# Help with heat exchanger for desalination

 Register Blogs Members List Search Today's Posts Mark Forums Read

 March 17, 2010, 17:22 Help with heat exchanger for desalination #1 New Member   wasil Join Date: Mar 2010 Posts: 6 Rep Power: 9 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.

 March 18, 2010, 13:00 #2 New Member   wasil Join Date: Mar 2010 Posts: 6 Rep Power: 9 basically its a pipe in a pipe situation with seawater in the inner pipe and water vapour on the outer. anyone??

 March 18, 2010, 13:52 #3 Senior Member     Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 13 If you have the mesh you have gained a lot! Now specify boundary conditions and start the simulation. __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

March 20, 2010, 04:10
#4
New Member

wasil
Join Date: Mar 2010
Posts: 6
Rep Power: 9
Quote:
 Originally Posted by sega 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?

March 20, 2010, 05:45
#5
Senior Member

Sebastian Gatzka
Join Date: Mar 2009
Location: Frankfurt, Germany
Posts: 729
Rep Power: 13
Quote:
 Originally Posted by redtilldead 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.
__________________
Schrödingers wife: "What did you do to the cat? It's half dead!"

 March 20, 2010, 06:13 #6 Senior Member     Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 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? __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

March 20, 2010, 07:18
#7
New Member

wasil
Join Date: Mar 2010
Posts: 6
Rep Power: 9
Quote:
 Originally Posted by sega 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.

 March 20, 2010, 09:21 #8 Senior Member     Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 13 What is really interessting to you? The water inside the tube, the surrounding vapor or both? __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

March 20, 2010, 09:25
#9
New Member

wasil
Join Date: Mar 2010
Posts: 6
Rep Power: 9
Quote:
 Originally Posted by sega 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

March 20, 2010, 09:41
#10
Senior Member

Sebastian Gatzka
Join Date: Mar 2009
Location: Frankfurt, Germany
Posts: 729
Rep Power: 13
Quote:
 Originally Posted by redtilldead 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
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.
__________________
Schrödingers wife: "What did you do to the cat? It's half dead!"

March 20, 2010, 09:50
#11
New Member

wasil
Join Date: Mar 2010
Posts: 6
Rep Power: 9
Quote:
 Originally Posted by sega 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

By the way, Im really appreciating your help here!

March 20, 2010, 10:55
#12
Senior Member

Sebastian Gatzka
Join Date: Mar 2009
Location: Frankfurt, Germany
Posts: 729
Rep Power: 13
Quote:
 Originally Posted by redtilldead 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 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 !
__________________
Schrödingers wife: "What did you do to the cat? It's half dead!"

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post enigma Main CFD Forum 2 November 1, 2009 23:53 saii CFX 2 September 18, 2009 08:07 Rogerio Fernandes Brito FLUENT 14 November 25, 2008 06:47 B.Simon CFX 3 October 28, 2008 19:53 Mark CFX 6 November 15, 2004 16:55

All times are GMT -4. The time now is 16:22.