BUBBLING TO JETTING TRANSITION AT SUBMERGED ORIFICES

Abstract

A distillation sieve tray serves to disperse an upward flowing gas phase into a liquid phase. Inter-mixing between the gas and liquid on an active sieve tray can occur by various mechanisms. It is well known that two significantly different flow regimes exist in the sieve-tray columns, the froth and the spray regimes. Investigations into these regimes have shown that different correlations are required for each regime, in which the hydrodynamic behaviour and mass transfer are different. In the spray regime, the predominant phase is gas while liquid exists in the form of droplets dispersed in the gas phase. In the froth regime, the phases are inverted and gas bubbles are distributed in a continuous liquid phase. The purpose of this thesis is to investigate the factors that determine whether submerged orifices will operate in the bubbling regime or in the jetting regime. A study of this transition is useful in explaining the more complicated phenomena of "froth" and "spray" regimes occurring on a sieve tray. Various experimental methods have been used to identify this transition. A novel technique involving an optical fiber probe was used for the instantaneous phase detection in the liquid above the orifice as the gas flowrate was increased up to and beyond transition. It was found that in all cases, the measured time averaged void fraction as detected by the probe attained a maximum value at transition. This technique is direct and reliable. Results were confirmed by visual inspection of highspeed video images and by statistical analysis of chamber pressure fluctuations. A new theoretical model of the transition between bubbling and jetting at a single submerged orifice has been developed. The model considers the momentum on a film of liquid adjacent to the gas core at the onset of jetting, and estimates the thickness of the film by drawing the analogy with surface waves in gas-liquid annular flow. The model predictions correlated very well with experimental data from the present work and from published sources.