Fluid Film Flow Model Reference

Film thickness comes from the mass equation. These equations are solved in a segregated manner, that is, one after the other. The energy equation, however, can either be solved segregated or as part of the coupled energy equation. A parabolic velocity profile is assumed in the direction normal to the wall, for laminar fluid film, and the fluid film turbulence model is based on the universal velocity profile approach.

Table 1. Fluid Film Flow Model Reference
Theory	See Fluid Film
Provided By	[film phase] > Models > Fluid Film Flow
Example Node Path	Continua > Physics 1 > Models > Fluid Film > Fluid Film Phases > [phase] > Models > Flow Model
Requires	A Fluid Film phase.
Activates	Physics Models	Equation of State Viscous Regime Fluid Film Segregated Energy
	Initial Conditions	Fluid Film Thickness Velocity
Properties	See Flow Model Properties.

Flow Model Properties

Maximum Volume Fraction

Sets the maximum volume fraction ( $α_{f, m a x}$ in Eqn. (2728)) for the fluid film phase. This property allows you to include the effect of the liquid film onto the gas. The volume that the film occupies is subtracted from the gas-filled cells next to the shell region.

To ensure stability, you are advised to set the maximum volume fraction to a value lower than 1. The default value is 0.8.

You are advised to set the maximum volume fraction to 0 in the following situations:

You want to deactivate the volume fraction effect.
The film forms due to droplet impingement.

Maximum Film Thickness

Sets the maximum film thickness ( $h_{f}$ in Eqn. (2721)) for the fluid film phase.

In some cases the film thickness can become very large. This behavior is typically physical (for example, the liquid film is pushed into a corner and it cannot escape). A very large film thickness (much larger than the thickness of the neighbor volume cell) can cause numerical instabilities in the simulation. Setting this property ensures that the film thickness does not exceed an appropriate maximum value at any location. The default value is 1.0 meters.

When the specified thickness is reached, any extra fluid is removed from the film and is lost from the simulation. In this scenario, mass conservation is not explicitly satisfied in the simulation.

Stabilised Film Thickness Equation

This option must be activated for steady simulations.

Activating this option improves the ability of the Fluid Film solver to handle high mass impingement upon very small fluid film cells. In cases where the impinging droplets are large (that is, a few times bigger than the cell size), the sudden addition of fluid affects adjacent cells. The residuals can increase dramatically and cause convergence problems. The convergence issues arise because the film fluxes depend on two dependent variables: the film thickness and the film velocity. That is, the mass conservation is influenced by both the velocity and the film thickness, and the simulation can develop oscillations of both.

To resolve this issue, a modified film thickness equation is used instead of Eqn. (2721). This option prevents the residuals from increasing out of control, keeping the simulation stable and allowing the solution to converge.

The film thickness equation has two stabilization mechanisms added:

Capillary Gravity Advection
To damp velocity-thickness-oscillations, a linearization of an artificial capillary-gravity advection term is added to the film thickness equation. For converged simulations, this extra term has no effect on the results.
Mass Conservation Residual
To further improve convergence, the mass conservation residual is added to the film thickness equation.