245 research outputs found
LES of Jets and Sprays Injected into Crossflow
No full textThe 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
Fast-ion diagnostic in fusion plasmas by velocity-space tomography
No full textVelocity distribution functions of ions in high-performance fusion plasmas can deviate substantially from simple Maxwellian distributions. They can be strongly anisotropic or have local maxima at velocities larger than the thermal velocities. The distribution functions of the fast ions in the plasma are often the key to understanding heating, current drive, and various plasma instabilities. However, the distribution functions are unfortunately not always predictable for plasmas with instabilities. These often cause fast-ion transport that is not fully understood. The measurement of fast-ion velocity distribution functions is therefore crucial to operate high-performance plasmas for eventually harvesting energy. However, until now this measurement could not be done. The velocity-space tomography approach put forward in this thesis allows this measurement, providing a new meeting ground between theory and observation. This allows studies of the behavior of fast ions in fusion plasmas at an unprecedented level of detail.Traditionally fast-ion measurements are presented in terms of spectra of a measured quantity that is particular to each diagnostic, e.g. the power density of radiation, photon or particle count rates, or the times-of-flight in a detector. These spectra in diagnostic measurables are then compared with synthetic spectra based on numerical simulations. The results of such comparisons are often hard to interpret since it is not immediately clear how to attribute any discrepancies between measurements and predictions to the fast-ion distributions. To exploit the rich information about fast ions contained in the spectra by traditional procedures, we need to consider hundreds of measurements, keeping in mind nuisance parameters and the complicated relationships between the measurements and velocity space. Borrowing from usual position-space tomography, velocity-space tomography provides a way to process this wealth of information at once. It provides a 2D image that is straightforward to interpret, is the best useful fit to hundreds of simultaneous measurements, combines data from different diagnostics, shows the fundamental quantity of interest rather than quantities of secondary interest, and accounts for nuisance parameters.Until today measurements by the diagnostics fast-ion D-alpha spectroscopy, collective Thomson scattering, neutron emission spectrometry, gamma-ray spectroscopy, and fast-ion loss detectors installed at the tokamaks ASDEX Upgrade, JET, MAST, DIII-D, and EAST have been interpreted using velocity-space tomography. We have measured strongly non-Maxwellian fast-ion velocity distribution functions in plasmas heated by neutral beam injection and by electromagnetic wave heating in the ion cyclotron range of frequencies. The interaction of energetic particles with plasma instabilities has revealed strong selectivity in velocity space. This has filled gaps in our yet incomplete understanding of the physics of the redistribution of fast ions due sawteeth, neoclassical tearing modes, and Alfvén eigenmodes. Fast-ion densities have been measured for the first time, and first movies following the time evolution of the fast-ion velocity distribution function in plasmas with instabilities have been presented. Velocity-space tomography based on the foreseen set of fast-ion diagnostics at the next-step fusion device ITER has revealed that co-passing and counter-passing particles, which are two out of three major classes of particles in tokamaks, cannot be told apart. An additional detector is now proposed that would allow this distinction.Velocity-space tomography has also revealed the density, drift velocity, and anisotropic temperatures of the thermal ions, has deblurred measurements with fast-ion loss detectors, and has provided a new approach to interpret gamma-ray measurements of runaway electrons. This thesis describes the development of velocity-space tomography and demonstrates its utility for the interpretation of fast-ion measurements.<br/
W-band waveguide bandpass filter with E-plane cut
No full textIn this paper, we present a design and measurements of a five-section bandpass filter with a passband from 96 to 106 GHz. The insertion loss is less than 1.4 dB in the passband, and the rejection is better than 40 dB in the range from 115 to 142 GHz. We use transmission line coupling theory based onTchebyscheff’s synthesis in order to provide an initial guess for the geometrical parameters of the filter such as cavity lengths and coupling widths. The filter is manufactured from brass in two halves in the E-plane cut topology. The S-parameters of the filter are measured and compared with the simulations.The measured passband insertion loss is approximately 0.4 dB worse than in the simulation, and the measured passband width is approximately 3.4% narrower. The measured filter attenuation roll-off corresponds well to the simulation. We also compare our S-parameter measurements of the E-plane filter with corresponding measurements of a very similar H-plane filter. The transmission and reflection characteristics of the E-plane filter are better than those of the H-plane filter
LES of Jets and Sprays Injected into Crossflow [Elektronisk resurs]
No full textThe 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
Consistency Issues of Lagrangian Particle Tracking Applied to a Spray Jet in Crossflow
No full textNumerical simulations are performed for multiphase jets in crossflow. The flow solver uses an Eulerian/Lagrangian approach. Turbulence in the gas phase is modeled in the framework of large eddy simulation. The dispersed phase is handled using Lagrangian particle tracking. The model assumptions of solvers for Lagrangian particle tracking are critically assessed for typical flow conditions of spray jets in crossflow. The droplets are assumed to be spherical and isolated. It is shown that several model assumptions are apparently inconsistent in larger portions of the flow field. Firstly, average Weber numbers can be so large that the model assumption to regard droplets as spherical is questionable, not only near the nozzle, but also in the far-field. Secondly, the average droplet spacing can be so low that droplets directly interact with each other, again also in the far-field. Thirdly, the average Stokes numbers in the jet region can be so large that the phase coupling between the dispersed and continuous phase is weak. Some remedies to these deficiencies are proposed. (C) 2006 Elsevier Ltd. All rights reserved
NO<sub>x</sub> reduction by ozone injection and direct plasma treatment
No full textNOx reduction by ozone injection and direct plasma treatment is investigated for different process parameters in a 6 m long serpentine reactor. Several aspects including the role of mixing scheme, water vapours, steep temperature gradient and time dependet NOx levels are taken into consideration. The process chemistry is monitored by FTIR, chemiluminiscence and absorbtion spectroscopy. The kinetic mechanism is also investigated in 3D simulations
Bi-Maxwellian, slowing-down, and ring velocity distributions of fast ions in magnetized plasmas
No full textWe discuss analytical fast-ion velocity distribution functions which are useful for basic plasma modelling as illustrated for the tokamak ITER. The Maxwellian is by far the most widespread model for ions and electrons in tokamaks and stellarators. The bi-Maxwellian and the drifting (bi-)Maxwellian are extensions allowing for anisotropy and bulk plasma flow, respectively. For example, fast ions generated by wave heating in the ion cyclotron range of frequencies are often described by bi-Maxwellians or so-called tail temperatures. The ring distribution can serve as a basic building block for arbitrary distributions or as a bump-on-tail in stability studies. The isotropic slowing-down distribution is a good model for fusion α-particles. The anisotropic slowing-down distribution occurs for anisotropic particle sources as is typical for neutral beam injection. We physically motivate these distribution functions and present analytical models in various coordinate systems commonly used by theorists and experimentalists. We further calculate 1D projections of the distribution functions onto a diagnostic line-of-sight to gain insight into measurements relying on the Doppler shift
On bias of kinetic temperature measurements in complex plasmas
No full textThe 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 withnormally distributed random displacements in consecutive frames. Faulty particle tracking results in a measurement bias of the deduced velocity distribution function and hence the deduced kinetictemperature. 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
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