets.

The profiled struts and the cylindrical struts shares the same maximum thickness (or diameter), which is of 10.2% of the inlet hydraulic diameter (blockage ratio of 16.2%) at the same axial location. Each the struts rings contains 11 struts. Figure 4 shows the geometries of the profiled struts. The profiled strut blades are set at a metal angle of 7°.

Table 2: Measured inlet flow angles (radially averaged)

0 7 14 21 35

NO STRUT

PROFILED

CYLINDRICAL

.7 6.8 14.5 22.4 35.8

.5 6.6 14.4 21.6 36.0

.3 6.8 14.5 21.8 36.0

Pressure Recovery Trends The overall pressure recovery for diffuser-only, profiled and circular cylindrical struts as a function of inlet angle is shown in Figure 6. Tests for configurations without struts were characterized by weakly increasing recovery trends as the swirl angle increased from 0° to 21°. Pressure recovery peaks at 14°, dropping rapidly as swirl increases from 21° to 35°.

Figure 4: Profiled strut geometry

RESULTS AND DISCUSSION

This general trend of optimum pressure recovery at moderate swirl (10° to 20°) followed by a rapid reduction at higher swirl (> 25°) agrees well with the data reported by Coladipietro et al. [15] and Hoadley and Hughes [1]. Pressure recovery for both profiled and cylindrical struts is lower at all angles when compared to the strutless configuration. This is attributed to the total pressure loss caused by skin friction and base drag on the struts as well as associated airfoil blockage effects. The profiled struts exhibited higher recovery than the cylindrical struts at

Diffuser Inlet Survey Previous testing [7] indicated the presence of circumferential non-uniformity at the inlet of the diffuser attributed to variation in the flowfield of the collector. Figure 5 shows the circumferential distribution of recovery for different strut configurations at an inlet swirl of 7 degrees. For all the cases the static pressure is high at the 180 degree location. Helix

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swirl angles up to 21°.This is due to the fact that even given the relatively low solidity, the profiled struts provide a certain amount of flow straightening leading to a reduction in kinetic energy loss.

Δ

At an inlet angle of 35°, the pressure recovery of profiled-strut configuration drops below that obtained for the cylindrical struts. The associated increased pressure loss is attributed to the effects of stall in the profiled strut passage due to excessive incidence. The profiled struts were set at a 7° angle to the meridional plane and designed with a large leading edge radius to accommodate a wide range of swirl. However, at an inlet angle of 35° the struts have clearly stalled, as indicated by the measured reduction in pressure recovery.

Figure 7: Recovery vs. Inlet Angle (diffuser-collector configuration)

Collector Exit Flow Field Assessment Δ

To understand how swirling flow exiting the diffuser might affect the collector, it is first important to understand the basic flow structures within the collector. It has been mentioned in many reports, such as Owczarek et al. [9], Zhang et al. [10] and Yoon et al. [11], that the flow structure inside an exhaust collector is dominated by a pair of counter-rotating vortices, resulting in local flow reversal and indicated by a significant reduction in measured pressure recovery. These structures can be clearly seen in Figure 8 which shows vector velocity maps obtained near the collector exit plane using a traversed five-hole probe for the diffuser-collector model without struts. The color map indicates the velocity component coming out of the traverse plane, and the vector field shows the in-plane components. Dark areas in the figures represent regions of back flow (negative vertical velocity). Exact evaluation of data in these zones was not possible because the probe was not calibrated to capture reverse flow.

Figure 6: Recovery vs. inlet angle (diffuser only configuration)

Figure 7 shows the overall pressure recovery of the diffuser-collector test models as a function of inlet angle for varying strut geometries. The diffuser-only (Figure 6) model exhibits higher pressure recovery for all strut geometries across the full range of swirl. As expected the implementation of a collector results in a reduction in pressure recovery due to the radial deflection of exhaust flow in a relatively confined space.

With increasing inlet angle, the diffuser-collector model with struts omitted exhibits a more diffuse right-side vortex as the left-side vortex becomes more compact and intense. Increased swirl in the flow exiting the diffuser amplifies this asymmetry. This is attributed to the swirl direction and the fact that more flow is being forced to the left side of the collector than the right side, which intensifies the out-of-plane vorticity due to spatial confinement of the vortical flux. This leads to the development of a large reversed-flow region within the right side vortex core. Figure 9 shows traverse data near the collector exit plane for the profiled struts at different inlet angles. For angles less than 21°, the counter rotating vortices remain approximately equal in size and strength. This indicates that the profiled struts improve pressure recovery by providing a certain amount of de-swirling of the flow as it exits the diffuser leading to more circumferential uniformity in the collector.

The general trend of the experimental results with the collector differ significantly from those of the diffuser-only tests. Most notably, both the strutless configuration and the cylindrical strut configuration showed a measurable reduction in recovery for even moderate inlet angle when the collector was added. The diffuser/collector configuration with profiled struts however, actually benefitted from angles up to 21°. It can be assumed that, for moderate inlet angles, the flow exiting the profiled struts is nearly axial. The configuration without struts as well as the one with cylindrical struts allow the swirl to more readily propagate through the diffuser into the collector. While this flow feature had little impact on the diffuser-only performance it seems to play a significant role once the collector is added to the system, suggesting that swirling flow exiting the diffuser adversely affects the collector.

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Back Wall

(a)

(b)

(a)

Missing data in the last row

Front Wall

(b)

(c)

(c)

(d)

(d)

Figure 8: Velocity in collector exit plane for strutless configuration: (a) 7° angle; (b) 14° angle; (c) 21° angle; (d) 35° angle.

Figure 9: Velocity in collector exit plane for profiled strut: (a) 7° angle; (b) 14° angle; (c) 21° angle; (d) 35° angle.

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Attached swirl flow Surface Oil Flow Visualization Surface oil flow visualization was used to support the findings of the five-hole probe traverse data. An image for the no-strut configuration shown in Figure 10 illustrates the mechanism by which swirl exiting the diffuser creates asymmetry in the collector.

Lift off line

Reverse swirl flow Impingement Line

Figure 11: Oil flow visualization - diffuser/collector model no struts at 21° inlet angle: hub surface

Oil flow traces for the profiled strut configuration clearly show a pattern on the back wall caused by the effects of the struts (Figure 12). In each of the eight wake zones the flow spreads radially after leaving the diffuser. This regime is dominated by axial flow, except on the left side where three wakes merge just before the flow area increase around the center-body, a minor swirl-driven effect.

Reverse Flow On Hub

Swirl Direction Figure 10: Oil flow visualization - diffuser/collector model / no struts at 21° swirl: back wall (forward looking aft)

At a swirl of 21° the flow exiting the diffuser clearly propagates towards the left side of the collector center-body. The flow develops into a vortex, increasing its strength as it moves radially through the collector.

Diffuser Exit Flow Radial On Back Wall

Meanwhile, the reduced flow on the right side of the collector transitions to a noticeably weaker vortex (Figure 8 (c)), creating a region of significant stagnation at the exit of the collector. The 240o to 300o region of the collector back wall shows disorganized flow in this region with low shear stress and multiple separation and attachment nodes and lines [17]. Oil flow visualization on the diffuser hub, shown in Figure 11, provides evidence of a significant hub separation bubble penetrating well upstream of the collector, limiting the potential pressure rise.

Merged Wakes

Figure 12: Oil flow visualization - diffuser/collector model / profiled struts at 21° inlet angle, back wall (forward looking aft)

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The 240o to 300o sector shows evidence of regions of reduced skin friction and localized reversed flow near the collector case. This provides a stark contrast to the same region seen in Figure 10. A significant reduction in reverse flow is shown at the hub surface of the diffuser-collector interface plane (Figure 13) when profiled struts are introduced into the configuration. This is a clear indication that the asymmetric flow structure in the collector is weakened by the presence of profiled struts for angles up to 21°.

(b)

Strut Ring Location Reverse Flow Due To Separation

Strut wakes

Figure 14: Oil flow visualization - diffuser/collector model / profiled struts at 35° inlet angle: (a) shroud view; (b) hub view

CONCLUSION Experimental studies were conducted on a 1/12th scale model of an industrial gas turbine diffuser/collector to investigate the effects of swirl and strut geometry. Pressure measurements with a three-hole probe at the diffuser inlet and a five-hole probe at the collector exit were used to determine pressure recovery. Oil flow visualization was employed to provide details of the near-wall flow in the diffuser. Tests on the diffuser-only configuration without struts, revealed that pressure recovery increased weakly with increasing swirl, peaking at 14° before rapidly dropping. Subsequent tests on the diffuser-collector model with no struts showed reduced pressure recovery over the full range of angles and a shift in peak recovery to lower swirl. With the implementation of profiled struts the diffuser-only configuration was characterized by reduced recovery across the range of swirl and a more-rapid drop in recovery beyond the peak. For the diffuser/collector configuration the addition of profiled struts revealed an expected reduction in recovery when compared to diffuser-only results. However a pronounced rising trend at lower swirl was discovered.

Impingement Points On Splash Plate

Figure 13: Oil flow visualization - diffuser/collector model / profiled struts at 21° inlet angle, hub surface

Subsequently, as the inlet angle approaches 35°, the flowfield is characterized by significant separation and reversed flow in the diffuser. Evidence of separation and stall on both endwalls around the struts is indicated by the oil flow visualization shown in Figures 14. The resulting drop in recovery is attributed to the associated increase in total pressure loss across the struts as well as the increased flow asymmetry in the collector.

(a)

Separation vortex

De-swirling effects associated with the application of profiled struts led to an increased operating range of the diffuser /collector configuration. Any decrease in the flow angle entering the collector leads to a reduction in flow asymmetry associated with smaller reverse-flow vortices initiating from the lower portion of the collector.

Profiled Strut

ACKNOWLEDGEMENT This work was sponsored by Solar Turbines, Inc. The authors wish to acknowledge the contributions of Brian Boehm and Hans Hamm.

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Impingement line

[14] Boehm B. P., 2012, “Performance optimization of a subsonic Diffuser-Collector subsystem using interchangeable geometries,” M.S. thesis, Department of Mechanical Engineering, Virginia Polytechnic Institute and State University. [15] R. Coladipietro, J. M. Schneider, K. Sridhar, 1974, “Effects of Inlet Flow Conditions on The Performance of Equiangular Annular Diffuser.” Trans CSME 3(2):75-82. [16] D. Hoadley, D. W. Hughes, 1969, “Swirling Flow in an Annular Diffuser.” University of Cambridge, Department of Engineering, Report CUED/A-Turbo/TR5 [17] Hunt, J.C.R., Abell, C.J., Peterka, J.A., and Woo, H., 1978, “Kinematical studies of the flows around free or surface-mounted obstacles; applying topology to flow visualization,” J. Fluid Mech., 88(1), pp. 179-200.

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