Learning Forum

[Fabio Ferrario]

I am doing diffusion evaporation simulations that use an UDF to govern the rate of evaporation (or vapour inlet, since I'm not simulationg the liquid phase). I have noticed that if I try to do this simulation without selecting the Inlet Diffusion option in the species model tab, I don't have molar fraction continuity next to the surface. This means that concentrations above the evaporating pool are different from what you would expect, heavily diluted compared to the saturation condition (e.g., it dilutes to less that 60% the expected value). Selecting Inlet Diffusion fixed the problem, but I am left wondering on why it does. I could not really find much info in literature on how Inlet Diffusion works, but I suppose it uses the molar fraction defined on the cells of the inlet and compares it to the molar fraction of the cells adjacent to it. From this, it uses the usual Fickian formulas to define a mass inflow.

My doubt is the following : If i have both UDF that does the same exact thing as the Inlet Diffusion, when both are activated, do the effects add up?

When comparing to experimental data, I have a sensible over prediction in the transient part of the process, but once it reaches a stationary condition, mass flow rates seem to align with experimental data (for the most part).

[Rob]

Inlet diffusion will diffuse material from the boundary into the domain: I can't comment much further as the manual isn't overly clear. Given you're adding mass at (probably) a very slow rate you'll see diffusion/convection take material away from the boundary depending on near surface mesh resolution and flowfield.