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Ray April 7, 2006 17:22

Radiation Model - Spectral Model
 
Hello,

I want to simulate a flame radiation with the Monte Carlo Model. But I don't know which and what spectral model I have to use. The grey is probable not good enough. How do I have to set the multiband frequencies or is the multigrey the model of choice? Plz somebody help me.


opaque April 7, 2006 17:49

Re: Radiation Model - Spectral Model
 
Dear Ray,

What are you trying to do? Gray is always the first cut for any radiation calculation since most do not have an accurate set of properties, or the mixture is so complex that is not easy to come up with a good set of data..

If your Gray model results are close to expected, and you want to refine the calculation; then, you should determine which species are real participating media, and which type of approximation is good for that type of mixture.. In combustion cases, usually models based on Weighted Sum of Gray Gases/Multigrey are the next approach.. There are more elaborate models, but they are not available in ANSYS CFX.

Using the Multiband model requires a lot of information which is available in research databases such as HITRAN (please check Dr. M. Modest Radiative Heat Transfer book).. You will require an enourmous amount of computational work using such databases.. That is why they invented models such WSGG (basically a reduced model for non-gray approximation)..I imagine you are using a reduced of reactions already to save some computer time, or because you are solving turbulent combustion flows..

Why MC on a flame calculation? Are P1 or DTM not good enough for your geometry?

Good luck, Opaque..


Ray April 8, 2006 03:03

Re: Radiation Model - Spectral Model
 
Hello opaque,

thanks for your reply. I want to use MC because wall to wall radiation is very important point in my case. Following the CFX manual MC is the best model for that.

I'm simulating a turbulent combustion with a reduced mechanism. I have CO, H20 and CO2 as products which have an enormous effect on radiation. Thats why I think the grey gas approach is not exact enough and as the manual says it it not recommended for combustion.

Do you know how to use WSGG or where I can get the coefficients for it? Why do you say it is not available in CFX? It is implemented in CFX 10.0.

opaque April 10, 2006 09:33

Re: Radiation Model - Spectral Model
 
Dear Ray,

OK.. Just keep in mind that if your gas is fairly absorbing in most of the bands/gray gases, the wall to wall radiation effect is minimal since the radiation is absorbed when traveling through the medium..

Also, in most combustion enviroments the walls are fairly cold compared to the flame itself; therefore, most of the radiative heat flux comes from the flame towards the wall, and no the other way around.

I did not say that CFX does not have WSGG.. I said "There are more elaborate models...".. I meant more elaborate than WSGG such the Full Spectrum K-Distribution, Spectral Line Weigthed, etc.. In fact, if you are basically burning CH4 with O2, and having CO, H2O, CO2 as products you can use the built-in template for WSGG in CFX-10.0..

When you start a new case in CFX-Pre, select library instead of General, and pick the multigrey_radiation.ccl library.. Continue your setup from there.. Or, read the template and extract what you need and paste it to your current setup..

Good luck, Opaque..

PS. I still rather get combustion results w/o radiation.. Go for a gray/WSGG using DT, and if the results look suspect, I will try Monte Carlo..



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