Experimental investigation of three-dimensional bubbly two-phase pipe flows

Modelling gas-liquid two-phase flow is a topic of constant relevance in nuclear thermal hydraulics. Gas-disperse two-phase flows occur in e.g. fuel elements in the reactor core, in pipes and components during pressure loss, sudden reflooding or other events. Due to the deformable gas-liquid interface and the complexity of heat, mass and momentum transfer across the interface, gas-liquid two-phase flow is very difficult to model and simulate. On the device scale it is common to use Euler/Euler multi-fluid approaches for CFD simulations, which require a good number of empirical correlations as closure models. Such models are commonly derived from experiments. Validation of the correctness of predictive simulations then also requires experiments, which must be simplified to a degree to allow provision of CFD-grade experimental data but complex enough to resemble real flow situations. The latter calls especially for investigations on flow fields in more complex three-dimensional domains, which are prototypical for e.g. bends, valves, T-junctions and rod bundles.
In this contribution the experimental investigation of generic three-dimensional two-phase flows will be presented. Experiments were performed at a vertical test section at the Transient Two-Phase Flow (TOPFLOW) facility at Helmholtz-Zentrum Dresden – Rossendorf (HZDR). The test section is a pipe with an inner diameter of 54 mm and a length of 5000 mm with a flow constriction at half lengths. For the latter a ring shaped diaphragm and a half-moon shaped diaphragm have been investigated. Experiments were performed for a wide range of superficial gas and liquid velocities in the bubbly flow regime. Besides conventional measurement techniques for mass flow rates, temperatures and pressure, the ultrafast X-ray tomography scanner ROFEX for the determination of bubble dynamics, as well as a specifically adapted thermal anemometer probe for determination of liquid velocities is employed. The two-phase flow in such geometry exhibits certain important structures. In the narrow obstacle passage the flow accelerates with accordingly high shear stress being visible in large bubble deformation and break-up. Downstream a dead zone with recirculation develops and bubbles are being captured, which is associated with increased gas hold-up and bubble coalescence. The high resolution measurements allow for the first time to study the two-phase dynamics in detail and disclose velocity distributions along with gas phase and bubble size data as a function of time and space.