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Question

The outside dimensions of a small cubical furnace constructed of fire clay bricks (K=2 W/mK) are 0.5 m x 0.5 m x 0.5 m, the walls are being 0.1 m thick. The temperatures at the inside and outside surface temperatures are 500oC and 50oC respectively. Also the heat transfer coefficient for top surface is 12W/m2K, bottom surface is 8W/m2K and side surface is 15 W/m2K with surrounding temperature taken as 35oC. The heat transfer from the cube to the surrounding is

A
0.6 KW
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B
9 KW
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C
0.3 KW
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D
12 KW
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Solution

The correct option is C 0.3 KW
The heat transfer from the cube to the surroundings = The total heat covected out from all the sides of the cube to the surroundings
Qtotal=Qconvection from top+Qconvection from bottom+Qconvection from sides

=htopAΔT+hbottomAΔT+hsides(4A)(ΔT)

=(htop+hbottom+4hside)×AΔT

=(12+8+4×15)0.5×0.5(5035)

= 300 W = 0.3 kW

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