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Solving heat equation for heatpulse in a point on the surface 
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#1
Jan1813, 08:12 AM

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Hi everybody, I'm trying to find a solution for the 3D heat equation for pulsed surface heating of a semiinfinte solid with insulated surface. I know the method of reflection is required, and that a point source in an infinite solid gives the following solution:
[itex]U(x,y,z,t)= \frac{Q}{8\sqrt{(πκt)^3}}*e^{\frac{x^2+y^2+z^2}{4κt}}[/itex] Where κ is thermal conductivity and Q is a measure for the strength of the heat source. However, I have only found a solution for a semiinfinite solid with surface temperature zero and a heat source inside the solid. In my case however, the heat source is on the surface, (let's say in point (0,0,0)), hence surface temperature cannot be zero, yet to make matters (a little less) complicated, let's assume a perfectly insulated surface with no heat transfer.. Thanks in advance for help, or tips for usefull literature 


#2
Jan1813, 10:09 AM

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#3
Jan2113, 02:35 AM

P: 8




#4
Jan2113, 08:26 AM

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Solving heat equation for heatpulse in a point on the surface
Think of it first as an infinite solid. You have a sudden instantaneous spherically symmetric injection of heat at the origin. Then you let the heat diffuse away. Half the previously injected heat goes up, and half the previously injected heat goes down. So there is symmetry of the temperature distribution with respect to the xy plane (z = 0). No heat crosses this boundary (after t = 0). This is exactly what your solution tells you. The upward heat flux at z = 0 is kdU/dz, but dU/dz is zero at all times after t = 0.



#5
Jan2113, 08:57 AM

P: 8

Thanks a lot for the extra information, your explanation looks correct indeed, and I understand that in this case my equation covers the problem since I cannot see a theoretical error in your explanation .
However, I still find it odd that there is no effect of the insulated boundary (although I might of course be mistaken): Isn't the heat that normally goes down in the z<0 area (when the solid is infinite), 'trapped' due to thermal insulation of the surface, resulting in the fact that it is dissipated to the other side, leading to a higher heat input in the z>=0 area? And if this is the case, does this only affect the factor Q (heat input) in the equation? 


#6
Jan2113, 09:07 AM

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If there is no heat flow through a surface, replacing that surface with an insulated boundary doesn't change the heat flow. 


#7
Jan2113, 09:16 AM

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Ok, thanks a lot! I didn't realise I had the correct solution lying around all this time :).



#8
Jan2113, 12:55 PM

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Chet 


#9
Jan2413, 11:37 PM

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#10
Feb2613, 03:20 AM

P: 8

About the insulation: in my question I assumed a perfect insulation with no heat transfer across the boundary, but what if we try to make the situation more realistic and insulation is not perfect.. So instead of a semiinfinite solid, we now actually have 'two semiinfinite solids with their surfaces in thermal contact' (or something like that). Does this change anything for the heat transfer?
Again, intuitively, one would assume it does, but I cannot rely on my intuition :). Furthermore, small differences are quite important here, because I'm also intrested in the heat propagation on the boundary plane. Hope this isn't too much to ask? 


#11
Feb2613, 06:35 AM

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Let me say this back so that I understand correctly. You now have 2 semiinfinite solids with an imperfect insulating material of thickness ?? between then, and you release the heat at a point on the surface of one of the solids, but not at the mirror image point on the other solid. And the thermal properties of the insulating material is different from that of the two semiinfinite solids. Correct?
Chet 


#12
Feb2713, 02:21 AM

P: 8

Ah no, sorry for the confusion, there is 1 semiinfinite solid, and the other semi infinite solid IS the insulating material.. So 2 materials in contact with eachother, with different thermal properties, and with a point heat source on the boundary surface between them. I hope this makes it more clear to you?



#14
Feb2813, 03:20 AM

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#15
Feb2813, 06:40 AM

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If the thermal diffusivities are unequal, there will be heat flow across the interface. I'm not sure whether this problem has an analytic solution. Of course, it can always be solved numerically. 


#16
Feb2813, 08:01 AM

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What confuses me is how come the OP's original equation does not have a diffusion term at all? Shouldn't k, Cp and density always occur in combination (i.e. Diffusivity) in the solutions of the heat equation?



#17
Feb2813, 08:18 AM

P: 8

And to return to the problem: the thermal diffusivities are NOT equal, but I would still like to find an analytical solution (if possible of course). Any suggestions on how/where to find it (maybe in literature, but my search has been fruitless untill now). 


#18
Feb2813, 08:33 AM

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