Abstract: Visual observations of the bottom reflooding process in an annular channel were carried out. From the visual study of the flow and heat transfer during reflooding phase, the flow regime and heat transfer mechanism transformations were studied. The comparison of phenomena during the reflooding under various conditions shows that the existence of the internal heat source in the heating rod has little effect on the characteristics of the flow and the heat transfer during the reflooding and that the inlet mass flow flux is a key factor that affects flow regimes and heat transfer characteristics. The greater the mass flux is, the easier the flow regime transition happens.