Close-clearance helical impellers: A physical model for newtonian liquids at low Reynolds numbers
✍ Scribed by V. V. Chavan
- Publisher
- American Institute of Chemical Engineers
- Year
- 1983
- Tongue
- English
- Weight
- 982 KB
- Volume
- 29
- Category
- Article
- ISSN
- 0001-1541
No coin nor oath required. For personal study only.
✦ Synopsis
With simple hydrodynamic concepts, a physical model is developed to describe the flow at the wall close to the impeller blade. The parameters of this model were evaluated from the large amount of data on power consumption and checked with some independent data on velocity profiles. Together with the physical insight provided by the model, it helped to obtain general relationships for power and heat transfer coefficients. Equations for the latter were developed using Lev6que's approximation. Agreement between the predicted results and the literature data of several independent studies was rather good. A method to analyze mixing or blending results is also suggested.
V. V. CHAVAN Process Engineering Consultant
Bomendijk 19
3181 RE Rozenburg
The Netherlands
SCOPE
For many industrial operations, such as mixing or blending of liquids, dispersing pigments into liquids and heat transfer, and for polymerization, impellers like ribbons, screws or combined ribbon-screws were often used. These are proved to be particularly efficient with the highly viscous liquids and thus are often operated at low Reynolds numbers (Re < 50).
Research into the hydrodynamics in such vessels has began as early as 1956 and general flow patterns have been known since that time. Quantitative studies to a greater extent were undertaken in early 70's in a number of laboratories. Most available information is, however, on the power consumption. Some data on circulation capacities, mixing times and heat transfer have also been collected. Data have often been empirically correlated; although a couple of semitheoretical attempts were made, they neither provided the complete physical picture nor did they obtain general relationships. A model capable of putting the most of experimental data in a proper perspective, general enough to describe more than one phenomena like power consumption, mixing and heat transfer, and simple enough to be readily usable was therefore highly desired. Development and application of such a physical model is the subject of this paper.
CONCLUSIONS AND SIGNIFICANCE
Most of the literature data on velocity profiles, circulation capacities, power consumption, mixing times and heat transfer in vessels agitated by ribbons, screws in draft tube and combined ribbon have been thoroughly analyzed in this paper. The proposed hydrodynamic model has been successful in providing a basis to analyze transport phenomena in these agitated vessels.
Further, general relationships have been developed for predicting power consumption and heat transfer and are also useful for scale-up and extrapolation. The model also helps to quantitatively analyze the effect of geometrical variables such as the impeller pitch and the gap between the impeller and the vessel on the physical quantities. Several conclusions of such effects are elaborated. In summary, the proposed model and analysis provides a better understanding of the phenomena in these vessels which should help design better mixers and scale-up complex situations such as polymerization or fermentation reactors.