Flow characteristics in a rod bundle under gas-liquid counter-current flows

Flow characteristics in rod bundles under counter-current gas liquid flows have not been well evaluated except for counter-current flow limitation (CCFL). In our previous studies, we had measured CCFL (falling liquid velocity J_L for updraft gas velocity J_G), pressure gradient dP/dz and void fraction α, and we obtained the wall and interfacial friction factors (f_w and f_i). The measured CCFL characteristics differed with the existing data arranged according to the gas and liquid velocities at the upper grid plate, but agreed with them when arranged according to the velocities in the rod bundle, and a CCFL correlation was obtained by using the velocities in the rod bundles. In this study, we evaluated correlations to compute α and dP/dz in an accident analysis for a nuclear reactor. As a result, we found the f_w value became negative due to multidimensional flows in the region of large J_G, hence correlations were obtained for f_i and the liquid film thickness δ (whose uncertainties were relatively small). When the f_w value was needed, we found that it could be computed by using the liquid film model with the f_i and δ correlations. The f_w values obtained from the f_i and δ correlations agreed well with the measured f_w including negative values. (author)