When the wall film approaches an expanding corner, the effect of its momentum may drive the film to separate from the wall surface. The separation criterion proposed by Wegener et al. [103] is employed in Ansys Forte. This criterion is based on a force balance analysis that considers the surface tension forces at the top and the bottom of the film, and the gravitational force. A critical force ratio is derived by solving the momentum equation of the film. For an expanding angle θ shown in Figure 6.10: Film separation from wall at an expanding corner, the critical force ratio is given as
 | (6–104) |
where 
is the liquid density, 
is the velocity magnitude of the film, h is film
thickness, σ is surface tension, g is
gravitational acceleration, and 
is a characteristic breakup length for the thin film breakup, which takes the
form of
 | (6–105) |
For this correlation, the Reynolds number of the film is defined as
 | (6–106) |
And the Weber number is based on the relative velocity between the gas and liquid film
 | (6–107) |
where 
is the gas density and 
is the gas velocity magnitude. When the critical force ratio becomes greater
than one, the inertial force is considered to be great enough to trigger the onset of film
separation.
