| The horizontal twin-shafts stirred tank for high viscosity fluid is widely used inchemical industry, food, especially in polymerization productions, and so on.However, theoretical study of such devices is still very scarce. Therefore it is verynecessaryto carry out the investigation of the performance of the stirer not only on thetheory but also equipment.In this paper, sensitivity of pressure and velocity fields to gravity under variousimpeller speeds was analyzed by means of computational fluid dynamics (CFD)described with the Realizable k-ε turbulence model and dynamic mesh method.Theresults showed that: in sufficiently small Froude number (Fr=2Di/g) conditions(<0.03104), the pressure is very sensitive to gravity; when larger than or equal to thatFroude number (0.03104), the pressure is dominated by inertia not gravity. Thevelocity fields are much more greatly affected by Reynolds number than the pressurefields do.Through the CFD method, and also by developing a statistical quantitativeevaluation method, studies of the distribution of pressure and three-dimensionalvelocity fields, and turbulent kinetic energy in the novel D-T horizontal twin-shaftstirred tank were conducted while concerning the impacts of impeller on them. Thesimulation results show that: The relative pressure increases systematically along thegravity direction in smaller impeller speed case; however, it becomes less uniform inlarger impeller speed cases. Besides, flow fields can be symmetrical only on conditionof symmetrical boundary conditions, even though the physical geometry of the stirredtank is symmetrical. What is more, the numbers of blades and bars have little effect onthe formation of turbulence. And the turbulence is strongly affected by impeller speed.Compared with the D-T horizontal twin-shaft stirred tank, a star style horizontal twin-shaft stirredtank was developed to overcome the shortage that lack of axial thrust of the former. Thenthrough the CFD method, the application of large-eddy simulation (LES) combined with thedynamic mesh method was used to carry out a detailed description of the pressure and thethree-dimensional velocity field in it. The impact of viscosity on them was also deeply analyzed.The results showed that: the relative pressure distributes very differently under various viscosityconditions. And a swirl flow is formed in the fluid with the rotation of the shaft. What is more, the rotating flows formed by the main shaft and the cleaning shaft interact with each other at the crosssections of the two shafts. Moreover, the axial flow is weakened with increasing viscosity.Using PIV techniques, the velocity field in a visualized horizontal twin-shaftstirred tank with star style mixing devices was studied experimentally by screeningsuitable tracer particles. Experimental results show that: clearer images can beobtained only by the use of fluorescent tracer particles, combined with cut-off filter.But better experimental results can be obtained only in low viscosities. And numericalsimulations have been done compared to the related experiments in this paper.Finally, the mixing time of the star style mixing equipment under the conditions of different liquidhold-up volume, viscosity, and stirring speed was investigated by the tracer method, combinedwith modern image processing technologies, for example, binarization technology, and so on.And then a general formula for mixing time calculation is obtained under experimental conditions.This can prepare the necessary parameters for the application of such devices. |
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