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-   -   Help with heat exchanger for desalination (https://www.cfd-online.com/Forums/fluent/73814-help-heat-exchanger-desalination.html)

 redtilldead March 17, 2010 17:22

Help with heat exchanger for desalination

Hello all,

Im a total beginner to the CFD world, and Im making a CFD model for the condensing tubes in a desalination stage.

Basically what i want to model is a 4 tubes carrying seawater over a certain distance surrounded by a warmer wall (consider a bigger tube).

The seawater flowing in the tubes will gain the heat from the wall as it flows. I have made the geometry in Gambit. Just need help with how to approach the CFD.

Then I will change the geometry of the tubes (from circular to elliptical) to show how the newer design is better for transferring heat to the seawater.

Can someone please guide me on how to do the CFD? doesnt have to be really advanced.

 redtilldead March 18, 2010 13:00

basically its a pipe in a pipe situation with seawater in the inner pipe and water vapour on the outer.

anyone??

 sega March 18, 2010 13:52

If you have the mesh you have gained a lot!

Now specify boundary conditions and start the simulation.

 redtilldead March 20, 2010 04:10

Quote:
 Originally Posted by sega (Post 250674) If you have the mesh you have gained a lot! Now specify boundary conditions and start the simulation.
how can i set the temperature for the vapour but not fixed?

 sega March 20, 2010 05:45

Quote:
 Originally Posted by redtilldead (Post 250891) how can i set the temperature for the vapour but not fixed?
Temperature is part of the solution process.
You mean temperature boundary conditions?

Switch on the energy equation and I'm sure you can even set functional (not fixed, if thats what you mean with that).
Have a look at the User Guide.

 sega March 20, 2010 06:13

Maybe you have a look at chapter 7.6 of the User Guide for profile-boundary conditions.

For anything else or strange you will have to use a User Defined Function (UDF).

What kind of "not fixed" temperature boundary condition do you want to use?

 redtilldead March 20, 2010 07:18

Quote:
 Originally Posted by sega (Post 250900) Maybe you have a look at chapter 7.6 of the User Guide for profile-boundary conditions. For anything else or strange you will have to use a User Defined Function (UDF). What kind of "not fixed" temperature boundary condition do you want to use?
basically i need 2 flows.

1) seawater flowing in a tube - velocity inlet temp is 373 K

2) water vapour is around the tube at 380 K. tube is made of nickel.

i want the vapour to give its heat to the water. the water should output around 376K.

The thing is, my velocity inlet and pressure outlet are set for the water. But i dont know how to set the temp for the vapour around it.

seems like a fairly simple problem, but for some reason it wont work.

 sega March 20, 2010 09:21

What is really interessting to you?
The water inside the tube, the surrounding vapor or both?

 redtilldead March 20, 2010 09:25

Quote:
 Originally Posted by sega (Post 250915) What is really interessting to you? The water inside the tube, the surrounding vapor or both?
Im actually interested in water, and how the temperature increases as it absorbs heat from the vapour.

I tried reading the profiles (chap 7.6) but its just way out of my league :o:D

 sega March 20, 2010 09:41

Quote:
 Originally Posted by redtilldead (Post 250917) Im actually interested in water, and how the temperature increases as it absorbs heat from the vapour. I tried reading the profiles (chap 7.6) but its just way out of my league :o:D
Forget the chapter. This is not what you need, I was just misunderstanding you.

What you need is this:
If you are interessted in the water inside the tube only, forget about the surrounding. Simply mesh the water and the tube.

Than I would recommend to use a thermal boundary condition of the 3rd kind on the outside of the tube.
Then you can take the effect of the surrounding vapor into account through the heat transfer coefficient.

 redtilldead March 20, 2010 09:50

Quote:
 Originally Posted by sega (Post 250920) Forget the chapter. This is not what you need, I was just misunderstanding you. What you need is this: If you are interessted in the water inside the tube only, forget about the surrounding. Simply mesh the water and the tube. Than I would recommend to use a thermal boundary condition of the 3rd kind on the outside of the tube. Then you can take the effect of the surrounding vapor into account through the heat transfer coefficient.
How can I "use a thermal boundary condition of the 3rd kind on the outside of the tube"? Any quick steps on that will be great. As I mentioned earlier, im still on a learning curve :o

By the way, Im really appreciating your help here! :)

 sega March 20, 2010 10:55

Quote:
 Originally Posted by redtilldead (Post 250921) How can I "use a thermal boundary condition of the 3rd kind on the outside of the tube"? Any quick steps on that will be great. As I mentioned earlier, im still on a learning curve :o By the way, Im really appreciating your help here! :)
Convective Boundary Condition.

If you want to simulate such a case you should make yourself familiar with the theoretical background on heat transfer: http://web.mit.edu/lienhard/www/ahtt.html !

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