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Extracting certain parameters from heat transfer models
Posted Dec 7, 2010, 10:51 a.m. EST Heat Transfer & Phase Change Version 4.1 9 Replies
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Dear Experts,
I managed to solve a heat transfer problem in COMSOL. Briefly, we have a microcapillary in which Water is filled and an IR laser is used to heat a part of the capillary (say, 100 microns length). Thus the laser spot heats a particular spot in the capillary (perpendicular to it's length), and the heats the neighbourhood also via diffusion/convection. The model closely matches the experimental results, but not perfect yet.
I have two issues,
1) The temperature slope on either side of the laser spot (where the T is max) is larger in the experimental case, however, smaller in the model. I tried to increase the T slope so as to match the expt range by increasing heat transfer coefficient, h, however, it didn't make any difference, I tried to play with surface emissivity too, however, not much use!. What might have gone wrong!?
2) In the existing model, how can I extract certain thermal parameters such as 'heat diffusion length' or any other similar useful parameters?
Please do advice ASAP and many thanks in advance.
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you might have a laser pulse distribution that is not square and rather a Gaussian with some tail (and energy in the wings).
i) Did you define the 100um heating region as a Gaussian distribution ?
ii) do you correctly couple the heat exchange between the glass (that is probably aslo absorbing some heat) and the fluid ?
for the postprocessing you can always repalce an internal COMSOL value by a combination of terms and calculate out any equations, either as a praph plot or as values (even more easily in v4)
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Good luck
Ivar
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Thanks for your quick response. I didn't define the heat source as Gaussian, rather as a square/cube where the energy is deposited. If it's due to 'wing-energy' as you mentioned, we probably should see the opposite effect., I guess.
I defined the heat transfer between water and glass , which is "h".
From glass to external surroundings, I used surface emissivity, e. Any combination of these parameters are unable to make the model to match the experiment, however, the discripancy is <10C. Noticeble yet!.
Could you please elaborate on "for the postprocessing you can always repalce an internal COMSOL value by a combination of terms and calculate out any equations, either as a praph plot or as values (even more easily in v4)"
Thanks and regards
Sagar
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emissivity (radiative exchange) is of importance if you get to high temperatures difference, else I would suspect you have some convection too, no ?
For the postprocesssing I was referring too the fact that you can not only plot the predefined values from COMSOL, but any equation or variable you define made up rom any combination of COMSOL defined or user defined variables/ operators. Just type in your desired equation.
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Good luck
Ivar
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Thanks again for this. I think I have some convection too, however, not sure how to set/varry the parameters for convection.Is there an option in the model (possibly) to to click on "convection ON" and vary the parameters?, in such case, what could be the most important parameter that would have a significant influence on heat transfer?
Thanks again
Sagar
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I use COMSOL 3.5a and, when I choose the general heat transfer application mode the system allows me to enable and disable convection in the subdomain settings.
But I do not know how it is done in version 4.1, you hed better check if it is possibble.
atilla
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check carefully the new doc, there are quite alot about minor but subtile changes in the HT doc. I have not tested all yet, so I'm not sure howto answer correctly
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Good luck
Ivar
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In 3.5a I was able to set the heat source for microwave heating as Qav_rfw in the heat source field. I do not know how to do this in 4.1.
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I see in 3.5a Qav_rfw = resisitive heating "time average
in 4.1 RF EMW physics you have emw.Qrh resisitive heating
But you have also the Microwave Heating physics MH
mh.Qtot = total heat source
among many other ...Q... heat loss variables
Use the CNTRL + ENTER short-cut when you are in a post-processing entry field to get the list of variables
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Good luck
Ivar
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