The effects of surfactants on the motion and transport mechanisms of a condensing droplet in a high reynolds number flow

Abstract

The motion and transport mechanisms of a condensing droplet initially contaminated with insoluble monolayer surfactant material are examined through a theoretical approach. The surface tension gradient force induced by the surfactant and the shear stress from the relative motion between droplet and its ambient vapor are evaluated on the droplet surface as major forces affecting the internal motion of the droplet. The strength of the internal motion ranges from one order of magnitude smaller than the free stream velocity for slight surfactant contamination to almost a complete stop in motion for high surfactant concentration. TAE-HO SCOPEIt has been indicated by many review articles that only a small fraction of the published experimental results in the area of bubble and droplet transport phenomena agrees with the existing theory. Experimental liquids (especially water) are very easily contaminated with surface active materials (surfactants), which alter the surface tension and, therefore, a surface tension gradient along the interface develops. The internal motion of a bubble or droplet then is retarded and the convective heat and mass transfer rates are reduced.Kintner ( A comprehensive understanding of the role played by the monolayer surfactants on the fluid motion and the transport mechanisms of bubbles and droplets under phase change in high Reynolds number flows is of fundamental importance. For example, it will shed light on the following processes: the absorption of pollutant species by the condensing raindrops in the atmospheric science area, the effects of surfactants on the formation and motion of vapor bubbles in boiling heat transfer, two-phase flow, combustion of fuel droplets, spray drying, and direct contact heat and mass transfer. The nuclear industry also benefits from this study, as it will contribute to the design of the nuclear reactor containment spray system where surfactants are likely to be present.In this study, the following effects have been examined specifically:1. The effect of monolayer surfactant on the surface conditions, i.e., surface equilibrium conditions and surface tension distribution.2. The effect of monolayer surfactant on the fluid motion inside the droplet.3. The condition for the formation of surfactant cap from monolayer surfactant on the drop surface. CONCLUSIONS AND SIGNIFICANCEThe effects of insoluble monolayer surfactants on the internal circulation of a condensing droplet in high Reynolds number flow have been investigated for droplet radii ranging from 100 to 1,000 pm. The surfactant surface diffusivity varies from to 10-3 m2/s.It is found that surfactants with lower surface diffusion coefficients are more effective in weakening the strength of internal circulation. This may be explained by the convectiondiffusion balanced transport mechanism on the surface. Droplets with more total amount of the same surfactant have slower internal motion because that much more surface area is under surfactant concentration gradients, which oppose the internal motion. The surf...

Modeling the gas absorption in a spray scrubber with dissolving reactive particles

“…Surface-active contaminants, however, can suppress the internal droplet recirculation (Levich, 1962;Clift et al, 1978;Chang and Chung, 1985), implying f u ¡ 1.…”