Search

Records of the time-varying temperature profile at flight relevant operating conditions are acquired at the exit of a combustion chamber fitted with a staged, lean-burn fuel injector using high-speed laser induced fluorescence (LIF) at a sample rate of 10 kHz. Temperatures are estimated from the concentration dependent fluorescence of the hydroxyl (OH) radical under the assumption of local equilibrium. Beyond the time-series analysis, the acquired data is correlated with simultaneously acquired OH chemiluminescence sampled in the primary zone near the fuel injector. These analyses reveal a strong influence from the precessing vortex core, originating in the primary zone, on oscillations in the temperature profiles measured at the exit of the combustor.

This contribution describes recent efforts leading toward the successful application of particle image velocimetry (PIV) in highly luminescent flames avoiding saturation of the second frame of commonly available double shutter PIV cameras, which is usually inevitable when using their interline-transfer CCD sensors. Information on fuel placement, reaction zone and temperature field among other quantities can be provided by frequently used spectroscopic techniques. The velocity information is of equal importance in providing insight into the convective transport of reactants and their products. This flow field data can be achieved by PIV using a dual sensor camera setup splitting the optical path with a beamsplitter cube. By exposing each sensor separately in the sub-microsecond range saturation due to flame luminosity can be sufficiently reduced to allow reliable measurement in pressurized combustion.

The contribution demonstrates the applicability of volumetric PIV in a highly loaded compressor cascade at M1 = 0.60. Under these operation conditions the secondary flow structures in the cascade are dominated by a passage vortex located at the base of the blade and near the suction side. The application of volume resolving thick-sheet PIV (or tomo-PIV) near the trailing edge of the cascades blades is intended to demonstrate the techniques potential of instantaneously resolving secondary flow structures within the separation region of the cascade and its ability to derive three dimensional statistical data of fluctuations of velocity in the turbulent flow region. The report describes various aspects of the adaption of the tomographic PIV setup to the restricted access on the cascade wind tunnel. A four camera setup is was used to resolve a measuring volume of 36x24x3:5 mm from which a total of 54x74x11 vectors at a spacing of 0.64 mm in x (chord-wise direction) and a spacing of 0.32 mm in y and z could be recovered. The stability of the camera setup is documented by evaluating the image shifts of stationary features due to vibrations of the wind tunnel. Three dimensional reconstruction of the imaged particle volume is achieved with the maximum entropy reconstruction technique (MENT, [2, 6]) and validated against synthetic data as well as conventional MLOS-SMART reconstruction. The reconstruction quality of experimental data is documented by plots of the reconstructed intensity along volume depth. The extraction the three-dimensional displacement field relies on multi-resolution, 3-D correlation processing with iterative volume deformation. The recovered three-dimensional velocity fields are compared at selected planes with (thin-sheet) stereo PIV data.

Three-dimensional, unsteady flow fields of a pitching low aspect ratio wing were measured by tomographic PIV [6] in air. The analysis of the vortex dominated flow field provides a deeper understanding of vortex interaction and three-dimensionality of the low Reynolds number (Re ? 10,000) flow. In order to recover the complete flow field the measurement set-up was designed to be traversable. A high spatial resolution and a large volume thickness could be achieved by a set of high sensitive sCMOS cameras. A specific feature of this campaign was to locate the measurement domain directly above the flat-plate-wing surface. Evaluation of the measurement data is performed by DLR in-house software. A selection of measurement results of this highly complex flow is presented in this paper.

A demonstration experiment and first assessment of a dual-volume and four-pulse tomo PIV system operating with two pairs of perpendicularly polarized laser light pulses and respective polarizing filters for separation of the scattered particle light is presented. The polarizing filters are placed in front of eight sCMOS double-frame cameras forming two independent tomo PIV systems consisting of four cameras each. The whole system is able to provide data-sets of four subsequent particle image distributions on separate frames with uncorrelated ghost particles in the reconstructed measurement volumes. The system can be applied in a four pulse mode to low and high speed air flows measuring particle displacements with enhanced accuracy in the same volume or alternatively in a mode in which particle doubleimages in two neighbouring measurement volumes are captured simultaneously or with any time difference. The used principle to separate the two tomo PIV systems relies on the fact that for chosen seeding particles, in our experiment small spherical liquid droplets of DEHS in the order of 1 ?m diameter, the mode of polarization does not change significantly during light scattering. The flow of investigation is a part of the separation and shear region of a turbulent boundary layer flow downstream of a backward facing step. After tomographic reconstruction of the volumetric particle distributions from four subsequently captured images of both systems a multi-frame pyramid-like evaluation has been applied with a 3D correlation technique. An assessment of sources of uncertainty using the proposed set-up within a wind-tunnel environment concerning vibrations and signal separation and a first comparison of the improvements of the velocity vector estimation based on multi-frame correlation with standard evaluation approaches is given.