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• Flow boiling characteristics were observed at different inclination. • A new mechanistic CHF model was developed. • The effect of flow channel configurations and inclination were considered. External reactor vessel cooling (ERVC) strategy has been used in China advanced passive nuclear power plant. For this concept, it is very important to remove the decay heat through two phase natural circulation flow between reactor pressure vessel (RPV) wall and insulation. A full height facility named REPEC-II was constructed to study the capability of the two phase natural circulation flow under IVR-ERVC conditions. In this experiment, the effect of flow channel geometry and subcooling on local critical heat flux under atmosphere pressure were studied. The high-speed photography (HSP) technique was applied to the investigation of boiling characteristics. In this paper, a theoretical model based on hydrodynamic model is developed to describe the hydrodynamic behavior of the macrolayer under two-phase boundary layer, which play an important role to initiate critical heat flux. [ABSTRACT FROM AUTHOR]