School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemWettability, surface tension and venturi scrubber performance(University of Melbourne, 1973)The addition of a wetting agent or detergent to the liquor of both model and industrial venturi scrubbers, markedly improves particulate collection efficiency. A study of the literature Indicated that an explanation for the improvement was not available. However, it would appear that the role of the wetting agent is to reduce surface tension which could either reduce the drop size or increase the drop-particle attachment or both. A reduction in drop size would improve efficiency if either the impaction or diffusion mechanism of aerodynamic collection was operating. A number of empirical equations have been developed to correlate the operating parameters of an air-liquid atomizer with the drop size distribution generated. These equations differ as to the role of surface tension and there has been only one detailed experimental study under conditions similar to those in a venturi scrubber. Changes in surface tension of water have not been studied as a single variable and it was decided to do this in a model venturi scrubber. The venturi was operated so that drops rather than clouds (composed of ultra-fine drops) were generated. The break-up of the liquid jet and the drops were photographed. The drop size distribtuions for water and water solutions of an ionic and non-ionic detergent giving minimum surface tensions were obtained. There was no statistical significance in the difference between the means of the three distributions and indeed little difference between the shape of the curves. It is concluded that reduction in surface tension of water on addition of a detergent does not influence the drop size distribution in a venturi scrubber. This is thought to be due to the fact that a detergent will only reduce the surface tension of water at the water surface. The inter-molecular attraction within the bulk of the water is not changed on addition of detergent. Thus, when a water jet is rapidly atomized the detergent has insufficient time to spread to or over the newly created surfaces. At the moment of atomization, the surface tension of the solution of detergent in water is effectively that of water alone and hence the drop size is not altered. Addition of detergent does improve venturi scrubber performance and it could do so by increasing the attachment of particles to drops. Alternatively, the bubbles which are formed only after the addition of detergent, may collapse into very fine drops which could act as highly effective collectors for sub-micron particles. Agreement between the median drop diameters obtained in this work and those predicted from the empirical equation of Ingebo and Foster was satisfactory.