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Pulsed Laser heating problem - won't cool to ambient
Posted 7 nov 2021, 16:52 GMT-5 Version 5.4 2 Replies
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Hi there,
I have attempted to create a simulation using the Heat Transfer In Solids package in COMSOL which involves the optical heating of a thin film ontop of a silicon wafer by a single laser pulse. I have modelled the pulse spatially by a Gaussian beam perpendicular to the surface of the 3D domain, and temporally by a piecewise step function, which behave correctly.
Problems arise however when I attempt to solve for time dependent solutions for the temperature in the centre of the irradiated area. No matter what fix I try I cannot get the finite domain to cool back to the ambient temperature as expected - instead I find that the temperature will drop steadily and then plateau at a value higher than the ambient temperature.
I guess this is something to do with the boundaries I have set but it seems to be the standard choice (even in the case of the demonstration Laser Heating of a Silicon Wafer application ID: 13835). At the surface boundary I have set Surface to Ambient, whereas in the case of the boundaries perpendicular to the irradiated surface I have set as open boundaries. My assumption is that what is going on is that the domain is insulated perpendicular to the beam incidence, and equilibriates to a temperature based on the heat deposited by the laser pulse.
Any help would be greatly appreciated! Please let me know if you need further elaboration.
Many thanks, Dan
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In this case:
1) I would multiply the heat transfer coefficient (h) by a large number (i.e. 1000) to see if the slow rate of cooling is the reason.
2) If you have defined the laser heating by a piecewise or interpolation function, make sure it is defined over the whole range of simulation time. Otherwise, COMSOL extrapolates it and the laser radation may not stop.
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Hi Amin,
Thanks for your response. I have tried to implent the increased h to no avail. Also defining the simulation range to be longer than the piecewise input does not result in the domain cooling to ambient even for time where t_sim>>t_pulse. I suspect that this is primarily a problem with the definition of the boundaries. Setting open boundaries provides a cheap but incorrect solution to my problem. In any case thank you very much for your support.
Best regards, Dan