Study On The Spontaneous Movement Of Liquid Drop During The Condensation Of Water-ethanol Vapor Mixture

During the condensation process of binary vapor mixture(e.g,water-ethanol),the pseudo-dropwise condensation(so called as Marangoni condensation)occurs since the surface tension difference on condensate surface,which is induced by the concentration difference of condensate corresponding to the temperature difference on the condensate surface.The Marangoni condensation is different from the ordinary dropwise condensation formed on the hydrophobic heat transfer surface.The formation of the drop mainly depends on the surface tension difference of the condensate,and the formation is independent of the hydrophilicity of the heat transfer surface.During the condensation process of some binary vapor mixture,the spontaneous movement of condensate drops is driven by the surface tension gradient,which is induced by the concentration gradient along the condensate surface that caused by the bulk temperature gradient on the horizontal heat transfer surface without any external force.On the other hand,when the heat transfer surface does not have bulk temperature gradient(heat transfer surface with uniform temperature),the condensate drops move while the temperature around the drops spontaneously forms an uneven distribution,resulting in surface tension imbalance,it can also cause the spontaneous movement of the drops.In this paper,the experimental study on the spontaneous movement of condensate drops on the heat transfer surface with uniform temperature distribution was carried out during the Marangoni condensation of water-ethanol vapor mixture.The Marangoni condensation experiment system of water-ethanol vapor mixture was designed and established.On the heat transfer surface,the temperature gradient region and the uniform temperature distribution region were adjacent and coexisting in this system.The condensate drops obtained the initial velocity in the temperature gradient region,and then the drops entered the uniform temperature distribution region.And the whole experimental system realized the measurement of the heat transfer characteristics of the vapor mixture condensation and the recording of the movement of the condensate drops.In this study,the spontaneous movement of condensate drops was initially confirmed on the heat transfer surface with uniform temperature distribution,during the condensation of water-ethanol binary vapor mixture.In this study,the movement of condensate drops on the heat transfer surface had two modes: with initial velocity and without initial velocity.The comparison and analyzation were performed on the characteristics of drop movement between the condensate drops with and without evident initial velocity on uniform temperature surface.It was confirmed that the spontaneous movement occurred when the condensate drops have evident initial velocity,while the random movement occurred when the drops didn't have evident initial velocity(no spontaneous movement),And its average velocity was close to 0 mm/s.When the concentration of vapor mixture was the same,the initial velocity of the drop obtained in the temperature gradient region decreased with the surface subcooling decreased,and the average drop velocity also decreased on the uniform temperature surface.Finally,it was verified the inference that the driven force for the spontaneous movement of condensate drops on uniform temperature surface was,the local temperature distribution and the resulting local imbalance of surface tension around the drops.It provided an important experimental basis for the clarification of the mechanism of the spontaneous movement of drops driven by the difference in surface tension during the Marangoni condensation.