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-   -   Emissivity for boundary condition (http://www.cfd-online.com/Forums/fluent/34214-emissivity-boundary-condition.html)

ereiss July 16, 2004 09:48

Emissivity for boundary condition
 
I am modeling the heat flow in a concrete floor heated with imbedded hot water pipes. For the top surface boundary condition I am using the mixed thermal conditions, where I can input the external emissivity and temperature for the radiative heat transfer. I can't find any place (such as "define materials") to input the emissivity of the concrete surface though. Even if I select radiation under "define models" I can't find any place to provide the surface emissivity. How does FLUENT calculate this? Does it assume an emissivity of the boundary of 1? Thanks for any information you can provide.

Bob July 16, 2004 12:13

Re: Emissivity for boundary condition
 
What Radiation model are you using?

ereiss July 16, 2004 12:59

Re: Emissivity for boundary condition
 
I am not using a radiation model. I am just using the energy model but, I can still use a mixed boundary condition that incorporates both convection and radiation. It is the radiation part that I don't know how FLUENT calculates.

nazeem July 25, 2004 04:48

Re: Emissivity for boundary condition
 
Dear ereiss ,you should take a global convection coeficient which includes conv+radiation

h=24.1*e-4*epsilon*pow(T,1.61)

epsilon :emissivity ,T temperature

and you write a UDF for this......

n7310889 August 10, 2012 01:27

External radiation
 
Quote:

Originally Posted by nazeem
;113968
Dear ereiss ,you should take a global convection coeficient which includes conv+radiation

h=24.1*e-4*epsilon*pow(T,1.61)

epsilon :emissivity ,T temperature

and you write a UDF for this......

Dear Nazeem,
Could you please write the equation a bit clearly? And is it possible for you to give a reference for that, so that I can make some study. Another request is, I am modeling a conjugate heat transfer problem (liquid coolant flow through a tube), in which I have to give three boundary conditions on the outer surface of the tube: (non-uniform) functional heat flux as heat input, radiation loss and convection loss. But I don't understand how can I ensure all of these 3 at a time:confused:.
Could you please suggest me? I'll be happy if you may text me in my email.

Regards
n7310889 (murad99me@yahoo.com)


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