Experimental investigation and CFD validation of horizontal air/water slug flow

In different scenarios of small break Loss of Coolant Accident (SB-LOCA), stratified two-phase flow regimes can occur in the main cooling lines of pressurized water reactors. Because these flow patterns cannot be predicted with the required accuracy and spatial resolution by the one-dimensional system codes, the stratified flows are increasingly modelled with computational fluid dynamics (CFD) codes. In CFD, closure models are required that must be validated, especially if they are to be applied to nuclear reactor safety. Because the structure of the interface is strongly connected with the momentum transfer between the phases, the water surface characteristics can be used for validation purposes. Experimental data suitable for CFD validation must satisfy special quality criteria, in particular the boundary conditions and the measurement resolution.

For the investigation of co-current two-phase flows at atmospheric pressure and room temperature, the Horizontal Air/Water Channel (HAWAC) was built at Forschungszentrum Dresden-Rossendorf. At the channel inlet, a special device was designed for a separate injection of water and air into the test-section. A blade separating the phases can be moved up and down to control the free inlet cross-section for each phase. This provides adjustable and well-defined inlet boundary conditions and therefore very good CFD validation possibilities.

The HAWAC facility is designed for the application of optical measurement techniques, which deliver the high resolution required for CDF validation. Therefore, the 8 m long acrylic glass test-section with rectangular cross-section provides good observation possibilities. High-speed video observation and particle image velocimetry (PIV) were applied during slug flow. The camera images show the generation of slug flow from the inlet of the test-section. An algorithm was developed to recognise the interface and the extraction of quantitative values, like water level and slug propagation velocity. The PIV measurements reveal the inner flow rotation inside a slug.

Parallel to the experiments, CFD calculations were carried out. The aim of the numerical simulations is to validate the prediction of slug flow with the existing multiphase flow models built in the commercial code ANSYS CFX. The Euler-Euler two-fluid model with the free surface option was applied to a grid of 600,000 control volumes. The turbulence was modelled separately for each phase using the k-ω based shear stress transport (SST) turbulence model. The results compare well in terms of slug formation, velocity, and breaking. The qualitative agreement between calculation and experiment is encouraging, while quantitative comparison show that further model improvement is needed.