液-液两相液层间传质过程的Rayleigh-Bénard-Marangoni对流特性

Rayleigh-Bénard-Marangoni convection characteristics during mass transfer between liquid layers

传质引发的Rayleigh-Bénard-Marangoni对流(RBM对流)对化工传递过程有着显著影响.但是,已有的相关研究多集中于气-液体系,并且有限的针对液-液体系的相关研究尚缺乏对RBM对流演化及其引发的界面扰动行为的深入分析.因此,本文基于阴影法设计搭建了竖直狭缝内液-液两相液层间传质过程的RBM对流特性可视化实验平台,并实验观测了水-甲苯-丙酮三元体系中丙酮组分扩散传质时出现的RBM对流结构以及其向下层水相主体的发展演变过程,探讨了水相丙酮初始浓度、甲苯相丙酮初始浓度以及甲苯层厚度对RBM对流特性和液-液界面形貌的影响.研究表明:在Rayleigh-Taylor不稳定性作用下,水相上层密度(重力)分层"界面"下凸沉降形成波浪形丘状"界面",并随着"界面"处密度与压力失调的加剧而演变成羽状流;因羽流区"界面"不同浓度梯度引起的传质特性差异,羽状流又可以演变成弱羽状流和强羽状流两种形态;当丙酮浓度梯度增大到一定程度后,近界面处短时间内产生大量RBM对流结构,且结构间相互影响增强而聚并成对流团,并随着传质过程的进行,逐渐演变成独立的强羽状流; RBM对流强度与上下液层丙酮浓度梯度大小呈正相关关系,且液-液界面粗糙度及其非稳态波动随着丙酮浓度梯度的增加而增大.

Rayleigh-Bénard-Marangoni convection(RBM convection)induced by the mass transfer has a great influence on the performance of real chemical engineering process.However,the researches of RBM convection characteristics during mass transfer across the interface in liquid-liquid system and their influence on the interface morphology are still limited.In this research,a visualization experiment via the amplified shadowgraph method is conducted to investigate the mass transfer in water-toluene-acetone system in a vertical slit.The convective structure of RBM and its evolution are visually observed.The effects of the initial acetone concentration of aqueous phase and toluene phase,and the thickness of toluene layer on the RBM characteristics and the morphology of the liquid-liquid interface are investigated.The experimental results show that these structures are induced by the interface tension difference along the interface and the vertical density difference caused by non-uniform mass transfer at the interface.As a result of the mass transfer at the interface,the density stratification occurs at the top of the aqueous phase,where the light liquid layer supports heavy one.In addition,non-uniform mass transfer produces perturbation at the top of the aqueous phase,which induces the Rayleigh-Taylor instability at the'interface'between the heavy and light liquid layer.Consequently,a wave-shaped-mound'interface'in the upper aqueous phase is formed as the heavy liquid comes down into the light one,and it can be further evolved into a plume flow with the enhancement of the imbalance between density and pressure at the'interface'.Due to the difference in mass transfer characteristic caused by different concentration gradients in the plume'interface',the plumes can also evolve into weak plumes and strong plumes.Under the large acetone concentration gradient,a number of RBM convective structures are generated near the interface in a short time and the convective cloud is formed due to the dramatic interaction and coalescence between these structures.With the weakening of mass transfer,the convective cloud disappears and the strong plume is gradually formed.In addition,the strength of RBM convection is demonstrated to be positively correlated with the acetone concentration gradient across the aqueous solution-toluene interface.In addition,the roughness of the interface and its unsteady fluctuation grow up with the increase of acetone concentration gradient across the interface.