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 onethird 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 (r₂r₁) 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
