In the same figures the analytical solution holding for the thermal entry problem in straight smooth wall tube [29] and the empirical correlation by Janssen and Hoogendoorn [26] holding for helical coiled smooth wall tubes under uniform wall heat flux boundary condition, are reported too. As expected, as a consequence of the fluid mixing due to the centrifugally induced secondary flow, the local Nusselt number, circumferentially averaged,reaches values higher than the ones obtained in a straight pipe for both the smooth and corrugated wall helically coiled tubes.