Heat Transfer Questions and Answers

Q: 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

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.

Identify the correct statement for conduction?

A.It is a macroscopic form of heat transfer.
B.In fluids, conduction occurs due to the collisions and diffusion of the molecules during their regular motion.
C.In solids, conduction may be attributed to atomic activity in the form of lattice vibrations.
D.The basic equation for thermal conduction is the Maxwell's law.

Exam Year(s)

DSSSB - 2023,

Answer: C

In solids, conduction may be attributed to atomic activity in the form of lattice vibrations, along with the movement of free electrons in metals. This is the statement the key accepts. Conduction is a microscopic mechanism and not a macroscopic form, in fluids it occurs from collisions and diffusion of molecules in their random motion, and the basic equation of conduction is Fourier's law, not Maxwell's law. The second option is also a textbook statement but the key names the third.

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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

A.3.75
B.16.67
C.37.52
D.60.21

Exam Year(s)

GATE AG - 2018,

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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