数值模拟静态混合器结构对PS/CO_2熔体温度的影响

利用专用CFD软件Polyflow对SMX型和Kenics型静态混合器中PS/CO_2发泡溶液进行数值模拟计算,分析比较不同板厚在不同元件个数条件下两种静态混合器消耗的压力损失,以及不同CO_2浓度对静态混合器压力损失的影响;并引入"离散系数"分析比较两种静态混合器出口温度均匀性的变化.数值模拟的结果表明:SMX型静态混合器冷却效果优于Kenics型静态混合器,并且SMX型静态混合器出口温度均匀性高于Kenics型静态混合器.     

Comparative analysis of different static mixers performance by CFD technique: An innovative mixer

The flow and mixing behavior of two miscible liquids has been studied in an innovative static mixer by using CFD,with Reynolds numbers ranging from 20 to 160. The performance of the new mixer is compared with those of Kenics, SMX, and Komax static mixers. The pressure drop ratio(Z-factor), coefficient of variation(CoV), and extensional efficiency(α) features have been used to evaluate power consumption, distributive mixing, and dispersive mixing performances, respectively, in all mixers. The model is firstly validated based on experimental data measured for the pressure drop ratio and the coefficient of variation. CFD results are consistent with measured data and those obtained by available correlations in the literature. The new mixer shows a superior mixing performance compared to the other mixers.     

Residence Time and Concentration Distribution in a Kenics Static Mixer

Static mixers, often referred to as motionless mixers, are in-line mixing devices that consist of mixing elements inserted into a length of pipe. Most of the experimental works in this field have concentrated on establishing design guidelines and pressure drop correlations. Due to experimental difficulties, few articles have been published on the investigation of the flow and mixing mechanisms. In this work, a Kenics KMX static mixer was utilized to study concentration and residence time distribution (RTD) and effect of Reynolds number on mixing. The static mixer had six mixing elements arranged in-line along the length of the tube, and the angle between two neighboring elements was 90°. The length of the mixer was 0.98 m with internal and external diameters of 5.0 cm and 6.0 cm, respectively. The main continuous fluid was water, and NaCl solution was used as a tracer. All experiments were conducted with three replications at three Reynolds numbers, Re = 1188.71, 1584.95, and 1981.19. A dispersion model was used to model the RTD data. The experimental results were compared with the model results and reasonable agreement was achieved.     

Laminar mixing of shear thinning fluids in a SMX static mixer

Flow and mixing of power-law fluids in a standard SMX static mixer were simulated using computational fluid dynamics (CFD). Results showed that shear thinning reduces the ratio of pressure drop in the static mixer to pressure drop in empty tube as compared to Newtonian fluids. The correlations for pressure drop and friction factor were obtained at Re_MR?100. The friction factor is a function of both Reynolds number and power-law index. A proper apparent strain rate, area-weighted average strain rate on the solid surface in mixing section, was proposed to calculate pressure drop for a non-Newtonian fluid. Particle tracking showed that shear thinning fluids exhibit better mixing quality, lower pressure drop and higher mixing efficiency as compared to a Newtonian fluid in the SMX static mixer.     

Numerical investigation of the performance of several static mixers

The performance of six static mixer (Kenics, Inliner, LPD, Cleveland, SMX and ISG) are compared using 3D numerical simulations in laminar creeping flow regime. Numerical pressure drop results are tested against experimental ones, showing overall a good agreement. Besides pressure drop, four criteria (extensional efficiency, stretching, mean shear rate and intensity of segregation) are chosen to compare the static mixers. It appears that Kenics, Inliner, LPD and Cleveland mixers are rather similar. The ISG mixer seems better than this first group of mixers, but pressure drop is too high compared to other advantages. From our numerical results, SMX appears to be the most efficient of the six compared static mixers.     

Improving mixing performance by curved-blade static mixer

This article addresses design modification to a flat-blade static mixer to enhance mixing performance. The static mixer elements used in this work consist of four blades with curvature made to intensify turbulent-like flow, while reducing the pressure drop. The blades were mounted on a cylindrical housing with 45° rotation relative to the axial direction. The mixer assembly was used in three different arrangements of 8, 10, and 14 elements for a range of Reynolds number between 600 and 7,000. The coefficient of variance (COV) of samples was used to measure the mixing quality. The curved-blade mixer provides considerable improvement in mixing quality compared with the flat-blade mixer and comparable to the SMX mixer. Compared with the flat-blade static mixer, the new design reduces the COV by up to about 50%. This effect is more pronounced when the number of mixing elements increases. Furthermore, the friction factors for the modified mixer, obtained at a wide range of Reynolds number, were apparently smaller than those for the flat-blade, SMX, and SMV mixers. The correlation presented for the friction factor, when all mixer arrangements and aspect ratios were considered, supports the experimental data with ±15% deviation.     

Drop Breakup in an SMX Static Mixer in Laminar Flow

The mechanism of drop breakup inside SMX static mixers in the laminar flow regime was studied using experimental observations and computational fluid dynamics (CFD). The deformation and breakup of a single drop was simulated using the volume of fluid (VOF) model. It was observed that drops break up after collision with the leading edges and cross‐points of the bars in the SMX static mixer. It was found that drop collision with the bar cross‐points of the SMX static mixer elements is most effective for drop breakup. Elongation and folding result in drop breakup at the cross‐points.     

Pressure drop and mixing behaviour of non‐Newtonian fluids in a static mixing unit

Static or motionless mixers have received wide application in chemical and allied industries due to their low cost and high efficiency. The pressure drop and mixing behaviour of such mixers have been widely studied. However, the available information for non‐Newtonian fluids is scanty. The results of pressure drop and mixing studies conducted with a locally made motionless mixer (MALAVIYA mixer) and four non‐Newtonian fluids—aq. CMC, PVA, and PEG solutions are reported in this article. The new mixer causes less pressure drop compared to some of the commercial mixers. Mixing behaviour of the unit is more closer to plug flow and a two‐parameter model correlates the dispersion data.     

A SIMULATION OF A MOTIONLESS MIXER

Continuous laminar mixing in segmented twisted-tape motionless mixers is considered. A solution to the steady isothermal creeping flow of a Newtonian fluid in a twisted-tape mixer has been obtained via two-dimensional numerical procedures. The developed flow within a section of the mixer has been solved in a helical coordinate system by an iterative scheme. The resulting solution is rigorously correct in the absence of entrance and exit flows at the junction between sections. An algorithm is presented for the modelling of these junction flows via two-dimensional procedures. Simulated cross-sectional mixing patterns have been generated for comparison with experimental results

The performance of twisted-tape mixers is simulated for various designs, beginning with the particular geometry of the Kcnics Static Mixer, and for different operating conditions Results suggest that the rate of mixing as a function of the total twist per section is optimized with respect to pressure drop when sections contain 80 degrees of twist. The capability for rational improvement in other design and operating parameters is illustrated. The mechanisms of laminar mixing are discussed and quantified; of primary importance is the tendency for interfacial area to assume an orientation within each section which is favorable to mixing in subsequent sections.     

Effects of geometry and flow rate on secondary flow and the mixing process in static mixers—a numerical study

A method based on computational fluid dynamics (CFD) for the characterization of static mixers using the Z factor, helicity and the rate of striation thinning is presented. These measures were found to be well-suited for the characterization of static mixers as they reflect the pressure drop, the formation of secondary flow, i.e. vortices, and their effect on the mixing process. Two commercial static mixers, the Kenics KM and Lightnin Series 45, have been characterized. In the mixers investigated, secondary flow is formed in the flow at the element intersections and due to the curvature of the mixer elements. The intensity of the vortices is higher in the Lightnin than the Kenics mixer due to edges in the middle of the Lightnin mixer elements. The formation of vortices affects the Z factor by an increase in the power requirement, and the rate of striation thinning by an increase in the stretching of the striations. The formation of vortices was observed at a Reynolds number of 10 in both mixers with aspect ratios of 1.5. However, the intensity of the vortices was greater in the Lightnin than the Kenics mixer, which was observed in not only the magnitude of the helicity, but also the Z factor, rate of striation thinning and the distribution of striation thickness.The distribution in striation thickness is shifted towards thin striations as the flow rate is increased from below to above the Reynolds numbers of which vortices were first observed, but some striations still pass the mixer elements almost unaffected, which can be seen in the skewness of the distribution of the striation thickness, which shifts from being negative to positive.     

不同结构静态混合器内熔体流动及混合效果的数值模拟

构建长度、直径相同,但具有不同扭曲率、分割次数的3种静态混合器,然后利用Polyflow软件模拟低密度聚乙烯(HDPE)熔体在静态混合器内的流动情况,得到熔体在静态混合器内的速度场、压力场、停留时间、分离尺度等参数,并通过分析示踪粒子在静态混合器内的分布情况,表征其混合效果。结果表明,扭曲率大的静态混合器,压降较大、横向速度分量较大、分离尺度较小、混合效果更佳;而分割次数多的静态混合器,压降增大,但是分割次数对横向速度、混合效果的影响较小。     

Characterization of mixing in an optimized designed T-T mixer with cylindrical elements

Passive micromixers are preferred over active mixers for many microfluidic applications due to their relative ease in integration into complex systems and operational flexibility. They also incur very low cost of manufacturing. However, the degree of mixing is comparatively low in passive mixers than active mixers due to the absence of disturbance in the flow by external forces and the inherent laminar nature of microchannel flows. Various designs of complex channel structures and three-dimensional geometries have been investigated in the past to obtain an efficient mixing in passive mixers. But the studies on mixing enhancement with simple planar geometries of passive mixers have been few and limited. The present work aims to investigate the possibility of mixing enhancement by employing simple planar type designs, such as T-mixer and T-T mixer with cylindrical elements placed in the mixing channel. The mixing performance has been evaluated in the Reynolds number range of 6 to 700. Numerical results have shown that T-T mixer with cylindrical elements performed significantly well and obtained very good mixing quality over basic T-mixer for the entire range of Reynolds number (6 to 700). The device has also shown better mixing as compared to basic T-T mixer and T-mixer with cylindrical elements. A larger pair of vortices formed in the stagnation area due to the presence of a cylindrical element in the junction. Cylindrical elements downstream caused significant enhancement in mixing due to splitting and recombining action. The size of the cylindrical element in the T-T mixer has been optimized to obtain better mixing performance of the device. Remarkable improvement in mixing quality by T-T mixer with cylindrical elements has been obtained at the expense of small rise in pressure drop as compared to other passive designs considered in this study. Therefore, the current design of T-T mixer with cylindrical elements can act as an effective and simple passive mixing device for various micromixing applications.     

SMX型静态混合器湍流特的数值模拟

以SMX型静态混合器为研究对象,运用计算流体力学软件Fluent对3种结构静态混合器管内湍流流场进行数值模拟.结果表明:SMX型静态混合器的混合元件对流体有切割和分散作用,可使径向速度与周向速度的最大值达到轴向平均表观流速的2～2.5倍;流体流经3、4个混合元件后流动基本达到稳定;混合器前3个混合元件对流体湍动强化作用...     

Influence of agitator design on powder flow

Design influences the flow within a powder mixer but quantitative guidance is lacking. Here the performance of mixers of different geometry was compared using positron emission particle tracking. One mixer had six long flat blades; the other carried short paddles. With the former, blade angle and number of axial compartments had little effect on agitation in the transaxial plan but axial dispersion was enhanced by longer axial compartments. A loop of circulation was found below the shaft. For the short paddle device, the transaxial agitation was more uniform, with a lower mean angular velocity and narrower ranges of velocities. The mixing elements inhibited the formation of the loop of circulation. In both cases, the axial flow had a cellular structure created by the radial supports for the blades but the short paddles mixer showed more chaotic behaviour, the axial dispersion coefficient being typically five times higher and increasing with fill rather than decreasing as seen with the six-blade device. A rationale for the design of powder mixers is thus emerging.     

Experimental characterization and multi-scale modeling of mixing in static mixers

Mixing in static mixers is studied using a set of competitive-parallel chemical reactions and computational fluid dynamics (CFD) in a wide range of operating conditions. Two kinds of mixers, a wide angle Y-mixer and a two jet vortex mixer, referred to as Roughton mixer, are compared in terms of reaction yields and mixing times. It is found that the Roughton mixer achieves a better mixing performance compared to the Y-mixer. The effect of flow rate ratio on mixing in the Roughton mixer has been studied as well and it is shown that the mixing efficiency is not affected by the flow rate ratio. Moreover, experimental results and model predictions are in good agreement for all mixer geometries and operating conditions. CFD is used to calculate absolute mixing times based on the residence time in the segregated zone and it is shown that mixing times of less than 1 ms can be achieved in the Roughton mixer. In addition, CFD provides insight in local concentrations and reaction rates and serves as a valuable tool to improve or to scale-up mixers.     

Experimental study on energy consumption characteristics of rotational–perforated static mixers

An ideal static mixer can achieve efficient mixing at low pressure drops. Owing to the excellent performance of the tridimensional rotational flow sieve tray (TRST) in a gas–liquid two-phase system, the TRST structure was modified into a rotational–perforated static mixer (RPSM) to enhance mixing in multicomponent liquid systems. The energy consumption characteristics of the RPSM were experimentally studied based on Reynolds numbers in the range of 986–7892, gap L = 0–80 mm, and relative angle γ = 0–45°. The effects of the element installation method, number, gap, relative angle, fluid Reynolds number, fluid properties, and other parameters on the RPSM pressure drop were also investigated. An interaction analysis of each factor was performed using the factorial design method and an empirical model of the RPSM Z-factor was established. Additionally, pressure drop in the RPSM was compared with those of other commonly used static mixers. Results show that, when the element is backward-installed, the pressure drop is higher than that in the forward direction because the fluid is constantly twisted. Moreover, the pressure drop increases with increasing element gap, and the average increase is 43.64% and 19.28% for the forward and backward installations, respectively. The influence of the relative angle on the pressure drop is mainly reflected when the gap L = 0. Subsequently, the degree of influence of each factor was determined, and the Z-factor was calculated and found to be consistent with the experimental values (relative error of less than 15%).     

FRICTION FACTOR IN STATIC MIXER AND DETERMINATION OF GEOMETRIC PARAMETERS OF SMX SULZER MIXERS

Pressure drops were determined in fluid flow through SMX Suizer static mixer of different sizes. In order to investigate a large range of Reynolds number, the experiments were performed with fluids of different viscosities. Pressure drops measurements in static mixer considered as a porous medium, are analysed with a capillary model for the determination of the geometric parameters of static mixers: pore diameter and tortuosity. These parameters allow the expression of pressure drops in terms of friction factor,f_c as a function of pore Reynolds number, Re_pA universal equation is obtained for the friction factor:f_c=16/Re_p+,0.194 which covers both viscous and inertial flow regimes.     

Formation of O/W emulsions by static mixers for pharmaceutical applications

Oil-in-water (O/W) emulsions produced by static mixers in the laminar flow regime are characterized for their oil drop size spectra. The emulsions are used in the first process step for the production of microspheres for pharmaceutical applications by the emulsion extraction method. However, emulsion generation by static mixers in the laminar flow regime is rarely discussed in the scientific literature. Here we deduce a non-dimensional correlation for predicting the Sauter mean oil drop size as a function of the static mixer operation parameters and the liquid properties. First, the material properties of the organic and water phases are characterized. Second, the oil drop size spectra of the emulsions are measured by laser diffraction. Dimensional analysis is used to describe the relationship between the process parameters of the static mixer and the Sauter mean oil droplet size. Emulsion production experiments using SMX static mixers with two different diameters are carried out with the mixing of the two liquids taking place in the laminar flow regime. We provide results covering a wide range of all process parameters, which were identified influencing the droplet size of the emulsion. The correlation achieved is related to the non-dimensional drop-size based Ohnesorge number of the emulsification process and allows for the prediction of the mean oil droplet size with good accuracy, which is an essential information about the emulsion properties relevant for the pharmaceutical application.     

Performance of Kenics static mixer over a wide range of Reynolds number

The present study deals with the numerical simulation of flow patterns and mixing behaviour in Kenics static mixer over a wide range of Reynolds number. Three different sets of Kenics mixer (aspect ratio = 1.5) comprised of 3, 9 and 25 elements each have been characterized. The Reynolds number was varied in the range of 1 to 25,000 (i.e., from laminar to turbulent flow regime). The numerical approach takes into account the aspects of the fluid flow at higher Reynolds number values including circumferential velocity profiles at different cross-sections within the Kenics mixer, which were neglected in previous studies. It was observed that cross-sectional mixing in the turbulent flow regime takes place up to 30% of each element length at element-to-element transition; beyond that velocity profiles were uniform. The experiments were also carried out to measure the circumferential and axial velocity profiles and pressure drop in three different Kenics Mixers using air as fluid. The pressure drop per unit element (ΔP/η) was found to be independent of the number of Kenics mixing elements used in the system. The total pressure drop across Kenics mixer obtained by CFD simulations were compared with the experimental pressure drop values and correlations available in the literature. The numerical results were found in good agreement with the experimental as well as the results reported in the literature. A new pressure drop correlation in the Kenics static mixer has been developed.     

Numerical characterisation of folding flow microchannel mixers

Micromixers have been considered in numerous recent studies with the aim of mixing different liquid streams for the common circumstance of non-inertial flow, i.e., in the Stokes flow regime. Under such conditions, the diffusion of momentum is dominant but the diffusion of species remains weak because the Schmidt number of liquids is large. Most mixers that have potential for application in the Stokes regime make use of a folding flow pattern that approximates the baker's transformation. In the work presented here, the general scaling of mixers of this type is developed from the exact equation for species transport and computations are made for a specimen mixer geometry to test the effectiveness of the resulting scaling. The scaling relation developed is found to give an excellent representation of the actual mixing characteristics of the specimen mixer over the entire range of Péclet number of practical interest. Finite volume computations are employed to solve the governing equations up to around Pe=10³. At higher Péclet numbers, where finite volume numerical solution becomes inaccurate with affordable mesh sizes, the species equation is solved using a Monte Carlo method instead. Finally, the scaling relation is used to develop the design relations needed to determine the number of mixing elements, the pressure drop incurred and the Péclet number of operation to achieve a given mixture uniformity within a specified mixing time.