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brajh11 November 18, 2011 17:56

Residence Time Distribution
Anybody know how is possible to implement in CFX Residence Time Distribution of flue gases in a combustion chamber?

ghorrocks November 19, 2011 06:14

You want to impose this on a simulation or extract this from a completed run as post-processing?

brajh11 November 19, 2011 07:54

I want extract this from a completed run as post-processing

ghorrocks November 20, 2011 06:31

If the simulation has already been run then put some streamlines on it (assuming it is steady state) and colour the streamlines by time. This will be your residence time distribution.

If the simulation has not run then add a user scalar with convection equation only, but a source term of 1 [s^-1]. If you do a plot of this variable it will be the residence time of that region of the flow. This will work in both steady and transient flows.

ghorrocks November 20, 2011 17:57

Here's some links to other posts on this issue. It is always worthwhile doing a search on the forum as many questions have been asked before.

Note both of these links use a source term of 1 [s/s] which then means your AV has the units of seconds which makes more sense than mine which is unitless.

Danial Q November 20, 2011 22:43

Modelling suggestions??
Anyone please suggest that if it is possible to model a molten droplet of metal at high temp say 2700 C hitting a slolid metal surface at some temp say 500C. this molten droplet solidifes and there lies an interface between the two. I have to model "this interface" which acts different with its variable thickness. while this thickness depends on many solid and molten droplet's properties. i have to model the heat transfer of this interface with its corresponding thickness along the surface.
Please make comment if it would be right to define the domain interface between the solid and fluid (molten droplet) domains as my required interface? i am confused about it because it would negate the idea of variable thickness of that interface modeling. someone told me that it is beyond the range of CFX modeling. he suggested me to solve it with explicit dynamics rather CFX. is dt true?
Your help would be highy appreciated :).


ghorrocks November 21, 2011 05:54

Talk to CFX support about solidification modelling. And please do not hijack threads, start a new thread.

Danial Q November 21, 2011 17:42

HI glen
sorry, it ws first time ..i dint realize dt..

emily.imdieke February 3, 2014 13:29

Am I correct in assuming that if the flow is transient you can't just simply color the streamlines and get the residence time distribution or does it work the same way?

ghorrocks February 3, 2014 17:17

You can colour streamlines by any local variable. So if you have a residence time distribution you can colour it by that.

emily.imdieke February 3, 2014 17:23

So basically if my timestep is 0.001, but if my streamline time distribution goes to 1.7 seconds according to CFX Post, it is just taking the streamlines that started at that individual timestep and calculating them out to completion at 1.7 seconds which is when they would leave the geometry?

ghorrocks February 3, 2014 17:34

Of course in a transient simulation streamlines are just what the instantaneous flow field looks like - a real particle will not necessarily follow that path as the flow field is changing in time. So be careful trying to read too much into streamlines in transient simulations.

Having said that, I think your description of how it works is correct. You should do some checks to be sure.

emily.imdieke February 3, 2014 18:06

Thank you for the response. Upon further looking at the other threads that you posted it seems as though someone else agrees as well that transient streamline times are difficult to interpret.

This being said. I did try the additional variable method and had mine set as a volumetric scalar with a source of 1. I was originally trying to look at this to find residence time but did not figure out how each of the different variables worked nor did I realize that graphing them would be a good option. My questions then are what would be the location that this graph would be plotted on, and what are the correct variables to be plotted? Such as what is the different between Particle Time and Particle Traveling Time?

ashtonJ November 9, 2014 19:17

Dear Glenn,
For residence time calculation, the unit of source term for AV is kg/m^3. How is it possible to define a source term for AV with unit [s/s]??

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