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Old   September 2, 2005, 06:20
Default Residence time
  #1
AlwaysLearning
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Hi Guys,

I am trying to calc the residence time in a simple domain. Cuboid, one inlet (supply) one outlet.

Isothermal, Ke and incompressible.

I am trying three things. No real joy.

To start with.

Path lines (set from interior (fluid)) gives a max rtd of 6.07 secs Particale tracks, (I set an injection of Carbon dioxide thoughout all fluid) gives 2.8 secs.

Why the difference?

I am also trying a UDS/UDF as suggested on the fluent site. Here I set a UDS. Set its value at 0 at inlet.

Copy the UDF and use that for the rt_source calc. as below.

DEFINE_SOURCE(rt_source,c,t,dS,eqn) { real source = C_R(c,t); dS[eqn] = 0.0; return source; }

Yet with this I don't get any RTD?

I get a uniform distribution of zero!

Ideas?

Thanks

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Old   September 7, 2005, 11:03
Default Re: Residence time
  #2
AlwaysLearning
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Anyone got any thoughts on this.

Thanks
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Old   September 8, 2005, 01:57
Default Re: Residence time
  #3
Rajeev Kumar Singh
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The best way to calculate residence time is that you use species transport. Using materials panel add a new fluid. Go to mixture template, in the names region go to edit and add the new species (look up the VOF simulation tutorial) one on spinning bowl of water on how it is done. Keep the tracer fluid as number1 and bulk fluid as number2. You can remove all others. Once you have set the two liquids in mixture. Make their properties same (density=same, viscosity-same, molecular wt=same) i.e you have the tracer fluid only different in name. then you track the surface avg)mass fraction of tracer coming out of outlet in unsteady state. You have the residence time curve

hope this helps Raj
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Old   September 8, 2005, 14:07
Default Re: Residence time
  #4
Newbie
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Thanks for this _ I will give it a try.

Some other thoughts whilst on.

Can I calculate the RTD for a steady state prediction? The method you gave only allows me, assuming I am understanding your words correctly, to calc the Res' Time at the outlet.

Can I get the RTD 'throughout' the domain. i.e. to look for stagnant areas of fluid inthe domain and/or look for short circuiting (i.e. inlet fluid going straight to outlet), and also quantify these regions?

Thoughts ?

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Old   September 9, 2005, 03:01
Default Re: Residence time
  #5
Rajeev Kumar Singh
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If you look up Chapter 9 of the O.Levenspiel's book it will give u a method of how to find out the dead volume, and mixed volume of any system based on RTD curves for the outlet. I do calculate for steady state flows. First I solve flow equations for steady state. Once flow is achieved. Stop solving flow and patch species and solve unsteady state for few hundred seconds (time can be found out from the average residence time) roughly 5 times the avg,. residence time. Once you get the curve anything above 2*theta is dead volume. Better go through Levenspiel's book
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