- Title
- Hydrodynamic characteristics of a plunging jet reactor
- Creator
- Atkinson, Bruce Winston
- Resource Type
- thesis
- Date
- 1994
- Description
- Professional Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis is concerned with the behaviour of a liquid jet which discharges into a two-phase mixture to form a "plunging liquid jet downflow bubble column". It has been found that the characteristics of the flow in the column are highly dependent on the liquid velocity. In general, it is possible to model the flow only if the liquid velocity is greater than the rise velocity of bubbles in the system, otherwise recirculation of gas from the base of the column affects the characteristics of the plunging jet. The variation of jet diameter with jet length was investigated, and the free jet characteristics were related to the gas entrainment rate. It was found that the gas entrainment data were best described by a film-wise gas entrainment model. The gas entrainment ratio was found to be a function of the jet velocity and the free jet length. The pressure in the head space in the top of the column above the twophase mixture, is an important indicator of the stability of the confined plunging jet system. It was found that the pressure in the head space could be predicted from a momentum balance. A mixing zone, in which the momentum of the plunging jet is essentially dissipated, has been defined. The total mixing length in the downcomer, given by the sum of the free jet length and the mixing zone length, was found to be related to the gas entrainment rate, the jet diameter, and the jet velocity. The mixing zone was delineated using a centre-line jet pressure profile, from which the centre-line velocity profile was calculated. An energy balance was used to determine the axial profile of energy dissipation, and it was found that the mixing loss was primarily due to the dissipation of jet kinetic energy. The region of highest energy dissipation rate was thus determined to be the region of maximum axial velocity gradient in the jet. The energy dissipation profile was used to estimate the maximum stable bubble size in the mixing zone, based on the principle of bubble break-up in shear flow, using a critical Weber number approach. The model was found to agree well with measured bubble size data. Bubble coalescence in the downcomer was found to be significant, despite the presence of surfactant. The bubble size distribution could be reasonably modelled based on a binary exponential coalescence model, or equally well using a cluster coalescence model. The void fraction in the uniform two-phase flow region of the downcomer was related to the gas-liquid flow ratio, and was correlated using the two dimensional drift-flux model. It was also found, that for moderate gas liquid flow ratios, the uniform two-phase flow region could be treated as the flow of liquid through an expanded bubble bed, where the bubbles were regarded as rigid spheres. This model failed at higher gas rates, presumably due to the formation of larger, deformable gas voids. The models resulting from this work provide a basis on which to design systems incorporating the plunging liquid jet downflow column.
- Subject
- jets; fluid dynamics; two-phase flow; bubbles; THESIS 1762
- Identifier
- http://hdl.handle.net/1959.13/1519647
- Identifier
- uon:57410
- Rights
- Copyright 1994 Bruce Winston Atkinson
- Language
- eng
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