Population balance model for the CFD simulation of adiabatic and diabatic two phase gas liquid flows

A generalized inhomogeneous Multiple Size Group (MUSIG) Model based on the Eulerian modeling framework was developed in close cooperation of ANSYS-CFX and Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and implemented into CFX. By simulating a poly-dispersed gas-liquid two-phase flow, the mass exchanged between bubble size classes by bubble coalescence and bubble fragmentation as well as the momentum exchange due to bubble size dependent bubble forces have to be considered. In a vertical pipe flow particularly the radial separation phenomenon of small and large bubbles, which was proven to be a key phenomenon for the establishment of the corresponding flow regime, is well described by this approach. Recently the approach was extended considering bubble shrinking or growing by condensation or evaporation. Size dependent bubble forces can at least be represented roughly assigning the size groups to few different dispersed gaseous phases having different velocity fields.
The derived model has been validated against experimental data from the TOPFLOW test facility at the HZDR. Numerous tests investigating air-water flow at 0.25 MPa and steam-water flow at stem pressures between 1-6.5 MPa and sub-cooling temperatures from 2 to 17 K in vertical pipes having a length up to 8 m and a diameter up to 200 mm were performed. The wire-mesh technology measuring local gas volume fractions, bubble size distributions and velocities of gas and liquid phases at different distances from the gas injection was applied.
For air/water flow the shift of the gas volume fraction profile from a wall peak to core peak could be reproduced. For steam water flow by varying the gas nozzle diameter the initial bubble size was influenced and the effect of the bubble size on the condensation rate could be shown. Due to the drop of hydrostatic pressure along the pipe, the saturation temperature falls towards the upper pipe end and for some tests in the upper part re-evaporation was reproduced.
Weaknesses in this approach can be attributed to the characterization of bubble coalescence and bubble fragmentation, which must be further investigated. A further topic is bubble induced turbulence.