A team from Siberian Federal University (SFU) together with their colleagues from the Institute of Computational Modeling of SB RAS presented a calculation describing the structure of flows and evaporation processes in the two-layer system with liquid - gas-vapour mixture. The main attention is paid to the influence of two factors, namely, gravity and the thickness of the liquid layer. The understanding of mechanisms that determine the nature of flows and vaporization (condensation) effects is important for the development of micro-sized cooling devices for satellites and life support systems of the orbital space station, as well as in material studies and chemical industry. The results of the study were published in the Microgravity Science and Technology journal.
Heat and mass transfer processes are observed everywhere. One of them is called convection, when fluxes of the matter and energy are transferred in flows. Permanent motion occurs in the liquid. Upper layers cool, get heavier, and go down, and liquid in the lower layers has higher temperature and rise to the top. One of the reasons for heat loss at the border between liquid and gas is related with evaporation. A part of molecules has higher energy in comparison with others, and it is enough to break intermolecular bonds and to be released from the surface. This is how liquid particles transfer to their gas state, i.e. become vapor. An opposite effect is called condensation and takes place when vapor molecules contact with cooled liquid surface.
"The described processes depend on internal and external conditions: properties of the medium (liquid and surrounding gas or gas-vapor mixture), temperature regime of the system, its geometry (e.g. the width of the channel in which liquid and vapor are located), and the effect of gravity. In our work we focused on describing the influence of the two latter factors," says Victoria Bekezhanova, Dr. habil. in physics and mathematics, Professor of SFU, and leading researcher in the Department of differential equations in mechanics at the Institute of Computational Modeling of SB RAS.
The presented mathematical model describes convective processes in the two-layer liquid and gas-vapor system. It is based on the most important equations of fluid dynamics (the Navier - Stokes equations) and takes into account several additional factors. For example, the thermodiffusion effect can lead to the formation of zones with different concentrations of molecules depending on the temperature. Under the influence of various factors a liquid can be stratified into warm and cold layers, whereby some physical properties of the medium (density, superficial tension, and so on) can be changed. A system in which upper layers are cooler than lower ones is unstable, because convective motion occurs in the fluid being in a gravity field (i.e. cold liquid goes down, and hot one goes up). This process can be controlled, for example, by means of changing the thickness of the liquid layer. With decrease of the height of the liquid layer, one can suppress convection due to the increasing influence of the thermocapillary effect that causes hot liquid to move along the liquid-gas interface.
"Traditionally, a need to explore convective processes is required for developments in thermal physics, chemical industry, material studies, and biological medicine. Our recent results may be of use for modifying liquid cooling systems in various micro- and mini-sized electronic devices, including those used on board of space stations or new generation satellites, and also for improving thermal control systems, thermal drying technologies, or applying coatings with given characteristics (thickness, form, mechanical or chemical properties, and so on," concludes Victoria Bekezhanova.
Caption: Structures on the surface of the evaporating liquids driven by co-current gas flow observed in the experiments (Lyulin and Kabov, 2014). Now, the formation of the patterns can be predicted with the help of the new theoretical approaches developed by siberian scientists.