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Master of Science

Thesis (MEng.) -- Stellenbosch University, 1988. / ENGLISH ABSTRACT: In this report various mathematical models for the thermal evaluation of evaporative coolers and condensers are presented. These models range from the exact model based on the work by Poppe [84P01] to the simplified logarithmic models based on the work of McAdams [54Mcl] and Mizushina et al. [67MI1], [68MI1]. Various computer programs were written to perform rating and selection calculations on cross-flow and counterflow evaporative coolers and condensers. Experimental tests were conducted on a cross-flow evaporative cooler to determine the governing heat and mass transfer coefficients. The experimentally determined coefficients were cqrrelated and these correlations are compared to the existing correlations. The two-phase pressure drop across the tube bundle was also measured and a correlation for two-phase pressure drop across a tube bundle is presented.

Thesis. 1976. Sc.D.--Massachusetts Institute of Technology. Dept. of Chemical Engineering. / Microfiche copy available in Archives and Science. / Bibliography: leaves 225-237. / Sc.D.

Aromatic hydrocarbons constitute an important fraction (~20%) of total volatile organic compounds (VOCs) in the urban atmosphere. A better understanding of the aromatic oxidation and its association in urban and regional ozone and organic aerosol formation is essential to assess the urban air pollution. This dissertation consists of two parts: (1) theoretical investigation of the toluene oxidation initiated by OH radical using quantum chemical and kinetic calculations to understand the mechanism of O3 and SOA precursors and (2) experimental investigation of atmospheric new particle formation from aromatic acids. Density functional theory (DFT) and ab initio multiconfigurational calculations have been performed to investigate the OH-toluene reaction. The branching ratios of OH addition to ortho, para, meta, and ipso positions are predicted to be 0.52, 0.34, 0.11, and 0.03, respectively, significantly different from a recent theoretical study of the same reaction system. Aromatic peroxy radicals arising from initial OH and subsequent O2 additions to the toluene ring are shown to cyclize to form bicyclic radicals rather than undergoing reaction with NO under atmospheric conditions.Isomerization of bicyclic radicals to more stable epoxide radicals possesses significantly higher barriers and hence has slower rates than O2 addition to form bicyclic peroxy radicals. At each OH attachment site, only one isomeric pathway via the bicyclic peroxy radical is accessible to lead to ring cleavage. Decomposition of the bicyclic alkoxy radicals leads primarily to formation of glyoxal and methyl glyoxal along with other dicarbonyl compounds. Atmospheric aerosols often contain a considerable fraction of organic matter, but the role of organic compounds in new nanometer-sized particle formation is highly uncertain. Laboratory experiments show that nucleation of sulfuric acid is considerably enhanced in the presence of aromatic acids. Theoretical calculations identify the formation of an unusually stable aromatic acid-sulfuric acid complex, which likely leads to a reduced nucleation barrier. The results imply that the interaction between organic and sulfuric acids promotes efficient formation of organic and sulfate aerosols in the polluted atmosphere because of emissions from burning of fossil fuels, which strongly impact human health and global climate.

Limited worldwide energy supplies demand the improved utilization of thermal energy, which is the dominant form of all primary energy sources used today. Large quantities of waste heat are routinely exhausted wherever thermo-mechanical energy conversion occurs, providing an opportunity to improve utilization. Two waste-heat-driven cycles are analyzed: an absorption/compression cascade cooling cycle and a coupled Rankine/compression cycle. The absorption/compression cascade provides an environmentally-sound option for a common approach to thermal energy recovery: the use of absorption cycles for cooling applications. To achieve cooling at temperatures below 0ºC, ammonia-water is the overwhelming choice for the working fluid. However, concerns about the toxicity and flammability of ammonia sometimes limit its application in sensitive arenas. In this study, a lithium bromide-water absorption cycle is coupled with a carbon dioxide vapor compression cycle to realize the benefits of high-lift cooling without the concerns associated with ammonia. This cycle utilizes a waste heat stream at temperatures as low as 150°C to provide cooling at -40°C. The topping absorption cycle achieves a coefficient of performance (COP) of about 0.77, while the bottoming cycle achieves a COP of about 2.2. The coupled Rankine/compression cycle provides a mechanical expansion and compression approach to achieve thermally activated cooling, again driven by waste heat. The power produced in the turbine of the Rankine cycle is directly coupled to the compressor of a vapor-compression cooling cycle to generate cooling to be utilized for space-conditioning. The refrigerant R245fa is used throughout the cycle. Even with low grade waste heat sources, a Rankine cycle efficiency of about 11-12 percent can be achieved. When coupled to the bottoming compression cycle with a COP of about 2.7, this yields an overall waste heat to cooling conversion efficiency of about 32 percent at nominal conditions.

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (p. 224-236).

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (leaves 173-183).

Not available / text

This work analytically solves the governing equations of the laminar film condensation from pure vapors in vertical tubes to find the condensate film thickness. The solution is then extended to turbulent flow conditions for steam. All other relevant quantities are derived from the film thickness solution. For laminar film condensation of quiescent vapors, an exact explicit solution and an approximate closed-form solution were found by using a new definition of the dimensionless film thickness, the Lambert W-function, and a logarithmic function approximation. For laminar mixed-convection film condensation with interfacial shear stress, an approximate closed-form solution was found by using a new definition of the pressure gradient. For turbulent film condensation of steam, correlations of the turbulent vapor and liquid viscosities were formed by asymptotically comparing this approximate laminar closed-form solution to a turbulent flow numerical solution. The present solutions compared very well to published numerical and experimental data.