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Old   April 11, 2014, 01:05
Default Modeling a mixture of water vapor, dry air and water droplets
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Santosh
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Hi All,

I am modeling a mixture of water vapor, dry air and water droplets, and need to account for phase change, ie. the vapor can condense (and nucleate) to form droplets or the droplets should evaporate, depending on the conditions. Can anyone suggest a good multiphase model to do this?
Is the Eulerian two-fluid model the best approach? or are there any there models that are used, especially in the steam turbine modeling community?

Any insight is much appreciated!
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Old   April 14, 2014, 07:32
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Hi Konangsh,

can you tell an order of scale of your box? In CFX, I have seen a droplet condensation model which is used. By the way you have to know the proportion of liquid to know what approach you can use (Lagrangian, Eulerian).

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Old   April 15, 2014, 06:39
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Hi Miki,

Thanks for your reply. The problem I am interested is more like a jet/plume. I would think the proportion of water vapor in dry air is fairly small, but very important to be included due to the latent heat release during condensation, etc. and for the formation of droplets. I am leaning towards to Eulerian model to avoid tracking every droplet. But my question is whether I should be solving the different governing equations for all three components. i.e.. water vapor, air and droplets? Do you have any better ideas?

Is the CFX model based on an Eulerian formulation?
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Old   April 16, 2014, 02:03
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Hi Konangsh,

Thank you for your reply. I would consider the Eulerian Approach too with two fluids, one is the mixture of two ideal gas (the water vapor and the dry air) so an ideal gas and the droplets.

I would also consider if there is some interaction with a wall (is there any impact of your jet? ).

In CFX, it is called the droplet condensation model, it is available with the Eulerian approach as a specific interphase mass transfer.

At least, the way to calculate depends also of which software you want to use (STAR-CCM+, Fluent, OpenFOAM, CFD++, ...) but maybe the best way to do is to evaluate the heat and mass transfer and to implement as a used-defined function/field function. As I know, the heat released by latent effect is not always calculated. But at low temperature ( under 5C), the error is really low.

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Old   April 17, 2014, 21:37
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Thanks, Miki, for the insight.

Quick question: If I am modeling the mixture of water vapor and air as a single gas phase, would I still be able to track the amount of vapor in the system? This is important as the vapor may decrease after condensing to form liquid droplets, or the droplets may evaporate to create vapor. Also, tracking the latent heat release or absorption in this process is critical to model the large scale structure of the flow.

I am leaning towards fluent or open foam, as I have access to these codes. But I can't seem to find any solid work on modeling a mixture of air (non-condensible gas) and vapor (condensible) along with phase change (nucleation, droplet growth etc.). Are you aware of any papers on this? Any leads would help immensely.

Thank you very much for your help! Really appreciate it!

PS. I don't have any wall interaction to consider. It's free jet/plume.
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Old   April 23, 2014, 05:40
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You can track the amount of vapor by the mass fraction term. Then, this term will change during evaporation/condensation.

With OpenFOAM, you have to check maybe the reactingFoam solver. There is also another topic about evaporation modelling on CFD-online
(evapPhaseChangeFOAM).

Concerning the papers, I just find some theories about the dropwise condensation (Paper from Rose is the reference), you can find it on sciencedirect.com for example. You can also find other documents on
Springer.

As you don't have to consider the wall, you just have a bulk condensation and so you can have a look on the spray.

I hope that's helps!

Miki
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Old   April 25, 2014, 01:41
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Thanks a ton, Miki! Your input has been very helpful!

I understand the problem better now, and will (hopefully) be able to get some work going

Are you working on something similar? Which university?
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Old   April 28, 2014, 03:04
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Hi Konangsh,

I hope too that you can solve your problem.

I'm currently simulating the begin of the dropwise condensation process on surface with OpenFOAM and STAR-CCM+. I want to know what's happen in terms of visibility.

I've trying several approaches but as my droplets do not seem te be hemispherical, it is more complicated.

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