Numerical simulation of falling film flow boiling along a vertical wall

Full text of publication follows: When a dryout occurs in film flows with heating from the wall, the wall surface being cooled is no longer in intimate contact with the liquid film. Consequently, the heat transfer will dramatically reduce and the corresponding wall temperature will rise rapidly up to the melting temperature of the heat transfer plate or pipe. It is very important to investigate the heat transfer characteristics of liquid films flowing along a heating wall and the dryout phenomena of the liquid films associated with increasing heat flux in the high heat flux component devices for chemical and mechanical devices and nuclear reactor systems. Many studies have been conducted on the dryout phenomena and it has been shown that the dryout conditions are influenced by several different flow conditions, for instance, subcooled and saturated liquid films and so on. The dryout process of boiling liquid films is different between them: in the case of subcooled liquid films, the process is caused by the local surface-tension variation along the film. On the contrary, in the case of saturated liquid films the surface temperature of boiling films is maintained at a saturation temperature and there can be no variation of surface tension along the film. The process in the case of saturated liquid films is caused by the reduction of film flow rate due to the flow imbalance. This reduction of film flow rate is promoted by the evaporation and the liquid droplets arising from the film surface due to the burst of vapor bubbles. Therefore, it is very important to predict the sputtering rate of liquid droplets and to understand the behavior of vapor bubbles in film flow boiling. In the present study, numerical simulations based on the MARS (Multi-interface Advection and Reconstruction Solver) developed by one of the authors have been performed in order to understand the dryout of film flow boiling. The film flows along a vertical wall are focused in the present study. According to the stability theory, The film flows along a vertical wall are always unstable, and various types of surface waves are appeared due to the nonlinear wave interactions such as solitary waves and capillary waves, etc. These surface waves have effect on the behavior of vapor bubbles in film flows. In order to investigate the effect of external disturbance on film flow boiling, the film flows are disturbed in time with artificial oscillation by external force; it is easy to control two-dimensional waves by means of this method. The frequencies of artificial oscillation are set to 13 Hz-60 Hz and the Reynolds number of film flows based on the equilibrium film thickness and mean velocity are set to 30 - 400, i.e., in the region of laminar flow. It will be clarified the relationship between flow conditions and heat transfer of film flows including flow boiling with various parameters, such as the frequency of surface waves, Reynolds number, physical properties of working fluids, and behavior of boiling bubbles. These data will be very helpful for the multiphase flow modeling in the future. (authors)