Novel venturi technology for the purpose of gas-liquid mass transfer

The introduction of a gas into a liquid occurs in many chemical and biological engineering processes which \nrequire a chemical or biological reaction to occur. In the case of aeration, air is introduced into water. The \naim of this thesis is to investigate the use of a novel venturi technology, termed the insert that can alleviate \nthe problems of existing technologies such as, restricted depth of use, mechanical wear and failure due to \nthe moving parts and problems with clogging and fouling, whilst providing high aeration efficiency. \nThe inserts tested comprise of a central hub surrounded by a number of aerofoil shaped vanes, which have \nair orifices located on their surfaces. The vanes create a number of discrete channels, which separate the \nflow and each channel is representative of a venturi. Three inserts and a regular venturi were tested. The \ninserts had different angles of attack and a blockage ratio of either 1.5 or 4. Three orientations of the air \norifices with respect to the vanes were considered. All inserts were compared to a regular venturi of \nblockage ratio 1.5, which was made to British Standard 5167-4:2003. \nTwo flow regimes were identified. The first is when a bubbly flow exists throughout the entire length of the \ndowncomer. The second is when a large ventilated cavity forms at the point of air injection, which is typical \nat low water and higher air flow rates and was more prominent with the lower blockage ratio inserts. The \nventilated cavity was seen to have a negative effect in terms of bubble size, specific power and mass \ntransfer performance. The results show that the bubble size produced depends on the air and water flow \nrates, the flow regime and the insert design. The average bubble size at fixed flow rates is essentially the \nsame (differences within ± 10 %), when a ventilated cavity is present. However, when a full bubbly flow is \npresent throughout the downcomer there were smaller average bubble sizes. Also inducing a swirl in \ncombination with a high blockage ratio resulted in coring of the air at the higher flow rates. Reducing the air \nto water velocity slip ratio at the throat, by increasing the blockage ratio and the amount of air orifices, \nreduces the length of a ventilated cavity. \nThis study also examines the hydrodynamic and mass transfer characteristics of the inserts. The inserts \nwere tested in a laboratory scale experimental setup within a 100 mm ID pipe, where they were located \nabove the liquid surface. The results show that increasing the blockage ratio of the insert promoted a \nsmaller mean bubble size, resulting in an increased mass transfer rate. However, the increased blockage \nratio results in significantly higher specific power consumption. The effect of insert design on the volumetric \nmass transfer coefficient was measured using a dynamic method outlined in the ASCE standard ASCE/EWRI \n2-06. The results confirmed that a reduced bubble size had a superior performance. The mass transfer \ncoefficient is observed to be up to 50 % larger with a higher blockage ratio at higher flow rates. \nComputational fluid dynamic simulations are validated against the laboratory scale experimentation to \nAbstract \niii \ndetermine the average bubble size, specific power consumption and the mass transfer coefficient, which \nwere found to be within 6, 15 and 25 % of the laboratory scale values respectively. In addition to with these \nvalidated models, geometric scaling was investigated for the one of the inserts, where it was geometrically \nscaled from 100 mm to 190 mm ID. It was found that the geometrically scaled insert had an increased \nSauter mean bubble size of 18.6 %, an increased pressure loss of 14.5 % and increased specific power \nconsumption of 18.5 %. \nAlong with the laboratory scale experimental work, hydrodynamic and mass transfer testing was conducted \non high strength wastewater and clean water with an insert geometrically scaled up to 150 and 190 mm \nrespectively. A number of key parameters were seen to affect the system performance, including the \nphysical properties of the water, such as dissolved solids, the upstream geometry and mixture outlet. \nIn conclusion it was found that the inserts in this experimental work have an improved aeration \nperformance in comparison to the regular venturi, at the lower air and water flow rates. However, above \nthese lower flow rates, the regular venturi has the best aeration performance. Areas for improvement have \nhowever been identified for the inserts, such as decreasing the air to water velocity slip ratio at the throat, \nwhere the performance of the redesigned insert can successfully be investigated using CFD simulations.