E-PolyLearning

26. Logarithmic mean area of the cylindrical tube is given as
a. 2πrm
b. πrml
c. 2πrml
d. 2rml
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Answer: (c).2πrml

27. A hot fluid is being conveyed through a long pipe of 4 cm outer diameter and covered with 2 cm thick insulation. It is proposed to reduce the conduction heat loss to the surroundings to one-third of the present rate by further covering with same insulation. Calculate the additional thickness of insulation
a. 11 cm
b. 12 cm
c. 13 cm
d. 14 cm
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Answer: (b).12 cm

28. The heat flow equation through a cylinder of inner radius r1 and outer radius r2 is desired to be written in the same form as that for heat flow through a plane wall. For wall thickness (r₂-r₁) the area will be
a. A1 + A2/2
b. A1 + A2
c. A2 – A1/ log e (A2/A1)
d. A1 + A2/2 log e (A2/A1)
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Answer: (a).A1 + A2/2

29. A cylinder of radius r and made of material of thermal conductivity k 1 is surrounded by a cylindrical shell of inner radius r and outer radius 2r. This outer shell is made of a material of thermal conductivity k 2. Net conductivity would be
a. k 1 + 3 k 2/4
b. k 1 + k 2/4
c. k 1 + 3k 2
d. k 1 + k 2
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Answer: (a).k 1 + 3 k 2/4

30. For steady state and constant value of thermal conductivity, the temperature distribution associated with radial convection through a cylinder is
a. Linear
b. Parabolic
c. Logarithmic
d. Exponential
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Answer: (c).Logarithmic

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