Effect mechanism of multiple obstacles on non-Newtonian flow in ceramic 3D printing (arcuate elements)

Direct ink writing (DIW), as an additive manufacturing method, can effectively mould ceramic parts. The single screw extruder is used here to extrude viscous SiC slurry. The deposits caused by the low viscosity and the agglomerations resulting from the nonuniform mixing, form the obstacles in the channel, which affect the normal theoretical flow of the slurry, and interaction with other printing parameters. Therefore, it is necessary to explore the effect mechanism of the obstacles on the flow. The obstacles are always irregular, which makes it difficult to directly analyse them. The irregular geometries are always composed of linear and/or arcuate elements; therefore, the obstacles can be simplified into regular geometries. In the previous work, linear elements have been analysed first. As the continuous work, arcuate elements are investigated in the current research. To conduct the required simulations, an improved MRT LBM (multiple relaxation time lattice Boltzmann method) with a pseudo external force is proposed. The above numerical method is combined with the rheological model to analyse cases with two obstacles, and the obtained results are used to reveal the general mechanism in cases with multiple obstacles. The results show that the central angles, radii, and positions of the obstacles are important factors affecting the flow. To obtain a stable and controllable slurry flow, it is recommended that the central angle and the radius should be small enough. The number of obstacles should be minimized, and the position of the last obstacle is expected to be far away from the outlet to avoid the negative velocity. In addition, the adjacent obstacles should maintain a certain distance to ensure the full development of the vortex and avoid affecting the following obstacles or back vortices.     

Numerical study and acceleration of LBM-RANS simulation of turbulent flow

The coupled models of LBM(Lattice Boltzmann Method) and RANS(Reynolds-Averaged Navier–Stokes) are more practical for the transient simulation of mixing processes at large spatial and temporal scales such as crude oil mixing in large-diameter storage tanks. To keep the efficiency of parallel computation of LBM, the RANS model should also be explicitly solved; whereas to keep the numerical stability the implicit method should be better for RANS model. This article explores the numerical stability of explicit methods in 2D cases on one hand, and on the other hand how to accelerate the computation of the coupled model of LBM and an implicitly solved RANS model in 3D cases. To ensure the numerical stability and meanwhile avoid the use of empirical artificial limitations on turbulent quantities in 2D cases, we investigated the impacts of collision models in LBM(LBGK, MRT)and the numerical schemes for convection terms(WENO, TVD) and production terms(FDM, NEQM) in an explicitly solved standard k–ε model. The combination of MRT and TVD or MRT and NEQM can be screened out for the 2D simulation of backward-facing step flow even at Re = 10~7. This scheme combination, however, may still not guarantee the numerical stability in 3D cases and hence much finer grids are required, which is not suitable for the simulation of industrial-scale processes. Then we proposed a new method to accelerate the coupled model of LBM with RANS(implicitly solved). When implemented on multiple GPUs, this new method can achieve 13.5-fold acceleration relative to the original coupled model and 40-fold acceleration compared to the traditional CFD simulation based on Finite Volume(FV) method accelerated by multiple CPUs. This study provides the basis for the transient flow simulation of larger spatial and temporal scales in industrial applications with LBM–RANS methods.     

多孔介质内流体流动的格子Boltzmann模拟

采用格子Boltzmann方法模拟多孔介质内的流动过程,通过预测渗透率,比较了单松弛模型、多松弛模型和熵格子模型在多孔介质计算中的优劣,为研究多松弛模型中各自由参数的影响,选择了12种组合进行模拟,此外,还将大涡模拟与格子Boltzmann方法相结合模拟了多孔介质内高Reynolds数下的流动及流型的转变。结果表明：单松弛模型和熵格子模型预测的渗透率随黏度逐渐增大,而多松弛模型得到的结果随黏度变化很小,另外,多松弛模型中不同松弛参数的组合对结果有较大的影响,通过比较推荐了模拟多孔介质时的最佳组合,计算结果与经验公式吻合较好。大涡模拟与多松弛模型结合较好地预测了多孔介质内流型的转变,Reynolds数越大,多孔介质内的涡越多,并且变大。     

多通道烧嘴水煤浆气化过程数值模拟

针对气化炉烧嘴在使用过程中出现的雾化效果差的问题,采用了DPM颗粒离散多相流模型,研究了同心三通道烧嘴使用过程中的水煤浆管内流动和雾化规律。改变多个影响因素并正交分析后得出:水煤浆雾化受烧嘴内外两气道气体流速变化的影响,气体流速增大时,水煤浆SMD值降低;当增加水煤浆流道锥角,使水煤浆与气体射流充分预混时,SMD值减小明显。此外,增加烧嘴出口节流同样会减小SMD值。综合比较各因素的影响,发现节流尺寸和内喷嘴气体流速对水煤浆的雾化效果影响最大。     

Novel ion mobility analyzers and filters

Various novel mobility analyzer (MA) designs useful for the separation of gas phase ions and charged particles according to their electrical mobilities are discussed. Traditional differential mobility analyzer (DMA) designs have mostly been restricted to two-elements (electrodes) of either parallel or coaxial cylindrical geometries, with the inlet and the outlet on different elements, between which is maintained a large voltage difference. Calculations of the performance of several MA designs free from some such restrictions are presented in the case of potential flows. They include, for example, devices in which (i) the inlet and outlet are on the same element; (ii) the inlet and outlet are at the same voltage (“isopotential devices”); (iii) more than two-elements exist; (iv) a fair fraction of the sheath gas flow passes through one of the elements; (v) all ions within a finite mobility range originating at an inlet point are focused on the outlet (mobility focusing). These latter devices may be more properly described as “Ion Filters” because only ions within a tunable mobility range can reach the outlet. By restricting the “bandwidth” through additional elements or auxiliary sheath gas flow suction/injection, these filters can be used as high resolution DMAs. Isopotential aspiration counters have been used previously as atmospheric ion counters.     

Consolidation grouting technology for fire prevention in mined‐out areas of working face with large inclined angle and its application

The phenomenon of linear flow of slurry in mined‐out areas of working face with large inclined angle is a crucial issue that hinders grouting for fire prevention. The traditional way of grouting exerts a poor effect on inhibiting coal spontaneous combustion, so it is in urgent need of a suitable grouting technology for fire prevention in mined‐out areas with large inclined angle working face. This paper brings up the consolidation grouting method for fire prevention for the first time and conducts a systematic study on the flow characteristics of slurry and sedimentation characteristics of slurry in mined‐out areas with large inclined angle working face. Consolidation slurry materials and their ratio are optimized by experiments. Test platform has been established, and effectiveness of the consolidation grouting for fire prevention has been simulated. The results show that settling velocity of yellow mud increases at first and then decreases as the concentration of consolidation slurry materials ascends. The optimum mass concentration is 0.2%–0.8%. The flow speed of mud is greatly reduced by laying consolidation slurry materials (The decreasing range is approaching 50%). The retention of yellow mud in mined‐out areas has significantly increased. Finally, technology in working site has been developed; meanwhile, compressed air and spray between frames have been used during the period of working face, and the consolidated way of expansion bags in wood crib to lay consolidation slurry materials has been adopted during the period of stopping work. Field application shows that CO concentration at upper corner decreases significantly and eliminates signs of spontaneous combustion. Besides, the flowing water becomes clearer, and effectiveness of the consolidation grouting is remarkable after laying consolidation slurry material band. Research results can provide theoretical guidance to technology for fire hazard prevention in mined‐out areas with large inclined angle. Copyright © 2016 John Wiley & Sons, Ltd.     

Numerical Simulation of the Two‐Phase Fluid Field in a Gas‐Around‐Liquid Spray Nozzle

A new gas‐around‐liquid spray nozzle (GLSN) was designed, and the two‐phase flow fluid field in this nozzle was simulated numerically. Flow characteristics under different structural parameters were obtained by changing the L/D ratio of the premixing chamber, incident angle, and inlet pressures. Increasing the L/D ratio and incident angle improved flow characteristics such as atomization flow, outlet velocity, and turbulence intensity. The nozzle performed optimally at an L/D ratio of 0.5 and incident angle of 60°. The atomization flow decreased with higher gas pressure and increased with higher liquid pressure. The outlet velocity mainly depended on the inlet gas pressure, not on the inlet liquid pressure. These results provide an indication for optimum structures and parameters of the GLSN.     

An alternative presentation of the design parameters for mass flow hoppers

The procedure for determining the critical hopper outlet dimension and wall slope for mass flow hoppers by the Jenike method is well established and documented. However, existing presentations relating the bulk solid flow properties of effective angle of internal friction and the kinematic angle of wall friction with the hopper wall slope and flow factor for mass flow are often inconvenient for manual use and present difficulties when included in design programs suitable for microcomputers.This paper details an alternative presentation of the original Jenike flow factor charts. These alternative charts have been abbreviated to display only the critical design values in the border region between mass flow and funnel flow. The charts eliminate the need for imprecise parameter interpolations by displaying the required design parameters in the form of contours of constant wall slope and critical flow factor as a function of the effective angle of internal friction and the kinematic angle of wall friction.An illustrative example is provided to demonstrate the inherent advantages of this presentation for the evaluation of mass flow hopper geometries.     

Use of Computational Fluid Dynamics to Evaluate Alternative Spray Dryer Chamber Configurations

Abstract

A computational fluid dynamic simulation study was carried out to compare the gas flow and temperature patterns as well as particle trajectories for a pressure nozzle atomized liquid slurry spray dried in four different chamber geometries. The gas and liquid conditions are held the same for all four chamber geometries. The gas flow is co-current and enters and exits centrally thus maintaining a two-dimensional axi-symmetric flow pattern. It is found that as a result of variations in the gas flow and thermal patterns in the four configurations examined (viz cylinder-on-cone, conical, hour-glass shaped, and lantern-shaped axi-symmetric) the droplet trajectories, residence times, and drying times vary. It was noted that the effective chamber volume occupied by drying particles depends on the chamber design.     

Particle Deposition in Laminar Crossflow Filtration of Power Law Slurry

Abstract

A theoretical model for predicting the probability of particle deposition in crossflow filtration of power law slurry is developed. The model is based on the critical angle of friction between depositing particles, which can be estimated by analyzing the forces exerted on the particles. The binding force between the particles due to polymer adsorption plays an important role in the particle deposition. The smaller the flow behavior index of the slurry is, the larger the binding force and the higher the probability of particle deposition will be. The effects of operating conditions such as the crossflow velocity of the slurry and the filtration rate on the probability of particle deposition are also discussed in depth. The calculated values of the probability of particle deposition agree fairly well with the experimental data. A program is designed to simulate the packing structure and the porosity at the cake surface. The porosity increases not only with the increase of the crossflow velocity, but also with the increase of the flow behavior index of the power law slurry.     

Orthogonal Curvilinear Cake Filtration

Abstract

Conventional analysis of cake filtration considers cakes formed under one-dimensional rectilinear flow conditions. Other cake geometries are often encountered in practice, and more generalized equations are needed. An orthogonal curvilinear filter cake equation is derived here for an arbitrary orthogonal cake geometry. This equation is applied to rectangular, cylindrical, oblate-spheroidal, and elliptic-cylindrical geometry cakes through the coordinate scale factors. Experiments are run on Lucite cakes in rectangular and elliptic-cylindrical coordinates. The constitutive parameter, Λ, is determined from experiments in rectangular coordinates and is used to predict the performance of an elliptic-cylindrical cake. Comparison of the predicted results with the experimental results of the elliptic-cylindrical cake shows the consistency of the equation for different geometries. The filter assembly used in the experiments was designed to be pressurized without being submerged within a larger slurry agitation vessel. This allows ready access to the assembly walls for placement of probes for measuring local pressure and porosity in rectangular and elliptic-cylindrical geometries.     

钙基吸收剂吸收CO2过程的格子Boltzmann模拟

近年来,利用CaO吸收烟气中的CO2越来越受到国内外研究者的关注。考虑到单松弛格子Boltzmann模型(LBGK模型)在模拟多孔介质内流动时存在的不足,采用了多松弛(MRT)格子Boltzmann方法来研究多孔CaO吸收CO2的过程,并分析了不同参数,如Damkohler数(Da)、摩尔体积比和CaO颗粒的粒径对CaO转化率的影响。研究结果表明：当Da越大或者摩尔体积比越大时,CaO的转化率会越来越低;另外,如果CaO颗粒的粒径越大,其转化率也将会越低。这些结论定性上与实验结果是吻合的。     

Lattice Boltzmann simulation of asymptotic longitudinal mass dispersion in reconstructed random porous media

To research macroscopic mass transport characteristics of porous media, a lattice Boltzmann method (LBM) approach was utilized to calculate asymptotic longitudinal mass dispersion. In this study, a D2Q9 model with multi‐relaxation‐time (MRT) collision operator, which is appropriate for incompressible flow with a high Péclet number without refining the lattice, was chosen. With respect to the microstructure of porous media, random placement (RP) method was applied to obtain randomly positioned particles. Based on the exhausted numerical results presented in the study, a new correlation of longitudinal mass dispersion was established. By comparing with available experimental data in the literature, reasonable agreements are observed in a wide porosity range from 0.3 to 0.7, indicating the validity of the proposed correlation. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2770–2780, 2018     

加热风前后湿法脱硫直排烟囱二次脱水流场的模拟计算

在安装有二次脱水装置的湿法脱硫直排烟囱内引入热风,可进一步降低烟囱出口含水率,有效解决烟囱飘雨问题。为考察热风对气流流动特性影响,进而优化脱水装置结构,使脱水效果与运行阻力达到最佳匹配,基于CFD方法,采用离散相模型、RNG k-ε双方程湍流模型和组分输运模型组合建立了描述直排烟囱内部流场的流体动力学模型,对其进行了数值模拟,获得了加热风前后烟囱内温度、速度、压强和含水率的变化规律。结果表明,加热风后,烟囱出口处温度上升3 K,速度增加0.6 m·s^-1,压强降低55 Pa,含水率减小29 mg·m^-3,旋流板最佳叶片角度是45°。模拟结果均处于工程实际监测数据波动范围内,验证了模拟合理性和可行性,为优化直排烟囱内部结构和指导工程实际现场取样点提供了理论依据。     

Einfluß der Wandreibung auf die Dimensionierung von Bunkern — Verfahrenstechnische und statische Gesichtspunkte

Influence of wall friction on the design of silos – Aspects of process technology and statics. Mass flow and funnel flow are distinguished when considering the discharge of stored bulk solids. In order to cause mass flow to occur, i. e. the complete bunker filling is in motion, hopper walls need to be steep. Conditions close to the outlet are decisive, a large value of wall friction requiring steep hopper walls. For a safe design of bunker walls with respect to mechanical strength, DIN 1055, part 6 is commonly applied; it is currently being revised. From this regulation a strong influence of the angle of wall friction can be inferred, small values of the angle being associated with high horizontal pressures. In case the angle of wall friction depends on pressure and other parameters and safety factors are included there may be a considerable difference between the angle used by a process engineer and by a civil engineer.     

Simulation of the polymeric fluid flow in the feed distributor of melt blowing process

The polymeric fluid flow in the feed distributor of melt blowing process is simulated using three‐dimensional finite element method. The numerical results are experimentally verified quantitatively and qualitatively using laser Doppler velocimetry and particle image velocimetry respectively. The effects of the distributor's geometric parameters on the uniformity of the transverse flow distribution are investigated. As the manifold angle increases, the flow distribution curve appears to transform gradually from a “hill” shape to a “bone” shape. The uniformity of flow distribution at distributor outlet, especially the fluctuation at the central part, will be improved by increasing the land height. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1570–1574, 2006     

PVC聚合釜温度自控改造与优化

通过测温装置测得聚合釜内浆料和冷却水出口的实时温度,采用串级PID控制方法控制冷却水流量,从而控制聚合釜内的浆料温度。     

水合物浆体在水平细管中的流动相变换热

实验测量了质量分数为0~19.2%的TBAB水合物浆体在恒热流条件下在小管径内流动时的流动传热特性。利用数值计算获得管内浆体的沿程温度分布,在此基础上得到水合物浆体在小管径内的强制对流传热特性。结果表明加热功率对小管径管道中的浆体传热特性有较大影响;在层流状态下,流速对传热系数的影响相比紊流时要小。在实验数据基础上总结出了TBAB水合物浆体在小管径管内的对流换热经验关联式。     

The effect of fin conductance on laminar heat transfer characteristics of internally finned tubes

An analytical solution for the temperature distribution and Nusselt number during fully developed laminar flow in internally finned tubes is obtained taking into consideration the realistic situation of fins with finite thermal conductivities. Axially uniform heat flux and circumferentially uniform temperature are assumed at the outer surface of the tube. For any tube geometry, it was found that the heat transfer characteristics are influenced by a single parameter K, which is directly related to the angle subtended by the fin, and the thermal conductivities of the fluid and the fin material. Nusselt numbers corresponding to different values of K are presented for a range of tube geometries     

细小尺度下潜热型功能热流体压降与传热特性

实验研究了相变微胶囊颗粒(囊芯材料为正十六烷,壳材为尿素-甲醛树脂)和去离子水混合制成的潜热型功能热流体流过等热流细小圆管的流动与传热特性,同时以去离子水作为传热工质在相同条件下进行了对比实验。得到了压降随质量流量的变化规律,实验段出、入口温度以及量纲1出口温度随Reynolds数变化规律,量纲1壁面温度沿轴向的分布规律,平均Nusselt数随Reynolds数的变化关系。结果表明,相变微胶囊颗粒的加入会导致流动压降增大,但随着流量增加,流动压降逐渐与单相液体的接近;出口温度及壁面温度要比相同条件下单相液体的低;含有较小相变微胶囊颗粒浓度的潜热型功能热流体的平均Nusselt数是相同条件下单相液体的2.0～4.0倍。