Analysis of Formation of Vapor Columns in Film Boiling

Boiling is an effective means of enhancing heat transfer from a heated surface. Above a critical superheat and heat flux (Leidenfrost point) stable film boiling is observed. In the film boiling regime, bubble formation occurs periodically in space and time. The site of bubble growth is called a node and the site between two nodes is called an antinode. The spacing between two consecutive nodes is given by the most dangerous Taylor wavelength (Berenson, 1961). An increase in the superheat results in an increase in the frequency of bubble formation and its subsequent detachment. Reimann and Grigull (1975) in their experiments observed that above a critical heat flux, bifurcation from the periodic bubble formation occurs and at the nodes, stable column formation takes place. However, at the antinodes, periodic bubble formation is still observed. On further increasing the heat flux, thin slender vapor columns form at the nodes and antinodes. Bubble formation and detachment in such cases take place from the tip of these stable columns. Numerical simulations of bubble growth in film boiling have been attempted using Volume of Fluid (VOF) methods as well Level Set (LS) methods. We here use a Coupled Level Set and Volume of Fluid (CLSVOF) method to perform simulations of saturated film boiling of water at near critical pressure (21.9MPa). In these simulations, we show that the initial film thickness plays a key role in the prediction of the critical superheat above which stable vapor columns are observed to be formed. Capillary pressure leads to snap-off of the leftover-stem of vapor after bubble detachment. Formation of stable columns is a result of balance between the inertial pressure due to the vapor influx into the column and the capillary pressure.