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No description available.

No description available.

No description available.

The phase-out of chlorofluorocarbons (CFCs) has resulted in an expanding new area of research in alternative ozone friendly propellants, for example hydrofluoroalkanes (HFAs). The HFA solvent system is unique in that many CFC soluble compounds behave differently in the HFA alternatives, such as HFA-134a and HFA-227. The reason for the difference in solubility is not fully recognized. This work investigates the solubility of 22 compounds in HFA-227 with the addition of ethanol as a cosolvent. The physical properties of both solute and solvent were investigated in order to determine the effects on solubility. The solubilities of 5 compounds in HFA-134a were also investigated. A thermodynamic approach was utilized in order to look at the enthalpic and entropic effects on solubility in the propellant. Due to the high vapor pressure of propellants, a liquid model was utilized, owing to its ease of use in characterizing solubility. The correlation between the liquid model 2H,3H-decafluoropentane (DFP) and the propellants HFA-134a and HFA-227 was examined.The solubilities in HFA-227 with ethanol ranged from 0.001 to 3.282 %w/w, where the solubilities always increase when ethanol was added. The experimental solubilities were compared to calculated values obtained from ideal solubility and regular solution theory models. A clear correlation with the ideal solubility (melting point) combined with an intercept term and two physical properties was noted. A regression approach was also used to predict the activity coefficient in HFA-227 with 0 - 20% ethanol. These equations were combined with the extended ideal solubility equation, creating a useful predictive equation with AAE values ranging from 0.32 to 0.36, or factor errors of 2.09 to 2.29. The equations shown in this work are useful for the prediction of solute solubility in HFA-227/ethanol mixtures.Results in the liquid model DFP with 0 - 20% ethanol show that a regression equation results in a useful predictive equation for the solubilities in both HFA-134/ethanol and HFA-227/ethanol systems, where the AAE values ranged from 0.3 to 0.56, or factor errors of 2.0 to 3.6.The solubilities of a series of chlorobenzene compounds along with a group of hydrogen donating and/or accepting compounds was examined in HFA-134a. The entropic effects appear to be the limiting factor in the solubility of these compounds. The compounds capable of hydrogen accepting and donating exhibited negative enthalpy of mixing values when placed in HFA-134a, a stark contrast to the values obtained for the chlorobenzenes. This suggests HFA-134a is able to strongly interact with solutes capable of donating or accepting hydrogen.

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A nonlinear stability analysis of a liquid sheet moving in an incompressible gas medium at rest subject to sinuous disturbances is presented. The first, second and third order governing equations have been derived along with appropriate initial and boundary conditions which describe the characteristics of the fundamental mode, and the first and second harmonics. It is found that the thinning of the sheet is caused by the growth of harmonic waves, and subsequent liquid sheet rupture occurs at every half wavelength interval. The amplitude growth rates of the disturbances are calculated at the dominant wavenumber for different initial amplitudes and are then compared with the predictions from the linear theory. The analysis also allows the determination of the breakup time and breakup length, and the effect of the flow parameters on the breakup time is also investigated. The present experimental study on the liquid sheet breakup explores the growth of liquid sheet instabilities and subsequent spray formation. Flow visualization reveals different flow regimes. Liquid sheet surface displacements, wavelengths and breakup lengths are measured using a photographic technique, and the drop sizes and velocities are measured simultaneously by the Phase Doppler Particle Analyzer (PDPA). The result of acoustic excitation for different regimes of liquid disintegration shows that for certain frequencies resonance and enhanced instability occur. However, in the spray regime where a fine spray is formed, although visually there are some effects of acoustic excitation on the wave development, the Phase Doppler Particle Analyzer (PDPA) measurements show no noticeable effect on the drop sizes and velocities. The theoretical predictions are compared with the experimental results for dominant wavelength and growth rate of the unstable waves on the liquid sheet surface. / Graduate

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A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 1998. / Experiments have been conducted at around 17000C to determine the whether carbothermic reduction of alumina is possible at these temperatures. Total pressure of the system was reduced to around 30 kPa and various metallic solvents such as copper, nickel, iron and tin were used to dissolve the metallic aluminium produced. The use of a solvent (and hence decreasing the activity of metallic aluminium) and a lower pressure are thermodynamic requirements to increase the extent of reduction under a given set of conditions. This enables the use of lower temperatures than are required under atmospheric conditions. The highest recovery of aluminium was achieved with the nickel solvent decreasing in order from iron, copper and tin. This ranking was also in accord with the extent of deviation from ideality in the respective binary solutions of these solvents with aluminiur, The nickel-aluminium system displays the largest negative deviation from ideality whereas the till, aluminium system showed a positive deviation. The rate and extent of the reduction was found to be highly dependent on temperature and pressure. The pseudo first order reaction rate was found to be the primary order for the reduction of aluminium in all the solvents used. It is also suggested that the reduction rate was controlled primarily by chemical reaction rate father than by transport processes. This is due to the extreme sensitivity of the rate and extent of the reaction to temperature. / AC2017