Experimental investigation and modelling of entrainment rate during bottom reflooding in a narrow rectangular channel

•Bottom reflooding experiments are conducted.•Entrainment rate under different experimental conditions is measured.•The factors affecting the entrainment rate during bottom reflooding are discussed.•An entrainment model for bottom reflooding in a narrow rectangular channel is developed and verified. Entrainment rate during bottom reflooding in a narrow rectangular channel is extremely important for heat transfer and reactor security when large break loss of coolant accident (LBLOCA) happens. In order to investigate the entrainment rate under different conditions, an experimental apparatus was established to simulate the process of bottom reflooding under different initial and boundary conditions, the volume of entrainment liquid was obtained by mass conservation according to the pressure drop of entrained water tank and the experimental results revealed that the entrainment rate increased with increasing inlet velocities, coolant temperature and the heating power of the experimental section, but decreased with the increase of system pressure. Furthermore, a new entrainment rate model was developed based on the experimental results combined with the liquid sheet breakup mechanism which was verified to occur in the post-CHF region. Experimental results were used to correct the model with the dimensionless number which was relative to the vaporization of the entrained liquid in the post dryout region. Comparison of the new model with experimental data in open literature and existing model was performed to validate the accuracy of the new developed model, which suggested that the new model could accurately predict the entrainment rate of bottom reflooding in a rectangular channel with the average relative error of 13.1%.