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Problems Calculating Residence Time [trace]

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Old   October 19, 2010, 21:19
Default Problems Calculating Residence Time [trace]
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Vitor de Miranda Henrique
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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.


Uploaded with ImageShack.us


I do not know what I'm doing wrong.

I would be grateful if someone could help me

Vitor Henrique
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Old   October 19, 2010, 22:13
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Michael P. Owen
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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."
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Old   October 19, 2010, 23:25
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Vitor de Miranda Henrique
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Quote:
Originally Posted by michael_owen View Post
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": Tracer Analysis in CFX

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
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Old   October 21, 2010, 10:28
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Michael P. Owen
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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.
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Old   October 21, 2010, 15:57
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Quote:
Originally Posted by michael_owen View Post
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
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Old   October 21, 2010, 16:39
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Michael P. Owen
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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.
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Old   June 24, 2012, 01:49
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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
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Old   June 24, 2012, 07:13
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Time is a variabel on streamlines, not a general simulation variable.
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Old   June 24, 2012, 18:39
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Thank you so much for reply.
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Old   November 8, 2014, 23:46
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Ashkan Javadzadegan
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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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