Imaging of Fluid Dynamics in a Structured Packing Using X‐ray Computed Tomography

For the investigation of fluid dynamics from computed tomography (CT) images, an evaluation method was developed that enables data acquisition directly from the recorded CT images. A segmentation algorithm was implemented in Matlab to assign every component a specific gray value. A comparison of weight experiments with CT scans and following segmentation demonstrates the good accuracy of this method. The liquid holdup in different Mellapak 500.Y packings was determined through CT measurements and compared to predictive correlations from the literature. Three‐dimensional visualizations of short packing sections enable an evaluation of the liquid distribution and the flow morphology of the liquid in the packing. The common assumption of an evenly wetted packing surface with a constant liquid film thickness could not be proved.     

冷却塔淋水填料性能试验台的研制

针对目前的冷却塔淋水填料性能试验台存在的不足,研制出了新的填料性能试验台。填料的高度和喷淋高度可调,以适应水量变化的要求;进塔空气状态通过多种组合方式进行调节;考虑了送风和淋水的均匀性,并考虑了壁流的问题;风量和水量都由变频器加以控制。试验结果表明,该试验台送风、淋水均匀度高,测试数据重复性高、可靠性高、精准度高。试验台具有灵活性和可靠性,能满足各种填料和不同运行工况的要求。     

散装填料塔载点气速的计算与应用

指导出了散装填料层的载点关联式,通过对6种填料的填料层载点气速的计算表明,本文关联式的计算值与实验值吻合较好,计算准确性优于文献公式。简述载点气速的应用。     

Binder jetting additive manufacturing of silicon carbide ceramics: Development of bimodal powder feedstocks by modeling and experimental methods

A limitation of binder jetting additive manufacturing is the low density of fabricated parts. Mixing powders with different sizes is a promising approach to increase powder bed packing density and, hence, printed part density. However, in previous studies mixed powder feedstock was prepared by trial and error method. In this research, both modeling and experimental methods were used to prepare the bimodal powder feedstocks. Analytical packing model was introduced for irregular powders. A bimodal powder was prepared by mixing two different-sized silicon carbide powders (i.e. coarse and fine) using ball mill, and their tap densities were measured. Silicon carbide plates were printed using the coarse and bimodal powders by a commercial binder jetting system. Results showed that the modeling method could predict the tap density of bimodal powders with high accuracy. The printed parts from bimodal powder achieved higher green densities than those from the unimodal powder.     

波纹填料层压降的新关联

导出了计算波纹填料层压降的半经验关联式，在包括泛点气速的多种气速下关联式对于波纹填料层压降的计算值与实验数据吻合较好，计算准确性优于文献公式。     

Sphere packing with a geometric based compression algorithm

An efficient algorithm for the random packing of spheres can significantly save the cost of the preparation of an initial configuration often required in discrete element simulations. It is not trivial to generate such random packing at a large scale, particularly when spheres of various sizes and geometric domains of different shapes are present. Motivated by the idea of compression complemented by an efficient physical process to increase packing density, shaking, a new approach, termed compression algorithm, is proposed in this work to randomly fill any arbitrary polyhedral or cylindrical domains with spheres of various sizes. The algorithm features both simplicity and high efficiency. Tests show that it takes 181 s on a 1.4-GHz PC to complete the filling of a cylindrical domain with a total number of 26,787 spheres, achieving a packing density of 52.89%.     

Cylindrical object contact detection for use in discrete element method simulations,Part II—Experimental validation

This article is the second of two focusing on cylindrical object contact detection. The first part presents algorithms for determining if contact occurs between a cylindrical object and a plane, and between two cylindrical objects, in addition to expressions for contact overlap, contact normal vector, and contact location. This article presents the tests and results used to quantitatively validate the algorithms implemented in three-dimensional discrete element method (DEM) simulations. In particular, four tests are performed. The first consists of a comparison against analytical expressions for post-collision translational and rotational velocities for a single cylinder impacting a flat wall. The second involves comparisons against experimental residence time measurements of a cylindrical particle moving within a baffled, rotating horizontal cylinder. The third test consists of comparisons against experimental bulk density measurements of cylindrical particles dropped into a cylindrical container. The last test compares experimental and computational values of the dynamic angle of repose in a rotating drum. In addition to true cylindrical objects, comparisons are also made in the first three tests against two cylindrical object approximations using glued spheres. The true cylinder model performs well in each of the tests while the glued sphere approximations perform poorly, especially when fewer spheres are used to represent the cylinder shape. These studies demonstrate that the cylinder contact algorithms presented in the first article perform correctly, and more accurately model real cylinder behavior than glued sphere approximations.     

规整填料内单相流的LDV实验研究

To date, many models have been developed to calculate the flow field in the structured packing by the computational fluid dynamics （CFD） technique, but little experimental work has been carried out to serve the vali-dation of flow simulation. In this work, the velocity profiles of single-phase flow in structured packing are measured at the Reynolds numbers of 20.0, 55.7 and 520.1, using the laser Doppler velocimetry （LDV）. The time-averaged and instantaneous velocities of three components are obtained simultaneously. The CFD simulation is also carried out to numerically predict the velocity distribution within the structured packing. Comparison shows that the flow pattern, velocity distribution and turbulent kinetic energy （for turbulent flow） on the horizontal plane predicted by CFD simulation are in good agreement with the LDV measured data. The values of the x-and z-velocity components are quantitatively well predicted over the plane in the center of the packing, but the predicted y-component is sig-nificantly smaller than the experimental data. It can be concluded that experimental measurement is important for further improvement of CFD model.     

Packing of Cylindrical Particles with a Length Distribution

The packing of cylindrical particles with log-normal and modified power-law length distributions has been experimentally studied. The results indicate that the packing density is heavily dependent on the parameters in the two distributions. However, this dependence cannot be predicted by the direct analogy to that for the packing of spherical particles. It is postulated that the packing of nonspherical particles be governed by two factors: the shape effect and the size effect, which respectively correspond to the unmixing and mixing states of a particle mixture and are quantified from the specific volumes of the two states. Analysis of the results suggests that the shape effect is dominant for the packing of cylindrical particles with a wide length distribution.     

An exact method for determining local solid fractions in discrete element method simulations

A novel solid fraction algorithm is presented which accounts for the partial volume of a sphere straddling cuboidal bin boundaries. The algorithm accounts for spheres intersecting a single plane (face), two perpendicular planes (edge), or three perpendicular planes (corner). Comparisons are made against the more common algorithm in which the solid fraction is determined by assigning the sphere's total volume to the bin in which the sphere's center of volume (COV) is located. Bin size‐to‐sphere diameter ratios >30 must be used to give errors <5% when using the traditional method when applied to simple cubic (SC) and hexagonal packing assemblies. Bin size‐to‐sphere diameter ratios larger than five are required for random sphere packings. Although time averaged solid fraction measurements are similar using either the exact or COV solid fraction schemes, the scatter in the COV method is much larger than for the exact method. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

XD-1 新型锥度螺旋填料的流体力学特性与机理

XD-1型锥度螺旋填料独特的锥度螺旋结构能使气液二相由逆流接触转变为错流接触,使之与常规填料有完全不同的流体力学特性。在80 mm的冷模实验塔内,用空气-水物系测定了填料的流体力学特性,还与θ环、拉西环和丝网波纹填料进行了比较。实验结果表明,在相同喷淋密度下,锥度螺旋、拉西环、θ环和波纹丝网4种填料的空隙率之比为:0.823∶0.781∶0.947∶0.855时,单位填料层压降之比为:0.58∶1.35∶1.08∶0.65,泛点气速之比为:0.725∶0.280∶0.658∶0.703。     

计算散装填料层泛点的新方法

在半理论分析的基础上，导出了散装填料层泛点气体速度的关联式，由实验数据计算了常用填料本文关联式的液泛填料因子。本文泛点关联式对8种填料泛点气速的计算值与实验数据的总平均计算误差为3.52%，新关联方法与实验数据吻合很好，计算准确性优于文献公式，且比文献公式计算简便。     

Errors implicit in digital particle characterisation

Factors affecting the accuracy of using digitised images of projected particles in order to determine particle properties such as shape, size and particle perimeter are defined. In particular, the inaccuracies in the representation of particles of known shapes as digitised particles under different resolutions due to loss of information at the particle boundary are quantified. The results are generalised for arbitrary shapes by the use of mathematical proof and implications are considered for relevant particle analysis techniques. An attempt is made to find relationships between resolution and accuracy, and between particle shape and accuracy. Accuracy here is defined as the deviation of the area of the digitised image from the actual area of the real particle. Implications for accuracy of 3D digitised representations of particles are highlighted and its relevance to predictions of 3D particle packing based on the digitised data.     

规整填料层压降的计算及与散装填料的比较

根据文献[6]导出的规整填料层压降关联式,由实验数据计算出9种规整填料本文关联式的压降常数,用空气-水物系和非水物系的实验数据进行了验证,并与Billet关联式计算值和实验值进行了比较;对规整填料层与散装填料的泛点和压降进行了比较。结果表明:该关联式与实验数据吻合较好,压降计算误差在±15％;规整填料力学性能优于散装填料。     

液膜性质的小尺度研究

Structured packing is a good candidate for CO₂ capture process because of its higher mass transfer efficiency and lower pressure drop.Now,the challenging problem of CO₂ capture and storage demands more and more efficiency equipment.The aim of the present study is to investigate the liquid film characteristics under counter current gas phase and throw some insight into the enhancing mechanism of mass transfer performance in structured packing.A high speed digital camera,non-intrusive measurement technique,was used.Water and air were working fluids.Experiments were carried out for different gas/liquid flow rates and different inclination angles.The time-average and instantaneous film widths for each set of flow parameters were calculated.It is shown that the effects of gas phase could be neglected for lower flow rate,and then,become more pronounced at higher flow rate.According to instantaneous film width,three different stages can be distinguished.One is the constant width of liquid film.The second is the slight decrease of film width and the smooth surface.This kind of character will lead to less interfacial area and deteriorate the packing mass transfer performance.For the third stage,the variation of film width shows clearly chaotic behavior.The prediction model was also developed in present work.The predicted and experimental results are in good agreement.     

波纹形规整填料的几何关系

由波纹形规整填料的几何形状,严格推导出其各几何量之间的关系和成形的填料板片与其成形前原始板片的纵向及横向收缩比,这将有利于填料制造厂商改进产品质量,和有依据地进行成本核算。并探讨了液滴在波纹板片上的流动轨迹,有助于对液体在这类填料层内分布作进一步研究。     

Numerically packing spheres in cylinders

A novel approach has been developed to numerically pack spheres in cylinders. The packing algorithm uses a simple sequential technique that is based on a dimensionless packing parameter. The dimensionless packing parameter includes both axial and radial variables in order to determine a sphere's sequential placement within a cylindrical packing structure. The numerical simulation has been applied to the loose packing of identical spheres in cylindrical containers with D/d ≥ 2.0. The predicted results for the mean porosity and the radial porosity distributions are validated against the available experimental data for spheres in cylindrical containers. The simulated results are highly accurate and the simple packing algorithm requires minimal computational prerequisites.     

Shape influences on the packing density of frustums

The shape of a frustum is a bivariate function of aspect ratio (w) and base radii ratio (c). To investigate the shape influences on the packing density of frustums, random packings of cones (c = 0), truncated cones and cylinders (c = 1) with various heights and diameters are studied through numerical simulations. An improved relaxation algorithm with assembly sphere models for non-spherical particles is applied in the numerical simulations, and the randomness of the packings of the frustums considered is verified. Base on the simulation results, the relationship between the packing density and shape parameters is illustrated, and an empirical formula is proposed to reflect the correlation. It shows that, for the particles having the same w, truncated cones can be packed denser than cones, but looser than cylinders. The packing density of truncated cones first increases and then decreases with the growth of w, while the packing density increases monotonically with the increase of c. No obvious peak is found on the curve of packing density versus c for the random packing of truncated cones, which is different from the reported results for ordered packing where a peak was identified. Furthermore, we observe that the packing density of truncated cones is a linear superposition of the c effect on w effect, and the effects of c and w on the packing density can be treated independently. The optimal aspect ratios of truncated cones, which give the highest packing densities, are all around 0.8. The highest packing densities of cylinders and cones are also the upper and lower bound of the random packing density of truncated cones.     

A new multi‐scale model based on CFD and macroscopic calculation for corrugated structured packing column

A multi‐scale approach with the combination of computational fluid dynamic (CFD) and macroscopic calculation methods has been proposed to predict the hydrodynamics behavior in the corrugated structured packing column. On the basis of the concept of the representative unit, the three‐dimensional (3‐D) volume of fluid (VOF) model of the structured packing is applied in the small scale simulation, and the stream split fraction coefficients and effective wetted area ratio are calculated. The unit network model, which is a mechanistic model, is applied in large scale calculation basing on the small scale results. The liquid holdup distribution in the entire column can be available by this multi‐scale method. A comparison between the simulation results and the experimental data of our previous work is given to validate the present model. The multi‐scale model is proved to be prospective to assist the analysis and design of structure packing columns in chemical engineering. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3119–3130, 2013