LES of Jets and Sprays Injected into Crossflow

The objective of this thesis is to numerically simulate a fluid jet injected into a crossflow of the same or another fluid, respectively. Such flows are encountered in many engineering applications in which cooling or mixing plays an important role, e.g. gas turbine combustors. The jet in crossflow (JICF) is used both for cooling and for injecting liquid fuel into the air stream prior to combustion. The numerical simulations regard three space dimensions and track also the flow dynamics by integrating the governing equations in time. The spatial and the temporal resolution are such that the large-scale flow structures are resolved. Such an approach is referred to as large eddy simulations (LES). The motion of the fuel droplets is treated by Lagrangian particle tracking (LPT) with the stochastic parcel method, along with submodels for evaporation, collision, breakup, and a novel submodel for aerodynamic four-way coupling: The particle drag is corrected depending on relative positions of the particles. Mixture fraction and temperature transport equations are solved to enable the modeling of droplet evaporation and the mixing of the gaseous fuel with ambient air. In the simulations of multiphase JICF, several computed results are shown to be inconsistent with the underlying assumptions of the LPT approach: The magnitude of the Weber numbers indicates that droplets are not spherical in large portions of the flow field in wide ranges of parameters which are relevant for gas turbine operation. The magnitude of the droplet spacing suggests that aerodynamic interaction (indirect four-way coupling) among droplets may be important. The LES with aerodynamic four-way coupling reveals significant effects compared to two-way coupling for monodisperse particles in a dense multiphase flow. For single-phase JICF, the impact of nozzle shape on the large-scale coherent structures and the mixing is studied. Effects of circular, square, and elliptic nozzles and their orientation are considered. It is demonstrated that square and elliptic nozzles with blunt orientation raise turbulence levels significantly. The scalar distribution in a cross-sectional plane is found to be single-peaked for these nozzles whereas circular and the nozzles with pointed orientation show double-peaked scalar distribution. It is the nozzles with a single-peaked distribution which are the better mixers. The differences and similarities of single- and multiphase JICF are compared, and it is demonstrated that the flow field solution for multiphase flow approaches the flow field solution of single-phase flow in the limit of small Stokes numbers

3D particle tracking velocimetry using dynamic discrete tomography

Particle tracking velocimetry in 3D is becoming an increasingly important imaging tool in the study of fluid dynamics, combustion as well as plasmas. We introduce a dynamic discrete tomography algorithm for reconstructing particle trajectories from projections. The algorithm is efficient for data from two projection directions and exact in the sense that it finds a solution consistent with the experimental data. Non-uniqueness of solutions can be detected and solutions can be tracked individually

On bias of kinetic temperature measurements in complex plasmas

The kinetic temperature in complex plasmas is often measured using particle tracking velocimetry. Here, we introduce a criterion which minimizes the probability of faulty tracking of particles with normally distributed random displacements in consecutive frames. Faulty particle tracking results in a measurement bias of the deduced velocity distribution function and hence the deduced kinetic temperature. For particles with a normal velocity distribution function, mistracking biases the obtained velocity distribution function towards small velocities at the expense of large velocities, i. e., the inferred velocity distribution is more peaked and its tail is less pronounced. The kinetic temperature is therefore systematically underestimated in measurements. We give a prescription to mitigate this type of error

Effects of aerodynamic particle interaction in turbulent non-dilute particle-laden flow

Aerodynamic four-way coupling models are necessary to handle two-phase flows with a dispersed phase in regimes in which the particles are neither dilute enough to neglect particle interaction nor dense enough to bring the mixture to equilibrium. We include an aerodynamic particle interaction model within the framework of large eddy simulation together with Lagrangian particle tracking. The particle drag coefficients are corrected depending on relative positions of the particles accounting for the strongest drag correction per particle but disregarding many-particle interactions. The approach is applied to simulate monodisperse, rigid, and spherical particles injected into crossflow as an idealization of a spray jet in crossflow. A domain decomposition technique reduces the computational cost of the aerodynamic particle interaction model. It is shown that the average drag on such particles decreases by more than 40% in the dense particle region in the near-field of the jet due to the introduction of aerodynamic four-way coupling. The jet of monodisperse particles therefore penetrates further into the crossflow in this case. The strength of the counterrotating vortex pair (CVP) and turbulence levels in the flow then decrease. The impact of the stochastic particle description on the four-way coupling model is shown to be relatively small. If particles are also allowed to break up according to a wave breakup model, the particles become polydisperse. An ad hoc model for handling polydisperse particles under such conditions is suggested. In this idealized atomizing mixture, the effect of aerodynamic four-way coupling reverses: The aerodynamic particle interaction results in a stronger CVP and enhances turbulence levels

Waveguide Bandpass Filters for Millimeter-Wave Radiometers

A fundamental requirement for most mm-wave heterodyne receivers is the rejection of the input image signal which is located close to the local oscillator frequency. For this purpose we use a bandpass filter, which for heterodyne receivers is also called an image rejection filter. In this paper we present a systematic approach to the design of a waveguide bandpass filter with a passband from 100 to 110 GHz and upper rejection bandwidth in the range from 113 to 145 GHz. We consider two non-tunable filter configurations: the first one is relatively selective with 11 sections (poles) whereas the second one is simpler with 5 sections. We used established design equations to propose an initial guess for the geometries of the filters, optimized the geometries, constructed the filters using two different milling methods, measured their transmission and reflection characteristics, and compared the measurements with numerical simulations. Measurements of both filters agree well with simulations in frequency response and rejection bandwidth. The insertion loss of the 11-pole filter is better than 10 dB and that of the 5-pole filter is better than 5 dB. The 11-pole filter has a sharper attenuation roll-off compared with the 5-pole filter. The upper out-of-band rejection is better than 40 dB up to 145 GHz for the 11-pole filter and up to 155 GHz for the 5-pole filter

Translational, rotational and vibrational temperatures of a gliding arc discharge at atmospheric pressure air

Gliding arc discharges have generally been used to generate non-equilibrium plasma at atmospheric pressure. Temperature distributions of a gliding arc are of great interest both for fundamental plasma research and for practical applications. In the presented studies, translational, rotational and vibrational temperatures of a gliding arc generated at atmospheric pressure air are investigated. Translational temperatures (about 1100 K) were measured by laser-induced Rayleigh scattering, and two-dimensional temperature imaging was performed. Rotational and vibrational temperatures (about 3600 K and 6700 K, respectively) were obtained by simulating the measured emission spectra of OH

Translational, rotational, vibrational and electron temperatures of a gliding arc discharge

Translational, rotational, vibrational and electron temperatures of a gliding arc discharge in atmospheric pressure air were experimentally investigated using in situ, non-intrusive optical diagnostic techniques. The gliding arc discharge was driven by a 35 kHz alternating current (AC) power source and operated in a glow-type regime. The two-dimensional distribution of the translational temperature (Tt) of the gliding arc discharge was determined using planar laser-induced Rayleigh scattering. The rotational and vibrational temperatures were obtained by simulating the experimental spectra. The OH A–X (0, 0) band was used to simulate the rotational temperature (Tr) of the gliding arc discharge whereas the NO A–X (1, 0) and (0, 1) bands were used to determine its vibrational temperature (Tv). The instantaneous reduced electric field strength E/N was obtained by simultaneously measuring the instantaneous length of the plasma column, the discharge voltage and the translational temperature, from which the electron temperature (Te) of the gliding arc discharge was estimated. The uncertainties of the translational, rotational, vibrational and electron temperatures were analyzed. The relations of these four different temperatures (Te>Tv>Tr >Tt) suggest a high-degree non-equilibrium state of the gliding arc discharge