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-   -   Access to path length CEL ¿variable? (https://www.cfd-online.com/Forums/cfx/92834-access-path-length-cel-variable.html)

 jrivera September 26, 2011 17:41

Access to path length CEL ¿variable?

Hello,

As i wrote in the title, i need to access to that variable. Specifically, i need the absorption coefficient of a gas, but it depends on a lot of things, so, i'll use curves of emissivity for different p*Leff of this gas to obtain its emissivity, replace this emissivity in the equation e = 1 - exp(kpL), (with e: emissivity, k: absorption coefficient, p: partial gas pressure, L:path length) getting the value of k, but to do this i need the value of L or at least the way of getting it from a CEL expression.
The gas is SO2, there aren't enough information about the radiative properties of this gas, so i have to do this from this way. If anyone have information about the absorption coefficient of this gas, i'll be very grateful.
I'm sorry for my bad english. Thanks for the future answers.

Jorge

 ghorrocks September 27, 2011 06:06

I think the equation you are quoting is for a finite thickness of gas, not a gas property which is based on a unit cell of gas. The gas property should not have path length in it, instead it uses absorption coeff and it seems you already have that.

 jrivera September 27, 2011 09:28

First, thanks for your answer.
I agree with you, the path length isn't a gas property, but if you look in the chapter 7.6.3 of the CFX Theory guide, you'll see that CFX asks you for the absorption coefficient to calculate the gas emissivity according to the equation of the last post. So, CFX calculates a path length to get emissivity but i don't know if it's an unique path length per unit cell of gas or the equation is resolved for every direction the unit cell of gas sees, so, there are many path lengths per unit cell of gas.
I don't have the absorption coefficient, that's the difficult value to get because it depends on temperature, bands and other things that are away from my knowledge.

 ghorrocks September 28, 2011 07:08

I think you should read up the CFX dosumentation on radiation models. There are a number of radiation models, some of which assume a opaque gas where the radiation only penetrates a short distance. Ray tracing is not done for the models as the radiation only affects a short distance from the radiating body. Is this what you are referring to?

If you want ray tracing with long path lengths you need to use discrete transfer or monte carlo approaches. Neither of these use path length as an input variable as they work out the path length.

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