Calculate the rate of heat transfer through the composite wall in the temperature difference is 25°C and the contact resistance between the sheets of wood is 0.005 m2 K/W
GIVEN
• Wood wall
? Two layers 0.025-m-thick each, nailed together
• Nail cross sectional area of nails = 0.5% of wall area
• Temperature difference (?T) = 20°C
• Contact resistance (A Ri) = 0.005 (m2 K)/W FIND
• The rate of heat transfer through the wall
ASSUMPTIONS
• One dimensional heat transfer through the wall
• Steady state prevails
SKETCH
PROPERTIES AND CONSTANTS
? Wood (Pine) (kw) = 0.15 W/(m K)
? Mild steel (1% C) (ks) = 43 W/(m K)
The thermal circuit for the wall with contact resistance is shown below.
the thermal resistance of the wood and the nails are
The combined resistance of the wood and the contact resistance in series is
The total resistance equals the combined resistance of the wood and the contact resistance in parallel
with the resistance of the nails
Therefore the rate of heat flow through the wall is:
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