用于界面对流模拟的格子Boltzmann方法

通过引入外力修正系数和界面对流强度,在介观尺度上运用格子Boltzmann方法模拟乙酸乙酯吸收二氧化碳传质过程的界面Rayleigh对流现象。基于实验测定的流场平均速度值来确定外力修正系数,在外力修正系数确定的条件下,进行了平均速度分析,其结果与实验相符;并且考查了平均浓度和界面对流强度随时间变化关系。

水吸收CO_2过程界面对流的激光诱导荧光观测

建立了采用激光诱导荧光技术观测水吸收CO_2过程中界面对流现象的实验装置,利用荧光素钠作为荧光剂,通过标定获得了CO_2在水中溶解浓度与荧光强度的关系,进而获得了定量测量水中CO_2浓度分布的实验方法。通过建立的实验装置和方法,测量了水吸收CO_2过程中液体近界面Rayleigh对流发生时的浓度分布演化过程。利用实验结果对文献中报道的临界Rayleigh数进行了验证;并对上述吸收体系的界面传质由分子扩散主导逐步转变为对流传质主导的过程,以及Rayleigh对流对吸收过程界面传质的强化进行了定量分析。

气相第2组分对水溶解CO2过程界面对流影响的LIF观测

采用激光诱导荧光(LIF)观测方法考察了在气相中分别添加乙醇和二氯甲烷分别对CO_2在水中溶解过程界面对流的影响,得到了液相中CO_2浓度分布及其演化的观测结果,通过物料衡算得到了相应条件下的液相传质系数。CO_2溶解过程会出现由密度梯度引起的Rayleigh对流。实验结果表明,当添加的乙醇含量小于8.47 mg·L-1时,Rayleigh对流会被增强,进而促进了CO_2的溶解;随着气相中乙醇含量的增大,Rayleigh对流反而被抑制;气相添加二氯甲烷会显著增强Rayleigh对流,提高了CO_2的传质速率,随着气相二氯甲烷含量的增大,CO_2在水中溶解过程的液相传质系数呈现先加强后恒定的趋势。

基于单胞孔隙结构的多孔泡沫界面对流换热系数研究

界面对流换热系数是采用局部热非平衡模型研究多孔泡沫流动传热过程的一个关键参数。本文采用体中心切球法构造多孔泡沫单胞孔隙结构来模拟多孔泡沫真实物理结构,在孔隙结构下进行流动换热过程的数值模拟,计算对流换热系数,并与文献中实验研究获取的努塞尔数对比以验证数值模拟准确性,然后对不同孔隙结构的多孔泡沫流动换热过程进行数值模拟计算,利用数据拟合建立界面对流换热系数准则关联式。基于建立的界面对流换热系数准则关联式,对多孔泡沫在局部热平衡及局部热非平衡两种假设下的流动传热过程进行宏观数值模拟,并与文献实验值进行对比,结果进一步验证了所建立关联式的准确性,说明基于单胞孔隙结构获取界面对流换热系数的方法是有效的,同时也验证了局部热非平衡模型能够更加精确地描述多孔泡沫的流动传热过程。

气相中乙醇对水吸收CO_2过程界面对流的影响

考察了在气相中加入少量乙醇对水吸收CO2过程中液相界面对流的影响。采用粒子成像测速仪(PIV)测定了这一气体吸收过程中液体瞬时流场。通过对液体流场信息进行处理,得到了不同条件下速度和湍动能的数据及其分布;根据流场内速度和湍动能及其分布结构,定量分析了气相中加入乙醇对气液传质过程界面对流的影响,发现气相中乙醇的加入导致了局部Marangoni效应,很大程度上增大了界面附近的对流,为促进气液传质提供了依据。

界面张力梯度驱动对流向湍流转捩的研究

作为空间自然对流热质输运的基本形式,界面张力梯度驱动对流是流动和传热强耦合的复杂非线性过程,也是一个多控制参数耦合作用的过程,表现出丰富的流动时空特征.界面张力梯度驱动对流是微重力流体物理的重要研究内容和学科前沿,同时在空间燃料输运过程和空间能源或热管利用等空间流体管理问题中均有重要应用.本文综述了界面张力梯度驱动对流向湍流转捩研究的背景意义、地面实验、空间实验及数值模拟的研究现状,重点介绍了从非线性动力学角度来研究转捩规律的具体方法,目前最常见的手段是对观测量的时间序列进行分析,通过频谱分析及相空间重构等方法计算时间序列的特征量,从而判断流动模式,这类方法理论成熟,计算简单,但需要对大量数据进行繁琐的处理;另一种方法是通过数值计算分岔来研究对流在时空中的转捩模式,这类方法可以直接计算出分岔点,但是复杂之处在于需要求解大规模的线性或非线性方程组,本文详细阐述了两种方法的理论背景,应用状况及局限性,探讨了将两种方法相互结合,在研究中互为补充的可能,并对今后的研究方向提出了建议.

Characterization of Rayleigh Convection in Interfacial Mass Transfer by Lattice Boltzmann Simulation and Experimental Verification

Concentration gradient induced Rayleigh convection can influence effectively interracial mass transfer processes, but the convection phenomena are known as mesoscopic and complex. In order to investigate this phenomenon, 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 absorptlon 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 convection 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 theRayleigh 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.

Hydrodynamic Instabilities Driven by Acid-base Neutralization Reaction in Immiscible System

The hydrodynamic instabilities driven by an acid-base neutralization reaction, in contact along a plane interface, placed in a Hele-Shaw cell under the gravitational field are reported. The system consists of the heavier aqueous tetramethyle-ammonium hydroxide below the lighter layer of organic phase with propionic acid as reacting specie. The effect of chemical composition on hydrodynamic instabilities during interfacial mass transfer accompanied by a neutralization reaction is investigated. Depending on the initial concentration of the reacting species, Marangoni convection in the form of roll ceils or trains of waves is observed. Mach- Zehnder interferometer is used to measure the change in base concentration at the time of instability formation. The results show that the instabilities resulted from the convection flow are more efficient to the mechanism of mass transfer and can drastically alter pattern formation in the system.

Free dendritic growth model considering both interfacial nonisothermal nature and effect of convection for binary alloy

Considering both the effect of the nonisothermal nature of the interface as well as the effect of forced convection,an extended free dendritic growth model for binary alloys was proposed.Comparative analysis indicates that the effect of convection on solute diffusion is more remarkable compared with the ignorable effect of convection on thermal diffusion at low bath undercooling,due to the fact that solute diffusion coefficient is usually three orders of magnitude less than thermal diffusion coefficient.At high bath undercooling,the effect of convection on the dendritic growth is very slight.Furthermore,a satisfying agreement between the model predictions with the available experiment data for the Cu70Ni30 alloy was obtained,especially at low bath undercoolings,profiting from the higher values of interfacial migration velocity predicted by the present model with nonideal fluid case than that predicted by the one ignoring the effect of convection.

Orientation-dependent Hydrodynamic Instabilities from Chemo-Marangoni Cells to Large Scale Interfacial Deformations

The interplay between chemistry and interfacial-tension-driven hydrodynamic instabilities has been studied experimentally. The system on hand consists of two immiscible liquids separated along an initially plane interface at which an interfacial reaction takes place to produce in situ a surfactant. It is identified that the dynamics of the system depends on the orientation of the Hele-Shaw cell with respect to the vector of gravity. If the nele-Shaw cell is placed vertically, Marangoni cells with vigorous convection develop in both phases along a nearly planar interface. However, if the Hele-Shaw cell is tilted off the gravity, the instabilities in the system are characterized by the large scale interracial deformation with a spatio-temporal periodicity together with the chemo-Marangoni convection. The focus is on the exploration of the transition from the cellular mode to the large scale interfacial deformation.

Convective heat and mass transfer in MHD mixed convection flow of Jeffrey nanofluid over a radially stretching surface with thermal radiation

Mixed convection flow of magnetohydrodynamic(MHD) Jeffrey nanofluid over a radially stretching surface with radiative surface is studied. Radial sheet is considered to be convectively heated. Convective boundary conditions through heat and mass are employed. The governing boundary layer equations are transformed into ordinary differential equations. Convergent series solutions of the resulting problems are derived. Emphasis has been focused on studying the effects of mixed convection, thermal radiation, magnetic field and nanoparticles on the velocity, temperature and concentration fields. Numerical values of the physical parameters involved in the problem are computed for the local Nusselt and Sherwood numbers are computed.

Three-dimensional simulation of interfacial convection in CO_2–ethanol system by hybrid lattice Boltzmann method with experimental validation

By using a hybrid lattice-Boltzmann–finite-difference method(hybrid LBM–FDM method),three-dimensional simulations of solutal interfacial convection were conducted for the process of CO2absorption into ethanol.A self-renewal interface model is adopted as an interfacial perturbation model.The simulation results revealed some three-dimensional features of the induced interfacial convection,such as the development of diverging cellular flow and Rayleigh plume-like convection in liquid phase.The concentration distribution of the simulation result is validated and found to be in well agreement with the Schlieren visualization results qualitatively.Additionally,the mass transfer enhancements by interfacial convection were investigated via both simulation and experiment for the absorption process,and the mass transfer is shown to be enhanced by the interfacial convection by about two-fold comparing with that by diffusion.

Group Theory Analysis of Free Convective BoundaryLayer Behavior at a Stretching Surface

In the Present study, free convection and heat transfer behavior of electrically conducting fluid in the boundary layer over a vertical continuously stretching surface is investigated. The effects of free convection, magnetic field, suction/blowing at the surface and the stretching speed of the surface on the flow and heat transfer characteristics are considered. By applying one-parametric group theory to analysis of the problem, a similarity solution is found. The governing equations of continuity, momentum and energy are solved numerically by a fourth-order Runge-Kutta scheme. The numerical results, which are obtained for the flow and heat transfer characteristics, reveal the influences of the parameters.