Discussion
Convective film heat transfer coefficient (h) of air (thermal conductivity =0.025 W m-1 K-1) in W m-2 K-1, trapped between two parallel glass panes in a window 1.5 mm apart, is
Answer: B
The film coefficient is about 16.67 W m-2 K-1.
In a thin gap the trapped air hardly moves, so heat crosses it by conduction, and the equivalent film coefficient is the conductivity divided by the gap width:
\[ h = \frac{k}{\delta} = \frac{0.025}{0.0015} = 16.67 \]
3.75, 37.52 and 60.21 do not follow from \(k\) and the 1.5 mm gap; a common slip is to leave the gap in millimetres, which gives 0.0167.
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