Flows ribbon stirrer

This graphical representation enables an interesting comparison to be made between the power characteristics of different stirrer types. In the first place it is evident that the Ne value for Re = 1, i.e. NeRe = const) increases with the surface area of the stirrer, it being with the anchor stirrer a factor of 10 greater and with the helical ribbon stirrer a factor of 25 greater than that of the propeller stirrer. The smaller the wall clearance of the stirrer, the longer laminar flow conditions remain. With gate and cross-beam stirrers these are maintained up to Re 10, with anchor stirrers and helical ribbon stirrers even up to Re 10. ... 

However, the flowed against front faces of the stirrers provide a far from proportional contribution to the stirrer power. Measurements on four helical ribbon stirrers with helices of different length confirm this (single and double helical ribbon stirrers with pitches of 0.5 and 1 helical ribbon area A ). The following relationship was found [611] ... 

In [120] power measurements are presented for tlie combination of turbine stirrer and helical ribbon stirrer in Newloniari liquids in the laminar range. It was found that the total Ne value with the combination was considerable larger than the sum of the individual Ne values NeRe - 70 for turbine stirrers, 150 for helical ribbon stirrers and 350 rather than 220 for the combination. This can be explained by the disturbance of the axial flow of the helical ribbon stirrer by the radial flow of the turbine stirrer in the lower vessel region. 

If the course of mixing with a helical ribbon stirrer in the represented laminar range is approximated with wfl = const, it can be said that the mixing time decreases directly proportionally with the stirrer speed. From about Re = 50 the forces of gravity begin to affect the flow pattern. Round the stirrer shaft a liquid cone remains under the surface, whose participation in the intimate mixing is retarded [611]. The helical ribbon stirrer is therefore only sensibly usable in the range Re < 100. 

In Fig. 3.13 below, the authors [363] plotted their own measurements for the helical ribbon stirrer for different wall clearances d/D. It shows (see the table below right), that the stirrer power of the helical ribbon stirrer decreased rapidly with increasing wall clearance and that, to a smaller extent, the nd values also decreased. This stirrer type, which up to now has proved to be the most suitable for homogenization in the laminar flow range, should if at all possible be utilized with d/D = 0.9. 

The helical ribbon stirrer is particularly favorable for homogenizing in the laminar flow range, but has two serious drawbacks, namely, the high cost of manufacture and the complicated introduction and installation in the tank. These disadvantages... 

In this study [614] the comparison between the anchor stirrer and a double helix ribbon stirrer with a pitch of 0.5 for the same d/D = 0.98 established, that the helical ribbon stirrer for Re l provided a Nm value which was approximately double that with the anchor stirrer, but that this superiority had already been lost at Re 100. Apparently, here the radial flow component is entirely sufficient for heat transfer. 

A considerable removal of the boundary layer from the inside wall of the tank is only possible with stirrers with wiper blades. (For the numerical analysis of the three-dimensional flow and thermal behaviour in a scraped-surface heat exchanger see [647].) A first review over such stirrers in [443] brought little of substance. Nagata [0.6] provided the following relationship for helical ribbon stirrers with wipers ... 

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