Investigation of Unsteady Flow Behavior in Transonic Compressor Rotors with LES and PIV Measurements

In the present study, unsteady flow behavior in a modern transonic axial compressor rotor is studied in detail with large eddy simulation (LES) and particle image velocimetry (PIV). The main purpose of the study is to advance the current understanding of the flow field near the blade tip in an axial transonic compressor rotor near the stall and peak-efficiency conditions. Flow interaction between the tip leakage vortex and the passage shock is inherently unsteady in a transonic compressor. Casing-mounted unsteady pressure transducers have been widely applied to investigate steady and unsteady flow behavior near the casing. Although many aspects of flow have been revealed, flow structures below the casing cannot be studied with casing-mounted pressure transducers. In the present study, unsteady velocity fields are measured with a PIV system and the measured unsteady flow fields are compared with LES simulations. The currently applied PIV measurements indicate that the flow near the tip region is not steady even at the design condition. This self-induced unsteadiness increases significantly as the compressor rotor operates near the stall condition. Measured data from PIV show that the tip clearance vortex oscillates substantially near stall. The calculated unsteady characteristics of the flow from LES agree well with the PIV measurements. Calculated unsteady flow fields show that the formation of the tip clearance vortex is intermittent and the concept of vortex breakdown from steady flow analysis does not seem to apply in the current flow field. Fluid with low momentum near the pressure side of the blade close to the leading edge periodically spills over into the adjacent blade passage. The present study indicates that stall inception is heavily dependent on unsteady behavior of the flow field near the leading edge of the blade tip section for the present transonic compressor rotor

Stereoscopic Measurements of Blade Deformation on a Shrouded Prop-Fan with Boundary Layer Ingestion by means of Image Pattern Correlation Technique

The material presented herein describes the implementation of optical blade deformation measurements based on the image pattern correlation technique (IPCT) in a stereoscopic imaging configuration applied to the first rotor of a counter-rotating shrouded fan stage (CRISPMulti) which has a diameter of about 1m. The correlation-based stereoscopic image analysis is discussed, as well as the difficulties with regard to the correlation-based stereomatching of two camera views for the highly curved blade surface involving pattern displacements up to 12% of the image height. Cross-correlation of speckle images at reference positionwith images of the displaced positions recovered the full 3c displacement field in the optically assessable area of the suction side. The three-dimensional displacements could be evaluated with triangulation errors below 0.5mm at blade tip velocities exceeding 250m/s (4830rpm). Comparative measurements using a third camera imaging the blade tip match the stereo IPCT results within deviations of 0.5mm for the axial component whereby the circumference component matches closely

The Challenge of Time-Resolved Flow Investigation of a One-Stage Centrifugal Compressor with a Non-Symmetric Volute

A single stage industrial centrifugal compressor including vaneless diffuser and a non-symmetric volute is investigated with regard to the development of flow instabilities and rotating stall. A deeper understanding of flow phenomena that might be a precursor for stall inception would help to predict unstable operation already during the design phase of a compressor stage, as today the design is commonly supported by CFD analysis providing performance data and flow information at an early stage of the design process. Due to the complex flow path geometry of the volute a sophisticated instrumentation concept was developed allowing for unsteady pressure measurement with synchronized high-resolution flow field measurement. This paper summarizes the challenges and the steps taken during rig design in terms of integrating non-intrusive, high resolution pressure measurement sensors along with access for optical techniques such as time-resolved PIV and L2F to investigate unsteady flow phenomena and stall inception occurring at reduced speed operation. Results of performance measurements and surge testing are presented and compared to CFD simulations. Exemplary results of the extensive database obtained by optical techniques in combination with dynamic pressure measurements are provided

K 1-6: an asymmetric planetary nebula with a binary central star

We present new imaging data and archival multiwavelength observations of the little studied emission nebula K 1-6 and its central star. Narrow-band images in H-alpha (+ [NII]) and [OIII] taken with the Faulkes Telescope North reveal a stratified, asymmetric, elliptical nebula surrounding a central star which has the colours of a late G- or early K-type subgiant or giant. GALEX ultraviolet images reveal a very hot subdwarf or white dwarf coincident in position with this star. The cooler, optically dominant star is strongly variable with a period of 21.312 +/- 0.008 days, and is possibly a high amplitude member of the RS CVn class, although an FK Com classification is also possible. Archival ROSAT data provide good evidence that the cool star has an active corona. We conclude that K 1-6 is most likely an old bona fide planetary nebula at a distance of ~1.0 kpc, interacting with the interstellar medium, and containing a binary or ternary central star. The observations and data analyses reported in this paper were conducted in conjunction with Year 11 high school students as part of an Australian Research Council Linkage Grant science education project, denoted Space To Grow, conducted jointly by professional astronomers, educational researchers, teachers, and high-school students.Comment: 13 pages, 5 figures, accepted by the Publications of the Astronomical Society of Australia (PASA

PIV Application for Investigation of the Rotor Blade Tip Interaction with a Casing Treatment in a Transonic Compressor Stage

This contribution describes the experimental investigation of the blade tip interaction with a casing treatment implemented to a transonic compressor stage using particle image velocimetry (PIV). The results obtained allowed for direct comparison with numerical simulations of the same compressor stage including the CT geometry, carried out using the DLR TRACE code following a new approach to efficiently perform time-accurate casing-treatment simulations. The single-stage transonic axial compressor was equipped with a casing treatment (CT), consisting of 3.5 axial slots per rotor pitch in order to investigate the predicted extension of the stall margin characteristics. Contrary to most other studies, the CT was designed especially accounting for an optimized optical access in the immediate vicinity of the CT, rather than giving maximum benefit in terms of stall margin extension. The nearly rectangular geometry of the CT cavities allowed one dividing bridge between two slots to be made of quartz glass with curvatures matching the casing. Thus the flow phenomena could be observed with essentially no disturbance caused by the optical access. Two periscope light sheet probes were specifically designed for this application to allow for precise alignment of the laser light sheet at three different radial positions in the rotor tip region (at 87.5%, 95% and 99% blade height). For the outermost radial position the light sheet probe was placed behind the rotor and aligned to pass the light sheet through the blade tip clearance. It was demonstrated that the PIV technique is capable of providing velocity information of high quality even in the tip clearance region of the rotor blades. Phase-constant measurements were carried out with a resolution of 8 phase angles per blade pitch in relation to the CT slots visible in the camera’s field of view. The chosen type of smoke-based seeding with very small particles (about 0.5 µm in diameter) supported data evaluation with high spatial resolution, resulting in a final grid size of 0.5 x 0.5 mm. The PIV data base established in this project forms the basis for further detailed evaluations of the flow phenomena present in the transonic compressor stage with CT and allows validation of accompanying CFD calculations using the DLR TRACE code. Based on the combined results of PIV measurements and CFD calculations of the same compressor and CT geometry a better understanding of the complex flow characteristics can be achieved

Entwicklung und Einsatz eines Sonden-basierten PIV Systems zur störungsfreien, dreikomponentigen Geschwindigkeitsmessung im Blattspitzenbereich transsonischer Turbomaschinen-Strömungen

In der vorliegenden Arbeit wird am Beispiel des Transsonik-Verdichters der Technischen Universität Darmstadt die Entwicklung spezieller Lichtschnitt-Sonden präsentiert, die durch wandbündige Integration in das Verdichtergehäuse weitgehend störungsfreie Particle Image Velocimetry (PIV) Messungen im Schaufelspitzenbereich des transsonischen Rotors ermöglichen. An der zusätzlich mit einem Casing Treatment (CT) im Gehäuse ausgestatteten Verdichterstufe wird die volle Einsatzreife der entwickelten PIV Konfiguration demonstriert. In der anschließenden, detaillierten Untersuchung werden flächige, örtlich hoch aufgelöste Geschwindigkeitsdaten gewonnen, die einen Einblick in die komplexe Interaktion der Rotorströmung mit dem CT gewähren. Im Rotorspalt werden sogar dreikomponentige Messungen mittels stereoskopischer PIV realisiert. Die Strömungsergebnisse dienen der Unterstützung und Validierung entsprechender numerischer Simulationen mit TRACE. Ein Vergleich von PIV und TRACE Ergebnissen wird in der Arbeit detailliert diskutiert. Zusätzlich werden aus dem Vergleich Kenntnisse über die spezifische Wirkungsweise des gewählten "bend-slot" CT gewonnen. Insbesondere die Beeinflussung des Spaltwirbels stellt sich als charakteristisches Merkmal für die Erweiterung des stabilen Verdichter-Arbeitsbereichs heraus

Selected applications of planar imaging velocimetry in combustion test facilities

The contribution provides an overview on the applicability of particle image velocimetry (PIV) and Doppler global velocimetry (DGV) in combustion test facilities and summarizes experiences gained in a number of applications that were performed in the past 6 years. Emphasis is placed on the experimental aspects of each application rather than the interpretation of the acquired flow field data because many of the encountered problems and chosen solution strategies are unique to this area of planar velocimetry application. In particular imaging configurations, seeding techniques, data acquisition strategies as well as pre- and post-processing methodologies are outlined. The paper describes four similar applications of PIV in atmospheric and pressurized combustors containing single burners that are operated either with kerosene or natural gas. Aside from providing adequate optical access to the respective facilities the most challenging issue has been the reliable supply of non-volatile and non-reacting solid particle seeding. Given the rather large mass flows of up to 1 kg/s inside the combustors, sufficient seeding quantities could best be delivered by fluidized bed particle generators, sometimes even operated in parallel. The devices have been continually improved to allow constant delivery rates only during data acquisition periods, thus preventing unnecessary collection of seeding in the facility. The choice and handling of the seeding particle powder was found to be crucial for the overall success of the measurements. The first application describes initial trials on a swirled kerosene spray fuel nozzle at 3 bars and illustrates problems encountered due to flame luminosity as well as strong light scattering from the fuel droplets. The second application summarizes the challenges faced in obtaining phase-resolved, three-component velocity data from a generic atmospheric gas combustor using stereoscopic PIV. A much larger, swirl stabilized gas burner as utilized in stationary power generation was the subject of the third described application. Here a novel methodology of data acquisition and post-processing provides phase resolved velocity maps even of flows that have non-constant oscillation frequencies. Finally, the fourth application describes how the combination of PIV with DGV can yield three-component velocity data in areas of limited optical access using only one viewing window and one separate window (or probe) for light sheet delivery

PIV Applied to a Transonic Centrifugal Compressor

In the present application PIV was chosen to analyze the complex flow phenomena inside a vaned diffuser of a new generation transonic centrifugal compressor, as this planar technique is capable of detecting unsteady flow structures and to resolve even high velocity gradients as well as unsteady shock configurations previously undetectable with point-wise techniques such as laser-2-focus velocimetry (L2F). Measurements were carried out at rotational speeds between 35,000 and 50,000 rpm. The compressor stage was designed for a pressure ratio of 6:1. Due to the advanced impeller geometry the diffuser section has a conical shape and a constant passage height of 8.1 mm. With a special light sheet periscope probe is was possible to adjust the light sheet to three vane span locations chosen for flow investigation: center plane (50%), one plane close to the hub (19%) and one close to the tip (74%)

Blattspitzenwechselwirkung mit dem Gehäuse und Gehäuseeinbauten

Im Rahmen des vorliegenden Vorhabens wurde eine einstufige Verdichterstufe, ausgestattet mit einer speziell auf eine optimierte optische Zugänglichkeit ausgelegten Gehäusekonturierung (Casing-Treatment, CT) sowohl numerisch als auch experimentell untersucht. Im Rahmen der experimentellen Untersuchungen kam dabei ein örtlich hoch auflösendes, planares Messverfahren (PIV) zum Einsatz. Die numerischen Untersuchungen erfolgten mit dem Verfahren TRACE, das zur effizienten Berechnung von Casing-Treatments (und anderen, beliebig anzuordnenden Nebengeometrien) weiterentwickelt wurde. Die durchgeführten Weiterentwicklungen umfassten dabei die Erweiterung der versetzt-periodischen (phaselag) Randbedingungen auf den Einsatz typischer CT Konfiguration. Die erweitere Methodik wurde umfassend dokumentiert, die Integration in das Verfahren beschrieben und überprüft und damit dessen Einsetzbarkeit nachgewiesen. Das derart weiterentwickelte Verfahren kam dann bei der Nachrechnung der untersuchten Verdichterkonfiguration zum Einsatz. Im Rahmen der experimentellen Untersuchungen wird die Adaption der Messtechnik an den Prüfstand der TU Darmstadt beschrieben. Bei der Auslegung des CT’s seitens der MTU wurde durch eine radiale Ausrichtung der CT-Schlitze bereits eine optimale optische Zugänglichkeit berücksichtigt, sodass die experimentellen Daten insgesamt einen hervorragenden Einblick in die komplexe Strömungsphysik der gehäusenahen Rotor-Slot Interaktion liefern. Messdaten wurden dabei im gehäusenahen Bereich für mehrere Betriebspunkte und unterschiedliche Rotorstellungen gewonnen. Die Vergleiche mit den numerischen Daten zeigten dabei eine bemerkenswert gute Übereinstimmung nahezu über den gesamten Betriebsbereich. Sowohl die lokalen Strömungsphänomene wie Stoßsysteme, Expansiongebiete als auch deren instationäre Wechselwirkung mit den im Gehäuse eingebrachten Schlitzen konnten schließlich anhand der qualitativ hochwertigen Messdaten studiert und im Detail mit den numerischen Daten verglichen werden. Darüber hinaus konnten die numerischen Daten - abgesichert durch das Experiment - schließlich dazu genutzt werden, die Wirkungsweise der Gehäusekonturierung im zu untersuchen

VERIFICATION OF THE THREE DIMENSIONAL SHOCK-STRUCTURES IN AN S-SHAPED TRANSONIC UHBR FAN-ROTOR

The German Aerospace Center (DLR) has designed and tested a 1:3 scaled model S-shaped fan rotor as an example for a medium pressure ratio propulsor with potential application to future UHBR aero engines. In the present study, the attention will focus on the tip region in which the flow field is subject to complex flow phenomena and to the impact of the Sshape feature on the radial shock structure. Steady numerical simulations with DLR in-house solver TRACE as well as measurements were carried out. The casing is instrumented with ten piezoelectric static pressure transducers over the rotor pitch. Particle Image Velocimetry (PIV) is used to catch the flow velocities at three radial blade positions. All experimental data require a phase-locked ensemble averaging procedure. The results include the global performance of the compressor and detailed comparisons between simulations and measurements to validate the shock structures as well as the highly three dimensional-design S-shape fan