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Heat transfer coefficient

Posted Aug 17, 2011, 10:40 a.m. EDT Version 4.0a 4 Replies

Maximilian Neusser
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Hallo,

im quite new to Comsol and i hope you can help me.

I try to calculate the convective heat transfer coefficient between a concrete ceiling and the air in the room.
It should be natural convection.

I read and tried to modify the tutorial example "free convection" but this doesent really work.

do you have any hints for approaching this problem?

thanks, best regards
Max
4 Replies Last Post Aug 18, 2011, 11:33 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Aug 17, 2011, 5:05 p.m. EDT
Hi

For me it is no obvious to understand exactly what you are trying to simulate.
I normally define a parameter for the heat convection, and I use 2, 5 or 10 [W/m^2/K] for respectively no air motion, natural motion or forced air motion, but this is for "pure surface" heat exchange when I do not bother to simulate the air via CFD.

if you have measurements for air and wall temperatures for given heat sources, and want to make a model to match experimental results, you need to declare your convection coefficient as a dependent variable for a least square regression (with or without the optimisation module)

Finally if it is to try out the advanced pre-defined wall convection models of COMSOL CFD/HT well I can only recommend to use the latest version and patch, as these are much more advanced in handling such cases, as well as the doc and the model library has nice examples to learn and start with

--
Good luck
Ivar
Hi For me it is no obvious to understand exactly what you are trying to simulate. I normally define a parameter for the heat convection, and I use 2, 5 or 10 [W/m^2/K] for respectively no air motion, natural motion or forced air motion, but this is for "pure surface" heat exchange when I do not bother to simulate the air via CFD. if you have measurements for air and wall temperatures for given heat sources, and want to make a model to match experimental results, you need to declare your convection coefficient as a dependent variable for a least square regression (with or without the optimisation module) Finally if it is to try out the advanced pre-defined wall convection models of COMSOL CFD/HT well I can only recommend to use the latest version and patch, as these are much more advanced in handling such cases, as well as the doc and the model library has nice examples to learn and start with -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Aug 17, 2011, 5:29 p.m. EDT
I used COMSOL for similar problems where I wanted to calculate effective natural/forced convection coefficients and it always worked but it usually required some problem-specific tuning in order to get an accurate converged solution. In addition to the example you mentioned, also check out the “gravity and boundary conditions” example as it applies the volume force as g*rho (instead of g*rho*alpha*(T-T_init)) and sets up a pressure gradient. Also if possible first check if the natural convective flow is expected to be laminar or turbulent. The relevant equation depends on geometry and can be found in heat transfer textbooks. Also make sure your mesh accounts for the boundary layers.

Nagi Elabbasi
Veryst Engineering
I used COMSOL for similar problems where I wanted to calculate effective natural/forced convection coefficients and it always worked but it usually required some problem-specific tuning in order to get an accurate converged solution. In addition to the example you mentioned, also check out the “gravity and boundary conditions” example as it applies the volume force as g*rho (instead of g*rho*alpha*(T-T_init)) and sets up a pressure gradient. Also if possible first check if the natural convective flow is expected to be laminar or turbulent. The relevant equation depends on geometry and can be found in heat transfer textbooks. Also make sure your mesh accounts for the boundary layers. Nagi Elabbasi Veryst Engineering
Maximilian Neusser
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Posted: 1 decade ago Aug 18, 2011, 4:19 a.m. EDT
thanks for the answers!

I tried to rebuild this kind of problem: www.comsol.com/papers/1696/download/Buhler.pdf
in a lager scale so a room can be modeld.
Then i want to make different ceiling structures and calculate the heat flux between the room and the ceiling.

i´m a student at the technical university of vienna so im working with a class kit license of comsol multiphysics, do you get more functions with the heat transfer module or is it inculded in the multiphysics package?!
because i cant open example files like this: www.comsol.com/showroom/gallery/1448/

best regards
thanks for the answers! I tried to rebuild this kind of problem: http://www.comsol.com/papers/1696/download/Buhler.pdf in a lager scale so a room can be modeld. Then i want to make different ceiling structures and calculate the heat flux between the room and the ceiling. i´m a student at the technical university of vienna so im working with a class kit license of comsol multiphysics, do you get more functions with the heat transfer module or is it inculded in the multiphysics package?! because i cant open example files like this: http://www.comsol.com/showroom/gallery/1448/ best regards
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Aug 18, 2011, 11:33 a.m. EDT
You're welcome!

The gallery example 1448 you mentioned actually has two models. The first one uses empirical estimates of the heat transfer coefficient. That needs the heat transfer module (unless you manually provide the heat transfer coefficient expression). The second model evaluates the heat transfer by solving the thermal flow problem. That model does not require the heat transfer module.

Nagi Elabbasi
Veryst Engineering
You're welcome! The gallery example 1448 you mentioned actually has two models. The first one uses empirical estimates of the heat transfer coefficient. That needs the heat transfer module (unless you manually provide the heat transfer coefficient expression). The second model evaluates the heat transfer by solving the thermal flow problem. That model does not require the heat transfer module. Nagi Elabbasi Veryst Engineering

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