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Forced external convective cooling

Posted Feb 7, 2011, 10:39 a.m. EST Fluid & Heat, Heat Transfer & Phase Change, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.1 5 Replies

Martin Fierz
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I have what must be a stupid question about cooling by external forced convection. I'm trying to model a metallic object that is locally heated on one surface and that will be cooled on the other surfaces by convection of air, and by radiative heat transfer.

I'm trying to model a case where there is an air current flowing in the room with a speed of U0 that is causing this cooling. In the heat-transfer physics mode, I choose "convective cooling" for all non-heated surfaces. In the heat flux settings, I set
- external forced convection
- plate, averaged transfer coefficient
- U_ext = U0
- external fluid: air
- pressure 1atm
- external temperature 293K

so far so good - but Comsol asks me for a "plate length" and this isn't explained anywhere in the user's guide as far as I can see. Is the plate length just the dimension of the object that I am modelling? Or is it something else?

thanks for an answer
Martin
5 Replies Last Post Dec 25, 2012, 5:32 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Feb 7, 2011, 10:59 a.m. EST
Hi

I suppose you are in 2D and then you need the "thickness" or depth or out of the papaer dimension =? length to go from absolute units to power densities

Does this make sens for you ?

--
Good luck
Ivar
Hi I suppose you are in 2D and then you need the "thickness" or depth or out of the papaer dimension =? length to go from absolute units to power densities Does this make sens for you ? -- Good luck Ivar
Martin Fierz
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Posted: 1 decade ago Feb 7, 2011, 11:37 a.m. EST

Hi

I suppose you are in 2D and then you need the "thickness" or depth or out of the papaer dimension =? length to go from absolute units to power densities

Does this make sens for you ?

--
Good luck
Ivar


I'm in 3D - my object is something like a cylinder (a bit more complicated, but basically a flattish cylindrical shape, see attached .png). the bottom of the object is heated, while the rest is exposed to the convective cooling. I'm afraid I don't know how to translate your answer to my problem - could you give me a hint?

thanks
Martin
[QUOTE] Hi I suppose you are in 2D and then you need the "thickness" or depth or out of the papaer dimension =? length to go from absolute units to power densities Does this make sens for you ? -- Good luck Ivar [/QUOTE] I'm in 3D - my object is something like a cylinder (a bit more complicated, but basically a flattish cylindrical shape, see attached .png). the bottom of the object is heated, while the rest is exposed to the convective cooling. I'm afraid I don't know how to translate your answer to my problem - could you give me a hint? thanks Martin

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Kai Herbrig
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Posted: 1 decade ago Feb 7, 2011, 12:20 p.m. EST
Hi,

I would say, the "plate length" is the "characteristic length" of your flow problem. Maybe the diameter should work. Don't forget: using average heat transfer coefficients is a coarse approximation. Consider that you don't have a plate and your are using coefficients for plates! But your results should not be too far away from reality. If you want to have exact results you must include air flow. But then it's gettting a bit more complicated.

Best regards
Hi, I would say, the "plate length" is the "characteristic length" of your flow problem. Maybe the diameter should work. Don't forget: using average heat transfer coefficients is a coarse approximation. Consider that you don't have a plate and your are using coefficients for plates! But your results should not be too far away from reality. If you want to have exact results you must include air flow. But then it's gettting a bit more complicated. Best regards
Zhifang Wang
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Posted: 1 decade ago Dec 25, 2012, 5:32 a.m. EST
i know some about the characteric length of surface:area/perimeter,but use it, i still cannot get the result i needed.

there are some pictures about my question, in the picture copy2 is the heating result as the initial value of cooling process ,two 3D cut points express one upper and another down surface,
for a flat plate model,the plate length and characteric length is same? and my way to express the characteric length is rigth or wrong?
so could you give me some advice??


thank you
i know some about the characteric length of surface:area/perimeter,but use it, i still cannot get the result i needed. there are some pictures about my question, in the picture copy2 is the heating result as the initial value of cooling process ,two 3D cut points express one upper and another down surface, for a flat plate model,the plate length and characteric length is same? and my way to express the characteric length is rigth or wrong? so could you give me some advice?? thank you
Zhifang Wang
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Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Dec 25, 2012, 5:32 a.m. EST
i know some about the characteric length of surface:area/perimeter,but use it, i still cannot get the result i needed.

there are some pictures about my question, in the picture copy2 is the heating result as the initial value of cooling process ,two 3D cut points express one upper and another down surface,
for a flat plate model,the plate length and characteric length is same? and my way to express the characteric length is rigth or wrong?
so could you give me some advice??


thank you
i know some about the characteric length of surface:area/perimeter,but use it, i still cannot get the result i needed. there are some pictures about my question, in the picture copy2 is the heating result as the initial value of cooling process ,two 3D cut points express one upper and another down surface, for a flat plate model,the plate length and characteric length is same? and my way to express the characteric length is rigth or wrong? so could you give me some advice?? thank you

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