Schlieren visualization of natural convection in binary gas–liquid systems

Abstract

Natural convection in 23 gas–liquid systems in a quiescent cell is visualized by Schlieren techniques. CO 2 absorption and desorption in 16 organic solvents and MEA aqueous solutions, and evaporation and condensation in six binary liquid mixtures are studied. Experimental data on densities and surface tensions of mixtures under investigation is presented. An empirical linear relationship between the density effect of CO 2 saturation and solvent density is obtained. Visual evidence is discussed in terms of coupled Rayleigh and Marangoni convection. Observed convection patterns are compared and grouped into four classes according to the signs of Ma and Ra numbers. In Rayleigh-unstable systems, density-driven plume convection dominates and interfacial turbulence is absent or weak. In Rayleigh-stable systems, cellular convection, accompanied sometimes by horizontal stratification and/or interfacial turbulence, is observed. Six distinct types of interfacial turbulence are found and explained by a combination of Marangoni instability and thermal effects. A special type of small-scale interfacial turbulence that occurs during Marangoni-stable CO 2 absorption in organic solvents is attributed to the coupling of thermal effects of gaseous absorption and solvent evaporation. Surface temperature histories are studied for CO 2 absorption into four solvents; thermal effects of absorption are responsible for the diverging flow that accompanies absorption. Intensive density-driven convection that takes place during chemical absorption of CO 2 into MEA aqueous solutions is photographed.