Bubbly flows with fixed polydispersity: validation of a baseline closure model

For practical applications the Euler-Euler two-fluid model relies on suitable closure relations describing interfacial exchange processes. In dispersed gas-liquid multiphase flow, closures are needed for bubble forces, bubble-induced turbulence, as well as bubble-coalescence and -breakup. The quest for models with a broad range of applicability allowing predictive simulations is an ongoing venture. Reasonable success has been achieved so far for flows that are amenable to a monodisperse approximation for the bubble size which limits the latter to no more than a few mm.
In the present work we extend the validation to flow in which bubbles with a broad distribution of sizes up to ~10mm are present, but the shape of the distribution remains unchanged during the flow development. The existence of such conditions which we term “fixed polydispersity” is deduced from the experimental data. For this kind of situation the complexity of the closure problem is reduced since a balance between bubble-coalescence and -breakup prevails that allows to neglect these processes and simply impose a fixed bubble size distribution. Conclusions towards best practice guidelines for modeling bubbly flows are drawn and needs for further research identified.