采用格子Boltzmann方法模拟解吸过程中的Rayleigh对流

应用二维非稳态格子Boltzmann方法研究了异丙醇-水溶液和丙酮-乙酸乙酯溶液解吸过程中Rayleigh对流的临界开始时间、流动特征及其对界面传质的影响,并与相关文献对比. 结果表明,临界开始时间随界面浓度增加呈先缓慢增大再迅速增大最后趋于稳定的变化趋势. Rayleigh对流结构经历了从有序到无序的发展过程,是不断更新的耗散结构. Rayleigh对流主要作用于液相主体,使液相主体具有较大的湍动速度(10-4~10-3 m/s). 液相主体中存在许多循环流动,促进了界面更新及界面与液相主体之间液体的交换与混合. 传质增强因子(介于2~6之间)表明Rayleigh对流能有效提高解吸过程传质速率,强化界面传质过程.     

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   

伴有Rayleigh对流的气液吸收过程数值模拟

Flow and concentration fields of liquid phase in a gas-liquid contacting system are simulated to show the Rayleigh convection by utilizing the finite-element method. The Schlieren images in C02-ethanol system provided direct visual verification of the present simulation, and the simulated results were well consistent with the experimental observation. The influence of the Rayleigh convection on mass transfer is analyzed qualitatively and quantitatively based on the simulated and the experimental results.     

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。     

伴有Rayleigh对流的气液传质理论

浓度梯度导致的Rayleigh对流会对界面传质产生重要影响。采用格子Boltzmann方法(LBM)对吸收过程伴有Rayleigh对流的气液传质理论进行了分析。针对表面更新理论,提出了将近界面处羽状对流结构的宽度作为特征尺度来求得表面更新时间。结果表明,Rayleigh对流的发生能够有效减小双膜理论中的液膜厚度;液相传质系数的LBM模拟值与表面更新理论预测值吻合较好。     

界面Rayleigh对流现象模拟的格子Boltzmann方法

采用格子Boltzmann方法中带有双分布函数的LBGK模型,在介观尺度上实现了对气液传质过程中界面对流现象的模拟,比较了不同Sc数、Ra数以及有局部浓度扰动的界面溶质扩散,对发生界面Rayleigh对流的临界条件进行了研究,发现临界Ra数在1.0×104—2.0×104之间。模拟结果和已有实验现象一致,证明双分布的LBGK模型适用于界面对流及扩散现象的模拟。     

吸收过程Rayleigh对流非稳态数值模拟

通过应用有限元方法,对气液吸收过程中出现的Rayleigh对流发生过程进行了数值模拟计算,得到了液相Rayleigh对流发生时的流场分布和浓度分布定量信息;利用激光纹影仪光学方法,获得了乙醇吸收CO2过程中Rayleigh对流的纹影图像,通过比较模拟计算结果和实验纹影图像,证实模拟计算结果的正确;在模拟计算和实验结果的基础上,对传质过程中Rayleigh对流对传质过程的影响进行了定性和定量的分析,揭示了界面传质导致的Rayleigh对流产生发展过程及对传质的影响.     

分形多孔介质传热传质过程的格子Boltzmann模拟

针对自然界中实际多孔介质具有的分形特性和随机性,利用中点替代算法和二值化处理构造统计上具有分形特性的随机多孔介质。分析了所构造的多孔介质盒维数与Hurst指数之间的关系。基于随机分形构造的原理,对二维实际多孔介质图像进行了重构。利用两点相关函数,分析了重构图像的结构相关性,并与实际目标多孔介质的结构特征进行比较。在与解析解对比验证的基础上,将基于二元混合理论的格子Boltzmann模型(LBM)用于模拟多孔介质内流体扩散过程。通过计算不同分形特性的二维多孔介质的有效扩散系数,研究了重构多孔介质的分形维数与有效扩散系数的关系。利用热耦合LBM模型计算多孔介质内传热过程,分析了不同的分形特性对多孔介质蓄热过程的影响。     

分形多孔介质传热传质过程的格子Boltzmann模拟

针对自然界中实际多孔介质具有的分形特性和随机性,利用中点替代算法和二值化处理构造统计上具有分形特性的随机多孔介质。分析了所构造的多孔介质盒维数与Hurst指数之间的关系。基于随机分形构造的原理,对二维实际多孔介质图像进行了重构。利用两点相关函数,分析了重构图像的结构相关性, 并与实际目标多孔介质的结构特征进行比较。在与解析解对比验证的基础上,将基于二元混合理论的格子Boltzmann模型（LBM）用于模拟多孔介质内流体扩散过程。通过计算不同分形特性的二维多孔介质的有效扩散系数,研究了重构多孔介质的分形维数与有效扩散系数的关系。利用热耦合LBM模型计算多孔介质内传热过程,分析了不同的分形特性对多孔介质蓄热过程的影响。     

并列和错列纤维过滤器稳态除尘过程的 格子Boltzmann模拟

大部分纤维捕集效率和压降的理论模型认为纤维性能仅取决于来流速度、颗粒粒径、纤维体积分数、过滤层厚度、纤维直径等因素。实际上,布袋除尘器的性能还与纤维配置方式直接相关。利用LB(lattice Boltzmann)两相流模型对多层纤维捕集颗粒物过程进行了数值模拟,研究了不同纤维配置方式下系统压降与捕集效率的变化。结果表明,错列纤维的性能参数优于并列纤维;纤维排列间距增大,压降增幅大于捕集效率,导致性能参数下降。通过比较不同位置纤维的捕集能力发现,在布朗扩散和拦截捕集机制主导下,前方纤维捕集能力略强于后方纤维;而在惯性碰撞捕集机制主导时,对捕集贡献最大的主要是前两排纤维,后方纤维对捕集效率的贡献非常小,可以忽略。这些研究结果可以对布袋除尘器的多层纤维配置方式的优化提供理论依据和工程建议。     

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.     

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.     

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.     

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。     

Absorption and desorption of carbon dioxide in several water types

This study investigated the effect of water type on the rate of CO₂ transfer from/to an aqueous phase with varying degree of water salinity. The absorption and desorption experiments were conducted on reverse osmosis product, brackish well, and brackish water reverse osmosis reject waters as well as seawater in a mechanically agitated tank. Results show that the direction of mass transfer has a major impact on the value of the volumetric mass transfer coefficient, k_La, with the absorption experiments always rendering higher values. Furthermore, k_La values always decreased with salinity in both absorption and desorption experiments until a certain critical salinity value was reached, beyond which mass transfer increased again. However, k_La values were found to decrease continuously with an increase in the water alkalinity in absorption experiments, while no clear conclusion could be drawn for the alkalinity effect in the case of desorption experiments. These observations suggest that the effect of alkalinity should be further investigated to elucidate its impact along with the salinity on the volumetric mass transfer rate.     

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...     

界面传质中Rayleigh对流的定量分析

通过纹影光路观察了特定气液传质装置中乙醇吸收CO₂过程所引发的Rayleigh对流在垂直界面方向上的发展过程。随着溶质吸收的进行,液层的流体稳定性变弱,扰动加剧气液界面失稳并发生湍动,进而发展为羽状流并逐步向液相主体发展,在此过程中伴随着对流胞的融合与增长。液层的浓度分布可通过对相应液层纹影图像进行定量分析获得。液层浓度分布和瞬时传质系数变化表征了Rayleigh对流的引发与发展及其对传质过程的强化效果,界面浓度分布及临界Rayleigh数解释了非均匀传质对湍动的引发机理。羽状流将高浓度液体快速带入主体,加速了近界面液层与主体液层的混合,增强了气液传质。