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Heat source units when working in 2D
Posted 15 feb 2011, 07:16 GMT-5 Heat Transfer & Phase Change, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 3.5a 5 Replies
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I'm using the conduction heat transfer mode to model a laser heating a block.
The laser hits the block and penetrates to a depth described by beer lamberts law (negative exponential), I am taking the absorptivity of the block to be 5000 per meter which is a standard value for the nylon block I am heating.
I am using the 'heat source' quantity in the subdomain settings to repersent the heat increase due to the laser.
My problem is that I have to use a huge value for Q to get any kind of heat rise in a reasonable time scale(say 0.00001K after 10 milliseconds).
I am wondering is the 'heat source' quantity what I should be using to repersent my laser.
Also why are the units for the heat source is 'watts per meter cubed' when I'm only working in 2D?
Thank you for your help.
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I believe its because COMSOL is always working in 3D, but it usually assumes that for 2D you have a 1[m] deep (Z) system and simplifies all equations accordingly. This gives the impression of a 1/m to much, but it really reflects that the values are to be considered "per meter thickness"
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Good luck
Ivar
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Thanks for the help.
Does this mean that if I wish to heat a 2D block with lengths of only a few micrometers I must scale up my laser power to account for the large depth of the block (relative to the size of the face)?
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in 2D you define a block thickness in the main physics node (it depends on the physics some are by 1[m], I agree this is, for me, not always clearly stated in the doc, apart from the units analysis) then it is assumed that you enter the heat in the full depth (volume) so you must take this into account, yes.
The easiest is to check systematically the units and to get these to be correct
Another way is check by some simple examples in 3D compared to 2D to get the right feeling and understanding
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Good luck
Ivar
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Why not? What unit does the density you entered has? It's always mass per volume. No matter if it's 1D, 2D or 3D. The (volume) heat source is just an other coefficient with "x" per volume. Clearly it has to be 'watt per meter cubed'.
Consider: if you want to model heating by a laser spot you can probably use 2D-axi. Otherwise (2D) you are modelling an unlimited enlarged thin laser line, no matter what you enter in the thickness field. If you don't have a symmetry axis, you can't use axial symmetry.
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