Combined study of heat exchange near the liquid-gas interface by means of Background Oriented Schlieren and Infrared Thermal Imaging

摘要

Experimental measurements of temperature field within liquid layer below the liquid-gas interface are performed simultaneously by means of Background Oriented Schlieren (BOS) and Infrared Thermal Imaging of the surface (IRTI). Completely different structures of the surface layer are observed in evaporating water and ethanol at room temperature. Ethanol has moving surface, whereas water surface is similar to stagnant elastic film. Transition from one type of the surface layer structure to another is observed in hot water or cold ethanol. The properties of the surface layer are examined with convective plume, coming from a heated wire and impinging the free surface from below. Heat propagation velocity along the surface is shown to be extremely sensitive to tangential velocity boundary condition at the interface. The stagnant state of liquid near the surface in water results in slow heat transfer. Direct comparison with numerical simulations shows that water flow corresponds to no-slip condition at the interface. In contrast, ethanol with moving surface exhibits fast heat propagation, described by boundary condition which takes into account Marangoni effect. Heat propagation velocity is over- or underestimated by factor of 5-15 if inappropriate boundary condition is used. Surface relief of the liquid layer heated with a wire is also simulated and discussed.