Characterization of the flow field inside a Ranque-Hilsch vortex tube using filtered Rayleigh scattering, Laser-2-Focus velocimetry and numerical methods

The design process of aero engines as well as stationary gas turbines is largely dominated by the cost and time efficient methods of Computational Fluid Dynamics (CFD). Over the past decade the CFD solver TRACE for Favre-averaged compressible Navier-Stokes equations has been developed at the Institute of Propulsion Technology and has been adopted for research as well as industrial applications. In the context of turbomachinery design, reliable modeling of the turbulent flow phenomena involved is a crucial aspect and one of the major subjects of numerical research in fluid dynamics. Novel approaches accounting for the anisotropy of the Reynolds stress tensor promise an improved accuracy in the simulation of industrially relevant configurations. One key aspect in the development strategy of turbulence models is the direct comparison of computational results with validation data produced from appropriate experimental setups with well-defined geometries and boundary conditions. The Ranque-Hilsch vortex tube (RHVT) was chosen in this respect due to its simple geometry with no moving parts on the one hand and its nevertheless complex 3D flow features on the other hand. To provide suitable experimental data the filtered Rayleigh scattering technique extended by the method of frequency scanning (FSM-FRS) was chosen to characterize the RHVT's averaged flow field, since it is capable of simultaneously providing planar information on temperature, pressure and flow field velocity (through the Doppler shift). As the reconstruction of a three component velocity field from FSM-FRS data would require the measurement plane to be observed from three independent directions, the point-wise Laser-2-Focus (L2F) technique is applied to provide 2C velocity profiles at discrete positions downstream from the cold exit

EFFECT OF LEADING EDGE EROSION ON THE PERFORMANCE OF TRANSONIC COMPRESSOR BLADES

Within this paper an experimental and numerical investigation about the effect of leading edge erosion at transonic blades was performed. The measurements were carried out on a linear blade cascade in the Transonic Cascade Wind Tunnel of DLR in Cologne at two operating points with an inflow Mach number of 1.05 and 1.12. The numerical simulations were performed by ANSYS Germany. The type and specifications of the erosion for the study were derived from real engine blades and applied to the leading edges of the experimental cascade blades using a waterjet process as well as detailed modelled and meshed within the numerical setup. Numerical simulations and extensive wake measurements were carried out on the cascades to evaluate aerodynamic performance. The increase in losses was quantified to be 4 percent as well as a reduction of deflection and pressure rise was detected at both operating points

The Current Gap Between Design Optimization and Experiments for Transonic Compressor Blades

The successful design of compressor blades by numerical optimization relies on accurate CFD-RANS solvers that are able to capture the general performance of a given design candidate. However, this is a difficult task to achieve in transonic flow conditions, where the flow is dominated by inherently unsteady shock effects. In order to assess the current gap between numerics and experiments, the DLR has tested the recently optimized Transonic Cascade TEAMAero at the Transonic Cascade Wind Tunnel. The tests were performed at a Mach number of 1.2 and with inflow angles between 145-147°. The results indicate satisfactory agreement across the expected working range, over which the cascade losses were consistently predicted within 3-6% error. However, some key differences are observed in the details of the wake and on the performance near the endpoints of the working range. This comparison helps validate the design process, but also informs its constraints based on the limitations of CFD-RANS solvers

Particle image velocimetry measurement in highly luminescent flames

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

Visualization and PIV Measurements of the Transonic Flow around the Leading Edge of an eroded Fan Airfoil

The influence of blade deterioration on the time-averaged and instantaneous flow around a fan airfoil is investigated by PIV, schlieren imaging and high speed shock-shadowgraphs in a transonic cascade windtunnel. In addition to a global characterization of the time-averaged flow using PIV, the instantaneous passage shock position was extracted from single shot PIV measurements by matching the tracer velocity across the normal shock with an exponential fit. From this the relaxation time of paraffine-ethanol seeding was verified to have a response time in the sub-microsecond range. The instantaneous shock positions, defined by the onset of the exponential decay, are assigned to a probability density distribution in order to obtain the average position and the width of fluctuations. Finally the distributions from the generic blunted edge geometry and the unaffected reference geometry are compared under near stall and under choked conditions. At similar back-pressure conditions the passage shock of the blunted edge geometry is located further upstream in comparison to the reference geometry. The amplitude of shock fluttering is significantly higher for the blunted edge. In order to extract the frequency range of the shock motion the direct shadow of the shock wave was tracked in high-speed shadowgraphs and spectrally analyzed. A comparison of the power spectra of the shock motion indicates that under choked conditions the blunted edge geometry exhibits a more continuous spectrum with a broader frequency range. The paper discusses details on the experimental implementation such as a specifically designed purged light sheet probe

Development of an optical multi-purpose sensor using filtered scattered light

We present the development of an optical sensor head which consists of a confocal backscatter arrangement of lenses and light guiding fibers. Using a molecular absorption cell which contains iodine vapor different scatter techniques are employed to obtain multiple flow field parameters in different media like gas and liquids. For velocity measurements filtered Mie scattering is used and validation experiments with a free jet yield an absolute error of 2m/s. In gaseous flows the sensor applies filtered Rayleigh scattering to determine temperature values with an accuracy of +/-3K and pressure values which show a systematic deviation of -2% from reference measurements. Finally filtered Brillouin scattering is introduced for measuring the temperature in liquids like tap water. This technique agrees within 1K when compared to a conventional thermo element measurement