The Earth's middle atmosphere at about 10-100 km has shown a substantial sensitivity to human activities. First, the ozone layer has been reduced since the the early 1980s due to man-made emissions of halogenated hydrocarbons. Second, the middle atmosphere has been identified as a region showing clear evidence of climate change due to increased emissions of greenhouse gases. While increased CO2 abundances are expected to lead to a warmer climate near the Earth's surface, observations show that the middle atmosphere has been cooling by up to 2-3 degrees per decade over the past few decades. This is partly due to CO2 increases and partly due to ozone depletion. Predicting the future development of the middle atmosphere is problematic because of strong feedbacks between temperature and ozone. Ozone absorbs solar ultraviolet radiation and thus warms middle atmosphere, and also, ozone chemistry is temperature dependent, so that temperature changes are modulated by ozone changes. This thesis examines the middle atmospheric response to a doubling of the atmospheric CO2 content using a coupled chemistry-climate model. The effects can be separated in the intrinsic CO2-induced radiative response, the radiative feedback through ozone changes and the response due to changes in the climate of the underlying atmosphere and surface. The results show, as expected, a substantial cooling throughout the middle atmosphere, mainly due to the radiative impact of the CO2 increase. Model simulations with and without coupled chemistry show that the ozone feedback reduces the temperature response by up to 40%. Further analyses show that the ozone changes are caused primarily by the temperature dependency of the reaction O+O2+M->O3+M. The impact of changes in the surface climate on the middle atmosphere is generally small. In particular, no noticeable change in upward propagating planetary wave flux from the lower atmosphere is found. The temperature response in the polar regions is non-robust and thus, for the model used here, polar ozone loss does not appear to be sensitive to climate change in the lower atmosphere as has been suggested recently. The large interannual variability in the polar regions suggests that simulations longer than 30 years will be necessary for further analysis of the effects in this region. The thesis also addresses the long-standing dilemma that models tend to underestimate the ozone concentration at altitudes 40-75 km, which has important implications for climate change studies in this region. A photochemical box model is used to examine the photochemical aspects of this problem. At 40-55 km, the model reproduces satellite observations to within 10%, thus showing a substantial reduction in the ozone deficit problem. At 60-75 km, however, the model underestimates the observations by up to 35%, suggesting a significant lack of understanding of the chemistry and radiation in this region.

With the depletion of petroleum based fuels and the corresponding concerns of national energy security issues, natural gas as an alternative fuel in IC engine applications has become an attractive option. Natural gas requires minimum mixture preparation, and is chemically stable, both of which make it a suitable fuel for homogeneous charged compression ignition (HCCI) engines. Compared to petroleum based fuels, natural gas produces less green-house emissions. However, natural gas is hard to auto-ignite and therefore requires a higher compression ratio, some amount of intake heating, or some type of pre-ignition. In addition, natural gas usually has large differences in fuel composition from field to field, which adds more uncertainties for engine applications. The current study determines the auto-ignition characteristics, engine performance, and nitric oxides emissions as functions of major operating parameters for a natural gas fueled HCCI engine, and determines differences relative to gasoline fueled HCCI engines which have been studied for many years. These tasks have been done using both zero- and multi-dimensional engine simulations. By zero-dimensional simulation, the effects of varying equivalence ratios, engine speeds, compression ratio, EGR level, intake pressure and fuel compositions are determined and analyzed in detail. To be able to account for the in-cylinder inhomogeneous effect on the HCCI combustion, multi-zone models coupled with cold-flow CFD simulations are employed in addition to the single-zone model. The effects of non-homogeneous temperature and equivalence ratio stratification on the ignition timing, combustion phasing, and emissions formation have been studied and discussed. Finally, the preliminary two-dimensional axial-symmetric CFD simulations have been conducted to study the in-cylinder temperature and the species distributions, which provide better visualization of the natural gas auto-ignition process.

No description available.

The photopolymerization of multifunctional acrylates leads to the formation of a complex and insoluble network due to cross-linking. This characteristic is a useful property for stereolithography applications, where solid parts of the desired shape are cured using a pre-determined energy exposure profile. Traditionally, the required energy exposure is determined using a critical energy--depth of penetration, or Ec--Dp, model. The parameters Ec and Dp, are usually fit to experimental data at a specific resin composition and cure intensity. As a result, since the Ec--Dp model does not explicitly incorporate cure kinetics, it cannot be used for a different set of process conditions without first obtaining experimental data at the new conditions. Thus, the Ec--Dp model does not provide any insight when a new process needs to be developed, and the best processing conditions are unknown. The kinetic model for multifunctional acrylate photopolymerization presented here is based on a set of ordinary differential equations (ODE), which can be used to predict part height versus exposure condition across varying resin compositions. Kinetic parameter information used in the model is obtained by fitting the model to double bond conversion data from Fourier Transform Infrared Spectroscopy (FTIR) measurements. An additional parameter, the critical conversion value, is necessary for determining the formation of a solid part of the desired height. The initial rate of initiation, Ri, combines all the factors that impact part height, and therefore, it is an important quantity that is required in order to find the critical conversion value. The critical conversion value is estimated using the Ri and Tgel value from microrheology measurements. Information about network connectivity, which can be used to get properties such as molecular weight, cannot be derived from models using traditional mass-action kinetics for the cross-linking system. Therefore, in addition to modeling the reaction using the ODE based model, the results from a statistical model based on Kinetic Monte Carlo (KMC) principles are also shown here. The KMC model is applicable in situations where the impact of chain length on the kinetics or molecular weight evolution is of interest. For the present project, the detailed information from network connectivity was not required to make part height predictions, and the conversion information from the ODE model was sufficient. The final results show that the kinetic ODE model presented here, based on the critical conversion value, captures the impact of process parameters such as initiator concentration, light intensity, and exposure time, on the final part height of the object. In addition, for the case of blanket cure samples, the part height predictions from the ODE model make comparable predictions to the Ec--Dp model. Thus, the ODE model presented here is a versatile tool that can be used to determine optimum operating conditions during process development.

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (p. 129-136).

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 225-227). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 225-227). Also available on the Internet.

Thesis (Ph. D.)--Institute of Paper Chemistry, 1982. / Bibliography: leaves 77-81.