SSC JE Mechanical Engineering 2017 Set 1 Paper

Question : 118 of 200

Marks: +1, -0

Critical radius of a hollow cylinder is defined as___.

Outer radius which gives maximum heat flow
Outer radius which gives minimum heat flow
Inner radius which gives minimum heat flow
Inner radius which gives maximum heat flow.

Solution:

Critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h.
The rate of heat transfer from the cylinder increases with the addition of insulation for

r_2 < r_{\cr}

, reaches a maximum when

r_2 = r_{\cr}

, and starts to decrease for

r_2 > r_{cr}

.
Thus, insulating the pipe may actually increase the rate of heat transfer from the pipe instead of decreasing it when

r_2 < r_{cr}

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