Hi all,

Do you know how to get the Absorption Co-efficient and Scattering Co-efficient values for air at 1000°C. I am modelling radiative heat transfer problem. Is there any online help available (or) any sujjested formulae to calculate. Please let me know if you have any idea.

Thanks & Regards

-mp

I will suggest you to see the paper

[1] Kaplan et al, Combustion and Flame 96:1-21 (1994).

In this paper Page8, there is a formula to get absorption co-efficient which is a function of mole fractions for non-luminous case. Also a formula there for soot (luminous) case. Hope this helps you.

Can I ask a question? What method are you using for radiation?

Regrads Manosh

Hi,

Air is transparent to radiative heat transfer. O2 and H2 molecules doesn't absorb or emit radiation. Therefore, absorption coefficient for air is zero. Then, wall to wall radiation is important NOT gas radiation. If the medium doesn't have particles its scattering coefficient is also zero.

In literature there are some polynimoal functions (for example Leckner model)for the calculation of absorption coef. of combustion products such as CO2 and H20.

Nuray kayakol

Nuray, Thanks for your help,

I wonder, is it the same case as you explained for air at higher temperatures as well, such as at around 1000°C, example, exhaust gases of an engine. Please let me know.

Manosh, well I am using the P-1 radiation model in FLUENT.

Thanks & Regards

-mp

Hi, First of all, exhaust gases of an engine is not air.You need to know the chemical composition of the outlet stream.

If medium is air use view factors for the calculation of radiative heat transfer. It is computatinally cheap.

If medium contains combustion products calculate absorption coeff. from any polynomial type gray gas radiation model. if it is less than 0.15 1/m P-1 is not recommended. Use discerete ordinate model of Fluent.P-1 is very old diffusion type radiation model.

Nuray kayakol

Hi Nuray,

Thanks a lot again,

I am able to follow your explanation. Now I understood why my simulation results deviate from the theoretical calculations. It may be because of using P-1 radiation model. Previously I used the default absorption coefficient value of 0.01 which is far less than 0.15 as well. Now I ll try again with DO model.

Since, I assume that medium is only air, I am interested in your explanation to use VIEW FACTORS for the radiative heat transfer. Infact, I am very new to this concept, could you please explain me more in detail how to use the VIEW FACTORS.

Thanks in advance

-mp

Hi,

View factor concept is used to analyse the radiation exchange between the surfaces when the medium is nonparticipating (absorption coeff. is zero)

I am not a fluent user. You can read more detail from http://www.shef.ac.uk/mecheng/staff/...heory/th11.htm

Any radiation text book (Modest, Ozisik, Seigel and Howell) is useful.

Nuray kayakol

Thanks again,

The DO model seems to work far better than P-1 model in Fluent in my case. But my question still remains the same, how can I calculate the absorption coefficient of air (until now for comparision purpose, I assumed the default value of 0.01 in fluent).

I remember from your previous explaination, Air is transparent to radiative heat transfer. O2 and H2 molecules doesn't absorb or emit radiation. Therefore, absorption coefficient for air is zero. Is it true at temperatures around 1000°C also. I assume pure and clean air as the medium. Hope you can help me further.

Thanks & Regards,

-mp

Hi Absorption coefficient for air is zero at high temperatures. You can define it as zero at 1000 C. DOM can handle wall to wall radiation.

Best Regards Nuray kayakol

Calculate the amounts of CO2 and H2O in your gas mixture, then you can use the Fluent domain based Weighted Sum of Gray Gases (WSGGM) method to have the solver calculate the absorption coefficient. Without CO2 or H2O your fluid does not participate in radiation as nuray explained, also not at higher temperatures, obviously.

Here is some useful references that gives some correlations which is function of the temperature, partial pressure of contributing gases and mean beam length:

(1) Leckner, Spectral and total emissivity of water vapor and carbon dioxide , Combustion and flame , Vol 19, 33-48, 1972.

(2) Smith et al, J Heat Transfer vol 104(4), pp.602-608 1982.

It is useful to mention technical papers as a source of information.

Nuray

[CFDfan]

Originally Posted by nuray kayakol
;23334

Hi,

Air is transparent to radiative heat transfer. O2 and H2 molecules doesn't absorb or emit radiation. Therefore, absorption coefficient for air is zero. Then, wall to wall radiation is important NOT gas radiation. If the medium doesn't have particles its scattering coefficient is also zero.

In literature there are some polynimoal functions (for example Leckner model)for the calculation of absorption coef. of combustion products such as CO2 and H20.

Nuray kayakol

I know this has always been assumed in the most of the CFD codes, but really doesn't the air around the radiator get warm/hot?

[LuckyTran]

Originally Posted by CFDfan

I know this has always been assumed in the most of the CFD codes, but really doesn't the air around the radiator get warm/hot?

Yes the air is hot but simply being hot does not necessarily mean that thermal radiation is emitted/absorbed. In order for thermal radiation to be emitted, the substance (air) must have an emissivity >0. You have to respect the physical chemistry.

Non-polar molecules do not interact with IR radiation and are therefore transparent. Hence, for dry-air, typically only CO2 is important and for wet-air CO2+water vapor because these are the only polar molecules in the mixture. At very high temperatures, we are not talking about IR and then some interactions might take place.

Hence, the hotness of the air near the radiator has little to do with radiation and is primarily due to advection/conduction of thermal energy.

[CFDfan]

Thank you LuckyTran for the explanation, but I still think it is too idealized. Higher-End CFD codes as scStream use not only View factor radiation method, that assumes no energy absorption/dissipation to/from the air (as explained by you), but Flux Radiation method as well. The Flux Radiation method allows for energy absorption/dissipation to/from the air but is computationally very intensive, which I think is the main reason for the View Factor radiation method to be much more popular. In addition, the difference in the thermal results from the two methods (especially in small air volumes) is very modest which makes the VF method quite reasonable approximation. The climate processors do not ignore energy absorption/dissipation to/from the air however.

[LuckyTran]

The view factor method is algebraic. The flux method involves a transport equation (which you can say is roughly 1-2 orders of magnitude more intensive than the view factor method). View factor method can also accommodate absorption by including a characteristic path length (which is not that much harder to calculate than the view factor). There are also really nice ray tracing methods which is even more computationally expensive which can be written in crappy matlab code, no need for "high-end" CFD codes.

Originally Posted by CFDfan

Thank you LuckyTran for the explanation, but I still think it is too idealized.

That's fine to have as an opinion. But at the end of the day you are using models and you have to live with these assumptions or reject them like you are. But for example: Despite global warming and CO2 emissions being such an important topic in political debate, we still define dry air as 78.12% N2, 20.86% O2 and .92 % Ar. Where is the CO2!?!?? In many academic settings, even the Ar is dropped. You see that, the way most people define air, it is impossible to have IR absorption. Changing the mode of thinking requires more effort.

Also we are not saying that there is no such thing as absorption/emission in the universe. We are saying that simply "hot" and "air" does not participate in radiation. It's not only the hot gas that makes it a participating media, it needs to be hot and able to participate. Cold gases also participate, just not as much. So when you say "hot air" you need to be much more specific about what you mean!

If you want to do these participating media calculations, make sure you can calculate the local concentration of CO2 and H2O in your model. And the make sure you have the absorption spectrum. In my opinion, data is severely lacking here.

[lieyna]

As you mention about view factor, however, it is not available of all radiation model. If i not mistaken, S2@ is account the view factor parameter and affect. So by using Do or P1 can i say that, the view factor can ignored in our simulation? and how much important and significant the view factor in our result simulation?

Originally Posted by nuray kayakol
;23338

Hi, First of all, exhaust gases of an engine is not air.You need to know the chemical composition of the outlet stream.

If medium is air use view factors for the calculation of radiative heat transfer. It is computatinally cheap.

If medium contains combustion products calculate absorption coeff. from any polynomial type gray gas radiation model. if it is less than 0.15 1/m P-1 is not recommended. Use discerete ordinate model of Fluent.P-1 is very old diffusion type radiation model.

Nuray kayakol