Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 32-34). Also available via the Internet.

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This thesis details the analyses of high resolution visible, infrared, and microwave spectra of gas phase ionic and neutral molecules. The visible and infrared spectra of several ions were measured using velocity modulation spectrometers developed in the present work. In each case the ions were generated in an electric discharge plasma. The microwave spectrum of vinyl iodide, CH₂=CHI, has been extensively measured and analysed. Visible Spectroscopy using Velocity Modulation: The (6,1) and (13,6) vibrational bands of the A²IIu — X²Σg⁺ electronic transition of N₂⁺ have been recorded in absorption at Doppler limited resolution. The rotational fine structure was fitted by least squares to standard expressions. The rotational and translational temperatures have been measured and indicate an equilibrium between translational and rotational motion in the He/N₂ plasma. Infrared Spectroscopy using Velocity Modulation: The infrared spectra of HCO⁺, H₃⁺ , HeD⁺, and N₂⁺ have been observed. Two previously unmeasured lines of the v₃ band of HCO⁺ and several previously measured lines of the v₂ band of H₃⁺ were used to adjust the spectrometer for maximum sensitivity. A new line in the rotational fine structure of the v = 1 ← 0 band of HeD⁺ was analysed using standard expressions. The rotational fine structure of the (2,5) vibrational band of the A²IIu — X²Σg⁺ electronic transition of N₂⁺ has been recorded and analysed in the region 2125 - 2205 cm⁻¹. Using the vibrational origin, T₂,₅ , obtained from this analysis combined with the origins, T₆,₁ and T₁₃,₆, obtained from the analyses of the visible spectra of N₂⁺ , it was possible to determine third order equilibrium vibrational coefficients for both the X²Σg⁺ and A²IIu states. Microwave Spectroscopy: The microwave spectrum of vinyl iodide, in its ground and first excited vibrational states, has been measured in the frequency range 20 - 108 GHz. The spectrum contains strong a-type transitions and very weak b-type transitions; all contain ¹²⁷I quadrupole hyperfine structure, with several large perturbations. A procedure specially devised for analysis of such spectra, which takes advantage of the perturbations, was applied to produce accurate values of constants that are otherwise unobtainable, and have permitted assignment of some b-type transitions. Also, as a result of this procedure, it was possible to measure both components of the dipole moment with relative ease. The centrifugal distortion constants and inertial defects have been compared with those calculated from a published harmonic force field, modified for the out-of-plane vibrations. A partial structure has been obtained. / Science, Faculty of / Chemistry, Department of / Graduate

The spout formed at the free surface of a gas-stirred liquid has received little attention even though it has both theoretical and practical significance. In steelmaking ladles, for example, the spout is the site of strong metal-slag-air mixing which affects: the kinetics of reactions at the slag-metal interface, the absorption of oxygen by the bath and the temperature drop of the bath. Notwithstanding its importance, the spout is usually neglected in flow models of gas-stirred baths because it has not been characterized quantitatively; assumption of a flat top surface, however, reduces the accuracy of the velocity and kinetic energy predictions, particularly close to the spout region. Thus in this study, the spout of upwardly injected gas jets in water was characterized experimentally in terms of gas fraction, bubble frequency and axial velocity distributions. The measurements were made with a two-element electroresistivity probe coupled to a microcomputer. Special hardware and software were developed to analyze the signals generated by contact of the bubbles with the sensor, in real time, for the turbulent flow conditions prevailing in the jet plume and spout. Correlations of the gas fraction with axial and radial position for different gas flow rates have been established from the measurements. The dimensions of the spout were obtained from time-exposure photographs; when compared with the gas fraction measurements, the spout boundary always corresponded to values ranging from 0.82 to 0.86. The radial profiles of bubble frequency at different levels in the spout have a bell shape; the bubble frequency decreases with increasing height. The velocity of the bubbles in the spout drops linearly with increasing axial position. Measurements of bath velocity near the walls of the vessel were also conducted with a laser doppler velocimeter for comparison to model predictions. The gas fraction data obtained for the spout then were incorporated into a mathematical model of turbulent recirculatory flow with which predictions of velocity, kinetic energy and effective viscosity in the bath were made. Predictions of the model were compared with the experimental measurements as well as with predictions assuming a flat bath surface (no spout); and the importance of incorporating the spout thus was demonstrated. The variation of the total kinetic energy in the spout with gas flow rate was determined. The energy increased with flow rate, as expected, but at a critical value, the rate of increase abruptly rose. Based on photographs taken of the gas/liquid dispersion, the increased spout kinetic energy appears to be related to the location of bubble break-up and possibly to gas channeling. At lower flow rates below the critical value, the bubble break-up occurs relatively close to the nozzle, whereas at higher flow rates bubble disintegration is nearer to the surface. At the lower flow rates the gas/liquid interaction was maximum which promoted the gas/liquid momentum transfer. Moreover, at the higher flow rates the gas dispersion was observed intermittently to be a continuous chain of large envelopes which could permit a fraction of the gas to channel through the bath for a considerable distance. The channeling phenomenon could lead to an inefficient gas/liquid energy transfer resulting in a reduced efficiency of bath mixing and enhanced energy release at the surface. These results can explain the observations of previous investigators who found that beyond a critical gas injection rate, the rate of decrease of mixing time with flow rate decreased. The metallurgical consequences of the spout and its influence on the flow field, especially in the near-surface region, have been highlighted, thus unveiling the practical bearing of the spout on the gas injection process. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

abstract: This dissertation studies two outstanding microscale fluid mechanics problems: 1) mechanisms of gas production from the nanopores of shale; 2) enhanced mass flow rate in steady compressible gas flow through a micro-conduit. The dissertation starts with a study of a volumetric expansion driven drainage flow of a viscous compressible fluid from a small capillary and channel in the low Mach number limit. An analysis based on the linearized compressible Navier-Stokes equations with no-slip condition shows that fluid drainage is controlled by the slow decay of the acoustic wave inside the capillary and the no-slip flow exhibits a slip-like mass flow rate. Numerical simulations are also carried out for drainage from a small capillary to a reservoir or a contraction of finite size. By allowing the density wave to escape the capillary, two wave leakage mechanisms are identified, which are dependent on the capillary length to radius ratio, reservoir size and acoustic Reynolds number. Empirical functions are generated for an effective diffusive coefficient which allows simple calculations of the drainage rate using a diffusion model without the presence of the reservoir or contraction. In the second part of the dissertation, steady viscous compressible flow through a micro-conduit is studied using compressible Navier-Stokes equations with no-slip condition. The mathematical theory of Klainerman and Majda for low Mach number flow is employed to derive asymptotic equations in the limit of small Mach number. The overall flow, a combination of the Hagen-Poiseuille flow and a diffusive velocity shows a slip-like mass flow rate even through the overall velocity satisfies the no-slip condition. The result indicates that the classical formulation includes self-diffusion effect and it embeds the Extended Navier-Stokes equation theory (ENSE) without the need of introducing additional constitutive hypothesis or assuming slip on the boundary. Contrary to most ENSE publications, the predicted mass flow rate is still significantly below the measured data based on an extensive comparison with thirty-five experiments. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2019

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The ionization and thermal balances are considered for a gas that is ionized by a dilute radiation field, taking into account the diffuse ionizing radiation produced by the gas. A number of models are constructed in which the electron temperature and the ionization of the elements H, He, C, N, 0, Ne, and Mg are determined for optically thin and optically thick gases ionized by ultraviolet synchrotron radiation under different conditions. Conclusions are then drawn about the general characteristics of ionization by synchrotron radiation. It is shown that, in an optically thin gas, because of the insensitive frequency- dependence of synchrotron radiation each element occupies a number of different stages of ionization at any one point in the gas. It is also shown that in an optically thick gas the heavy elements remain ionized to much greater distances from the source than hydrogen and helium, and that the gas becomes thermally unstable when H and He have become almost completely neutral. In addition, observations of the emission -line intensities of the Crab Nebula are compared with a model of this object. Considerable disagreement exists between the observed and predicted intensities, and possible reasons for the discrepancy are discussed.

It is well established that rarefied flows cannot be properly described by traditional hydrodynamics, namely the Navier-Stokes equations for gas flows, and the Fourier’s law for heat transfer. Considering the significant advancement in miniaturization of electronic devices, where dimensions become comparable with the mean free path of the flow, the It is well established that rarefied flows cannot be properly described by traditional hydrodynamics, namely the Navier-Stokes equations for gas flows, and the Fourier's law for heat transfer. Considering the significant advancement in miniaturization of electronic devices, where dimensions become comparable with the mean free path of the flow, the study of rarefied flows is extremely important. This dissertation includes two main parts. First, we look into the heat transport in solids when the mean free path for phonons are comparable with the length scale of the flow. A set of macroscopic moment equations for heat transport in solids are derived to extend the validity of Fourier's law beyond the hydrodynamics regime. These equations are derived such that they remain valid at room temperature, where the MEMS devices usually work. The system of moment equations for heat transport is then employed to model the thermal grating experiment, recently conducted on a silicon wafer. It turns out that at room temperature, where the experiment was conducted, phonons with high mean free path significantly contribute to the heat transport. These low frequency phonons are not considered in the classical theory, which leads to failure of the Fourier's law in describing the thermal grating experiment. In contrast, the system of moment equations successfully predict the deviation from the classical theory in the experiment, and suggest the importance of considering both low and high frequency phonons at room temperature to capture the experimental results. In the second part of this study, we look into the gas-surface interactions for conventional gas dynamics when the gas flow is rarefied. An extension to the well-known Maxwell boundary conditions for gas-surface interactions are obtained by considering velocity dependency in the reflection kernel from the surface. This extension improves the Maxwell boundary conditions by providing an extra free parameter that can be fitted to the experimental data for thermal transpiration effect in non-equilibrium flows. The velocity dependent Maxwell boundary conditions are derived for the Direct Simulation Monte Carlo (DSMC) method and the regularized 13-moment (R13) equations for conventional gas dynamics. Then, a thermal cavity is considered to test and study the effect of these boundary conditions on the flow formation in the slip and early transition regime. It turns out that using velocity dependent boundary conditions allows us to change the size and direction of the thermal transpiration force, which leads to marked changes in the balance of transpiration forces and thermal stresses in the flow. / Graduate