Radiation - Emissivity

crevoise · December 18, 2013, 12:09

Hello

I wish to simulate the heat losses and the radiation of an exhaust pipe.

For now, I am using the 'convection' option for the heat losses. The radiation is thus mobilized with the surface emissivity.

I wish to try the 'environment' option for the heat losses.
Thus, for the radiation, in addition to the 'surface emissivity', appears the 'external emissivitiy'.
What is the difference between the surface and the external emissivities?
Does this external emissivity is relevant in such a case?

Thank you for your help and explanations

siara817 · December 19, 2013, 09:54

External emissivities can be important if the heat exchange between the outer walls of your domain and surrounding environment is significant. I think in your case that you are simulating an exhaust pipe, you can not simulate the whole world around the exhaust pipe. Instead you can use this environment model and external emissivity to consider the heat radiating to environment.

crevoise · December 19, 2013, 10:13

Hello Siamak

Thank you for your answer.
There is indeed huge different in the results computations, when activating the 'external radiation'.

Compared to StarCD, I did not find any 'external radiation' option, but only the 'surfacic radiation' one.
Do you know if there is a way to implement such 'external radiation' in StarCD?

Thanks again for your help

Zeppo · December 20, 2013, 12:42

Take a look at the documentation supplied with the software. It shows how this boundary condition can be used in a particular case with car windows.

December 18, 2013, 12:09	Radiation - Emissivity	#1
crevoise Member Join Date: Oct 2011 Location: Thessaloniki, Greece Posts: 75 Rep Power: 15	Hello I wish to simulate the heat losses and the radiation of an exhaust pipe. For now, I am using the 'convection' option for the heat losses. The radiation is thus mobilized with the surface emissivity. I wish to try the 'environment' option for the heat losses. Thus, for the radiation, in addition to the 'surface emissivity', appears the 'external emissivitiy'. What is the difference between the surface and the external emissivities? Does this external emissivity is relevant in such a case? Thank you for your help and explanations

December 19, 2013, 09:54		#2
siara817 Senior Member siamak rahimi ardkapan Join Date: Jul 2010 Location: Copenhagen, Denmark Posts: 220 Rep Power: 18	External emissivities can be important if the heat exchange between the outer walls of your domain and surrounding environment is significant. I think in your case that you are simulating an exhaust pipe, you can not simulate the whole world around the exhaust pipe. Instead you can use this environment model and external emissivity to consider the heat radiating to environment. __________________ Good luck Siamak

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
wall emissivity for radiation problem: emitter is also receiving when it shouldn't	Young	FLUENT	14	January 7, 2016 07:08
Error finding variable "THERMX"	sunilpatil	CFX	8	April 26, 2013 08:00
Modeling both radiation and convection on surfaces - Ansys Transient Thermal R13	s.mishra	ANSYS	0	March 31, 2012 05:12
Multiphase radiation in CFX	Luk_Fiz	CFX	5	March 17, 2009 04:51
Radiation and interfaces	Frank	CFX	2	June 22, 2005 12:26

December 19, 2013, 10:13		#3
crevoise Member Join Date: Oct 2011 Location: Thessaloniki, Greece Posts: 75 Rep Power: 15	Hello Siamak Thank you for your answer. There is indeed huge different in the results computations, when activating the 'external radiation'. Compared to StarCD, I did not find any 'external radiation' option, but only the 'surfacic radiation' one. Do you know if there is a way to implement such 'external radiation' in StarCD? Thanks again for your help

December 20, 2013, 12:42		#4
Zeppo Senior Member Sergei Join Date: Dec 2009 Posts: 261 Rep Power: 22	Take a look at the documentation supplied with the software. It shows how this boundary condition can be used in a particular case with car windows.