Power Consumption in the Turbulent Regime for a Coaxial Mixer

The power consumption of a new coaxial mixer composed of an anchor impeller and a pitched‐blade turbine impeller, and a series of rods operated in a contra‐rotating mode has been characterized experimentally in the turbulent regime. It is shown that both the power curve and the turbulent power number vary significantly with the speed ratio between the impellers. Likewise with single impeller mixers, the transition regime starts at a Reynolds number above 100 and the turbulent regime between 10³ and 10⁴ irrespective of the definition of the Reynolds number used.     

新型同心双轴搅拌器功率与混合特性的数值模拟

基于同心双轴搅拌器的结构与运行特点,建立了兼顾其流动、混合过程的三维数学模型,并以过程工业应用较多的两种不同尺寸双层组合桨作为内桨、框式桨作为外桨构成的同心双轴搅拌器为研究对象,数值模拟了其在中高黏牛顿流体中同向及反向转动模式的功率特性、流场特性及混合特性。模拟结果表明,同向转动模式下,整个系统的搅拌功耗更小、混合效率更高;外桨功耗受内桨影响较大,一般随内桨转速的增大,恒速外桨的功耗同向转动时会减小、反向转动时会增大;对由桨式搅拌器构成的组合式内桨而言,当内桨直径与釜体直径之比为0.35左右时,相同Reynolds数下的单位体积混合能更小;中高黏牛顿流体中,同心双轴搅拌器的内桨采用上层六斜叶桨+下层六直叶桨的组合形式时更高效节能,仅在体系Reynolds数小于36时,上层二斜叶桨+下层二直叶桨的内桨组合形式才具有相对优势。     

多层桨搅拌槽内的微观混合特性

在直径0.476 m的多层桨搅拌槽内,采用平行竞争反应工作体系,就不同的多层桨型组合、进料时间、搅拌转速及进料位置对产物分布的影响规律进行了系统的实验研究,并采用涡旋卷吸模型就加料位置等操作条件对产物分布的影响进行了模拟计算,模拟值与实验值吻合. 结果表明,对于多层桨搅拌体系,在液面处加料时产物分布主要由上层桨的桨型决定,底层桨的排出流区加料时主要由底层桨的桨型及功率决定. 卷吸模型能够较好地描述搅拌槽内的微观混合过程.     

Coaxial Mixer Hydrodynamics with Newtonian and non‐Newtonian Fluids

The performance of several combinations of a wall scraping impeller and dispersing impellers in a coaxial mixer operated in counter‐ and co‐rotating mode were assessed with Newtonian and non‐Newtonian fluids. Using the power consumption and the mixing time as the efficiency criteria, impellers in co‐rotating mode were found to be a better choice for Newtonian and non‐Newtonian fluids. The hybrid impeller‐anchor combination was found to be the most efficient for mixing in counter‐rotating or co‐rotating mode regardless of the fluid rheology. For both rotating modes, it was shown that the anchor speed does not have any effect on the power draw of the dispersing turbines. However, the impeller speed was shown to affect the anchor power consumption. The determination of the minimum agitation conditions to achieve the just suspended state of solid particles (N_js) was also determined. It was found that N_js had lower values with the impellers having the best axial pumping capabilities.     

Agitation of Herschel–Bulkley fluids with the Scaba–anchor coaxial mixers

The design of the coaxial mixers depends on many interrelated parameters including the geometry and dimensions of the mixing vessel, the location and type of the impellers, speed ratio, impeller diameter, rotation mode, and fluid rheology. No study has been reported in the literature regarding the mixing performance of the coaxial mixers in the agitation of yield-pseudoplastic fluids. Thus, the main objective of this study was to evaluate the performance of a Scaba–anchor coaxial mixer (a novel configuration) in the mixing of xanthan gum solutions (yield-pseudoplastic fluids). The Herschel–Bulkley model was used to describe the rheological behavior of the xanthan gum solutions. To develop new correlations for the generalized Reynolds and power numbers of the coaxial mixers employed in the agitation of this class of non-Newtonian fluids, we utilized numerous experimental and computational fluid dynamics (CFD) data. The new correlations were tested successfully at different operating conditions (e.g. speed ratio, fluid rheology, and operation mode).     

Micromixing performance in a rotating bar reactor

A rotating bar reactor (RBR) was designed for adjusting micromixing efficiency. It consists of two concentric cylinders of inner rotating and outer fixed cylinders. Two feeding modes, named as radial and tangential feeding modes, were applied to modify the feeding at the bottom section of the RBR. The influence of feeding modes on the micromixing performance was measured with different rotational speeds, flow rates, viscosities, volumetric flow ratios, and hydrogen ion concentrations. Experimental results reveal that the RBR with tangential feeding mode has better micromixing performance than that with radial mode. The higher rotational speed has a positive impact on the segregation index (X_s). The values of X_s increased with the decrease of the flow rate, and the increase of the viscosity, volumetric flow ratio, and hydrogen ion concentration. The micromixing time, estimated by using an incorporation model, was in the range of 5.00 × 10⁻⁶-1.13 × 10⁻⁵ seconds in the RBR with a tangential feeding mode, which indicates a promising prospect of the RBR for process intensification of fast and complex reactions.     

Effect of Impeller Spacing on the Flow Field of Yield-Pseudoplastic Fluids Generated by a Coaxial Mixing System Composed of Two Central Impellers and an Anchor

The three-dimensional flow field generated by a coaxial mixer composed of double Scaba impellers and an anchor in the mixing of the xanthan gum solution, a non-Newtonian yield-pseudoplastic fluid was investigated using the computational fluid dynamics (CFD) technique. The mixing time measurements were performed by a non-intrusive flow visualization technique called electrical resistance tomography (ERT). To evaluate the influence of the impeller spacing on the hydrodynamics of the double Scaba-anchor coaxial mixer, the upper impeller submergence was set to 0.140?m while the lower impeller clearance and the spacing between two central impellers were changed within a wide range. The experiments and simulations were conducted for both co-rotating and counter-rotating regimes at different impeller spacing. The analysis of the collected data with respect to the power number, flow number, mixing time, and pumping effectiveness proved that the co-rotating mode had superiority over the counter-rotating regime. Furthermore, the impact of the impeller spacing in the co-rotating mode was assessed with respect to the mixing time, power number, and mixing energy. The results demonstrated that a coaxial mixer with the impeller spacing of almost equal to the central impeller diameter (C₂?=?0.175?m) and the impeller clearance of C₃?=?0.185?m was the most efficient configuration compared to the other cases. Additionally, the influence of the impeller spacing on the flow pattern was assessed in terms of the radial velocity, tangential velocity, axial velocity, shear rate, and apparent viscosity profiles. When the impeller spacing (C₂) was varied, the merging flow and parallel flow patterns were observed.     

Effect of hydrodynamic heterogeneity on micromixing intensification in a Taylor–Couette flow reactor with variable configurations of inner cylinder

Effect of hydrodynamic heterogeneity on micromixing intensification in a Taylor–Couette flow (TC) reactor with variable configurations of inner cylinder has been investigated by adoption of a parallel competing iodide-iodate reaction system. Two types of inner cylinder, circular inner cylinder and lobed inner cylinder (CTC and LTC), were used to generate hydrodynamic heterogeneity, focusing on the effects of the Reynolds number, the acid concentration, and the feeding time on the micromixing performance. Segregation index (Xs) was employed to evaluate the micromixing efficiency. It is revealed that Xs decreases with the increase of Reynolds number and feeding time but increases with the increase of acid concentration for both the CTC and LTC. However, the LTC does present a better micromixing performance at various operating conditions than that of the CTC as affirmed by both the experimental and computational fluid dynamics simulation results.     

Synergistic and interference effects in coaxial mixers: Numerical analysis of the power consumption

This paper is concerned with the design and application of coaxial mixers with the aid of analysis of interaction between each individual impeller.Two types of coaxial mixers pitched blade turbine (PBT)-helical ribbon (HR) and inner-outer HR operated in laminar regime were studied experimentally and numerically.The interaction implies synergistic and interference effects,which was revealed through the investigation of axial circulation rate,energy dissipation rate and power consumption.The influence factors including rotational speed ratio,rotating mode and impeller configuration were explored systematically.Quantitative analysis of power consumption involves three parameters:rate of variation in power consumption,interactive mode and ratio of power consumption.Analysis indicated that some important properties were embodied in the power curve.These properties are one-way and two-way interactions,critical speed ratio and dominant impeller.Finally,a new suggestion for power estimation was given.     

A new investigation of the metzner-otto concept for anchor mixing impellers

Anchor impellers are commonly used for the homogenization of non-Newtonian fluids, often in association with a set of coaxial turbines. The optimal design of such mixers relies on the knowledge of power drawn by the individual impellers. In non-Newtonian mixing, this can be readily obtained using the Metzner-Otto (1957) concept. In this article, the Metzner-Otto concept and the determination of the constant K_s for anchor impellers have been revisited using numerical and experimental techniques for the case of shear-thinning and shear-thickening fluids. Contrary to literature findings, it is shown that the constant K_s does not vary strongly with the power law index and that, for mixer design purposes, the use of a constant value of K_s for each of the rheological behaviors considered is adequate.     

THE GROWTH OF CAVERNS FORMED AROUND ROTATING IMPELLERS DURING THE MIXING OF A YIELD STRESS FLUID

The growth of caverns, formed around rotating impellers in a yield stress fluid during mixing in a stirred vessel, has been studied by observing impeller speeds at which fluid motion was first observed at the vessel's wall and base, and at the free liquid surface. The effect of impeller geometry has been studied with a disk turbine (DT), a two bladed paddle (2BP), a pitched blade turbine (PBT) and a marine propeller (MP).

The presence of four baffles (10%) was found to increase the impeller speed at which the cavern reaches the vessel wall by 9% on average over that observed without baffles. After the cavern has reached the vessel walls, impeller type had a small effect upon the vertical expansion of the cavern with increasing impeller speed. Radial flow impellers (DT and 2BP), on average, performed better than an axial flow impeller (MP), with a mixed flow impeller (PBT) in between. Baffles significantly reduce the rate of this vertical expansion of the cavern. Clearance of the impeller from the vessel base had little effect upon the growth of the cavern above the impeller.     

Gas-liquid mass transfer studies: The influence of single- and double-impeller configurations in stirred tanks

The influence of impeller structure on the mass transfer characteristics was studied with the steady-state method for gas-liquid volumetric mass transfer coefficient (k _L a). The single-impeller configurations included eight impeller types (three radial flow impellers, four axial flow impellers and one mixed flow impeller), and the doubleimpeller included three configurations (RT+RT, RT+WH _D , WH _D +WH _D ). For single-impeller, the gas-liquid mass transfer rates of radial flow impellers were better than those of axial flow impellers under the same rotation speed and gas flow rate. The mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) of radial flow impellers were also better than that of axial flow impellers. With the same kLa value under a certain gas flow rate, the local bubble size distribution between radial flow impeller and axial flow impeller was similar. As for double impellers, RT+RT provided the highest mass transfer rate under certain rotation speed and gas flow rate, while WH _D +WH _D gave the highest values of gas-liquid mass transfer coefficient with the same power consumption.     

Numerical study on solid suspension characteristics of a coaxial mixer in viscous systems

A coaxial mixer consisting of an anchor and a Rushton turbine was selected as the research object, whose solid suspension characteristics were studied with the help of Computational Fluid Dynamics (CFD) method. Based on the Eulerian-Eulerian method and modified Brucato drag model, the just-suspension speed of impeller was predicted, and the simulation results were in good agreement with the experimental data. The quality of solid suspension under different rotation modes was also compared, and the results showed the coaxial mixer operating under co-rotation mode could get the best performance, and a larger anchor speed was beneficial to solid suspension by enhancing the turbulent intensity at the bottom. Compared with the anchor, the inner Rushton turbine played a dominant role in solid suspension due to its high rotational speed, whereas an extremely high inner impeller speed would make the uniformity of solid distributions become worse. Additionally, the effects of solid phase properties were also investigated, the results revealed that the higher the overall solid volume fraction and the smaller the Shields number, the worse the performance of solid suspension, meanwhile the solid suspension was more susceptible to solid density compared with particle diameter within the same Shields number gradient.     

Characterisation of three different impellers using the LDV-technique

A characterisation of three commonly used impellers was made in this study by measuring local mean velocities and the fluctuations of these velocities with the LDV technique. The data was used to estimate volumetric flow, velocity fluctuations and turbulent intensity in the impeller region of the tank. The impellers investigated were a high flow impeller, a pitched blade turbine and a Rushton turbine. The cylindrical vessel used was made of Perspex, had a dished bottom (DIN 28013), was equipped with four baffles and had an inner diameter of 0.45 m. It was found that the bulk velocities could be scaled with the tip-speed of the impeller (ND). The flow rate at constant impeller speed increased in the order high flow impeller — Rushton turbine — pitched blade turbine. The corresponding order for the turbulence fluctuation is: high flow impeller — pitched blade turbine — Rushton turbine. The velocity profile of the flow out from the high flow impeller was furthermore, not as smooth as could be expected.     

24种搅拌器的功率曲线

文章对有挡板条件下常用的桨式、涡轮式、折桨式、推进式搅拌器采用桨槽径比为0.4—0.6,大于传统的1∶3的结构参数,还有双层桨的形式,进行搅拌功率曲线的测绘。另外对桨叶上开孔、管形桨、弧面桨、半管形桨的正、反两面进行搅拌器的功率曲线测绘,共24条曲线。详细介绍了功率曲线测绘设备的结构形式和各种参数比,以供设计搅拌器时使用;说明了较大的桨槽径比及双层桨叶在实际生产应用中的重要意义,对不同结构搅拌器的功率准数进行了对比,并说明了应用NP-Re曲线的注意事项。     

Power consumption and flow field characteristics of a coaxial mixer with a double inner impeller

A coaxial mixer meeting the actual demand of a system with high and variable viscosity is investigated. It has an outer wal-scraping frame and a double inner impeller consisting of a four-pitched-blade turbine and Rushton turbine. The power consumption and flow field characteristics of the coaxial mixer in laminar and transitional flow are simulated numerically, and then the distribution of velocity field, shear rate and mass flow rate are analyzed. The simulation results indicate that the outer frame has little effect on the power consumption of the double inner impeller whether in laminar or transitional flow, whereas the inner combined impeller has a great effect on the power consumption of the outer frame. Compared with the single rotation mode, the power consumption of the outer frame will decrease in co-rotation mode and increase in counter-rotation mode. The velocity, shear rate and mass flow rate are relatively high near the inner impeller in all operating modes, and only under double-shaft agitation wil the mixing performance near the free surface be improved. In addition, these distributions in the co-rotation and counter-rotation modes show little difference, but the co-rotation mode is recommended for the advantage of low power consumption.     

Improved power and mass transfer correlations for design and scale-up of multi-impeller gas-liquid contactors

The impeller power and volumetric mass transfer coefficient were measured in a pilot-plant single-, double- and triple-impeller vessels of inner diameter 0.6 m. The experimental conditions corresponded with those used earlier in geometrically similar laboratory scale vessel of inner diameter 0.29 m [Fujasová et al., 2007, Chem. Eng. Sci. 62, 1650-1669]. The same impeller types and their combinations were used as well as the experimental techniques and forms of the data treatment/correlations, which distinguish bottom and upper section behaviour. Concretely, 23 combinations of the following impeller types were used: Rushton turbine (RT), six-pitched-blade impeller pumping upwards (PBU) and downwards (PBD), Lightnin A315 (LTN) impeller, and Techmix 335 pumping upwards (TXU) and downwards (TXD). Distilled water, representing a low-viscosity coalescent batch, was used as the liquid phase.It was found that the correlations established on the basis of the laboratory scale data might be used to describe the transport characteristics in the pilot-plant vessel. The more precise correlations, based on the data from both the laboratory and the pilot-plant scale vessels have also been established. The specific powers dissipated by impellers under gassed conditions (P_g) were within the interval from 10 to 8500 W m⁻³ in the experiments. General correlations of the relative power down under aeration (P_g/P₀) are presented separately for the bottom and upper sections of the vessel. k_La were measured by dynamic pressure method in the individual vessel sections simultaneously. Their values moved within the interval from 0.002 to 0.21 s⁻¹. The best fit provided correlating the single- and the multi-impeller (double and triple) vessels data separately. Correlation of the k_La data measured in the middle height of the triple-impeller vessel, the method often used in literature, is also included.Of the triple-impeller configurations, 3RT gave the best mass transfer performance. The configurations utilizing the same impeller type have shown that the radial flow impellers provide higher (20 up to 50%) mass transfer coefficients than the axial flow impellers. The combined configurations (i.e., those with an RT impeller in the bottom section) do not achieve the mass transfer performance of 3RT. The k_La values produced by RT+2PBD and RT+2PBU were only 15-20% lower than those achieved using 3RT at the same power input. The 3LTN and RT+2LTN configurations provided the poorest mass transfer coefficients at the same power input, both being up to 40% lower than those of 3RT.     

Transitional Mixing of Shear‐Thinning Fluids in Vessels with Multiple Impellers

The mixing efficiency of shear‐thinning fluids was evaluated using carboxymethylcellulose sodium salt (Na‐CMC) aqueous solutions of varying mass concentrations and three types of impellers (Rushton turbine (RT), six‐flat‐blade turbine (FBT), six‐pitched‐down‐blade turbine (PBT)) which were mounted on a common shaft in combinations of three, four, and five impellers. The mixing time proved to be dependent on the number of impellers as well as on the distance between. The Reynolds number has a significant influence on the mixing time for all studied systems. The results of power consumption allowed to choose the impeller system with the best efficiency.     

THE GROWTH OF CAVERNS FORMED AROUND ROTATING IMPELLERS DURING THE MIXING OF A YIELD STRESS FLUID

The growth of caverns, formed around rotating impellers in a yield stress fluid during mixing in a stirred vessel, has been studied by observing impeller speeds at which fluid motion was first observed at the vessel's wall and base, and at the free liquid surface. The effect of impeller geometry has been studied with a disk turbine (DT), a two bladed paddle (2BP), a pitched blade turbine (PBT) and a marine propeller (MP).

The presence of four baffles (10%) was found to increase the impeller speed at which the cavern reaches the vessel wall by 9% on average over that observed without baffles. After the cavern has reached the vessel walls, impeller type had a small effect upon the vertical expansion of the cavern with increasing impeller speed. Radial flow impellers (DT and 2BP), on average, performed better than an axial flow impeller (MP), with a mixed flow impeller (PBT) in between. Baffles significantly reduce the rate of this vertical expansion of the cavern. Clearance of the impeller from the vessel base had little effect upon the growth of the cavern above the impeller.