Gas entrainment via vortex

As regards gas entrainment via vortex, the author is only familiar with his own work [609]. The apparent absence of other papers is probably due to the difficulty in finding an appropriate definition for this process. Upon intensive mixing in unbaffled vessels, the stirred tank is converted into a centrifuge. Consequently, a large proportion of the gas entrained in the liquid escapes in the immediate proximity of the stirrer. Therefore it is only partially justifiable to speak of gas throughput. 

In contrast to gas circulation in the stirrer plane, the gas stream which escapes above the stirrer from the liquid vortex, can be easily determined, if it can be ensured that it is not entrained by the stirrer again and mixed back into the liquid. In [609] a vertically displaceable tube, concentrically positioned round the stirrer shaft, was air-tight mounted on the vessel lid and dipped into the liquid vortex (see sketch in Fig. 1.22). A thin, smooth disk was attached to the lower end of the tube, which dipped into the liquid vortex, without disturbing the flow pattern, thereby separating the vortex into two zones. The lower zone is connected via the tube with the atmosphere, and the gas stream escaping into the upper zone can now be determined with a gas-meter. 

Measurements were carried out with a turbine stirrer (d = 60 mm) in a vessel (D = 200 mm) at H/D = 1 and h/d = 1 in a water/air system. Preliminary experiments showed that the diameter of the separating disk (55 60 70 mm) had no influence on the results. 

The dependence of the entrained gas throughput q upon the distance A between the separating disk and the upper edge of the stirrer was determined at only one stirrer speed (u = 2000 min Fr = 6.8 = constant). The following correlation was found  

Micro-mixing, on the other hand, describes the process of homogenization of liquid balls with their surroundings on a molecular level. This process takes place within the micro-scale A and is not scale-dependent. 

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As soon as the vortex reaches the stirrer head of a rapidly rotating stirrer (e.g. propeller or turbine stirrer) or the outer stirrer side of a slowly rotating stirrer (e.g. blade stirrer), gas is entrained in the liquid. Gas entrainment via vortex was... 

Fig. 1.22 Gas entrainment via vortex in comparison to the volume aeration by self-aspirating hollow stirrers (type "three-edged stirrer ) from [609]. 

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