Heat Transfer  GATE practice papers  Mechanical
Here, you can read Heat Transfer multiple choice questions and answers with explanation.
1) Two layers of different metals are made composite connecting parallel. The heat flow takes place as shown in figure. Thermal resistances of the two layers are R1 = 0.036 K/W and R2 = 0.05 K/W What is the thermal resistance of the system? (Marks: 02)
 Published on 19 Oct 15
a. 0.086 K/W
b. 47.778 K/W
c. 0.0209 K/W
d. none of the above
Answer
Explanation

ANSWER: 0.0209 K/W
Explanation: In a composite wall when layers are connected in parallel, the thermal resistance of the system is given by,
(1 / R) = (1 / R2) + (1 / R2)
R = (R1 × R2) / (R1 + R2)
R = (0.036 × 0.05) + (0.036 + 0.05)
R = 0.0209 K/W


2) A copper plat has dimensions 1m × 2m × 0.08m. The one of the largest surfaces is kept at 100 °C and other one is at 0 °C. Thermal conductivity of the copper plate k is 400 W/mK. What is the rate at which energy flows through the plate and the thermal resistance of the plate? (Marks: 02)  Published on 19 Oct 15
a. 800 kW, 1 × 10^{– 3} K/W
b. 800 kW, 1 × 10^{– 4} K/W
c. 80000 W, 1 × 10^{– 3} K/W
d. 80000 W, 1 × 10^{– 4} K/W
Answer
Explanation

ANSWER: 800 kW, 1 × 10^{– 4} K/W
Explanation: According to the Fourier's law of heat conduction, Rate of heat flow is given by,
qk = ( k A ΔT ) / l
= (400 × 1 × 2 × (80 – 0)) / 0.08 =800 kW
And thermal resistance is given by,
R_{t} = l / kA
= 0.08 / (400 × 1 × 2) =1 × 10^{– 4} K/W

