Heat transfer rate through the wall of a circular tube with convection acting on the outer...

Heat transfer rate through the wall of a circular tube with convection acting on the outer surface is given per unit of its length by

where i refers to the innertube surface and o the outer tube surface. Increasing r0 will reduce the heat transfer as long as

(a) r0

(b) r0 = k/h

(c) r0> k/h

(d )r0> 2k/h

(e) Increasing r0 will always reduce the heat transfer.