基于格子Boltzmann方法模拟纳米流体强化传质过程

使用Boltzmann方法对纳米流体中的传质过程进行了模拟,给出了修改后的Boltzmann方程和计算纳米流体中扩散系数的方程,通过两种方法,基于有限体积颗粒的LBE方法和基于点源颗粒的LBE方法进行了模拟,并与宣益民的实验进行了对比。最后计算了CO2在纳米流体中的扩散系数,计算结果表明,纳米流体因为其纳米颗粒的微扰动对传质有着很大的强化效果,为CO2吸收提供了一种新思路,并且证明了纳米流体的强化传质主要是靠对流传质。     

Pore-scale study based on lattice Boltzmann method of density driven natural convection during CO2 injection project

Saline aquifers are chosen for geological storage of greenhouse gas CO₂ because of their storage potential. In almost all cases of practical interest, CO₂ is present on top of the liquid and CO₂ dissolution leads to a small increase in the density of the aqueous phase. This situation results in the creation of negative buoyancy force for downward density-driven natural convection and consequently enhances CO₂ sequestration. In order to study CO₂ injection at pore-level, an isothermal Lattice Boltzmann Model (LBM)with two distribution functions is adopted to simulate density-driven natural convection in porous media with irregular geometry obtained by image treatment. The present analysis showed that after the onset of natural convection instability, the brine with a high CO₂ concentration infringed into the underlying unaffected brine, in favor of the migration of CO₂ into the pore structure. With low Rayleigh numbers, the instantaneous mass flux and total dissolved CO₂ mass are very close to that derived from penetration theory (diffusion only), but the fluxes are significantly enhanced with high Ra number. The simulated results show that as the time increases, some chaotic and recirculation zones in the flow appear obviously, which promotes the renewal of interfacial liquid, and hence enhances dissolution of CO₂ into brine. This study is focused on the scale of a few pores, but shows implications in enhanced oil/gas recovery with CO₂ sequestration in aquifers.     

Simulation of interfacial Marangoni convection in gas-liquid mass transfer by lattice Boltzmann method

Interfacial Marangoni convection has significant effect on gas-liquid and/or liquid-liquid mass transfer processes. In this paper, an approach based on lattice Boltzmann method is established and two perturbation models, fixed perturbation model and self-renewable interface model, are proposed for the simulation of interfacial Marangoni convection in gas-liquid mass transfer process. The simulation results show that the concentration contours are well consistent with the typical roll cell convection patterns obtained experimentally in previous studies.     

Rayleigh对流及其对界面传质影响模拟的格子Boltzmann方法

针对乙醇吸收CO₂过程中,由CO₂通过界面向液体乙醇传递所导致的Rayleigh对流现象的模拟,建立了二维格子Boltzmann方法（LBM）。采用浓度分布函数和流体质点密度函数的双分布模型格子Boltzmann方法,同时引入由浓度差导致的重度差作为外加力,实现了流体中浓度场与速度场的模拟。应用所建立的LBM方法,对界面具有多个离散CO₂扩散源的二维区域液相Rayleigh对流现象进行了模拟,结果显示,模拟得到的浓度分布结构与文献中实验结果相一致。通过考察Rayleigh对流和浓度分布结构,分析了Rayleigh对流存在条件下的传递规律。通过采用LBM方法对浓度场模拟可以定量给出液相界面瞬时传质通量。计算结果表明,瞬时传质通量随时间的增长先增加后减小,这种变化与相似文献   

界面传质中Rayleigh对流的格子Boltzmann模拟和实验验证(英文)

Concentration gradient induced Rayleigh convection can influence effectively interfacial mass transfer processes, but the convection phenomena are known as mesoscopic and complex. In order to investigate this phe-nomenon, a two-equation Lattice Boltzmann Method (LBM) is proposed to simulate the velocity and the concentra-tion distributions of Rayleigh convection generated in the CO2 absorption into ethanol liquid. The simulated results on velocity distributions are experimentally verified by PIV (particle image velocimetry technique) measurements. In order to simplify the analysis, the convection in the simulation as well as in the experiment, the Rayleigh convec-tion was manipulated into a single down flow pattern. The simulated results show that the concentration contours agree qualitatively with the schlieren images in the literature. The experimental and simulated results show that the Rayleigh convection under investigation is dominated by the flow in the downward direction and impels exchange of the liquid between the interfacial vicinity and the liquid bulk promoting the renewal of interfacial liquid, and hence enhances mass transfer. The comparison between the simulated and experimental results demonstrated that the proposed LBM is a promising alternative for simulating mass transfer induced Rayleigh convection.     

REV尺度多孔介质格子Boltzmann方法的数学模型及应用的研究进展

综述了多孔介质表征体元尺度(REV)格子Boltzmann模型的研究进展,根据对多孔介质处理方式主要分为部分反弹模型和阻力模型两类,分析归纳了各类模型的优缺点。由于阻力模型中渗流的广义格子Boltzmann方程(GLBE)的作用力是基于GUO等的作用力模型,可以准确得到宏观方程,不存在离散误差,且模型的平衡分布函数和作用力项中都包含反应介质特性的孔隙率,因而应用最为广泛。本文还重点介绍了REV尺度多孔介质LBE模型在流动、传热、传质、化学反应及相变等过程中的具体应用,认为REV尺度多孔介质内的三传一反数学模型中需要加入孔隙尺度因素,在更大工程尺度上应该考虑过程参数的各向异性,展望了REV尺度多孔介质LBE模型的发展和应用前景。     

不可压热流体中气体传质扩散过程的数值模拟

控制流体流动中溶解的气体分子浓度能有效控制流动过程中的化学反应,而由热产生的自然对流能够加强气体分子的传递,因此研究气体分子在热流体流动中的扩散混合过程有重要意义。应用格子Boltzmann方法,耦合热效应和扩散效应,数值模拟了一个简化的容器中随着自然对流的发展,溶解的氧气分子在整个容器中的扩散过程。首先建立了二维9速模型的双扩散模型来模拟热量和质量的双扩散对流。为了考察不同自然对流流动对气体分子传递的影响,设计了3种不同给热条件,对不同热流动的形成过程和气体分子扩散过程进行了模拟,与文献结果吻合良好。通过详细分析热边界如何影响流动和传质过程,证实了模拟的速度场与文献数据差异的合理性,同时为控制气体传质过程提供给热条件的设计依据。     

超临界反溶剂过程传质数学模型

建立了有机溶剂(甲苯)液滴与超临界反溶剂(超临界CO2)之间的传质模型,用于模拟超临界反溶剂制备微纳米粉体材料的传质过程。该模型考虑了双向传质过程,既有反溶剂向溶液的扩散过程,又有溶液中的溶剂向反溶剂的“汽化”过程。液滴的传质行为是影响颗粒形态和尺寸分布的关键因素。假定传质是在一个孤立的微小液滴与包围着它的反溶剂连续相间进行的,利用描述液滴内和液滴外某一点行为的连续方程、扩散方程、能量方程和动量方程,及界面上的守恒条件进行耦合,从而建立传质过程的数学模型,并给出求解方程和求解的边界条件和初始条件,进行数值求解。     

运动颗粒对传质过程影响的格子Boltzmann模拟

颗粒的主动运动对传质过程有重要影响.以表面恒浓度的二维球形颗粒为研究对象,采用耦合传质的格子Boltzmann方法(LBM)模拟了颗粒在自旋和振动两种情况下的相间传质过程.选择浸入运动边界法和非平衡态外推法处理运动颗粒边界,研究了颗粒自旋速度、颗粒振幅及振动频率对传质过程的影响.结果表明,中等雷诺数的自旋颗粒绕流中,随...     

A lattice Boltzmann simulation of mass transport through composite membranes

Composite membranes with a porous support layer and a dense skin layer have been extensively used in gas separation processes. A new approach, a mesoscale Lattice Boltzmann Simulation approach, is proposed and used to model the pore‐scale gas flow and mass transfer in the inhomogeneous membrane matrixes studied. Only physical forces are considered. Chemical forces are equivalently converted to physical forces through the relaxation time. Selective permeation of moisture through a composite membrane is modeled. The overall permeability is evaluated. It is found that mass transfer inhomogeneity exists not only in the porous media but also in the seemingly uniform dense skin layer. Increasing the diffusivity in the skin layer is more effective than decreasing the skin layer thickness in optimizing the overall membrane performance. The new approach gives more detailed insights into the directions for future design of composite membranes for gas separations like air dehumidification. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3925–3938, 2014     

烧结床层的热质分析

基于烧结生产的复杂物理化学过程,建立了烧结床层传热、传质和流动的二维非稳态数学模型,考虑了孔隙率、物料颗粒当量直径等床层结构影响参数的变化,并对气固传热系数进行了修正。通过数值计算,获得了烧结床层的温度场、结构变化和烟气的流场、温度场、浓度场等。烟气出口温度、床层总压降与生产实测值吻合较好,验证了数学模型的正确性。进一步分析了燃料配比、风量和给料温度等操作参数对烧结过程的影响。研究结果表明：燃烧带的厚度、最高温度随着烧结过程的进行而逐渐增加。床层孔隙率、颗粒当量直径的变化主要发生在燃烧带的熔融、冷凝阶段。料层压损最大的是燃烧熔融层,其次是混合料带,最小的是烧结矿层。增加焦粉含量、提高烧结混合料的初温,有利于提高成矿质量;风量过大时,会造成成矿质量下降、生产成本提高。     

流化床液固两相传质过程的模拟研究进展

液固两相流化床具有液固相接触效率高、传质和传热性能好、颗粒分布均匀等优点,已被广泛应用于众多工业过程中。然而,流化床中与传质过程耦合的颗粒流化的复杂非线性特征及其湍动特性,使得对传质过程特性的研究十分困难。且仅依靠实验观测和理论预测难以揭示多相流相互作用规律,无法获得全面和详细的速度场和浓度场分布情况。近年来,数值模拟的快速发展为深入探索流化床中液固两相流动行为及其与传质过程耦合问题提供了重要的途径。本文对流化床液固两相流动与传质过程模拟方法进行了综述,并对其未来研究趋势进行了展望。借助于计算传质学理论可以更精确地预测局部浓度的分布情况,进而可以深入分析液固两相流化床中的传质过程规律与传质特性,为液固两相流化床的设计和优化提供理论基础。     

表面张力对传质过程的影响

表面张力对传质过程的影响主要是影响传质界面积,当物系为正物系时,对板式塔有较小较稳定的汽泡,对填料塔和湿壁塔,有较稳定的液膜,结果使二者均有较大的传质界面积,传质效果较好;负物系的情况相反。当液体混合物的平均表面张力较小时,也有利于形成较小较稳定的汽泡,从而有利于传质。表面张力对传质过程的影响机理相当复杂,这方面的研究工作有待深入开展。     

An experimentally validated heat and mass transfer model for wax deposition from flowing oil onto a cold surface

A transport model is proposed for wax deposition onto a cold finger from flowing wax-containing oils. The model solves transient energy and mass balances simultaneously for a reversible first-order kinetic rate for precipitation of pseudo-single-component wax, and the effects of yield stress using a critical solid wax concentration to withstand flow-induced stress at the deposit-fluid interface. The model can predict the time evolution of the deposit thickness, and the spatial and temporal evolution of temperature and wax concentration as validated using cold finger experiments. It was found that for high wax content oils, deposit thickness growth is dominated by heat transfer. For low wax content oils that are unable to gel, the thickness growth is slow and accompanied by occasional sloughing. Regardless of the mechanism controlling the growth, mass transfer cannot be neglected as wax diffusion into the deposit continues to take place after the deposit has stopped growing.     

Investigation of mass transfer model of CO2 absorption with Rayleigh convection using multi-relaxation time lattice Boltzmann method

CO₂ absorption into absorbents is a widely used method to reduce carbon emissions, in which the concentration gradient near the gas-liquid interface may induce Rayleigh convection (RC). Once RC occurs, the mass transfer rate will be significantly enhanced. Therefore, it is necessary to explore the mass transfer enhancement mechanism further and develop a penetration/surface divergence hybrid mass transfer model. In this study, we conduct research on the process of CO₂ absorption into ethanol with RC. Firstly, we use a multi-relaxation time lattice Boltzmann method to simulate the absorption process and obtain the flow and concentration fields. And we also verify the reliability of the numerical simulation results by comparing with the experimental results. Then, we analyze the characteristics of non-uniform flow and concentration fields in RC. Moreover, we divide the near-interface region into diffusion-dominated and convection-dominated mass transfer zones by checking whether the horizontal average velocity is greater than 1.0×10^-4 m·s^-1. Furthermore, based on the differences in mass transfer mechanisms of the aforementioned two zones, we propose a penetration/surface divergence hybrid model to predict the instantaneous mass transfer coefficient. The prediction results demonstrate that the hybrid model can precisely predict the instantaneous mass transfer coefficient of the entire CO₂ absorption process. Our proposed hybrid model provides a promising way to deal with the complex mass transfer problems with non-uniform flow and concentration fields.     

气液传质理论研究进展

文内对气液传质的理论研究进行了评述，并预测了未来的发展方向     

功能性多孔介质中气泡输运调控的格子Boltzmann分析

为提高毛细蒸发海水淡化技术中的蒸发效率,多孔介质层需要维持一定的毛细压力,同时还要确保气泡能够快速通过。基于此背景,本文建立了多孔介质参数化模型,探究了气泡穿越多孔介质间隙过程的运动特征,研究在保持一定孔隙毛细压力的同时,通过调控孔道尺寸及排布从而使气泡能够更快速地通过多孔介质层。基于格子Boltzmann伪势模型分析了多孔介质孔隙率、壁面润湿特性、孔道排布及气泡水平方向初速度等对气泡形貌、上升速度、与壁面平均接触面积及孔隙毛细力的影响,获得了多孔介质的孔隙率设计范围,骨架润湿特性调控以及孔道排布方式的选择依据。同时还获得了在实际蒸发过程中,可以使气泡存在一定的水平方向初速度,从而能够更快地脱离多孔介质的策略。     

活性焦传热系数的测定

为研究活性焦传热性能,把床层径向传热简化为径向有效导热,将轴向传热看成轴向有效导热和流体流动传热2项组成,自制传热系数测定装置,改变扰流气体流速,测定床层轴向和径向温度分布,利用Matlab软件,采用正交配置法进行数据处理,通过赋予初值,采用最优化方法求解模型偏微分方程,得出径向有效导热系数Ker及壁给热系数Hw。试验结果表明:活性焦有效导热系数约为1.39 W/(m·K),壁给热系数随着颗粒雷诺数的增加而增加。试验测定的壁给热系数与低传热系数材料的壁给热系数经验公式较吻合。     

A study of ionic transfer in electrodialysis process with laminar flow

The concentration profile in a laminar flow of the diluent of a parallel-plate electrodialysis stack is derived by assuming operation at the limiting current density. The Nusselt mass transfer number is found to depend on the Reynolds number, the Schmidt number and the dimensionless hydraulic equivalent diameter according to (Nu)_theo. = 3.7 · (Re · Sc · de/L)^1/3. An empirical correlation of the Reynolds number, the Schmidt number, and the shape factor of the electrodialysis cell, with the ionic mass transfer rate in ion exchange membrane electrodialysis is obtained by measuring the limiting current densities at various conditions. The resulting empirical mass-transfer correlation, (Nu)_exp. = 3.91 · Re^0.333 · Sc^0.328 · (de/L)^0.352, holds for sodium acetate as dialysate at concentrations ranging from 0.0106 to 0.0508 kmol/m³, viscosities ranging from 0.608 to 0.926 mPa · s, temperatures ranging from 15 to 55°C, axial velocities ranging from 0.341 to 4.55 cm/s, and for cell thicknesses of 0.32 and 0.94 cm, and is found to be in agreement with the equation obtained from theory. The effects of concentration, flow rate and temperature on the limiting current density are also studied. The relationship between the thicknesses of the diffusion layer and the Reynolds number has been analyzed.