Flotherm radiation simulation doesn't meet with experiment

759599290 · June 8, 2015, 02:08

Hi everyone,
I am a thermal engineer and usually use Flotherm. However, I found that the radiation simulation result of Flotherm did not meet experiment and I am strongly doubted by my boss now. For deeper exploration, I compared the simulation results using single-radiating and Sub-divided radiating, and the results show great difference, the temperature of the box shell differs from 52 to 64 centigrade, which single-radiating corresponding the lower value.Does anyone know the difference of these two models?
If the result shows such a difference, than one of these two models must be inproperate, but why flotherm gives the user two choices (in which there is a wrong one)?

Boris_M · July 13, 2015, 08:36

There are no models that are wrong, there is a reason for every option the user is provided.
The difference between the two options are the single radiationg option is where a surface is treated as a single radiating surface and the sub-divided option is where the surface is split into several radiating surface elements, if this option is used the surface will be splot into many surface elements that are radiating instead of just one large surface.

I'm sure there is something wrong in your setup so you get wrong results. I'm not an expert in FloTHERM but in most cases either the thermal boundary conditions of the components are not correct, the environment radiation/temperature conditions don't match or material and radiation properties are not correct.

Boris

zdunol · October 13, 2015, 10:05

So, did you resolve the problem Leon?

In my opinion I would look for discrepancies between model and real life simulation. Unfortunately I'm far from being expert in radiative heat transfer but what about surface roughness? Flotherm also gives default values of emissivity, reflectivity and absorbance, maybe those are/were wrong? I can also imagine that in Flotherm your geometry do not feel any radiation from bodies around it, and all the radiation leaves the computational domain too, but it's a long shot :P

Boris_M · October 13, 2015, 10:57

Indeed it is important not only to consider the radiation of the component itself but all surrounding components that influence it with their own radiation or are blocking the radiation. Since the amound radiated by a body is P=epsilon*sigma*A*(delta T)^4
with epsilon being the emissivity, sigma the Stefan Boltzmann constant and A the surface area. Delta T is the temperature difference between the two bodies or between the body and the environment and if the other body that is blocking the radiation is hotter than the environment but colder than the hot body then the emitted radiation is lower.

If you want to learn more about radiation you can have a look at the webinar Automotive Lighting 201 on the 28th October at the link below. It will also be available as an archived video afterwards on the website if you miss it. It will specifically be on radiation for automotive lighting applications and FloEFD rather than FloTHERM but it explains the radiation physics in detail with reflection and refraction etc. So maybe interesting to watch and learn more about radiation in general:
https://www.mentor.com/products/mech...e-lighting-201

Regards,
Boris

June 8, 2015, 02:08	Flotherm radiation simulation doesn't meet with experiment	#1
759599290 New Member Leon Chen Join Date: Aug 2012 Location: ShenZhen, China Posts: 9 Rep Power: 13	Hi everyone, I am a thermal engineer and usually use Flotherm. However, I found that the radiation simulation result of Flotherm did not meet experiment and I am strongly doubted by my boss now. For deeper exploration, I compared the simulation results using single-radiating and Sub-divided radiating, and the results show great difference, the temperature of the box shell differs from 52 to 64 centigrade, which single-radiating corresponding the lower value.Does anyone know the difference of these two models? If the result shows such a difference, than one of these two models must be inproperate, but why flotherm gives the user two choices (in which there is a wrong one)?

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July 13, 2015, 08:36		#2
Boris_M Disabled Join Date: Jul 2009 Posts: 616 Rep Power: 23	There are no models that are wrong, there is a reason for every option the user is provided. The difference between the two options are the single radiationg option is where a surface is treated as a single radiating surface and the sub-divided option is where the surface is split into several radiating surface elements, if this option is used the surface will be splot into many surface elements that are radiating instead of just one large surface. I'm sure there is something wrong in your setup so you get wrong results. I'm not an expert in FloTHERM but in most cases either the thermal boundary conditions of the components are not correct, the environment radiation/temperature conditions don't match or material and radiation properties are not correct. Boris

October 13, 2015, 10:05		#3
zdunol Member zduno Join Date: Dec 2013 Posts: 55 Rep Power: 12	So, did you resolve the problem Leon? In my opinion I would look for discrepancies between model and real life simulation. Unfortunately I'm far from being expert in radiative heat transfer but what about surface roughness? Flotherm also gives default values of emissivity, reflectivity and absorbance, maybe those are/were wrong? I can also imagine that in Flotherm your geometry do not feel any radiation from bodies around it, and all the radiation leaves the computational domain too, but it's a long shot :P

October 13, 2015, 10:57		#4
Boris_M Disabled Join Date: Jul 2009 Posts: 616 Rep Power: 23	Indeed it is important not only to consider the radiation of the component itself but all surrounding components that influence it with their own radiation or are blocking the radiation. Since the amound radiated by a body is P=epsilonsigmaA*(delta T)^4 with epsilon being the emissivity, sigma the Stefan Boltzmann constant and A the surface area. Delta T is the temperature difference between the two bodies or between the body and the environment and if the other body that is blocking the radiation is hotter than the environment but colder than the hot body then the emitted radiation is lower. If you want to learn more about radiation you can have a look at the webinar Automotive Lighting 201 on the 28th October at the link below. It will also be available as an archived video afterwards on the website if you miss it. It will specifically be on radiation for automotive lighting applications and FloEFD rather than FloTHERM but it explains the radiation physics in detail with reflection and refraction etc. So maybe interesting to watch and learn more about radiation in general: https://www.mentor.com/products/mech...e-lighting-201 Regards, Boris