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UBC Theses and Dissertations

Diffusiophoresis under turbulent conditions Whitmore, Peter John

Abstract

This thesis describes both theoretical and experimental studies of diffusiophoresis, which is the movement of aerosol particles when they are suspended in a diffusing gas mixture. The phenomenon was studied under conditions of turbulent flow. Previous workers have demonstrated the efficacy of diffusiophoresis in continuously removing particles of the order of one micron (10⁻⁶m) in diameter from laminar gas streams. Such particles are difficult to remove by conventional gas cleaning techniques. However, any industrial application would probably require that the gas be in turbulent flow, to enable limitation of the equipment to an economic size. This observation provides the rationale for the current work, which can be divided into four major parts. The first part is the derivation of an expression for the diffusiophoretic velocity of a large particle (i.e.,a particle whose radius is large compared to the mean free path of the gas) in an undisturbed diffusing gas mixture. The continuum mechanics' equations were solved in a more rigorous manner than was hitherto available. Previous derivations all implied the existence of diffusion slip at the particle surface, but this was shown to contravene the law of energy conservation. With zero slip, the particle was found to adopt the mean mass velocity of the fluid. This result, which was used in subsequent theory, differs from those of earlier workers. The second part involves the development of a theory to predict the diffusiophoretic particle removal caused by diffusional mass transfer from a multicomponent gas stream. Two separate derivations were made, the more rigorous of which is based solely on the non-steady-state forms of the continuity equations for the particles and for the gas mixture. It is therefore independent of the patterns of mass transfer or gas flow. Theoretical predictions were derived for three possible values of the local particle velocity. These were the local mean mass velocity of the fluid, the local mean molar velocity, and a velocity suggested by Schmitt and Waldmann (1960). The third part consists of experimental studies of particle removal by diffusiophoresis from turbulent gas streams. A binary gas mixture containing aerosol particles was passed up through a wetted wall column (0.0254 m I.D. and 0.77 m in length) countercurrent to a flow of water. One component was insoluble, while the other was partially absorbed. The resulting particle removal was determined by measuring the inlet and outlet aerosol number concentrations in the gas. The soluble gases tested were ammonia and trimethylamine, while the insoluble gases were helium methane, nitrogen, argon, and freon 12 (dichlorodifluoromethane). Uniform latex particles of 0.50, 0.79, 1.011, 2.02, and 5.7 microns in diameter were used. The Knudsen numbers for the particles ranged from 0.015 to 0.30. The experimental results indicated that the theory relating particle removal to gas mass transfer was substantially correct. However, none of the theoretical predictions based on the three velocity expressions agreed with the experimental results for every case. It was concluded that this was because the particle diameters were sufficiently close to the mean free path of the gas that the particles fell into the transition regime, in which none of the velocity expressions apply precisely. The fourth part is a preliminary investigation into the practical application of diffusiophoresis for removing particulate matter from industrial gas streams. The importance of the type of equipment and the gases employed was demonstrated. It was concluded that diffusiophoresis used alone would not be economic except in special circumstances, but that it might be valuable when used in conjunction with other removal mechanisms.

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