Analysis of permeability and effective viscosity by CFD on isotropic and anisotropic metallic foams

Hydrodynamic properties of open-cell metallic foams are analyzed at the pore scale on a microtomographied sample from creeping flow to unsteady inertial flow. The influence of the anisotropy, obtained by shearing the initial sample, is also analyzed. The simulations show that the Darcy-Forchheimer law is valid at Re_dl>1 and that the inertial coefficient tensor can be asymmetric as proved by Whitaker [1996. The Forchheimer equation: a theoretical development. Transp. Porous Media 25, 27-61] and checked by the lattice Boltzmann method. Despite this property, the eigen vectors are nearly orthogonal to each other and their orientation follows the shear angle. To evaluate the accuracy of the results, the analysis of the REV leads to a REV size lower than 2.5d_p for the velocity and is lower than 4.5d_p for the inertial flow permeability tensor. It is shown also that the CFD approach, validated by MRI investigation [Graf von den Schulenburg et al., 2007. Flow through an evolving porous media-compressed foam. J. Mater. Sci. 42 6541-6548], matches the 3D-Brinkman-Forchheimer model if the effective viscosity is close to two.     

A parameter characterizing the geometry of expanded metal. I. A general mass transfer correlation for flow-by electrodes of expanded metal and wire cloth

A dimensionless parameter characterizing the geometry of expanded metal is presented and successfully applied in the correlation of mass transfer results obtained by other authors with flow-by electrodes of this material. The geometric term seems to apply also to nets arranged parallel to the flow.     

New predictive correlation for mass transfer coefficient in structured packed extraction columns

Developments in the area of packed columns, particularly structured packed columns, are ongoing, specifically in the area of liquid–liquid extractions in different industries. In the present study, mass transfer coefficients have been obtained experimentally in a structured packed extraction column to develop a new correlation for prediction of continuous phase Sherwood number. The experiments were carried out for toluene/acetic acid/water and n-butyl acetate/acetic acid/water systems with counter current flow in different heights of column. A new dimensionless parameter, d₃₂/h, is introduced in proposed equation. This number considers the effect of column height (h) and mean drop diameter (d₃₂) jointly. The main advantage of this approach is that the principal effect of column height is considered in correlation without which the experimental data could not be fitted with a acceptable accuracy.     

Swelling of crosslinked polyacrylates in isotropic and anisotropic solvents

The purpose of this study was to examine the swelling and deswelling of photochemically crosslinked poly(n‐butylacrylate) networks in isotropic and anisotropic solvents. The phase diagrams were established in terms of composition and temperature for five isotropic solvents, acetone, cyclohexane, methanol, tetrahydrofuran, and toluene, and two low‐molecular‐weight nematic liquid crystals, 4‐cyano‐4′‐n‐pentyl‐biphenyl and an eutectic mixture of cyanoparaphenylenes. Networks were formed by ultraviolet curing in the presence of 0.5 wt % difunctional monomer (hexane diol‐di‐acrylate) and 0.5 wt % photoinitiator (Darocur 1173). Immersion in excess solvent allowed us to measure the solvent uptake by weight and to determine the size increase by optical microscopy in terms of temperature. We calculated weight and diameter ratios considering the swollen‐to‐dry network states of the samples. Phase diagrams were analyzed with the phantom network model according to the Flory–Rehner theory of rubber elasticity, and for the anisotropic solvents, modeling was supplemented with the Maier–Saupe theory of nematic order for free energy. The polymer–solvent interaction parameter was deduced as a function of temperature, but the values were in discrepancy with Fedors's model of solubility parameters, which overestimated the interaction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1–9, 2004     

不同固体除湿方式的热质交换过程分析及性能比较

介绍了固体除湿转轮、热泵型内冷固体除湿床以及与热泵结合的多级固体除湿装置3种固体除湿装置的工作原理,建立了相应的传热传质模型,并通过实验验证了模型的准确性。通过模拟方法,对比了3种除湿装置的除湿效果。结果表明,热泵型内冷固体除湿床的传热传质过程最优,转轮的近似等焓的空气除湿过程最差,与热泵结合的多级固体除湿装置通过分级和内冷改进了转轮的近似等焓的空气处理过程。除湿转轮的再生温度一般在80℃以上,其他两种除湿装置的再生温度均在50℃以下;在达到相同送风含湿量时,除湿转轮的除湿与再生过程近似沿等焓线变化,送风温度很高;而其他两种除湿装置的送风温度比较低,COP能达到4以上。     

Mixing length equation for high schmidt number mass transfer at solid boundaries

A simple modification of the Van Driest damping factor for mixing length calculations of mass and/or heat transfer coefficients at solid boundaries is proposed to increase mass eddy diffusivities in the near-wall region, without affecting velocity predictions. Calculated mass transfer rates with a constant Sc_t = 0.85 are in good agreement with present and previous experimental results obtained in pipe and duct flows at high Schmidt numbers.     

A general correlation for the minimum spouting velocity

A new general correlation is presented for the minimum spouting velocity based on the annular pressure gradient at the top of the bed at minimum spouting. The correlation fits a wide range of experimental data to 11.6% on average. The normalized minimum spouting velocity ratio. u_mS/u_mF, is shown to be a function of three dimensionless parameters.     

Liquid/solid mass transfer in an air-lift loop reactor with a dispersed solid phase

In processes with immobilized cells mass transfer across the boundary layer surrounding the support often plays an important role. Relatively little is known about external mass transfer as a function of the superficial gas velocity in bioreactors such as air-lift loop reactors. In this work ion-exchange resins were used as a solid phase to determine the mass-transfer coefficient in such a reactor. Relations between the Sherwood number and the superficial gas velocity were derived and compared with relations from the literature. Relations in which the Sherwood number is a function of the energy-dissipation rate and relations in which the relative particle velocity is calculated from the rate of free fall of the particle were compared. It was shown that the Sherwood numbers that were functions of the energy-dissipation rates were higher than could be calculated on the basis of the rate of free fall. The Sherwood number obtained was used to calculate the k_l,s of carrageenan gel beads as a solid phase in an air-lift loop reactor. © 1998 SCI.     

The normal spectral emissivity of isotropic and anisotropic materials

The correlation of structural anisotropy with the normal spectral emissivity of pyrolytic graphite is discussed relative to polycrystalline graphite and to isotropic single crystal nickel. Results reported include data in the near infrared (1.5–13μ) at temperatures between 800 and 1200°C in a hydrogen atmosphere. Photomicrographs (up to 1000×), interference photographs, and X-ray orientation patterns were taken of all surfaces before and after the experiments to insure that no surface changes had taken place.The pyrolytic graphite C-face (c-axis perpendicular to the surface) exhibits a spectral dispersion similar to that predicted theoretically for the same orientation of single crystal graphite. The normal spectral emissivity of the C-face is on the average a factor of 2 to 3 lower than the A-face (c-axis parallel to the surface) and similar to a metal in its variation with wavelength. The Hagen-Rubens relation developed for metals in the infrared appears to hold qualitatively for pyrolytic graphite in that the lower emissivity face corresponds to the lower electrical resistivity direction.Normal spectral measurements on a series of cross-sections between the two major orientations of pyrolytic graphite have been made and shown to have similar trends as calculated and observed from visible reflectivity data for single crystal graphite. High purity and dense polycrystalline graphite shows a spectral dispersion similar to the A-face pyrolytic graphite, but with generally higher emissivity values due to the effect of the pores invariably present. The emissivity of pyrolytic and polycrystalline graphites are essentially temperature independent in contrast to the observed orientation dependence of the former.     

A correlation for heat transfer in liquid fluidized beds

A relationship, of the Colburn j-factor type⁽⁹⁾, is derived for heat transfer in fluidized beds. This correlation (10) successfully accounts for the existence of the characteristic maximum heat transfer coefficient at a particular porosity. The derived relationship also predicts the experimentally observed variation of h with change of particle size and density. These variations are mainly a function of the hydrodynamics of the system.     

Liquid–solid mass transfer for cocurrent gas–liquid upflow through solid foam packings

This article presents the liquid–solid mass transfer characteristics for cocurrent upflow operated gas–liquid solid foam packings. Aluminum foam was used with 10, 20, and 40 pores per linear inch (PPI), coated with 5 wt % Pd on γ‐alumina. The effects of gas velocity (u_g = 0.1?0.8 m m s^?1) and liquid velocity (u_l = 0.02 and 0.04 m m s^?1) are studied using the Pd/Bi catalyzed oxidation of glucose. The volumetric liquid–solid mass transfer coefficient, k_lsa_ls, is approximately the same for 10 PPI and 20 PPI solid foams, ranging from 2 × 10^?2 to 9 × 10^?2 m m s^?1. For 40 PPI solid foam, somewhat lower values for k_lsa_ls were found, ranging from 6 × 10^?3 to 4 × 10^?2 m m s^?1. The intrinsic liquid–solid mass transfer coefficient, k_ls, increases with increasing liquid velocity and was found to be proportional to u. Initially, k_ls decreases with increasing gas velocity and after reaching a minimum value increases with increasing gas velocity. The values for k_ls range from 5.5 × 10^?6 to 8 × 10^?4 m m s^?1, which is in the same range as found for random packings and corrugated sheet packings. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

固体热载体法褐煤热解过程中的传质传热特性

为揭示在固定床反应器中固体热载体法快速热解褐煤工艺过程中的热、质传递机理,建立了固体热载体法褐煤热解过程中的传质传热模型。模型包括球型颗粒的一维非稳态导热方程和基于分布活化能模型的动力学模块,分别采用有限容积法与Matlab软件中遗传算法工具箱对二者进行数值计算。通过呼伦贝尔褐煤热重实验数据与温度测定实验数据分别验证了预测的动力学参数及颗粒传热模型结果。研究发现,热、质变化在固体热载体法褐煤热解工艺中呈现复杂的耦合特性。此外,考察了在不同初始温度、热载体进料比与煤颗粒半径条件下,褐煤在热解过程中颗粒内部温度场在径向上随时间的变化规律,并分析了产物释放速率与温度场的关联性,结果表明热历程改变是工艺条件对热解产物分布造成影响的根本原因。     

Liquid–solid mass transfer in a three-phase stationary catalytic basket reactor

Measurements of the liquid–solid mass transfer coefficient k_S were performed in a three-phase multiple impeller and stationary catalytic basket reactor, called a Robinson–Mahoney reactor. Local coefficients were determined using naphtol particle dissolution in water and n-heptane with or without gas flow. Experiments have shown that local coefficients k_{S loc} depend on the particle position in the basket, agitation speed, and liquid properties. A correlation linking k_S to liquid velocity was established in a reactor simulating hydrodynamics in a piece of catalytic basket. Liquid velocity values were validated using a particle image velocimetry technique in a Robinson–Mahoney reactor, using water. © 2005 American Institute of Chemical Engineers AIChE J, 2005     

Liquid—solid mass transfer at vertical screens in bubble columns

The effect of gas sparging on the rate of mass transfer at a vertical single screen was studied by an electrochemical technique which involved measuring the limiting current of the cathodic reduction of potassium ferricyanide. Variables studied were screen characteristics, physical properties of the solution and gas superficial velocity. Screen characteristics, e. g., mesh number and wire diameter were found to have little effect on the rate of mass transfer. The data at different screens were correlated in terms of the superficial gas velocity by the equation St Sc_0.5 = 0.095 (Re Fr)^?0.272, where St is Stanton number, Sc is Schmidt number, Re is Reynolds number and Fr is Froude number. Comparison of the present data with previous results obtained with different transfer geometries has shown that screens produce higher rates of mass transfer than other geometries. The importance of the present work to catalytic and electrochemical reactor design was noted.     

A model for mass transfer in packed towers: mass transfer with controlling resistance in the gas phase

A model for mass transfer in the liquid phase in packed towers, proposed in a previous work[5], is employed in the case of mass transfer with controlling resistance in the gas phase. On taking into account the semistagnant liquid pockets by the liquid residence time distribution function, relationships are established between the volumetric mass transfer coefficients in different operations: evaporation, physical absorption, chemical absorption. The calculated values of the mass transfer coefficients agree well with experimental data.     

Reticulated ceramic foams from alumina-chromia solid solutions: A feasibility study

Open cellular ceramic foams were manufactured from plain and chromia-doped alumina, with a chromium concentration ranging between 1.25 mol% and 5.0 mol%. The (AlCr)₂O₃ starting powders were prepared by precipitation of a chromia precursor onto the surface of an alumina powder and subsequent calcination. Characterization of the starting powders as well as the foam samples made therefrom were carried out with respect to the chromium concentration in the alumina phase and the influence of the dopant on the cellular structure and sintering behavior of the doped material. While no positive effect on the compressive strength of the ceramic foams was found, the dopant influences the sintering behavior resulting in an increased shrinkage and in a reduction of total porosity.     

Heat transfer in metal foams and designed porous media

We present the characterization of heat transfer in commercial metal foam filled tubular reactors in comparison to a designed laser sintered device. The investigations are performed at empty tube Reynolds numbers ranging from 600 to 7600. Volumetric heat transfer performances up to 4.5 MW/(m³ K) were estimated by means of a simple calorimetric measurement setup, which is 3 orders of magnitude higher compared to conventional batch reactors. The heat transfer was found to increase with the ligament diameter ascribed to the enhanced turbulent kinetic energy induced. The fixed wall connection of the fully sintered device, realized by the applied manufacturing method, leads to 30% improvement of the heat transfer compared to no connection. Selective laser sintering was found to be an efficient tool for the design of continuous heat exchanger reactors covering a wide range of applications by simply adapting the geometry.