PREDICTION OF VOID FRACTION AND VELOCITY PROFILES OF BUBBLY FLOWS IN VERTICAL PIPES

Adopting a similar approach to Beyerlein et al. (1985), void fraction distributions in turbulent two-phase bubbly air/water upflows and down flows in vertical pipes were analyzed using a simple transport model w hich was based on the assumptions that the lateral shear-induced lift force acting on bubbles (Thomas et al., 1983) is balanced by bubble di spersion, and that bubbles in the Bow are conserved i.e. no bubble bre akup or coalescence occurs. The model shows the importance of consider ing the lateral lift force experienced by bubbles as they move relativ e to the liquid phase in a non-uniform velocity field. This force caus es the bubbles to accumulate near the wall forming a high concentratio n for upward Bow, while the concentration increases toward the centre of the pipe for downward Bow. The eddy diffusivity, as widely used in calculation of single-phase flow, can be extended to include the effec t of pseudo-turbulence (Lance and Bataille, 1991) due to bubbles, and thus can be linked with the bubble dispersion coefficient. It is also demonstrated that the transverse or radial pressure gradient induced b y the Reynolds stress exerts a lateral force on the bubbles, and thus affects their distribution in the Bow. A comparison of the model predi ctions with experimental data from Serizawa et al. (1975) for upflows and Wang et al. (1987) for both upflows and downflows shows that our m odel predicts void fraction peaking near the wall for upflows and cori ng at the centre-line for downflows. Compared with similar investigati ons (e.g., Drew and Lahey, 1982; Lopez de Bertodano et al., 1990) of t he same problem, our model approach appears to be simpler and more sui table for engineering calculations.