CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Help with mass transfer in heat and water vapor exchanger

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 19, 2011, 16:04
Default Help with mass transfer in heat and water vapor exchanger
  #1
New Member
 
Join Date: Dec 2010
Posts: 26
Rep Power: 6
mullenc525 is on a distinguished road
I've been studying heat and water vapor exchangers for some time. I understand there is a close relationship between heat transfer and mass transfer, so initially sought to understand heat transfer in the device. My geometries consist of channels on the order of 1mm rectangles with 10-20mm length, with periodic inlet/oulet.

Initially I studied one species (air) with an inward heat flux at one wall and a balancing energy sink over the entire domain. The results were as expected.

I am having trouble attempting to accomplish the analogue with species transfer instead of heat transfer. I have entered air and water vapor properties at the temperature and pressure of interest, and initialized the domain with the volume fraction appropriate for my device. One wall has a water vapor source, and the entire domain has a source with a negative constant to balance mass over the periodic domain. The negative source is extracted with velocities u,v,w, as I don't want it to affect the momentum of the air.

The rms residuals for species volume fraction are on the order of 0.02-0.04 after 60 iterations with a 10x timescale factor in a simple geometry (rectangular duct with appropriate inlet profile). Basically it is not converging, and of course the solution is nothing like expected.

Under fluid pair models I'm asked to enter information for interphase transfer, and drag coefficient. I don't know what this means or if its appropriate, it sounds to me like information for a 2-phase model.

Not sure what other details may be appropriate: steady state, laminar, both fluids continuous.

Does anybody have any experience with diffusion/advection of one gas in another?

edit - I just noticed another problem; the water vapor does not have the same velocity as the fluid average, as it is diffusing perpendicular to the stream. That makes it difficult to choose what velocity to remove it at without impacting the momentum of the other air.

Last edited by mullenc525; January 19, 2011 at 20:25.
mullenc525 is offline   Reply With Quote

Old   January 24, 2011, 13:25
Default
  #2
New Member
 
Join Date: Dec 2010
Posts: 26
Rep Power: 6
mullenc525 is on a distinguished road
bump - surely somebody must have done a problem with water vapor diffusing into air?
mullenc525 is offline   Reply With Quote

Old   January 24, 2011, 17:41
Default
  #3
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Are you looking purely at diffusion of vapour or does the vapour change physical properties much (ie condense or evaporate, or even just have a significant pressure/temperature change)?
ghorrocks is offline   Reply With Quote

Old   January 24, 2011, 17:41
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Simple advection/diffusion can be done very easily with an additional variable. Don't use multiphase unless you need to (and you know how to use it).
ghorrocks is offline   Reply With Quote

Old   January 24, 2011, 18:10
Default
  #5
New Member
 
Join Date: Dec 2010
Posts: 26
Rep Power: 6
mullenc525 is on a distinguished road
Hi Glenn,

Purely diffusion of vapor, no change in state, pressure or temperature. I noticed another thread where someone was looking at ink diffusion into water, and the consensus was to use additional variables. However, a caveat mentioned was that it was only appropriate if you wanted to make the assumption the ink properties were the same as water and mixing ratios low.

As we know, water vapor and air are substantially different, and I am working with mixing ratios of about 0.24 (not insignificant).

Are additional variables appropriate?
mullenc525 is offline   Reply With Quote

Old   January 24, 2011, 18:20
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
You can make the properties of the fluid a function of the amount of water vapour to some extent. This should work for a limited range of mass fractions, linked to viscosity, density.

Rather than a multiphase approach you might consider a single phase but multi-species approach using mass fractions. If you can't get the range of properties you need you should consider this approach as it will be able to handle pure air to pure vapour and everything in between.
ghorrocks is offline   Reply With Quote

Old   January 25, 2011, 18:40
Default
  #7
New Member
 
Join Date: Dec 2010
Posts: 26
Rep Power: 6
mullenc525 is on a distinguished road
Thanks Glenn,

I've gone the multi component route instead of using an additional variable; it's amazing how easy it is when you know what you are looking for!

I've selected water vapor to follow the transport equation with air using the constraint equation. Simply because I know the diffusivity of water in air and this is what somebody else did. Is this appropriate, and how does one select which to use in general?

Curtis
mullenc525 is offline   Reply With Quote

Old   January 26, 2011, 00:26
Default
  #8
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,651
Rep Power: 84
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
I am no expert on multi-component models but I don't think it matters which one you set up as the constraint. I would try it either way and see what the difference is.
ghorrocks is offline   Reply With Quote

Reply

Thread Tools
Display Modes

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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
heat and mass transfer durin water evaporation from wet surface yigitktu FLUENT 0 September 22, 2010 08:04
Constant velocity of the material Sas CFX 15 July 13, 2010 08:56
mass flow in is not equal to mass flow out saii CFX 2 September 18, 2009 08:07
Water vapour condensation in CFX-5.7.1 hdj CFX 1 November 27, 2005 08:15
Convective Heat Transfer - Heat Exchanger Mark CFX 6 November 15, 2004 16:55


All times are GMT -4. The time now is 17:34.