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how to integrate a heat source, problem with units

Posted 16 dic 2010, 03:42 GMT-5 Heat Transfer & Phase Change Version 4.0a 3 Replies

Jaques Kahl
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hey,

my name is jaques.
i try to solve a problem including little heating modules. these modules have a heat output of 25W each and their dimensions are 2.7 x 1.5 x 1.5 cm.
my questions are:
is this a general or a linear source in that case and how do i convert these 25W into W/m³ or W/m³*K.

thanks for your help, even if this perhaps seems to be a total beginners question.

yours truly
jaques
3 Replies Last Post 17 dic 2010, 10:45 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 16 dic 2010, 05:06 GMT-5
Hi

if you have a heat source of 25[W] for a given volume, the average heat density you should apply is for a 3D domain

Q[W/m^3] = 25[W]/(your_ domain_volume[m^3])

the your_domain_volume is obtained by a integration variable (or volume operator in V4) of "1" as integrand.

What you must catch with COMSOL is that the values you enter in the GUI fields are defined "per FEM element" of size dV=dx*dy*dz (in Cartesian) so you enter body forces in [N/m^3] and heat in [W/m^3] etc with the assumption that you integrate over the applicable domains.

for 2D nothing changes, but COMSOL assumes its working in 3D with a depth of ".d" or ".thickness" (by default = 1[m] for most physics) so then you must multiply you integrand (on a surface by 1*solid.d for a ST case.

In 2D-axi it is the "loop length" that matters which is "2*pi*r"

--
Good luck
Ivar
Hi if you have a heat source of 25[W] for a given volume, the average heat density you should apply is for a 3D domain Q[W/m^3] = 25[W]/(your_ domain_volume[m^3]) the your_domain_volume is obtained by a integration variable (or volume operator in V4) of "1" as integrand. What you must catch with COMSOL is that the values you enter in the GUI fields are defined "per FEM element" of size dV=dx*dy*dz (in Cartesian) so you enter body forces in [N/m^3] and heat in [W/m^3] etc with the assumption that you integrate over the applicable domains. for 2D nothing changes, but COMSOL assumes its working in 3D with a depth of ".d" or ".thickness" (by default = 1[m] for most physics) so then you must multiply you integrand (on a surface by 1*solid.d for a ST case. In 2D-axi it is the "loop length" that matters which is "2*pi*r" -- Good luck Ivar
Jaques Kahl
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Posted: 1 decade ago 17 dic 2010, 07:42 GMT-5
hey ivar,

thanks for you quick reply. theoretically i understand what you told me, but i have a problem in v4 to find the volume operator you mentioned, because i'm new at v4

thanks for your help and have a nice weekend!

jaques
hey ivar, thanks for you quick reply. theoretically i understand what you told me, but i have a problem in v4 to find the volume operator you mentioned, because i'm new at v4 thanks for your help and have a nice weekend! jaques
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 17 dic 2010, 10:45 GMT-5
Hi

the coupling variables are in v4 defined as "Definitions Model Couplings Iintegration" on a domain

--
Good luck
Ivar
Hi the coupling variables are in v4 defined as "Definitions Model Couplings Iintegration" on a domain -- Good luck Ivar

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