Investigation of Unsteady Effects in Transonic Turbomachinery Flows using Particle Image Velocimetry

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1 Investigation of Unsteady Effects in Transonic Turbomachinery Flows using Particle Image Velocimetry J. Woisetschläger, H. Lang, B. Hampel, E. Göttlich Institute for Thermal Turbomachinery and Machine Dynamics, Graz University of Technology, Inffeldgasse 25A, A-8010 Graz, Austria; Phone: , Fax: , Abstract Within two projects funded by the European Union and the Austrian Science Foundation unsteady effects in transonic turbomachinery flows were investigated to optimise these turbine stages used for power generation. A continuously operated cold-flow test rig with up to 2.8 MW shaft power and rpm was used. This work focuses on the application of stereoscopic Particle Image Velocimetry (PIV) for velocity and vorticity studies in combination with Laser Vibrometry (LV) and Laser-Doppler-Anemometry (LDA). With these techniques the wake flow between stator and rotor blades was investigated at subsonic and transonic Mach numbers. Special emphasis was put on phase-locking effects between stator vortex shedding and rotor blade passing frequency at transonic flow conditions. 1 Introduction In turbomachinery the inherent unsteadiness of the flow field caused by statorrotor interaction has a significant impact on efficiency and performance. For industrial gas turbines the market demands for machines of higher efficiency at lower costs per kw shaft power. Efficiency can be improved by advanced 3-D aerodynamics design and higher cycle temperatures. To meet the cost objective it is advantageous to reduce the number of stages, leading to higher pressure ratios per stage and therefore transonic flow conditions. The aerodynamics of these machines is dominated by pronounced pressure gradients (e.g. shocks), secondary flows (e.g. passage vortex, corner vortices, horseshoe vortices) and boundary layer material separating from the trailing edge of turbine blade profiles, forming so-called von Kármán vortex streets. PIV has become a powerful tool to investigate these unsteady processes in turbomachinery flows. PIV and stereo-piv have been applied to flow studies in transonic compressors [2,3,9,11], subsonic turbine flows [1,5,12,13] and transonic turbine flows [6].

2 230 Session 4 This paper focuses on the interaction of stator wake and rotor blades in a transonic turbine flow, where phase locking between vortex shedding from the trailing edge of the stator blades and the period of rotor blade passage was observed. A previously done investigation of vortex shedding behind a linear blade cascade (non-rotating arrangement of turbine blades in a wind-tunnel) using PIV preceded this work [14]. 2 Experimental Set-up 2.1 Test section The transonic test turbine at Graz University of Technology is a continuously operating cold-flow open-circuit facility which allows the testing of turbine stages with a diameter up to 800 mm in full flow similarity. Pressurized air is delivered by a separate 3 MW compressor station. The shaft power of the test stage drives a 3 stage radial brake compressor. This brake compressor delivers additional air mixed to the flow from the compressor station and increases the overall mass flow. The air temperature in the mixing chamber (turbine stage inlet) can be adjusted by coolers between 313 K to 458 K. The maximum shaft speed of the test rig is limited to rpm. Depending on the stage characteristic a maximum coupling power of 2.8 MW at a total mass flow of 22 kg/s can be reached using an additional suction blower in the exhaust line (750 kw). Detailed information on the stage used are given in Fig. 1 and Table 1. To grant optical access to the flow field the outer casing between vane exit and diffuser inlet was equipped with one large plane-concave window (HERASIL quartz glass; anti-reflection coating). Instead of traversing the PIV system circumferentially, the stator blades (mounted on a turnable ring) were rotated to adjust the PIV measurement planes or LDA positions. Fig. 1. Test section (all measures in mm).

3 Applications 231 Table 1. Test conditions in the transonic turbine stage (see also Fig.1) number of stator blades 24 number of rotor blades 36 rotor tip clearance [mm] 1 shaft power [MW] 1 Total pressure inlet [Pa] 250,000 pressure ratio stage p tot,in /p out 2.6 rotational speed [rpm] 9660 inlet total temperature T tot,in [K] 367 Trigger for the PIV system was provided by the turbine s control system (Bently Nevada) with respect to the rotor blade position. One TTL pulse per rotation was used to define the zero position between stator and rotor (trigger zero, see Fig.1) Conventional probe measurements (total temperature, total pressure, static pressure) were done in the mixing chamber, upstream of the stator, between stator and rotor, downstream of the rotor and in the diffuser using PSI 9016 network scanners and National Instruments Field Point modules together with thermocouples and PMP 4000 pressure scanners. For reasons of comparison a seven-hole probe was used to determine total pressure, pitch and yaw flow angle. 2.2 Stereoscopic particle image velocimetry To record flow velocity and vorticity for flow sections between stator and rotor at radii R222 and R237 stereoscopic PIV was used. The system used a pulsed double cavity Nd:YAG laser (120 mj / pulse) to provide double pulses at a pulse separation of 0.7 µs. The light was guided through an articulated arm to a light-sheet optics consisting of a spherical lens (600mm focal length), a cylindrical lens (-10 mm focal length) and a prism to illuminate a plane section of the flow. This section was imaged through the window by two DANTEC 80C60 HiSense cameras (1280x1024 pixel) and seeded by a PALAS-AGF 5D particle generator using DEHS oil droplets injected 500 mm upstream of the stator. A DANTEC FlowMap 110 PIV processor was used to control the recordings (one recording consisted of two images for each of the two cameras) triggered by the position of the rotor blades. For each camera two images (one recording) were evaluated using a crosscorrelation technique resulting in the vector field of displacements within the 0.7 µs delay time between the two laser pulses (interrogation area size was 64x64 pixel with 50% overlap, resulting in a grid of 40x32 vectors) Finally, a range validation and a moving validation filter was applied to reject erroneous vectors [8].

4 232 Session 4 Fig. 2. PIV setup (left), light sheet probe (center) and seeding pipe (right). To relate the single displacements in the single recordings to physical length and combine them into a three-dimensional vector field, a calibration through the curved observation window was performed using a special calibration target (100x100 white plate with a square grid of black dots, dot spacing 2.5 mm, dot size 1.5 mm). This calibration procedure resulted in a polynomial function relating the single displacement fields recorded by camera A and camera B into one single velocity field for the overlap area of both cameras [6]. By averaging velocity fields (180 for each rotor-stator position) mean values and vorticity fields were calculated using: v u ω z = (1) x y with u and v the in-plane velocities, x and y the in-plane coordinates and ω z the vorticity with z direction perpendicular to the in-plane components, all defined in the three-dimensional velocity field. 2.3 Other laser-optical diagnostics tools To record the frequency spectra of density fluctuations in the flow field a Laser Vibrometer (LV) was used (Polytec OVD 353 with OVD-3001 controller and OVD 02 velocity decoder). Widely applied in vibration analysis, these interferometer type systems detect minute changes in the optical path of a laser beam emitted by and reflected back into the system. In common applications, these changes are caused by surface vibration. Here, the optical path changes due to density fluctuations caused by flow turbulence and unsteady flow phenomena (pressure and temperature fluctuations) along the laser beam. In the turbine investigated surface vibrations occurred in the low frequency range (up to 4 khz) while these density fluctuations were dominant in the frequency range above 10 khz [12, 14]. The LV beam was fixed to a position in the wake of the stator (6 mm axial and 27 mm circumferential distance from the tip of the trailing edge) and reflected by the polished surface of the stator blade platform.

5 Applications 233 Fig. 3. PIV records (left) and particle displacements (right, after validation) for a given rotor stator angle (2.4 from zero position). For each rotor-stator angle 180 of these recordings were averaged. 1 is the leading edge of the rotor blades, 2 is the trailing edge of the stator blades, 3 the gap between rotor and stator. Additionally a Laser Doppler Anemometry (LDA) was used to record instantaneous velocity bursts at single positions caused by tracer particles passing the measurement volume formed by the laser beams (two dimensional DANTEC fibre flow system with Burst Spectrum Analyzer). This system was used to record axial and circumferential velocities and was triggered by the rotor position. A high number of individual bursts ( ) was recorded for each position traversed. These instantaneous velocity values (single bursts) were sorted within 40 windows within the blade passing period. Mean velocities, turbulence level and cross-moments were calculated within each window. Fig. 4. Oil film visualization (stator) and comparison between LDA and pneumatic probe measurements (behind rotor) to estimate the influence of the seeding injection.

6 234 Session 4 PIV needs good seeding. This means that a large mass flow of relative cold air has to be fed in the turbine as close as possible in front of the test zone. To estimate the influence of this injection, oil film visualization and a comparison between LDA and pneumatic probe measurements were done using a seven-hole probe (Fig. 4; yaw angle was averaged over one stator pitch for a position behind the rotor, positive angle is counter-clockwise to x-axis in Fig.1). Using the seeding pipe shown in Fig. 2 for the LDA measurements the change in flow angle was within the accuracy of the pneumatic probe measurements (no seeding pipe). An increase in turbulence level from 8 to 9% was observed in the streamline behind the seeding injection when the LDA was used to scan the field. 3 Results and Discussion Fig. 5 shows the flow field between stator and rotor at four different rotor positions slightly below the mid-span (R222) as recorded by stereoscopic PIV. These data (absolute values of velocity in Fig.5a) are already averaged values of approx. 180 instantaneous recordings at four rotor-stator positions. Clearly visible is the shock front which is strongest at the pressure side (lower side) of the stator wake, starting at the suction side of the neighboring stator blade. The passing rotor blades have significant influence on this front especially at the suction side (upper side) of the wake. With the rotor blades a velocity defect migrated through the flow field and the wake was bent back and forth (e.g. t=3/4t, Fig.5). When looking at the vorticity (Fig.5b), areas with good phase-locking between vortex shedding and rotor phase, as well as, areas with little or no phase-locking can be identified. In all positions along the wake where no clear relation in phase between the rotor passing and the vortex shedding existed, the mean value and the vortex centres were smeared during the process of averaging of the 180 single recordings taken in the same stator-rotor position (e.g. t=4/4t). On the other hand, when there was a clear phase relation between shedding and rotor passing, the vortex field was steady for a given stator-rotor position and the averaging process resulted in pronounced vortex centres (e.g. t=1/4t). Such a phase locking effect of vortex shedding to the blade passing frequency was predicted by other authors [10]. When the shedding process was phase-locked to the rotor passing, the vorticity observed in the averaged fields (Fig.5b) was about 60% of the vorticity values in the single images, providing clue to a highly stable vortex shedding during such intervals in the rotor passing period. Generally, the vorticity values slightly decreased with the distance from the trailing edge of the stator blade. In the single recordings, as well as in the averaged fields, the pressure side vortices had higher amplitudes than those shedding from the suction side. To confirm these PIV recordings LDA measurements were done for single positions in the same radial height within the flow channel. Triggered by the rotor position instantaneous velocity values (single bursts) were recorded for 120 positions along a line in the flow field behind the stator blades, then sorted

7 Applications 235 within the blade passing period (40 windows) and averaged. Fig. 6 gives the axial velocity component and its variance for all positions during one blade passing period. Fig. 5. Results for a single plane at R222 as recorded by stereoscopic PIV for one blade passing period T. a) velocity, b) vorticity. Fig. 6. Axial velocity and variance of axial velocity as recorded by LDA. The single bursts were sorted within the blade passing period by the rotor trigger signal and averaged. For better comparison one PIV result (Fig.5, t=1/ 4 T) is superimposed. While each of the PIV recordings represents the whole flow field at a given rotor-stator position, the LDA recordings give the development of velocity in a single position in space during rotor blade movement. In PIV and LDA recordings the flow velocities are determined only through the tracer particle movement. Using the particle size, the particle density and the frequency, the particle response can be estimated [7,8]. Assuming acceleration forces and friction the left plot in Fig.7 gives the particle response for DEHS droplets

8 236 Session 4 between 0.5 and 1 µm. Although the data sheet of the particle generator specifies a mean particle size of 0.2 µm under optimum conditions agglomeration during injection has to be taken into account. From our experience we expect a mean particle diameter closer to 0.7 µm. Such particles could follow a fluid velocity oscillation in air with a frequency of approximately 50 khz at an amplitude of at least 93% of the fluid oscillation amplitude. Fig. 7 compares these particle response functions to the frequency spectrum of density fluctuations recorded by the LV at 3300 rpm (0.45 MW, stage pressure ratio 1.2) and 9660 rpm. While at 3300 rpm the vortex shedding occurs at app. 38 khz (subsonic flow condition), at 9660 rpm it links to the 7 th harmonics of the blade passing frequency at 47 khz (transonic flow condition). First numerical simulations of these high frequency processes indicated shock reflections from the leading edge of the rotor blades moving back and forth through the flow field, triggering vortex shedding at the stator s trailing edge [4]. From these simulations it is currently believed that the diagonal line crossing the axial velocity plot (Fig.6, centre image) is such a reflection. This change in velocity is followed by number of pronounced vortex structures in this averaged result. Fig. 7. Particle response function and frequency spectrum of density fluctuation (recorded by LV) The density fluctuations recorded by the LV indicated higher harmonics in the vortex shedding process (Fig.7, right image). In this frequency range the tracer particles already acted as low pass filter so that these higher harmonics were only partly present in the tracer particle movement. As a result the vortex shedding movement appeared more sinusoidal in the PIV recordings than in preliminary numerical simulations [4]. The vortex cores transport boundary layer material which was less seeded (compare Fig.3). This led to a lower number of validated recordings in the wake. Together with the error due to calibration a maximum error of 8 m/s in the wake was estimated.

9 Applications Summary Investigating unsteady effects in a cold-flow transonic turbine stage of real size PIV enabled rapid recording of flow structures and the identification of vortex structures in high frequency shedding processes. Limitations in the investigation of this type of flows arise from the fact that the tracer particles used start to act as low pass filter. To estimate the influence of this effect to the results an additional technique was used, directly recording the density fluctuations in the flow. As a result, phase locking of vortex shedding behind the stator blade to the 7 th harmonic of the rotor passing frequency was observed. Preliminary numerical simulations [4] indicate that this effect is triggered by shock reflections propagating through the flow field and influencing the boundary layer shedding. Acknowledgements This work was made possible by the Austrian Science Fund (FWF) within the grant Y57-TEC Non-intrusive measurement of turbulence in turbomachinery. Graz University of Technology is member of the thematic network PIV-NET funded by the European Union (EU). References [1] Chow Y, Uzol O, Katz J (2002) Flow nonuniformities and turbulent hot spots due to wake-blade and wake-wake interactions in a multi-stage turbomachine, ASME J Turbomach, 124: [2] Copenhaver W, Estevadeordal J, Gogineni S, Gorell S, Goss L (2002) DPIV study of near-stall wake-rotor interactions in a transonic compressor, Exp Fluids, 33: [3] Estevadeordal J, Gogineni S, Goss L, Copenhaver W, Gorell S (2002) Study of wakeblade interactions in a transonic compressor using flow visualization and DPIV, ASME J Fluids Eng, 124: [4] Heitmeir F, Woisetschläger J, Sanz W, Jericha H (2003) Transonic turbine testing, In Štástný M, Sieverding K (eds) 5th European conference on Turbomachinery, paper TM01/79 [5] Hill DF, Sharp KV, Adrian RJ (2000) Stereoscopic particle image velocimetry measurements of the flow around a Rushton turbine, Exp Fluids, 29: [6] Lang H, Mørck T, Woisetschläger J (2002) Stereoscopic particle image velocimetry in a transonic turbine stage, Exp Fluids, 32: [7] Mei R (1996) Velocity fidelity of flow tracer particles, Exp Fluids 22:1-13 [8] Raffel M, Willert C, Kompenhans J (1998) Particle image velocimetry, a practical guide, Springer, New York [9] Sanders AJ, Papalia J, Fleeter S (2002) Multi-blade row interactions in a transonic axial compressor: Part I stator particle image velocimetry (PIV) investigations, ASME J Turbomach, 124:10-18

10 238 Session 4 [10] Sondak DL, Dorney DJ (1999) Simulation of vortex shedding in a turbine stage, ASME J Turbomach, 121: [11] Tisserant D, Breugelmans FAE (1997) Rotor blade-to blade measurements using particle image velocimetry, ASME J Turbomach, 119: [12] Uzol O, Chow Y, Katz J, Meneveau (2002) Experimental investigation of unsteady flow field within a two-stage axial turbomachine using particle image velocimetry, ASME J Turbomach, 124: [13] Wernet MP (2000) Development of digital particle image velocimetry for use in turbomachinery, Exp Fluids, 28: [14] Woisetschläger J, Mayrhofer N, Hampel B, Lang H, Sanz W (2003) Laser-optical investigation of turbine wake flow, Exp Fluids, 34: