(535i) Experimental Study on Rayleigh-Bénard-Marangoni Convection Induced By Mass Transfer through Liquid-Liquid Interface

Mass transfer through the liquid-liquid interface, which significantly impacts the production efficiency and product quality, is very common in chemical engineering process. During this process, it is often accompanied with Rayleigh-Bénard-Marangoni (RBM) effect. Therefore, the in-depth study of the RBM convection mechanism is of great significance for practical applications. However, in the previous researches on RBM convection, most of the attentions are paid on the qualitative observation of the convective structure at the interface, so that the evolution process and internal mechanism of the convective structures are ignored, and quantitative researches on the phenomenon of interface turbulence are still limited.

In this research, the water-toluene-acetone ternary system is used in a visual experiment based on the amplified shadowgraph method. The RBM convective structure and its evolution process are observed when the acetone component transfers cross the aqueous solution- toluene interface in a vertical slit. The influence of the initial acetone concentration of aqueous phase, the initial acetone concentration of toluene phase and the thickness of toluene layer are considered to quantitatively investigate the RBM convective. After the acetone is transferred from the aqueous phase to toluene, the Rayleigh-Bénard effect will be arisen owning to the naturally formed vertical density gradient inside the aqueous phase. Under the influence of the Rayleigh-Bénard effect, the mass transfer at the interface is non-uniform. Since the interface tension is related to the mass concentration, and then the Marangoni effect appears near the interface due to the non-uniform interface tension. These two effects can interact with each other to promote the RBM effect inside the aqueous phase. Under the influence of RBM convection, the concentration of each component is heterogeneous, so that the non-uniform refractive index is formed in the aqueous phase and it can be reflected on the projection screen with the amplified shadowgraph method. Therefore, the convection characteristics of RBM can be studied easily by investigating the characteristics and development laws of the projected image.

The visual experiment results show that the density stratification is formed near the aqueous solution- toluene interface when the acetone transfers from the aqueous phase to the toluene phase. Therefore, there is a light liquid layer supporting the heavy one at the top of the aqueous phase. However, due to the non-uniform mass transfer of acetone, perturbations are produced at the “interface” between the light liquid layer and the heavy one. As a result, Rayleigh-Taylor instability is induced, and a wave-shaped-mound “interface” is formed in the upper aqueous phase as the heavy liquid comes down into the light one. Then, with the enhancement of the imbalance between density and pressure at this “interface”, it can be further evolved into a plume flow. When the initial acetone concentration is large, with the drastic mass transfer of acetone, the plume develops into a strong plume which is shaped as an inverted mushroom-cloud. If the initial acetone concentration is small, the lower part of the plume may disappear and evolves into a weak plume. In addition, under the condition of the large acetone concentration, a lot of RBM convective structures generate near the aqueous solution- toluene interface in a short time. Due to the dramatic interaction and coalescence between these convective structures, there is not enough space for them to develop separately, so as to merge into a larger convection cloud. In the later stages of the experiment, the convection clouds can evolve into independent strong plumes gradually duo to the weakening of mass transfer.

Furthermore, accompanying with the increasing of acetone concentration gradient across the interface, more convective structures are produced and develop downwards faster. Due to the strong mass transfer process, intense disturbance appear near the liquid-liquid interface, which result in a rougher liquid-liquid interface. At the same time, the types of structures are more abundant and with higher frequent. Therefore, the convective structures become more complicated and the fluctuations in quantity are strengthened, resulting in larger numerical fluctuations in interface roughness. Under a thicker toluene layer, the concentration gradient between aqueous phase and toluene phase can be maintained in a high level for a longer period of time, thereby enhancing the RBM convection intensity, which in turn has a greater influence on the aqueous solution -toluene interface.