Joachim Klinner

Pulsed operation of high-power light emitting diodes for imaging flow velocimetry

High-powered light emitting diodes (LED) are investigated for possible uses as light sources in flow diagnostics, in particular, as an alternative to laser-based illumination in particle imaging flow velocimetry in side-scatter imaging arrangements. Recent developments in solid state illumination resulted in mass-produced LEDs that provide average radiant power in excess of 10 W. By operating these LEDs with short duration, pulsed currents that are considerably beyond their continuous current damage threshold, light pulses can be generated that are sufficient to illuminate and image micron-sized particles in flow velocimetry. Time-resolved PIV measurements in water at a framing rate of 2 kHz are presented. The feasibility of LED-based PIV measurements in air is also demonstrated.

Combined PIV and DGV applied to a pressurized gas turbine combustion facility

This paper provides an overview of flow field measurements on a pressurized generic combustor that shares typical features of realistic gas turbine combustors. Both Doppler global velocimetry (DGV) and particle image velocimetry (PIV) were applied in parallel to achieve volumetric, three-component velocity data sets of the reacting flow field at pressures of 2 and 10 bar with 700 K pre-heating. Limited optical access to the mixing zone required a combination of PIV and DGV to obtain averaged three-component velocity data from a single viewing direction. The acquired volume data sets of the time-averaged flow in the mixing zone contain about 40 parallel planes spaced at 2 mm with a spatial resolution of 1.2 × 1.2 mm2 each. Difficulties encountered in the application of stereoscopic PIV to a simple atmospheric generic combustor illustrate the advantage of the combined PIV–DGV technique.

Performance and accuracy investigations of two Doppler global velocimetry systems applied in parallel

Two Doppler global velocimetry systems were applied in parallel to assess their performance in wind tunnel environments. Both DGV systems were mounted on a common traverse surrounding the glass- walled 1.4×1.8 m 2 test section of the wind tunnel. The traverse normally supports a three-component forward-scatter laser Doppler velocimetry system. The reproducible vortical flow field generated by the blunt tip of an airfoil was chosen for this investigation and was precisely surveyed by LDA just prior to the DGV measurements. Both DGV systems shared the same continuous wave laser light source, laser frequency stabilization and fiber optic light sheet delivery system. The principle differences between the DGV implementations are with regard to the imaging configuration. One configuration relied on a single camera that observed three successively operated light sheets. In the second configuration, three camera views simultaneously observed a single light sheet using a four-branch fiber imaging bundle. The imaging bundle system had all three view points in a forward scattering arrangement which increased the scattering efficiency but reduced the frequency shift sensitivity. Since all three light sheet observation components were acquired onto the same image frame, acquisition times could be reduced to a minimum. On the other hand, the triple light sheet − single camera system observed two light sheets in forward scatter and one light sheet in backscatter. Although three separate images had to be recorded in succession, the image quality, spatial resolution and signal-to-noise ratio was superior to the imaging bundle system. Comparison of the DGV data with LDV measurements shows very good agreement to within 1-2 m/s. The remaining discrepancy has a variety of causes, some are related to the reduced resolving power of the fiber imaging bundle system (graininess, smoothing), exact localization of the receiver head with respect to the scene, laser frequency drift or background influences. The extensive data base available allows a more detailed study of the influences of each of these factors.

Detailed Flow Investigation Using PIV in a Rotating Square-Sectioned Two-Pass Cooling System with Ribbed Walls

In a 2-pass cooling system the pressure driven air flow distribution is investigated experimentally using the non-intrusive PIV Technique. The generic model as part of a complex and sophisticated cooling system consists of two square-sectioned ducts with a length of 20 diameters and an inherent 180 degree bend. The system has been investigated basically with smooth walls (case 0) and, later on, with two different kinds of ribbed walls in both legs. Ribs are applied to enhance the cooling performance; they are placed on two opposite walls of both legs in a symmetric (case A) and an asymmetric manner (case B), respectively. The ribs are inclined with an angle of 45 degrees versus the duct axis (i.e. main flow direction). The applied rib lay-out is well-proved and optimized with respect to heat transfer improvement and the inherent pressure drop increase. The system rotates about an axis orthogonal to the centreline of the straight passes. The configuration was analyzed with the planar the two-component Particle Image Velocimetry (2C PIV), which is capable of obtaining complete maps of the instantaneous as well as the averaged flow field even at high turbulence levels, which are typically present within duct turns, near ribs and, above all, during rotation. The presented investigations were conducted in stationary and rotating mode. Especially in the bend region separation phenomena and vortices with high local turbulence are apparent. The presence of ribs changes the fluid motion by generating additional vortices impinging the side walls. Flow visualization with injected oil smoke using the laser light sheet visualization technique was helpful to detect vortex structures and separations. Especially in the bend area separation regions and vortices with high local turbulence are apparent. The results shown in this paper demonstrate the effect of the 180 degree bend in combination with the two rib turbulator geometries for isothermal flow conditions excluding any buoyancy with and without rotation. Turbulent channel flow was investigated at a Reynolds number of 50,000, derived with the hydraulic diameter of the pass, non-rotating and at a rotation number of 0.02 which was chosen still moderate. Engine relevant rotation numbers are in order of .1 or higher. A reconstruction of model mountings will allow higher values for the next tests. Future work will expand to higher rotational speed and, also, will include buoyancy effects. This investigation shall help to clarify the complex flow phenomena due to the interaction of several vortices, present in two-pass cooling systems. The flow maps obtained with PIV are of good quality and high spatial resolution and therefore provide a test case for the development and validation of numerical simulation tools like the DLR flow solver TRACE which is not a topic of this paper.Copyright

Selected applications of planar imaging velocimetry in combustion test facilities

This chapter provides an overview on the application of particle image velocimetry (PIV) and Doppler global velocimetry (DGV) in combustion test facilities that are operated at pressures of up to 10 bar. 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 velocimetry application. In particular, imaging configurations, seeding techniques, data-acquisition strategies as well as pre- and postprocessing methodologies are outlined.

On the development of Doppler global velocimetry for cryogenic wind tunnels

A specially designed Doppler global velocimetry system (DGV, planar Doppler velocimetry) was tested in a high-speed cryogenic facility at Mach 0.3 to Mach 0.8 and pressures between 1.2 and 2.5 bar. The necessary seeding was achieved by injecting a mixture of gaseous nitrogen and water vapor into the dry and cold tunnel flow which then immediately formed a large amount of small ice crystals. As operational and access conditions are quite restrictive with respect to other facilities, DGV is currently considered the best choice for the non-intrusive measurement of flow fields. A comparison of DGV to the more wide-spread particle image velocimetry technique (PIV) is also given.

Extensive characterisation of a high Reynolds number decelerating boundary layer using advanced optical metrology

ABSTRACT Over the last years, the observation of large-scale structures in turbulent boundary layer flows has stimulated intense experimental and numerical investigations. Nevertheless, partly due to the lack of comprehensive experimental data at sufficiently high Reynolds number, our understanding of turbulence near walls, especially in decelerating situations, is still quite limited. The aim of the present contribution is to combine the equipment and skills of several teams to perform a detailed characterisation of a large-scale turbulent boundary layer under adverse pressure gradient. Extensive particle image velocimetry (PIV) measurements are performed, including a set-up with 16 sCMOS cameras allowing the characterisation of the boundary layer on 3.5 m, stereo PIV and high resolution near wall measurements. In this paper, detailed statistics are presented and discussed, boundary conditions are carefully characterised, making this experiment a challenging test case for numerical simulation.

Messung der Strömungsverteilung am Austritt der Bipolarplatten eines Brennstoffzellen-Stacks

Zusammenfassung Es werden zwei experimentelle Ansätze vorgestellt, um die Strömungsverteilung eines luftdurchströmten 8–12 KW Brennstoffzellen-Stacks unter realistischen Massenströmen nahe am anodenseitigen Austritt zu erfassen. Mit Hilfe einer Visualisierungstechnik kann die Eindringtiefe der Freistrahlen in das Sammelleitungsgebiet dargestellt werden. Durch Anwendung von zweikomponentiger Particle Image Velocimetry (2C-PIV) wird das Geschwindigkeitsfeld in der gesamten Höhe des Stacks hochaufgelöst gemessen. Neben der Implementierung des experimentellen Setups wird auf die PIV Datenauswertung eingegangen. Abstract This paper presents two different experimental approaches which concentrate on capturing the flow distribution close to the anode exit header of a prototype 8–12 KW fuel cell stack operated with air at realistic flow rates. The first approach intends to visualize the penetration depth of millimeter-sized jets towards the exit manifold. The second one is focused on obtaining the exit jet velocity field downstream of the bipolar plate exit header across the entire stack height by repeated 2C-PIV measurements on densely spaced adjacent light sheet planes. An overview of the experimental setup and the data evaluation is given.