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31.
A shear thinning fluid (1% carboxymethyl cellulose) was used to investigate mixing under laminar flow conditions in an unbaffled vessel. The effects of impeller modification in addition to eccentricity were studied. Quantitative measurements such as percentage of uncovered area and coefficient of variance (CoV) of a tracer solution distributed inside the vessel were obtained using planar laser-induced fluorescence (PLIF) method. Increased eccentricity was found to be more effective than increasing rpm alone in reducing isolated mixing regions size (determined by the percentage of uncovered area). The dual-flow pitched blade turbine (DF-PBT), which was the modified version of a standard pitched blade turbine (PBT), was designed to provide both upward and downward flow at the same time to induce more chaotic flow. Though numerical analysis showed this type of flow generated, DF-PBT did not return lower values for the percentage of uncovered area and CoV than PBT did. Power consumption data were also compared between the two impeller types and eccentric locations. Further analyses focusing on the interactions between the impeller blades and fluid rheology is needed to improve laminar mixing in stirred vessels by impeller modification. 相似文献
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The influence of impellers and baffles on the mixing of gas‐liquid floating particles in agitated vessels was investigated. Fifty‐two kinds of impeller combinations and twelve types of baffle arrangements were used. The associated power, gas holdup and solids concentration at the vessel bottom were measured. It is concluded that the mixing characteristics of three‐stage impellers were superior to those of two‐stage impellers for aspect ratios larger than 1.6. The optimal combination of impellers and baffles was proposed. The correlations of the relative power and the gas holdup for the optimal combination of impellers under all types of baffles were obtained. 相似文献
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The effect of impeller height relative to the vessel bottom was studied by measuring the drop size distributions of kerosene dispersions in water at two positions inside a stirred tank. Measurements were taken at 1/3, 1/ 2, and 2/3 of total vessel height for rotational speeds 250, 300, 350 and 400 RPM and for hold‐up fractions 0.02 and 0.04. Results show an influence of impeller height on drop sizes ranging from a Sauter mean diameter decrease of 7.8 % to an increase of 35 % relative to the ones obtained with the impeller at the center of the vessel 相似文献
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A new mixed down pumping impeller was designed and characterized with both Newtonian and non‐Newtonian fluids in terms of the power consumption and mixing times. A non‐intrusive color‐discoloration technique based on a fast acid‐base reaction was used to determine mixing times and to reveal the presence of both segregated and dead zones at low speed. This new geometry gives similar mixing times to those obtained with a radial turbine but with the power requirements of an axial flow impeller. It was demonstrated that segregated regions formed below the impeller are readily destroyed by the pumping action of this new geometry, while the regions formed above the impeller are destroyed by radial discharge, so that shorter mixing times are obtained. The use of the proposed geometry appears to be a good alternative for mixing applications requiring a good dispersion combined with low power consumption. 相似文献
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CFD modelling of the turbulent heat transfer was performed for a stirred tank equipped with a Rushton turbine impeller and four standard baffles. Eight different turbulence models, i.e. the standard k‐?, RNG k‐?, realizable k‐?, Chen‐Kim k‐?, optimized Chen‐Kim k‐?, standard k‐ω, k‐ω SST and Reynolds stress models, were used during the modelling. In all investigated cases, the boundary flow at the vessel wall was described by the standard logarithmic wall functions. The CFD modelling values of the local heat transfer coefficient were compared with the corresponding experimental data. The best agreement was obtained for the standard k‐?, optimized Chen‐Kim k‐? and k‐ω SST models. 相似文献
36.
The study of the loading/complete dispersion transition is of great importance especially in processes with enhanced mixing requirements. In the present work, new data and correlations concerning the dispersion characteristics in gas‐liquid contactors agitated by single and dual Rushton turbine systems are reported. The maximum amount of gas which can be completely dispersed, in the presence of gross, well defined recirculation patterns of gas at a given stirrer speed is predicted. Under these conditions, an increase of flow number with increasing Froude number could always be estimated. With decreasing impeller diameter, d, the same gas amount could be dispersed at higher stirrer speeds. At impeller spacing ΔH = 2 d, for d equal to 0.06 and 0.08 m, and ΔH = 1.54 d, for d = 0.10 m, the complete dispersion conditions of the dual impeller systems were slightly better than the corresponding conditions of the single impeller systems. 相似文献
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Mariano Martín Francisco J. Montes Miguel A. Galán 《Chemical engineering science》2008,63(12):3212-3222
The study of the hydrodynamics generated by impellers and its effect on the generation of bubbles and on their rising and dispersion is of key importance to improve the knowledge about the contact between phases and the mass transfer rates, particularly in cases where it is the limiting step. CFD simulations and high-speed video techniques are used to study the hydrodynamics developed by five different impellers, each located at three different positions above the dispersion device. Furthermore, two dispersion devices with one and two holes, respectively, are also used. The effect of the impellers on the characteristics of the bubbles and of the dispersions generated has been analysed. Bubbles generated under stirring are smaller than those generated in stagnant fluids. It is also shown that the initial bubble size at the orifice determines the contribution of the impeller and the perforated plate to the Sauter mean diameter. Although bubble formation is chaotic, the formation period is predictable based on three variables: the location of the impeller, its rotational speed and the gas flow rate. Bubble mean diameter was correlated to classical equations based on Kolmogorov's theory. Only when impellers are capable of breaking the bubbles, Kolmogorov's theory is completely verified. 相似文献