Learning Forum

[Ami Bha]

Hi all,

I am trying to simulate evaporation from liquid water to vapor by blowing hot air at 80 degress Celsius from a velocity inlet on a water surface (evaporation through convection)

Below 100 degrees Celsius (= T_sat) using the Lee model, I cannot see any evaporation occuring in my simulation. Can someone please help me which models with what settings I can use to achieve evaporation below 100 C without using UDFs? 

[Rob]

It does work, how long has the model been running? If you plot water volume fraction and monitor mass how do those plots look? 

[Ami Bha]

I simulated this case for 60 s and there was no change in volume, the water just got warmer. I was thinking this is because the drying air from the inlet is at 80 C not 100 C, which is the saturation temperature in Lee model. In reality water evaporation does occur below 100 C when you have an air flow above the water surface (through convection). But I cannot see that happening in my simulation.

I observed in a different simulation of mine, that the second the water reaches a temperature of 100 C, only then vapor develpment occurs.

I am using VoF Multiphase model and using air (primary phase), water-liquid and water-vapor as my phases. Mass transfer mechanism is evaporation-condensation model with T_sat at 100 C and the modal constants kept at 0.1 each (default values).

 

 

 

[Rob]

VOF and phase change can be a little challenging if you're not moving much mass. I tend to favour Eulerian, but Mixture may also make sense. 

What pressure are you at, and you may need to review the saturation settings. 

[Ami Bha]

Hi Rob,

The operating pressure is set to 101325 Pa (per default in the operating conditions). I didn’t set/change a pressure value anywhere. 

So does it mean that evaporation through convection is not possible with VoF model if my temperatures are not over 100 C? Or does it only work for large water volumes? 

By reviewing saturating settings, do you mean the T_sat conditions in the evap-condensation mass transfer mechanism? ( I am doing a 2D simulation btw) I'm not sure what to change the saturation setting to if not a constant value which is at 100 C at the moment...

 

[Rob]

I've always used Eulerian for this sort of problem. Just checking the model(s), if you have multiple species there are also options for phase transfer once that is set up. Check if that'll help. 

[Ami Bha]

Hi Rob,

Eularian model has Lee and Thermal phase change methods. Is thermal phase change a better option here? If yes, then why? Because it also uses a saturation temperature, so how would it suddenly allow vapor development below 100 C?

[Rob]

It's a saturation value, but I think you need to use polynomials or similar for the saturation condition.