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Originally Posted by Cathar
Hmmm, surely spreading resistance can't be 0. If it were zero, then the source of the heat flux would have to be the same size as the (flat) convectional area.
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Well, my thinking is as follows. Take a very small heat source at the end of a very thin wire length L, a 1D situation with L/k*A . Zero Spreading resistance.
Then add a 10mm thick plate. Imagine the plate is comprised of a large number of thin wires radiating from the heat source, each getting longer as their angle from the original thin wire increases in order to reach the surface of the plate. (Each one is also increasing in CS area, which offsets the increasing L somewhat) and the sum 1/Rspr =1/R1 + 1/R2 + 1/R3... + 1/Rlarge number is lower than the 1D resistance.
This impact from the increasing L is what I think of as the spreading resistance. Wrong?
In a spherical situation the L remains constant, 1/Rspr = 1/R1 x large number = very small number, in a hemisphere, 1/Rtotal = 1/R1 x large number/2. A very small number x 2.
Am I being stupid?
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R=k*A/L Edit: Oops that should be R=L/kA)