E-PolyLearning

16. Three metal walls of same thickness and cross sectional area have thermal conductivities k, 2k and 3k respectively. The temperature drop across the walls (for same heat transfer) will be in the ratio
a. 3:2:1
b. 1:1:1
c. 1:2:3
d. Given data is insufficient
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Answer: (a).3:2:1

17. A composite wall is made of two layers of thickness δ₁ and δ₂ having thermal conductivities k and 2k and equal surface area normal to the direction of heat flow. The outer surface of composite wall are at 100 degree Celsius and 200 degree Celsius. The minimum surface temperature at the junction is 150 degree Celsius. What will be the ratio of wall thickness?
a. 1:1
b. 2:1
c. 1:2
d. 2:3
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Answer: (c).1:2

18. Let us say thermal conductivity of a wall is governed by the relation k = k0 (1 + α t). In that case the temperature at the mid-plane of the heat conducting wall would be
a. Av. of the temperature at the wall faces
b. More than average of the temperature at the wall faces
c. Less than average of the temperature at the wall faces
d. Depends upon the temperature difference between the wall faces
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Answer: (b).More than average of the temperature at the wall faces

19. Heat is transferred from a hot fluid to a cold one through a plane wall of thickness (δ), surface area (A) and thermal conductivity (k). The thermal resistance is
a. 1/A (1/h₁ + δ/k + 1/h₂)
b. A (1/h₁ + δ/k + 1/h₂)
c. 1/A (h₁ + δ/k + h₂)
d. A (1/h₁ + δ/k + 1/h₂)
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Answer: (a).1/A (1/h₁ + δ/k + 1/h₂)

20. A pipe carrying steam at 215.75 degree Celsius enters a room and some heat is gained by surrounding at 27.95 degree Celsius. The major effect of heat loss to surroundings will be due to
a. Conduction
b. Convection
c. Radiation
d. Both conduction and convection
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Answer: (c).Radiation

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