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-   -   wall condensation (http://www.cfd-online.com/Forums/cfx/119636-wall-condensation.html)

Benfa June 20, 2013 16:00

wall condensation
 
Hi,

I am modelling a cavity that will be filled up by steam. It is a transient simulation with the goal to obtain the cavitys wall temperature over time. The other interesting physic will be the condensing of the steam on the initialy cold walls. therefore I will use the "wall condensation model". But there is something that makes me nervouse. The condensating steam is tracked by a trasnport euqation and another gas (for example) air is uses as constraint to fill up the near wall cells where the steam will vanish. But if I fill up my closed cavity the steam will push out the initial air and therefore there will be no air with increasing time. when the cavity is filled with steam the condensing water should be substituted by steam. but it is not possible to define a steam material as constraint because it can not condense. Am I missundesratnding anything or is the model not suitable for my application?

Thanks

ghorrocks June 20, 2013 18:43

You are not defining the simulation properly. Does the cavity initially contain pure steam? Then there is no air anyway. Or is it a mixture of steam and air? You need to know the initial condition.

And as it progresses and steam condenses, what happens? Is the cavity sealed, which means its pressure will drop? Or does air creep in from outside somehow so the proportion of air will increase over time?

You need to be able to define your initial condition and the boundary conditions correctly to model anything.

Benfa June 21, 2013 10:00

Sorry for not being clear.

Initially the cavity is filled only by air. Then pure steam will be blown into the cavity. the air will be pressed out of an outlet by the steam over time. after a specific time steam will also flow out of the outlet.

I use a variable composition mixture of air and steam in the gas phase. There is an additional homog. binary mixture of the steam and liquid water that takes into account the phase transition by antione eq. The gaseous steam is the transported variable while the air is constraint. But everytime the steam condenses the mass fraction of steam near the wall drops (that is correct) and the missing mass fraction is replaced by air (constraint). so everywhere where condensation occurs I get a "air sublayer" at the wall. but in reality there should be not only air. there should be an mixture of both

ghorrocks June 23, 2013 06:38

What happens if you model this with steam everywhere, and condensing on the walls? That is, no air present?

Benfa June 24, 2013 09:27

Quote:

Originally Posted by ghorrocks (Post 435430)
What happens if you model this with steam everywhere, and condensing on the walls? That is, no air present?

I will try asap! Then I will have only steam as tranport variable. Lets see what cfx will do if condensation starts and steam vanishes. I will post the message as soon as I get results.

ptbs March 10, 2014 07:39

Was it solved?
 
Hi Benfa.

Arriving a bit late on this topic, but I am very interested with this problem of condensation of a phase A (eg water) in a gaz B (eg.air ) along walls as function of temperature and speeds. Moreover, I intend to work in a compressible regime (Ma > 0.2).

Did you solve the problem and could you explain me which solver you used and how you procceeded. Is it a compressible solver? Did you solved heat tranfert? Is there a tuto on this topic or a place to find more info?

Thanks.

Best regards


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