In a tube flow problem with contant wall heat flux or wall temperature, the nusselt number is constant in the fully-developed region for both the temperature and velocity fileds, which means the effect of convective heat transfer is not significant. why is that? what problem will be created when the viscous term is neglected in the momenturm equation?

Awall of 0.5m thickness is to be constructed from a material which has average thermal conductivity of 1.4 w/mk. the wall is to be insulated with a material having an average thermal conductivity of 0.35 w/mk so that heat loss per square meter shall not exceed 1450 w. assume inner wall surface temperature of 1200°c and outer surface temperature of the insulation to be 15°c. calculate the thickness of insulation required.