Experimental Study On Interfacial Convection In The Gas-Liquid Mass Transfer Process

The differences in the physical properties of the fluid at the interface caused by interfacial mass transfer process may produce interfacial convection. Depending on different driving force, interfacial convection divides into Marangoni convection driven by the interfacial tension gradient and Rayleigh convection driven by the density gradient. The interfacial convection can promote update of the fluid element at the interface and enhance the interfacial mass transfer and its further research is significant important for mass transfer and separation process such as desorption, absorption, extraction and distillation.In this article, a modified Schlieren system and a gas-liquid mass transfer apparatus were used to visualize Marangoni and Rayleigh convection generated in the mass transfer process of different systems. A large number of Schlieren images describing the initiation, development and fading process of interfacial convectional structure were observed.The results of Schlieren observation indicate that interfacial convection is caused by Marangoni and Rayleigh effect, and its structure is rather complex for the differences in the physical properties and thickness of the fluid observed in the experiments. Before interfacial convection happened there is a period of time that mass transfer is in the form of diffusion and the interface is relative stable, and its length depends on the experimental conditions. The fluid at the interface spreading to the surrounding shapes the Marangoni convectional structure, and this structure may be affected by convection formation time and physical properties of the fluid concerned. During the process of CO₂ desorption from ethanol, the convectional structure is round at the beginning, and it turns into polygonal for the result of space restriction. During the process of ethyl acetate desorption from water, the convectional structure is great and its shape is random because the convection of this system is intense. For the Rayleigh instability, the interface changes from the calm into polygons which differs the polygonal Marangoni convectional structure, then these polygons broken and the convectional structure turns into chaotic.In addition, information such as space structure, initiation process and interface deformation about Marangoni convection was studied in detail. Through the observation of top view and front view of Marangoni convection, the movement locus of the fluid element and the space structure of Marangoni convection were attained. By the analysis of single Marangoni convectional structure, its area and radius ratio time histories were studied. The initiation process of Marangoni convection combining with interface deformation was described in detail. Also relative height profile of initial Marangoni convectional structure was studied through quantitative Schlieren, and it's the result of Marangoni convection and inhibited factors.