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-   -   Problems Calculating Residence Time [trace] (http://www.cfd-online.com/Forums/cfx/81204-problems-calculating-residence-time-trace.html)

vitormhenrique October 19, 2010 21:19

Problems Calculating Residence Time [trace]
 
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

michael_owen October 19, 2010 22:13

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."

vitormhenrique October 19, 2010 23:25

Quote:

Originally Posted by michael_owen (Post 279897)
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

michael_owen October 21, 2010 10:28

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.

vitormhenrique October 21, 2010 15:57

Quote:

Originally Posted by michael_owen (Post 280162)
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

michael_owen October 21, 2010 16:39

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.

sepidecent June 24, 2012 01:49

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

ghorrocks June 24, 2012 07:13

Time is a variabel on streamlines, not a general simulation variable.

sepidecent June 24, 2012 18:39

Thank you so much for reply.

ashtonJ November 8, 2014 23:46

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


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