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 mp February 18, 2003 05:16

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

 Manosh February 18, 2003 08:14

Re: Absorption & Scattering Co-efficients, Radiati

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?

 nuray kayakol February 18, 2003 10:19

Re: Absorption & Scattering Co-efficients, Radiati

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

 mp February 18, 2003 10:56

Re: Absorption & Scattering Co-efficients, Radiati

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

 nuray kayakol February 18, 2003 11:14

Re: Absorption & Scattering Co-efficients, Radiati

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

 mp February 18, 2003 12:56

Re: Absorption & Scattering Co-efficients, Radiati

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.

-mp

 nuray kayakol February 18, 2003 13:14

Re: Absorption & Scattering Co-efficients, Radiati

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

 mp February 19, 2003 11:10

Re: Absorption & Scattering Co-efficients, Radiati

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

 nuray kayakol February 19, 2003 11:20

Re: Absorption & Scattering Co-efficients, Radiati

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

 Erwin February 24, 2003 07:01

Re: Absorption & Scattering Co-efficients, Radiati

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.

 nuray kayakol March 3, 2003 10:48

Re: Absorption & Scattering Co-efficients, Radiati

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

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