Validation of a generalized model for bubble coalescence and breakup in MUSIG approach

Flow fields in the safety research of nuclear reactors are usually complex and often involve two-phase (gas-liquid) flows, where one of the phases is continuous and the other phase consists of disperse bubbles. It is well-known that bubble coalescence and breakup can lead to significant variations in the bubble size distribution, which influences the relative motion of bubbles, i.e. the redistribution of gas-liquid interface. To model the dynamic evolution of the disperse gaseous phase, the population balance equation (PBE) has to be solved together with the classical hydrodynamic Euler/Euler simulation. The Inhomogeneous MUSIG (MUltiple-Size-Group) model implemented in the CFX 12.0 commercial CFD code is one kind of efficient method for the solution of the PBE [1, 2]. However, the closure models for bubble coalescence and breakup were diagnosed as one weak point in the application of this approach [3, 4].
In the previous work, a generalized model was proposed for the modeling of bubble coalescence and breakup, which considers all important mechanisms, e.g. turbulent fluctuation, laminar shear, wake entrainment and eddy capture and interfacial slip velocity. The first test in a 1D Test Solver has shown that the new model is capable of tracing the evolution of bubble size distribution and radial gas volume fraction in vertical pipe flow [5, 6].
In the present study, the new model was implemented into CFX 12.0 through user FORTRAN subroutines and serves as new constitutive relations of the MUSIG approach. Two-dimensional axisymmetric multi-fluid simulations were performed for air-water flows in a large vertical pipe (DN 200). Simulation results for the evolution of bubble size distribution, radial gas volume fraction, Sauter mean bubble diameter as well as gas velocity were compared to the TOPFLOW experiment. The extents of the calculation grid are, x: 0~0.1m, z: 0.221~1.021m. The flow is in the z-direction.