Continuous Mixing of Fine Particles

The efficiency of reactors fed with particulate mixtures is often reduced by segregation of solids. Placing continuous mixers directly ahead of a reactor may be a solution to this problem. The performance of such mixers can be monitored for appropriate binary solids systems with an optical in-line measuring system. The tracer concentration (SiC or Irgalite) in Al(OH)₃ was registered with high time resolution, thus making possible an extended statistical analysis of mixing processes using the power density spectrum. Experimental mixers with a maximum -throughput of 300 kg/h were fed with a fluctuating tracer concentration and the variance reduction ratios were determined. A model was developed that takes into account feeding constancy, residence time distribution and the limited homogeneity of particulate mixtures. Diagrams for continuous solids mixing processes are derived therefrom. They demonstrate the importance of high-accuracy feed-rate control.     

Application of k- and q-space encoding NMR techniques on granular media in a 3D model fluidized bed reactor

A combination of PFG-NMR imaging and velocity encoding methods was applied to investigate the dynamic behavior of a bed of poppy seeds subjected to air flow, representing a model setup for fluidized bed reactors. The particle motion is described both from a statistical point of view, by determining propagators and dispersion coefficients representing an average over the whole bed volume, as well as combined with spatial resolution by generating velocity maps. Velocity images of different horizontal slices in the bed confirm the notion of a toroidal particle flow pattern inside the shallow granular bed. Despite the need of considerable averaging due to the random motion of the relatively few particles in the bed, quantitative velocity images and statistical information about the random particle motion can be obtained from monitoring the fluid component in the seeds by conventional spin-echo techniques.     

The Flow Patterns and Wall Stresses in a Louvered‐Wall Moving Granular Filter Bed of Quartz Sand

The flow patterns and wall stresses in a two‐dimensional louvered moving granular filter bed of quartz sand were investigated. The flow pattern histories of granular solids in the filter bed were recorded using a digital camcorder and a pressure gauge for simultaneously measuring the normal stresses as well as shear stresses of the granular solids. The static wall stress distributions produced by the granular solids were measured, and agreed with a theoretical prediction formed using the differential slice and Runge‐Kutta (order four) methods. The variations in the dynamic wall stresses with time in a moving granular filter bed were obtained and the effect of the louver angle upon the flow patterns and wall stresses was also investigated. Employing the results obtained by stress measurements and image processing, the wall pressure pulsation phenomena in a moving granular filter bed may be further understood. The results reported here provide fundamental information for the design of moving granular filter beds to act as high‐temperature flue gas cleanup filters.     

Coexistence of two granular temperatures in binary vibrofluidized beds

An investigation into the granular temperature distributions of a binary vibrofluidized granular bed has been conducted using positron emission particle tracking. By repeating each experiment with the tracer selected in turn from the two size components, the granular temperature and packing fraction distributions for each phase were determined. It was found that, for a range of size fractions, the granular temperature of the larger particles was higher than that of the smaller diameter grains, a result which was supported by a simple theoretical analysis based on the steady state energy equation.     

滚筒内非等粒径二元颗粒体系增混机理研究

提出了在圆形滚筒内设置十字形内构件的增混方式,并采用离散单元方法对设置不同大小内构件的滚筒内非等粒径二元颗粒体系的混合进行了数值模拟试验.通过模拟结果重点分析了内构件对混合的影响,讨论了内构件的尺寸对混合效果的作用,分析和探讨了滚筒内构件对二元颗粒体系的增混机理.研究发现,当滚筒内无内构件时,对流、扩散和离析三种作用机制对颗粒体系的混合和分离都起到了重要作用;当滚筒内含内构件时,颗粒的混合则只受到颗粒对流和扩散机制的作用,而颗粒的离析效应得到了很大程度的抑制.十字形内构件很大程度上会破坏滚筒内的自由表面流,从而使发生在自由表面流中的颗粒分离不能发生,最终可有效地增加颗粒之间的混合.对于采用在滚筒内设置十字形内构件的方式来增加颗粒间的混合,存在一个最优的内构件尺寸,内构件过小或过大都不利于颗粒间的混合. 关键词： 分离 混合 离散单元法     

Mixing and Segregation in Multi‐component Liquid Fluidized Beds

The mechanism causing mixing and segregation of multi‐component fluidized systems consisting of both density and size variant particle mixtures is presented. The validity of the model is demonstrated with the experimental results of three‐component (ternary) liquid fluidized systems. Ternary bed fluidized systems separated into different mixed layers and a pure layer of excess component. Location of the stratified mixed layers and pure layer can be predicted from the mixture bulk density evaluation. The volume fractions of solids and fluid corresponding to each mixed layer can be theoretically predicted from the force balance applied to the particles in that layer knowing the systems physical properties.     

Granular Leidenfrost effect in vibrated beds with bumpy surfaces

The effects of subjecting a bed of granular materials to horizontal vibrations by a bumpy oscillating surface have been investigated computationally in this study. The behaviour of the granular bed is determined by the vibration conditions applied which include the vibrating frequency and amplitude as well as the bumpiness of the oscillating surface. Under sufficiently vigorous vibration conditions, the granular Leidenfrost effect whereby the entire granular bed is levitated above the vibrating base by a layer of highly energetic particles may be observed. Granular temperature profiles of systems that exhibit the granular Leidenfrost effect indicate an unequal distribution of energy between particles near the vibrating base and those in the bulk. A bumpy oscillating surface was also observed to be more effective at introducing perturbations and transferring energy into a granular bed. The granular Leidenfrost effect can be induced by the application of larger grain sizes of particles constituting the bumpy vibrating base under vibration conditions that are normally insufficient for the onset of the effect. Lastly, a phase diagram which can be utilized for predicting the behaviours of granular beds that are subjected to oscillations by various types of bumpy surfaces has been constructed based on the simulation results obtained.     

Magnetization transfer contrast imaging of the human leg at 0.1 T: a preliminary study.

Magnetization transfer contrast imaging is an MR technique that capitalizes on interactions between the protons of mobile and macromolecularly bound water molecules. Studies to date, conducted primarily on 4.7 T and 1.5 T MR systems, have yielded results unique from conventional T1- and T2-weighted imaging studies. In this study, performed on a 0.1 T device, a section of lower leg was imaged in 20 normal human subjects and one patient with muscular dystrophy, using both a standard 500/22 gradient-echo sequence and a 500/22 gradient-echo sequence combined with off-resonance radio frequency irradiation designed to elicit magnetization transfer contrast. Results of the two techniques were compared. Our findings suggest that magnetization transfer contrast imaging is feasible at 0.1 T, and that this technique allows reproducible tissue characterization and improves contrast between certain tissues.     

Open problems in active chaotic flows: Competition between chaos and order in granular materials

There are many systems where interaction among the elementary building blocks-no matter how well understood-does not even give a glimpse of the behavior of the global system itself. Characteristic for these systems is the ability to display structure without any external organizing principle being applied. They self-organize as a consequence of synthesis and collective phenomena and the behavior cannot be understood in terms of the systems' constitutive elements alone. A simple example is flowing granular materials, i.e., systems composed of particles or grains. How the grains interact with each other is reasonably well understood; as to how particles move, the governing law is Newton's second law. There are no surprises at this level. However, when the particles are many and the material is vibrated or tumbled, surprising behavior emerges. Systems self-organize in complex patterns that cannot be deduced from the behavior of the particles alone. Self-organization is often the result of competing effects; flowing granular matter displays both mixing and segregation. Small differences in either size or density lead to flow-induced segregation and order; similar to fluids, noncohesive granular materials can display chaotic mixing and disorder. Competition gives rise to a wealth of experimental outcomes. Equilibrium structures, obtained experimentally in quasi-two-dimensional systems, display organization in the presence of disorder, and are captured by a continuum flow model incorporating collisional diffusion and density-driven segregation. Several open issues remain to be addressed. These include analysis of segregating chaotic systems from a dynamical systems viewpoint, and understanding three-dimensional systems and wet granular systems (slurries). General aspects of the competition between chaos-enhanced mixing and properties-induced de-mixing go beyond granular materials and may offer a paradigm for other kinds of physical systems. (c) 2002 American Institute of Physics.     

Fluidization of Nano-size Particles

Fluidization and collapsing bed experiments were performed with 'Tullanox', 10nm diameter fumed silica. The minimum fluidization velocity was determined to be 0.0115m/s at the unusually low volume fraction of solids of 0.0077. The solids volume fraction was measured using a -ray densitometer. Fluidization was without large bubbles, with a high bed expansion ratio. The highest granular temperature was of the order of that of Geldart B particles, as measured by Cody et al. (1996). The sedimentation process was simulated using a two-fluid hydrodynamic model. The input into the model was a measured solids stress modulus and an agglomerate size determined from the settling curves. With these two rheological parameters, there was good agreement between the sedimentation theory and the experiment. This study shows that the standard collapse bed experiment used in industry is a good test of rheological properties of particles.     

Determining uterine blood flow in pregnancy with magnetic resonance imaging

Objective

The purpose of this study is to determine the feasibility of measuring total uterine blood flow in pregnancy using magnetic resonance imaging (MRI) technique.

Methods

Uterine blood flow was determined in pregnant women in whom MRI was being carried out to assess a fetal anomaly. A two-dimensional time-of-flight magnetic resonance (MR) angiogram sequence was performed. Scout images and a peripherally gated phase contrast MR sequence were planned to study simultaneous blood flow in the uterine and ovarian arteries.

Results

The MR pelvic angiogram sequence was completed in 13 women. The uterine arteries were visualized and their cross-sectional area determined. The complexity of the pelvic blood supply prevented the calculation of blood flow velocity and, thus, total uterine blood flow.

Conclusion

The measurement of total uterine blood flow during pregnancy was not possible using our MR technique. The ovarian vessels were not consistently visualized. Doppler ultrasonography remains the best modality by which to estimate total uterine blood flow in pregnancy.     

MR digital subtraction angiography with three-dimensional data acquisition in the diagnosis of brain tumors: preliminary experience

MR digital subtraction angiography (DSA) visualizes intracranial vasculature using a rapid T1-weighted sequence and a bolus injection of gadolinium. Although two-dimensional sequences are most frequently used, we applied a three-dimensional technique in combination with a fast method of k-space filling to improve both the temporal and spatial resolutions. In this preliminary study, we assessed the feasibility of using this technique for the diagnosis of brain tumors in 21 patients by reviewing the obtained images and, in 10 patients, comparing the images with conventional angiograms. MR DSA visualized a tumor stain in 11 patients, a tumor mass effect in 9, and tumor-related vessels in 5. In 9 of the 10 patients for whom conventional angiograms were available, the two kinds of angiograms corresponded well. Three-dimensional MR DSA using our technique is a useful adjunct to conventional MR imaging for the visualization of tumor hemodynamics and, in some cases, tumor-related vessels and mass effects.     

Correlated displacement-T2 MRI by means of a Pulsed Field Gradient-Multi Spin Echo Method

A method for correlated displacement-T2 imaging is presented. A Pulsed Field Gradient-Multi Spin Echo (PFG-MSE) sequence is used to record T2 resolved propagators on a voxel-by-voxel basis, making it possible to perform single voxel correlated displacement-T2 analyses. In spatially heterogeneous media the method thus gives access to sub-voxel information about displacement and T2 relaxation. The sequence is demonstrated using a number of flow conducting model systems: a tube with flowing water of variable intrinsic T2's, mixing fluids of different T2's in an "X"-shaped connector, and an intact living plant. PFG-MSE can be applied to yield information about the relation between flow, pore size and exchange behavior, and can aid volume flow quantification by making it possible to correct for T2 relaxation during the displacement labeling period Delta in PFG displacement imaging methods. Correlated displacement-T2 imaging can be of special interest for a number of research subjects, such as the flow of liquids and mixtures of liquids or liquids and solids moving through microscopic conduits of different sizes (e.g., plants, porous media, bioreactors, biomats).     

MR imaging and cervical fixation devices: Evaluation of ferromagnetism,heating, and artifacts at 1.5 tesla

The purpose of this study was to assess ferromagnetism, heating, and artifacts for cervical fixation devices exposed to a 1.5 T MR system. Cervical fixation devices (three halos, one tong and two halo vests) were evaluated for compatibility with MR procedures. Ferromagnetism was determined using a previously described technique. Heating was evaluated by measuring temperatures at various positions on the cervical fixation devices while applied to a volunteer subject before and during the use of various pulse sequences, including an magnetization transfer contrast (MTC) sequence. Artifacts associated with routine clinical MR imaging of the cervical spine were qualitatively evaluated with the cervical fixation devices applied to a volunteer subject. None of the devices displayed attraction to the magnetic field. The temperature changes were ±1.5°C in each instance. The MTC pulse sequence produced a sensation of “heating” the skull pins that may have been caused by vibration of the cervical fixation device. The MR images of the cervical spine were obtained without apparent artifacts using each routine, clinical pulse sequence. The lack of ferromagnetism, negligible heating, and capability of obtaining diagnostically acceptable studies of the cervical spine indicate that MR imaging performed at 1.5 T or less may be conducted safely in patients with each of the cervical fixation devices tested using conventional pulse sequences.     

A four-element phased array coil for high resolution and parallel MR imaging of the knee

A four-element phased array coil for MR imaging of the knee was designed, built and tested for clinical use at 1.5 Tesla. In routine imaging, it provides over twofold increase in signal-to-noise (SNR) compared to two commercially available knee coils, and supports higher spatial image resolution. The phased array knee coil was also tested for its compatibility with parallel MR imaging that reduces imaging time by several folds over conventional MR technique. Results obtained using SiMultaneous Acquisition of Spatial Harmonics (SMASH) technique shows that our phased array knee coil can be used with parallel MR imaging. These improvements may enhance knee diagnosis with higher image quality and reduced scan time.     

Magnetic resonance imaging in human lymphedema: comparison with lymphangioscintigraphy.

Magnetic resonance (MR) imaging and isotope lymphography (lymphangioscintigraphy, LAS) was done in 32 patients with peripheral lymphedema (19 primary and 13 secondary). MRI characteristically showed diffuse dermal and subcutaneous edema, a nonedematous, occasionally hypertrophied skeletal muscle compartment, variability in regional lymph node size and appearance depending on the underlying clinical disorder, serpiginous "channels" or "lakes" consistent with dermal collateral lymphangiectasis and sequestered lymph, and increased subcutaneous fat. In contrast, LAS showed dermal diffusion ("backflow"), cross-over with retrograde tracer backflow (reflux), delayed tracer transport, and depending on the cause of lymphedema (i.e., primary or secondary), discrete or poorly defined lymph trunks (tracer "bands") and delayed or nonvisualization of regional lymph nodes. Although not a first-line clinical test, MR particularly in conjunction with LAS noninvasively provides accurate anatomical definition of the peripheral lymphatic system. In contradistinction to LAS, MR can visualize lymph trunks, nodes, and soft tissues proximal to sites of lymphatic obstruction. Together these imaging modalities may substitute for conventional oil contrast lymphography in the evaluation of the pathogenesis and evolution of most lymphologic disorders.     

窄振动颗粒床中的运动模式

实验研究了竖直振动情况下,窄容器中颗粒的运动模式.发现运动模式与颗粒床厚度及振动加速度有很强的依赖关系.实验表明横向尺寸较小的容器可以抑制对流卷及拱起现象.对于足够厚的颗粒床,即使振动加速度很大,颗粒床下部仍然存在着颗粒聚集态.出现聚集态时,颗粒床对容器底的冲击力是倍周期分岔的.实验表明倍周期分岔点与颗粒床厚度无关.对于较薄的颗粒床,颗粒可以是聚集态或对流卷,视颗粒尺寸而定.如果使用尺寸分布非常窄的球形颗粒,可以观察到颗粒的有序排列.出现同心的圆筒形“壳”结构,每个“壳”上的颗粒是二维六角密排列的. 关键词： 颗粒物质 倍周期分岔 颗粒聚集态 球堆积     

In situ magnetic resonance measurement of conversion,hydrodynamics and mass transfer during single- and two-phase flow in fixed-bed reactors

In recent years there has been increasing interest in applying magnetic resonance (MR) techniques in areas of engineering and chemical technology. The science that underpins many of these applications is the physics and chemistry of transport and reaction processes in porous materials. Key to the exploitation of MR methods will be our ability to demonstrate that MR yields information that cannot be obtained using conventional measurement techniques in engineering research. This article describes two case studies that highlight the power of MR to give new insights to chemical engineers. First, we demonstrate the application of MR techniques to explore both mass transfer and chemical conversion in situ within a fixed bed of catalyst, and we then use these data to identify the rate-controlling step of the chemical conversion. Second, we implement a rapid imaging technique to study the stability of the gas-liquid distribution in the low- and high-interaction two-phase flow regimes in a trickle-bed reactor.     

数值模拟颗粒旋转对流化床内气固两相流动特性的影响

流化床内颗粒自旋转将影响颗粒相的流动特性.本文运用基于颗粒动理学理论的欧拉-欧拉气固多相流模型,考虑颗粒自旋转流动对颗粒碰撞能量交换和耗散的影响,数值模拟流化床内气体颗粒两相流动特性.计算结果表明颗粒的自旋转使得床内更容易形成气泡,颗粒浓度分布变化增大.颗粒自旋转运动将导致床内非均匀结构更明显.