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steam condensation on water film

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Old   July 29, 2018, 05:18
Default steam condensation on water film
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ali pourahmadian
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i'm trying to simulate water steam counter current flow with condensation in deaerator.
the deaerator function is increase the water temperature by direct contact condensation of saturated or superheat steam on that in order to decline the oxygen solubility below 7 ppb.
the geometry is a cylinder which consists some stage of trays. trays are horizontal plates with slot holes and their main function is to hold the water droplets and make water film. i have some questions about this simulation
1) is it true to choose both water and steam in continuous? the phenomena is steam condensation on water film between trays so i cant choose water as dispersed because in this situation i have to enter the mean diameter.
2) if i choose both phases as continuous i have to enter heat transfer coefficient for at least one of the phases. i couldn't find any appropriate scale for that. also i don't know if the default drag coefficient for interphase is suitable or not.
3) should i active the free surface mode as i have water film in my domain or mixture model can be use in this condition too?
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Old   July 29, 2018, 06:16
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Glenn Horrocks
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This sounds like a very complex simulation. It is going to take a lot of development work to get something like this working. I trust you are an experienced CFD operator because if you are a beginner you have no chance of getting a simulation like this working.

On your questions:
1) That depends on how you model this. There are many ways of modelling this in combinations of continuous and discrete. And how you model this depends on exactly what you are modelling, the droplet size, whether it forms continuous fluid regions, whether bubbles, foam or mist is present and so on.
2) You need to make sure you have the right physical model (continuous, discrete, MUSIG, ASM, free surface etc) before you consider details of the models. Resolve Q1 first.
3) Again, resolve Q1 first. But if this model contains liquid water in both droplet form and continuous fluid form then this model has just gone extremely difficult to almost impossible.
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Old   July 29, 2018, 06:55
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thanks for your reply.
of course this is my master thesis and i have to do it anyway. fortunately for oxygen dissolution from water i can use henry law which exists in cfx and fluent.
my question is that what is the difference between continuous and dispersed morphology. i read the help but i couldn't get the meaning. and also there is another option "droplet phase change"
can i use it?
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Old   July 29, 2018, 07:04
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"continuous" means the phase forms largely continuous regions, "disperse" means the phase forms largely dispersed regions. The terms are quite self-explanatory. For instance:

A few air bubbles in water: air = disperse, water = continuous
A cup full of water: air = continuous, water = continuous, and with a free surface interface model
A few water drops in air: air = continuous, water = disperse
Water/air foam: This is not handled by the available CFX multiphase models, this would be a topic of research
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