Developing upward flow in a uniformly heated circular duct under transitional mixed convection

Upward flow of perfluorohexane (FC-72) in the entry zone of a uniformly heated circular duct was studied in a regime of transitional combined forced and free convection. Heat transfer coefficients were measured along the heated length at different values of flow rate and heat flux. Heat transfer impairment in the developing region due to laminarisation of the turbulent flow was observed and characterised by means of two well-known dimensionless groups: the Grätz number and the Grashof number based on the wall heat flux.