Interfacial friction factors in vertical circular pipes under flooding conditions at the bottom end

In our previous study (Goda et al.), we measured the void fraction α, pressure gradient dP/dz, and countercurrent flow limitation (i.e. relationship between superficial velocities, J_G and J_L) in vertical pipes (diameter D = 20 and 40 mm) under flooding conditions at the square bottom end and working fluids of air and water to obtain the wall friction factor f_w and the interfacial friction factor f_i based on the annular flow model. Measurements of α in vertical pipes under flooding conditions are few. In this study, therefore, we evaluated f_i from the measured dP/dz under flooding at the square bottom end reported by Bharathan et al. with D = 6.4 to 152 mm and air-water and by Ilyukhin et al. with D = 20 mm and working fluids of steam and water at pressures of P = 0.6 to 4.1 MPa. As a result, we found that the f_i values obtained from the measured dP/dz and the f_w correlation by Goda et al. were well correlated in terms of the gas Kutateladze parameter K_G^* and the dimensionless diameter D^*. The effect of fluid properties was expressed by a revision term with the viscosity ratio of gas and liquid phases (μ_G/μ_L). (author)