Experimental Measurement Of Gas-liquid Interfacial Rayleigh-Bénard-Marangoni Convection And Mass Transfer

In the gas-liquid and liquid-liquid interfacial mass transfer process, surface tension gradient and density gradient caused by temperature gradient and concentration gradient can generate interfacial instability and ultimately induce interfacial turbulence, which may significantly affect the mass transfer. The interfacial turbulence, also known as interfacial convection induced by surface tension gradient is referred to as Marangoni effect and that induced by density gradient is referred to as Rayleigh-Benard effect. In this desertation, the interfacial turbulence phenomena:the Marangoni effect and the Rayleigh-Benard effect are investigated.Firstly, the Marangoni and Rayleigh-Benard interfacial convections on a horizontal gas-liquid interface are experimentally measured by Particle Image Velocimetry (PIV). In the experiments, a simulator of horizontal gas-liquid interfacial mass transfer process is established. By making use of PIV, quantitative measurements of the velocity distributions are made for the Marangoni and Rayleigh-Benard interfacial convections generated in the liquid phase of a gas-liquid mass transfer of binary systems. The results indicate that Marangoni convection has effect mainly on the vicinity region of the interface, while Rayleigh-Benard convection has effect mainly on the bulk liquid. With both effect of Marangoni and Rayleigh-Benard convections, interfacial liquid is forced by both gravity in vertical direction and surface tension in horizontal direction, where an obvious roller eddies are generated.Secondly, Rayleigh convection and its effect on mass transfer are studied by vortex average velocity, characteristic scale and other statistical analyses. The results indicate that Rayleigh convection induces obvious turbulent vortexes in the liquid bulk. Mass transfer coefficient and the enhancement factor are estimated and they are indicated that Rayleigh convection promotes surface renewal and intensifies mass transfer significantly. Two new methods are proposed to estimate mass transfer coefficient. One is by calculation with the surface residence time using characteristic scales and surface velocity, and the other is by the concentration distribution predicted by velocity vector with fluid dynamic models. And both results agree with the experimental data.Thirdly, the thermal Marangoni flow fields in sessile droplets are measured by PIV, which encompass a wide range of droplet volatilities. There are converse thermal Marangoni convectional directions for different droplet evaporation rates. An approximate semianalytical solution is using to solve the flow field in the drying droplet in the present of Marangoni stress neglecting the effect of convection due to temperature distribution. Different simulation methods are used to predict the thermal Marangoni flow in the drying droplet with and without droplet distortion because of evaporation.And lastly, solutal Marangoni convection induced by ethanol desorption from a sessile ethanol-water bicomponent droplet is investigated experimentally by different methods and different measurement plane. A digital microscope qualitative measurement shows Marangoni convection sensitivity to surface concentration and viscosity. Micro-PIV quantitatively measures the velocity distribution on the top sheet of the droplet parallel to the glass slide. Velocity vector graphs display Marangoni convection fluctuant and inordinance. Experimental results indicate that convectional intensities reduce with the decrease in concentration and with the increase in viscosity. Mass transfer coefficients produce corresponding variations in flow velocities that indicate Marangoni convection influence on mass transfer with droplet interface breakdown and renewal and the interfacial oscillation will inevitably enhance mass transfer efficiency. Velocity measurement in the plane perpendicular to the substrate indicates solutal Marangoni flow in dominant with higher ethanol concentration. And statistical analysis of fluctuant velocity shows no obvious period in Marangoni flow.