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Total power in 2D heat transfer model

Posted Apr 25, 2018, 9:51 p.m. EDT Heat Transfer & Phase Change, Geometry Version 5.3 1 Reply

Bruce Ding
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Hi,

I'm new to COMSOL, and I'm doing a 2D heat transfer with a constant heat source (constant power density). The power density has a 3 dimensional unit (W/m^3), so even though I defined the geometry (xy) in 2D, I still need the depth (z) to get the actual total power. The model assume it is 1 m but I don't know why my temperature profile doesn't change at all if I change that number. Can anyone teach me the way to obtain the total poewr?

Thanks, Bruce

1 Reply Last Post Jun 11, 2018, 9:20 a.m. EDT
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Hello Bruce Ding

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Morgane
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Posted: 6 years ago Jun 11, 2018, 9:20 a.m. EDT

Hello,

I guess the Total Power is only define in 3D in COMSOL. If you want it on a surface, just multiplie it by the depth you had to enter (1m by default).

My (non expert) explanation for why your temperature does not change depending on the depth is the following : if you bild a 3D model, you can expect the Temperature to depend on all the dimentions and to be time dependent T(t,x,y,z). However, if you look at the heat equation and at the placement of your boundary conditions (T=T0 for exemple), you will see that the heat flux "can not always go in every direction". Therefore it does not depend on all three coordinates (x,y,z).

I have not seen your model but because it is in 2D, you did not impose any temperature difference along the z axis. Therefore ther is no reason for the temperature to vary along the z axis, whatever the depth is.

You can see an illustration of that in this COMSOL video around 10 mins. In the cylinder, the temperature only depends on the z coordinate because the top and the bottom have a different temperature. It does not depend on the radius. You can change the radius, the temperature profile won't change.

Hope it helps !

Hello, I guess the Total Power is only define in 3D in COMSOL. If you want it on a surface, just multiplie it by the depth you had to enter (1m by default). My (non expert) explanation for why your temperature does not change depending on the depth is the following : if you bild a 3D model, you can expect the Temperature to depend on all the dimentions and to be time dependent T(t,x,y,z). However, if you look at the heat equation and at the placement of your boundary conditions (T=T0 for exemple), you will see that the heat flux "can not always go in every direction". Therefore it does not depend on all three coordinates (x,y,z). I have not seen your model but because it is in 2D, you did not impose any temperature difference along the z axis. Therefore ther is no reason for the temperature to vary along the z axis, whatever the depth is. You can see an illustration of that in this [COMSOL video](http://www.comsol.fr/video/intro-to-modeling-heat-transfer-with-comsol-multiphysics) around 10 mins. In the cylinder, the temperature only depends on the z coordinate because the top and the bottom have a different temperature. It does not depend on the radius. You can change the radius, the temperature profile won't change. Hope it helps !

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