| Summary: | The physical behaviour of air-water plumes during upward injection in ladle-shaped vessels has been investigated. The study involved the experimental determination of the spatial distribution of the flow parameters characterizing the behaviour of the gas phase : gas fraction, bubble frequency, mean bubble velocity and pierced length, and the spectrum of the bubble velocity and pierced length.
A computer-aided electroresistivity sensor was developed to determine simultaneously the several local parameters. In particular the accurate measurement of the bubble rise velocity and bubble pierced length, under the turbulent conditions studied, necessitated an instrument capable of ensuring that sequential voltage pulses originating at the probe tips corresponded to a single bubble travelling axially and undisturbed between the tip contacts. To achieve this, special electronic instrumentation and software was built to analyse, in real time, the signals produced by the contact of the bubbles with the sensor. The extensive evaluation of the measuring system indicated a high accuracy and reproducibility of the results.
The plumes were investigated under various conditions of air flow rate, orifice diameter and bath depth. The measurements indicate that the radial gas fraction profiles, at different axial positions in the plume, exhibit similarity. The reduced gas fraction profiles can be approximated by a single Gaussian distribution for all the conditions studied. Thus a full description of the spatial distribution of gas could be obtained through the correlation of the axial gas fraction and half-value radius with the modified Froude number. The development of flow in gas-liquid plumes is evidenced by changes in the bubble frequency, mean bubble velocity and mean pierced length distributions. In the region close to the injection point, there is a steep change radially in bubble velocity and the motion of the bubbles is strongly affected by the gas injection velocity. Measurements of bubble frequency and pierced length indicate that bubble break-up occurs in this zone before a dynamic process of break-up and coalescence establishes a nearly constant bubble size distribution. In the region of fully developed flow in the plume, the mean bubble velocity and the standard deviation of the bubble velocity spectrum exhibit relatively flat radial profiles and the bubbles affect the flow only through buoyancy. The spectra of bubble pierced length and diameter in this zone can be fitted to a log-normal distribution. Injection conditions have only a slight Influence in determining the size of the bubbles in this region. Close to the bath surface a third zone is identified in which bubble velocity decreases more rapidly as liquid begins to flow radially outward from the plume. A mathematical model proposed in the literature for bubble plumes has been used for comparison with the experimental results. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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