GENERAL SEMINAR: Dr Milica Ilic, “Vinca” Institute of Nuclear Sciences National Institute of the Republic of Serbia, University of Belgrade
Abstract:
The peculiarity of boiling phenomenon is its multi-scale nature with time and length scales spanning up to nano-size, i.e. up to the molecular level. The investigations of mechanisms at the nano-scales is of importance for improvement of corresponding closure laws in macro- and meso- / micro- scale boiling models, but also for understanding of heat and mass transfer processes in innovative nano-devices. Due to limitations of contemporary experimental methods, the Molecular Dynamics (MD) simulations currently represent the only appropriate approach for getting insight into nano-scale boiling mechanisms.
This presentation deals with explosive boiling, which is a particular mode of boiling phenomena arising whena thin liquid film is heated by a solid substrate at extremely high temperature. Here the explosive boiling of water films on hot copper substrate is considered. MD simulations are conducted by free open source codeLAMMPS. The simulations are performed for thickness of water film in range 7.5-25.5 nm and temperatureof solid substrate in range 600-800 K.
The analyses of obtained MD results have shown that the occurrence of explosive boiling is governed by
complex interplay of thermal and mechanical mechanisms. Thermal effects are associated with intensive
increase of temperature in water layer next to the heating substrate. The mechanical mechanisms comprise formation and propagation of acoustic pressure waves (compressive or expansive) across the liquid film. Thermal mechanisms are dominant in cases with high substrate temperature and thick liquid films. In these configurations the temperature of water layer adjacent to solid substrate reaches spinodal value and explosive boiling occurs before expansion wave reaches the near wall region. In cases when water in the near wall region is strongly overheated (but significantly below spinodal temperature), mechanical effects contribute dominantly to triggering of explosive boiling through occurrence of tensile stresses which lead to spalling of liquid film and vapour generation. Finally, in cases with lower substrate temperature and thin liquid films, a convoluted interaction of thermal and mechanical mechanisms leads to explosive boiling. Besides these, the disjoining pressure seems to play a role in cases with thin liquid films.