Steam bubble condensation in polydispersed flow - Experiments and CFD simulations

Many activities were done in the last years to improve the modeling of adiabatic bubbly flows in the frame of CFD. In this case models for momentum transfer between the phases are most important. Usually they are expressed as so-called bubble forces. Experimental investigation as well as Direct Numerical Simulations (DNS) showed, that these bubble forces strongly depend on the bubble size. In addition to the well known drag force also virtual mass, lift, turbulent dispersion and wall forces have to be considered. The lift force even changes its sign in dependence of the bubble size (Tomiyama, 1989). In consequence large bubbles are pushed to the opposite direction than small bubbles if a gradient of the liquid velocity perpendicular to the relative bubble velocity exists (Lucas et al. 2001, Prasser et al. 2007). To simulate the separation of small and large bubbles more than one momentum equation is required (Krepper et al. 2005). For this reason recently so-called Inhomogeneous-MUSIG (MUlti SIze Group) model was implemented into the ANSYS-CFX code (Frank et al. 2008, Krepper et al. 2008). It allows the consideration of a number of bubble classes independently for the mass balance (for a proper modeling of bubble coalescence and breakup a large number of bubble groups is required) and for the momentum balance (only very few classes can be considered due to the high computational effort, criteria for the classification can be derived from the dependency of the bubble forces on the bubble size, e.g. the change of the sign of the lift force). In the presently implemented version of the Inhomogeneous MUSIG model only transfers between the bubble classes due to bubble coalescence and breakup can be modeled. In case of flows with phase transfer additional transfers between the single classes and the liquid and transfers between bubble classes caused by growth or shrinking of bubbles have to be considered. The equations for the extension of the MUSIG models are derived in Section 2. of this paper (see also Lucas et al. 2009). They were recently implemented into the CFX code and are presently verified (see Section 4).