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Old   January 25, 2008, 13:09
Default Lewis number in docs?
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Patti
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I'm looking for references in the Phoenics documentation about the molecular diffusivity models. Specifically, I want to know if the "unity Lewis number" approximation is used, and, if so, how to set the Lewis number to less than unity for very light species. Can anyone please point me in the right direction? Thank You Very Much, Patti
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Old   January 29, 2008, 17:10
Default Re: Lewis number in docs?
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Joe
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From my understanding of the documentation, I think unity Lewis number is used. The setting you're looking for is PRNDTL(phi) and PRT(phi). In the documentation they are stated as the ratio of kinematic viscosity divided by diffusion coefficient, in the case of species. So that would be the Schmidt number, Sc = Le*Pr. PRNDTL(phi) defaults to 1.0 for most variables.

I hope that helps
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Old   February 7, 2008, 21:23
Default Re: Lewis number in docs?
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Patti
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Thank you very much, Joe. That is not good to find out but good to know. The Lewis number for H2 should be about 0.3 and that for H ~0.1. I hope I can figure out how to set these. I've been digging through the encyclopedia, but so far no joy for the Schmidt or Lewis numbers.

Patti
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Old   February 8, 2008, 11:45
Default Re: Lewis number in docs?
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Joe
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Look for PRNDTL in the Phoenics encyclopedia, or PRT.

In the Phoenics instruction language, assigning Prandtl or Schmidt numbers is done with the command PRNDTL(phi)=X. When phi is TEM1 or H1, you're setting a Prandtl number, when the phi is a mass fraction like H2, you're setting the Schmidt number.

If you are simulating combustion of H2, hydrogen, with constant properties, in Phoenics you might set PRNDTL(H1 or TEM1) = 0.7; this is the Prandtl number. Then you would give the Phoenics command PRNDTL(H2)= 0.3*0.7, setting the Schmidt number for H2 to Le * Pr.

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Old   February 9, 2008, 16:38
Default Re: Lewis number in docs?
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Patti
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Thank you so much, Joe! I might be able to try that. Normally I set PRNDTL(TEM1)=-GRND9 so CHEMKIN to calculate accurate thermal conductivities of my gas mixture. To try your suggestion I'd need to retrieve the value it calculated (say, VAL) then set PRNDTL(H)=0.1*VAL. (Perhaps CHAM can comment here on how to do this in version 3.4?)

The CHEMKIN subsystem docs show that I can calculate species diffusivities by PRNDTL(H)=-GRND9, but if I can explicitly set individual species diffusivities, how is it that the Unity Lewis Number assumption is active?

So far all I really know is that if I set PRNDTL(H)=-GRND9 my results are VERY different from not setting it, seem physically unreasonable (compared to experiment), and I'm hoping for an explanation from CHAM.

THANKS!! Patti
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Old   February 11, 2008, 16:08
Default Re: Lewis number in docs?
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Joe
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If you're using GRND9 for density, ENUL, PRNDTL(TEM1), and PRNDTL(species), then you would be using Chemkin's transport libraries to calculate properties, so the case wouldn't be using a unity Lewis number assumption at that point. It would (essentially) have an individual Lewis number for each specie in each cell, based on the tranport properties Chemkin calculates.

Possibly you could get Phoenics to store the local Lewis number for you using In-form. Definitely using some Fortran programming in Ground would do it. That might let you figure out why you're getting unreasonable results.

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Old   May 6, 2008, 15:35
Default Re: Lewis number in docs?
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PattiMichelle Sheaffer
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Thanks for your help! I was able to get everything operating correctly (version 3.4) except for the Soret effect. The Enhanced Diffusion model seems to work OK. Now I notice in the Phoenics2006 code that the Enhanced Diffusion model was broken (i.e., in version 3.4), but apparently fixed in Ph2006. So I'm in the process of trying to get my models ported to 2006. The downside is that the Soret effect seems to be still borken.

THANKS! Patti
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