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-   -   model evaporation (http://www.cfd-online.com/Forums/flow-3d/69237-model-evaporation.html)

tfe October 16, 2009 08:49

model evaporation
 
Hello,

I want to model the evaporation of droplets on a substrate with the simulation software FLOW 3D. But in modelling this item I have some problems. The model I used is thermal bubbles with phase change. And I also also used the full energy equation.
But no volume has been vaporized after the simulation?
Could someone give me some advice whether it is better to use another model? Or maybe I forgot some parameters?

Regards

MuxaB October 19, 2009 23:55

All sounds good. Here is one detail that is important. In that model, the bubbles are assumed to contain only the vapor and no other gas, such as air. So when setting the void pressure, it must be the vapor's partial pressure. Also make sure you set the initial void temperature as well.

Have you defined the properties for the phase change: saturation temperature and pressure, and latent heat?

Michael

tfe October 20, 2009 02:36

Thanks Michael. The first time I wanted to simulate the evaporation of a droplet of water on a substrate. But I was wondering which temperature to take for the saturation temperature. Just the boiling temperature of water (100 degree)? Because the temperature of the surrounding is not above 25 degree I don't know whether this makes sense... And pressure is just the atmospheric pressure. I also filled in the latent heat of vapor/ gamma/ accomodation coefficient/ exponent for TP curve and the vapor specific heat.
Do you know whether I forgot something? Cause there is no volume evaporating and the volume of fluid of the droplet is not decreasing.

Besides that I got a warning

** warning if Clapeyron equation is used for saturation curve the equation will fail if pressure exceeds p= 4.73367E+12... But the pressure is not exceeding this value.

Hope you could help me a bit.
Hans

MuxaB October 20, 2009 11:29

You have to use Kelvin here, not Celsius. The ambient pressure must be the partial pressure of the vapor in the air, and not one atmosphere because that includes the air itself which is not part of the model.

the saturation pressure can be defined at the boiling point for water: 373K and 1 atmosphere.

Don't worry about the warning, it is just FYI.

tfe October 26, 2009 09:21

Thanks for that Michael.
But I still think there is a fault in my model. The volume of fluid decreases the same as the convective volume error % lost.

I think I did something wrong with the ambient pressure. Do you define the pressure of the air for the surrounding of the droplet? Or do you define only the partial pressure of the vapor in the air (in this case I mean the parameters pvoid, presi and pvrd)? I used for these parameters the saturation pressure of water at 298 Kelvin (room temperature) which is 3,11e3 Pa.
But in that case no surrounding air pressure is taken into account.

Regards,
Hans

MuxaB October 26, 2009 23:59

Hi Hans,

yes, this sounds about right. You need to set pvoid and tvoid for the void, presi and tempi for the fluid. If there are no open pressure boundaries, it should be enough. If the droplet heated or just sitting there and evporating?

What version of FLOW-3D are you running?

tfe October 27, 2009 02:54

Thanks Michael.
The droplet is in the beginning 303 Kelvin and lies on a substrate of 298 Kelvin, there is no extra heating in this simulation. The droplet is just sitting there and evaporating. I am running FLOW-3D version 9.3.
I think I should use the partial pressure of the vapor in the air for the pvoid and for presi the fluid pressure.

Regards,
Hans

MuxaB October 29, 2009 00:28

Yes, I agree.

tfe October 30, 2009 05:09

Despite these settings nothing happens with the droplet. No mass is evaporating and the volume of fluid is only leaving because of the convective volume error % lost. If I plot the volume of fluid against the volume error these graphs are exactly the same.

Maybe I did something wrong with the other settings of the model. The only parameters I used are:

- Bubble and phase change --> thermal bubbles with phase change.
- Heat transfer --> Fluid internal energy advection (second order monotonicity preserving) and Fluid to solid heat transfer (full energy equation)
- Surface tension model is activated
- Viscosity options --> the newtonian viscosity is used.

Is there somewhere an option to activitate the evaporation? Or do I maybe have to specify something else to activate the mass flux?

Hope you could give me some advice.
Hans


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