Experimental study of heat transfer at the transition regime between the natural convection and nucleate boiling: Influence of the heated wall tilt angle on the onset of nucleate boiling (ONB) and natural convection (ONC)

Abstract

The works carried out in this study aimed to get a better understanding of the heat transfer in natural convection and nucleate boiling as well as the transition between these two regimes. An experimental test set up was built to generate the liquid boiling on the wall using a boiling-meter. This sensor was developed in order to investigate the heat transfer and its dependence with time in well controlled conditions. Temperature, pressure and heat flux required for the measurements were implemented. Experiments were focused on determining the characteristic curves of heat transfer in the natural convection regime and the nucleate boiling regime. The influence of the orientation of the wall relative to the gravity on the heat transfer is investigated. Characteristic heat transfer curves were obtained in several operating conditions and wall orientation with regard to gravity.At the transition regime between natural convection and nucleate boiling regimes, it was highlighted a competition between these two regimes. It was found, that the transferred heat flux differs depending on the orientation of the wall and the degree of liquid superheat in the nucleate boiling regime. In the natural convection regime, the measured heat fluxes exhibited a slight variation with the tilt angle of the wall. While in the nucleate boiling regime, strong variations were observed on the onset of nucleate boiling as well as for the heat flux. At very low heat flux, the heat transfer increases with the tilt angle while it decreases for moderate heat flux. The onset of nucleate boiling (ONB) and the onset of dominant natural convection (ONC) were found to decrease as a function of the tilt angle of the wall with different variation laws. The wall superheats values for the ONB and ONC are getting closer when the tilt angle tends to 180°.