LAUSR

Abstract This paper has studied the flow regimes and their transitions in upward gas-liquid two-phase flows in vertical rod bundle flow channels. The past experimental observations on the flow regimes have been extensively reviewed and analyzed. Various flow regimes with the observer-dependent names are re-classified into 5 flow regimes, namely, bubbly, finely dispersed bubbly, cap-bubbly, churn and annular flows according to their different flow characteristics in the vertical rod bundle flow channels. In view of the problems in the available flow regime transition criteria and the drift-flux correlations, this paper has proposed the mechanisms for the bubbly to cap-bubbly flow transition, the cap-bubbly to churn flow transition, the transition to annular flow and the transition to the finely dispersed bubbly flow and developed the corresponding flow regime transition criteria to predict the five flow regimes based on these mechanisms. This paper has further improved Han et al. correlations for the drift-flux parameters of the distribution parameter and the drift velocity to predict the void fractions in the 5 flow regimes, together the newly-developed transition criteria, in the vertical rod bundle flow channels. The newly-developed flow regime transition criteria and the improved Han et al. correlation for the drift-flux parameters of the distribution parameter and the drift velocity have been examined with the experimental data. The examination has concluded that the newly-developed flow regime transition criteria can correctly predict 82.9% of the 451 observed flow regime data in the vertical rod bundle flow channels and the drift-flux model using the improved Han et al. correlation and the newly-developed flow regime transition criteria can give satisfactory prediction for the 959 experimental data of the void fraction with the mean relative error of 0.0766.