CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > ANSYS > FLUENT

Gas Radiaition Properties, Absoprtion Coefficent

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Search this Thread Display Modes
Old   June 8, 2015, 11:03
Default Gas Radiaition Properties, Absoprtion Coefficent
New Member
James O
Join Date: May 2015
Posts: 4
Rep Power: 10
MH_Engineer is on a distinguished road

I am trying to calculate the absorption coefficient for H2O and CO2 mixture for a radiation problem in Fluent.

According to the Fluent manual they want it in the units 1/m.

Past posts bring up such references as Hottel Charts or using text books such as Thermal Radiation Heat Transfer by Howell & Siegel.

Using Hottel Charts, you need to provide mean beam length which is shape dependent and the coefficient it calculates is unitless. Should I assume a 1m sphere when using these charts to get my coefficient in units 1/m???

Or in Thermal Radiation 5th Ed., there is the Leckner equation which is an algebraic equation of Hottel's charts. Once again it solves the coefficient as unitless, so should I just plug in 1m as Le to get the coefficient on a 1/m units?

The equations and charts aren't linear so I am having a hard time trying to represent them in units of 1/m.
MH_Engineer is offline   Reply With Quote

Old   December 30, 2015, 08:21
New Member
Join Date: Oct 2014
Posts: 5
Rep Power: 11
maciekstp is on a distinguished road
Mean radiation beam length can be calculated approximately with formula:


where V is a volume, and A is a surface area of your geometry. For a simple "flow in a duct" case, you can get that L=0.9*diameter.

Using Hottel charts, you can estimate gas emissivity for your case. As you said, Leckner equations can be used to calculate the emissivity.

Emissivity is unitless and it is equal to absorptivity. However, absorption coefficient is something different, and it is expressed in units [m^-1].

For a grey gas model, there is a popular equation:


where k is an absorption coefficient. After doing some math, you can obtain:


I am working in CFX and I did it by defining some expressions to calculate absorption coefficient. I tried to simulate simple flow of combustion gas in a duct. To verify my results, I calculated this case analytically. I estimated emissivity and absorptivity from Hottel charts and I used this formula:

q_rad=sigma*(gas emissivity*T_gas^4 - gas absorptivity*T_wall^4)

to calculate wall radiative heat flux. Unfortunately, value from CFX is much bigger than value from analytical solution. Does anyone have a similar problem? I will be grateful for any advice,

Best regards,
maciekstp is offline   Reply With Quote

Old   January 28, 2016, 17:13
New Member
Join Date: Sep 2013
Posts: 2
Rep Power: 0
Runtowhere is on a distinguished road
Hi, I think the q_rad formulation that you used for calculating the radiative heat flux is not
appropriate for this problem.
You can use the Fluent with the WSGG model to solve this problem to see if there is any difference between it with CFX.
Hope this works for you.
Runtowhere is offline   Reply With Quote


absorption coefficient, radiation model

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
mass flow in is not equal to mass flow out saii CFX 12 March 19, 2018 05:21
error message cuteapathy CFX 14 March 20, 2012 06:45
Constant velocity of the material Sas CFX 15 July 13, 2010 08:56
Two-Phase Buoyant Flow Issue Miguel Baritto CFX 4 August 31, 2006 12:02
Gas pressure question Dan Moskal Main CFD Forum 0 October 24, 2002 22:02

All times are GMT -4. The time now is 19:22.