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This thesis describes a method of determining the recombination rate coefficient experimentally, which does not depend on a specific model of the recombination process. With this method established, results are presented for the recombination rate coefficient measurements at 44.6 Torr.

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Theoretical investigations of electron-inert gas scattering are challenging because of the complex target structure. The electron-Helium system has been the most studied both at low and intermediate energies by sophisticated R-matrix and coupled channels methods. For the other inert gases, few calculations have been attempted at the same level sophistication. One problem is that general target-structure codes provide different forms of wave functions that must be interfaced with the scattering equations. The theoretical work presented in this thesis is based on the momentum-space coupled channels equations. For this formalism only one and two-electron atoms have been studied and purpose-built routines, specific to these atoms, have been developed. For the inert gases however such a task is much more formidable and a more practical approach is to use existing structure codes that have taken several man-years to develop. The framework of this thesis comprises of two parts. In the first part we discuss the need for, and the way to, modify the existing close-coupling code developed by Berge & Stelbovics in order to interface with other atomic structure packages in the literature. Two mainstream packages, an atomic structure package of Charlotte Froese Fischer and an atomic structure of Alan Hibbert are discussed. Methods to extract the wave functions for Helium and Neon targets using Hibbert's package are given. In the second part, various options and strategies for the calculation of the target structure, including frozen-core and configuration-interaction wave functions, using analytic Slater, Laguerre or numerical orbitals are considered for the Helium target. Hibbert's structure code wave functions are shown to be correctly interfaced into our momentum-space coupled channels code. The pros and cons of the various target structure descriptions are given and applied for lowenergy elastic and inelastic scattering of electron from Helium.

The objective of this work is to assess the capabilities of two modeling approaches, Reynolds-Averaged Navier Stokes (RANS) and Large Eddy Simulation (LES), to investigate turbulent buoyant helium plumes for the purpose of studying the dynamics of buoyancy driven plumes in the near source region. In this case, the velocity and plume concentration of the plume are predicted. RANS was applied to a model planar wall plume and predictions were compared to experimental data gathered in the self-preserving region. It was also used to model an axisymmetric plume with results compared to experimental data gathered in the near source region. The Simple Gradient Diffusion Hypothesis (SGDH) and the Generalized Gradient Diffusion Hypothesis (GGDH) were implemented in the standard k-epsilon model for the planar plume. The CFX buoyancy model in the commercial code, as well as SGDH and GGDH, were applied in predicting the characteristics of the axisymmetric plume. For the planar plume, good agreement with the experimental data was found when the SGDH approach was used. Both spreading rates and maximum values of velocity and mixture fraction were well predicted. Larger discrepancies between predictions and data were noticed with the GGDH model. Both models showed minimal sensitivity to the model constant, C3epsilon. In the case of the axisymmetric plume, all models were highly sensitive to the buoyancy constant. The GGDH model yielded the best results. In particular very good agreement was achieved for the radial profiles of the streamwise velocity. The axisymmetric plume was also simulated using LES. Initially the Smagorinsky constant was set at the default value of 0.2 and the grid size was varied to determine the dependency of the time averaged and rms quantities for velocity and plume concentration on the grid spacing. The two finest meshes tested produced similar time averaged values for velocity and plume concentration indicating that they were less sensitive to grid spacing. These time averaged results also showed that values for streamwise velocity and plume concentration along the central axis were significantly over predicted compared to the experimental results. Time-averaged centerline streamwise velocity plots showed that the streamwise velocity from the numerical results continue to increase while the experimental values begin to decrease after 0.69 m. This indicated that the transition from laminar-toturbulence was poorly predicted. Rms quantities remained sensitive to the mesh spacing even at the finest mesh tested; however the accuracy of the rms results appeared to improve as the grid was refined. Setting the Smagorinsky constant to zero produced a more accurate time averaged predictions but the rms quantities worsened.

The objective of this work is to assess the capabilities of two modeling approaches, Reynolds-Averaged Navier Stokes (RANS) and Large Eddy Simulation (LES), to investigate turbulent buoyant helium plumes for the purpose of studying the dynamics of buoyancy driven plumes in the near source region. In this case, the velocity and plume concentration of the plume are predicted. RANS was applied to a model planar wall plume and predictions were compared to experimental data gathered in the self-preserving region. It was also used to model an axisymmetric plume with results compared to experimental data gathered in the near source region. The Simple Gradient Diffusion Hypothesis (SGDH) and the Generalized Gradient Diffusion Hypothesis (GGDH) were implemented in the standard k-epsilon model for the planar plume. The CFX buoyancy model in the commercial code, as well as SGDH and GGDH, were applied in predicting the characteristics of the axisymmetric plume. For the planar plume, good agreement with the experimental data was found when the SGDH approach was used. Both spreading rates and maximum values of velocity and mixture fraction were well predicted. Larger discrepancies between predictions and data were noticed with the GGDH model. Both models showed minimal sensitivity to the model constant, C3epsilon. In the case of the axisymmetric plume, all models were highly sensitive to the buoyancy constant. The GGDH model yielded the best results. In particular very good agreement was achieved for the radial profiles of the streamwise velocity. The axisymmetric plume was also simulated using LES. Initially the Smagorinsky constant was set at the default value of 0.2 and the grid size was varied to determine the dependency of the time averaged and rms quantities for velocity and plume concentration on the grid spacing. The two finest meshes tested produced similar time averaged values for velocity and plume concentration indicating that they were less sensitive to grid spacing. These time averaged results also showed that values for streamwise velocity and plume concentration along the central axis were significantly over predicted compared to the experimental results. Time-averaged centerline streamwise velocity plots showed that the streamwise velocity from the numerical results continue to increase while the experimental values begin to decrease after 0.69 m. This indicated that the transition from laminar-toturbulence was poorly predicted. Rms quantities remained sensitive to the mesh spacing even at the finest mesh tested; however the accuracy of the rms results appeared to improve as the grid was refined. Setting the Smagorinsky constant to zero produced a more accurate time averaged predictions but the rms quantities worsened.

Thesis (Ph. D.)--University of Colorado, 1960. / eContent provider-neutral record in process. Description based on print version record. Bibliography: p. 153-158.

Thesis (Ph. D.)--Florida State University, 2003. / Advisor: Dr. James S. Brooks, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed Sept. 29, 2003). Includes bibliographical references.

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