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-   -   Advice setting up high quality steam with water droplets entrainment (http://www.cfd-online.com/Forums/cfx/126501-advice-setting-up-high-quality-steam-water-droplets-entrainment.html)

FreeFall79 November 19, 2013 11:53

Advice setting up high quality steam with water droplets entrainment
 
Hi All

I am trying to set up my fluid (dry steam), which has a mass fraction 99%, with the balance being water droplets. My intention is to have the water droplets extracted from the steam using a porous media domain where I will need to set up flow resistances. I am not sure how I start to set this problem up. I did some searching but most steam references deal with phase change. I am modeling a drying process and is an isothermal study. Any advice is much appreciated.

Thanks in advance

FF

ghorrocks November 19, 2013 18:57

Just because it is isothermal does not mean that phase change is not occurring.

If you are just trying to extract droplets from the flow then just call then droplets an inert particle. But I doubt this is really correct but I do not know what is important in this flow.

So what is important in this flow? Why are you modelling it? What do you hope to learn from it? We need to understand what you are doing to help you.

FreeFall79 November 21, 2013 16:34

I am trying to simulate steam being dried in a separator. I have a porous media domain in which steam/water droplet pairing will be forced through.

Like you said I need a means of extracting droplets via the porous media. Ideally I would like a model that could capture any entrainment be it droplets going with the flowing steam through the porous media or steam bubbles going down the drain channel. I hope that helps

Right now I have the droplets modeled as disperse fluid with the steam being a continuous fluid.

ghorrocks November 21, 2013 17:44

But what are you doing this model to tell you?

There are many approaches - from assuming the droplets are inert particles through to a full phase change model - but until I understand what is important in your case I have no idea what model is suitable.

FreeFall79 November 22, 2013 14:13

the intent of the study is to understand the drain capacity of the of this dryer unit. So the droplets extracted from the flowing steam is collected into a tank which also has a drain. I want to see if the water level is stable at certain massflows of steam and what would make the system overflow. I hope that helps.


I think since I want to collect the droplets I cannot model this as particles. Is that a fair assumption?

Thanks for probing for more information i am in way over my head at the moment.

ghorrocks November 23, 2013 05:41

If you are interested in the drain capacity - then isn't it just how much water flows out the drain under a given head of collected water? This can be modelled as a single phase flow. MUCH simpler.

FreeFall79 December 4, 2013 16:02

I was assigned this project and it was not very fleshed out.

The model is a bit more complicated in that head is unknown. We do not have a water level to initialize with and I eventually need to track water level under various transients. The input in the drain is coming from flow vanes that collect water as the droplets collide surface of the vanes. So this is why my model is much larger since I need to get entry and exit points of the steam and need some sort of droplet entrainment model to capture the effects.

Again thank you for probing for the pertinent info, I am very much a novice.

ghorrocks December 4, 2013 17:33

Have you considered de-coupling this? You can do this by running a series of steady state simulations at various driving pressures (which is the different free surface heights) and then you get a curve of flow rate versus pressure. Then run a simple ODE solver where the flow rate out of the collected droplets is the flow rate versus pressure curve.

This is a much simpler simulation, and will be able to run various transient scenarios in seconds rather than weeks like I suspect your "let's model everything" approach will take.

dingsheng1206 December 4, 2013 21:36

Quote:

Originally Posted by ghorrocks (Post 464879)
Have you considered de-coupling this? You can do this by running a series of steady state simulations at various driving pressures (which is the different free surface heights) and then you get a curve of flow rate versus pressure. Then run a simple ODE solver where the flow rate out of the collected droplets is the flow rate versus pressure curve.

This is a much simpler simulation, and will be able to run various transient scenarios in seconds rather than weeks like I suspect your "let's model everything" approach will take.

dear ghorrocks, i noticed that you are from australia, right? and i am from china, and we are in the neighboring time zone, right? and your post here was in the morning 5 AM,

so are you on the forum all the time? it is really king of you that you made so many sacrifices. salute to you!

ghorrocks December 4, 2013 21:58

Yes, I am from Australia, so yes we are in similar time zones. But my post was not at 5am, but 8:33am Sydney time. I do keep the forum up often at work, but I do not monitor it at 5am in the morning!

FreeFall79 December 10, 2013 15:44

hmmm I will have to follow up with my tech lead on this. Makes sense to me. The modeling everything approach was taken since he could not get a convergent solution simply modeling the trough and drain. There are three troughs connected to a single drain and he stated he would get constant oscillations. He directed me to expand the model to include more components to get a stable solution.

maybe I should revisit the simpler case. Since I cannot even get steam by itself to converge.

ghorrocks December 10, 2013 17:57

Agreed. It is a common mistake to jump into the model everything approach when you can learn far more with simpler models of sub-systems and achieve it in reasonable time frames. And if you cannot model the sub-systems accurately then what hope have you got for modelling the full system? So I strongly recommend modelling the individual sub-systems first as simple and achievable stand-alone models. In my experience these sub-system models tell you all you need to know in 99% of cases and then the full system mega-simulation is not required.

End of rant :) But I cannot count the number of times I have seen people fall into this trap.


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