# Problems Calculating Residence Time [trace]

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 October 19, 2010, 21:19 Problems Calculating Residence Time [trace] #1 New Member   Vitor de Miranda Henrique Join Date: Sep 2010 Posts: 6 Rep Power: 8 Sponsored Links Hello, I'm trying to calculate the residence time of a Plug Flow reactor, this is my steps: 1: Create a Variable: Tracer, Specific, with no units; 2: Enable the variable in the reactor domain; 3: Create a TracerInjection with the units kg/s My equation is step((t - 1[s])/1[s])*1[kg s^-1]*step((2[s] - t)/1[s]) this equation give a step between the times of 1 to 2 seconds; 4: Create a monitor equation TracerOut = areaAve(Tracer)@outlet; 5: Put all inicial conditions; The problem is when I solve the simulation, the CFX-solver give me a wrong chart of de DTR (including up to negative times) I am attaching a picture of the graph i get. The green curve is the step, and the others are of the DRT. http://img403.imageshack.us/i/tracer.jpg/ I do not know what I'm doing wrong. I would be grateful if someone could help me Vitor Henrique Last edited by wyldckat; September 3, 2015 at 18:01. Reason: disabled embedded images

 October 19, 2010, 22:13 #2 Senior Member   Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 10 How is this supposed to calculate the residence time? There are two methods you could use. Either use an additional variable with dimensions of time initialized to 0[s] and with the inlet boundary value set to zero and use an AV source to increment it at a rate of 1s/s, or skip the source and set the inlet value equal to the current simulation time, so each parcel of fluid knows when it was "born."

October 19, 2010, 23:25
#3
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Vitor de Miranda Henrique
Join Date: Sep 2010
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Quote:
 Originally Posted by michael_owen How is this supposed to calculate the residence time? There are two methods you could use. Either use an additional variable with dimensions of time initialized to 0[s] and with the inlet boundary value set to zero and use an AV source to increment it at a rate of 1s/s, or skip the source and set the inlet value equal to the current simulation time, so each parcel of fluid knows when it was "born."
Hello Michael,

I followed this "tutorial": http://www.cfd-online.com/Forums/cfx...lysis-cfx.html

how can I do this other method? i create a variable, with unitis of [s] then I set transport equation?
How can i create this AV source?

Sorry about the questions, but i'm new in CFD.

Tanks,

Vitor Henrique

 October 21, 2010, 10:28 #4 Senior Member   Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 10 Vitor, That thread is about injecting a tracer into the flow. That isn't what you want to do; you're trying to calculate residence time. Essentially that thread has absolutely nothing to do with what you want, as far as I can tell. You should use one of the two methods I described. You could also just plot the time on streamlines in Post, but for transient simulations the interpretation of this is tricky and may not yield what you want.

October 21, 2010, 15:57
#5
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Vitor de Miranda Henrique
Join Date: Sep 2010
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Quote:
 Originally Posted by michael_owen Vitor, That thread is about injecting a tracer into the flow. That isn't what you want to do; you're trying to calculate residence time. Essentially that thread has absolutely nothing to do with what you want, as far as I can tell. You should use one of the two methods I described. You could also just plot the time on streamlines in Post, but for transient simulations the interpretation of this is tricky and may not yield what you want.

Michael

I know how to create a variable, and how to set the units.But how can i use an AV source to increment it at a rate of 1s/s ? What is a AV source?

Thank you

Vitor Henrique

 October 21, 2010, 16:39 #6 Senior Member   Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 10 Insert a subdomain scoped to the entire domain. On the Sources tab, check sources, and then input a source for your additional variable. All of this is in the documentation.

 June 24, 2012, 01:49 #7 New Member   sepideh Join Date: Jul 2011 Posts: 21 Rep Power: 8 Hi If the situation is steady state can I still use streamline and plot time?because I dont have time variable! I have a rectagular box with a slot at the top of the box,that is inlet,I dont have any outlet and the approach is E-E. Regards, Sepi

 June 24, 2012, 07:13 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,725 Rep Power: 106 Time is a variabel on streamlines, not a general simulation variable.

 June 24, 2012, 18:39 #9 New Member   sepideh Join Date: Jul 2011 Posts: 21 Rep Power: 8 Thank you so much for reply.

 November 8, 2014, 23:46 #10 Senior Member   Ashkan Javadzadegan Join Date: Sep 2010 Posts: 248 Rep Power: 9 Dear all, I am trying to use the suggested method for calculating residance time. The only problem is the unit of source term for AV. the unit is kg/m^3; however, as it is mentioned here, the unit of source term should be [s/s]. Does any body know how to make the unit of source term [s/s]? Thanks

September 27, 2016, 11:13
#11
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Prashanth
Join Date: Jun 2016
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Quote:
 Originally Posted by michael_owen Insert a subdomain scoped to the entire domain. On the Sources tab, check sources, and then input a source for your additional variable. All of this is in the documentation.
Hii,

i have to calculate particle residence time and mass concentration in the cell.

am feeding steady particles in the transient flow using DPM method.

How can i come up with this.

Thank you

 Tags problem, residence, rtd, time, tracer

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