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biplab93
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Homework Statement
Two insulating materials of thermal conductivity K and 2K respectively are to be used as two layers of insulation for lagging a pipe carrying hot fluid. If the radial thickness both the layers is to be same, then -
i) the first material (thermal conductivity K) should be the inner layer, and the second material should be the outer layer
ii) the second material should be the inner layer
iii) the order of the materials is irrelevant
iv) numerical values required to give a specific answer
Homework Equations
The heat transfer in this case would be given by the following equation, I suppose:
q=[itex]\frac{2*pi*ΔT}{1/h1r1 +ln(r2/r1)/K1 + ln(r3/r2)/K2 +1/r3h3}[/itex]
q= radial heat transfer per unit time per unit length of pipe
ΔT= temperature drop across the pipe thickness i.e. hot fluid temperature inside the pipe - ambient temperature outside the pipe
h1= heat transfer coefficient of the hot fluid
h3=heat transfer coefficient of the ambient air outside the pipe
r1=inside radius of first layer
r2=outside radius of the first layer=inside radius of the second layer=r1+t, t=radial thickness of one layer of insulation
r3=outside radius of the second layer=r2+t=r1+2t
K1=thermal conductivity of the first layer
K2=thermal conductivity of the second layer
The Attempt at a Solution
In the book where I found the question, (i) is the given answer, but I don't understand how that is. Looking at the equation, I guess ln(r2/r1)/K1+ln(r3/r2)/K2 should be maximum, but I don't know how to reach the conclusion that K1=K and K2=2K is the optimum option.