The increase in insulation thickness enlarges the outer surface
area which in turn increases the heat flux rate continuously until
the diameter becomes equal to the critical radius. This can be seen
in Fig. 6(b) where the heat flux starts to decrease beyond a given
diameter [21]. This behaviour of heat flux is understood by the fact
that the thermal resistance to convection heat transfer is minimum when the cylinder outer diameter is equal to rcr as shown in Fig. 7.
However, due to the small size of the thermocouple wire, the effect
of wire curvature on the convection heat coefficient h around the
wires should be considered [24]. This case has been discussed in
detail in Case 2 belo