For industrial applications, the two-fluid model is preferred due to its  efficient modelling of small-scale interfaces. Whereas, a thin film model,  based on a long wave approximation, is used for the unresolved interfaces to  obtain the film features by solving the 2D Navier-Stokes equations for wall  films. Within the project, the target is to develop an experimentally validated  3D-CFD model to investigate the separation efficiency of droplet separators for  fuel cell systems. A hybrid model is developed, which couples the two-fluid  model with a thin film model via mass transfer terms for droplet deposition,  droplet entrainment and film separation. A two-way coupling between droplets  and the thin film is established using mass and momentum source terms, derived  analytical and from available experiments. The droplet separator is an essential  component of an automotive fuel cell system that segregates a significant amount  of liquid fractions from the air-water mixture. The flow dynamics inside a  droplet separator consist of a dispersed gas and liquid with a wall adhered  thin liquid film. The modelling is divided into the following stages due to the  complex fluidic phenomenon inside a generic droplet separator:

Droplet deposition model,
Film separation model,
Film transition model, and
Population balance model.

In order to systematically validate numerical models and methods that predict  the characteristics of films and the separation efficiencies of droplet  separators, high-quality experimental data must be carefully acquired. For the  experimental investigations an air-water two-phase flow loop was set up. The  flow loop is extensively instrumented in order to provide precise data on the  respective operating conditions such as mass low and pressure drop. The  following advanced measurement techniques have been applied:

HZDR's flow microscope to investigate droplet flow,
HZDR's advanced microfocus X-ray tomograph to visualize the liquid films, and
radioscopic imaging to investigate dynamic flow processes.

The generic droplet separator was extensively tested under varying operating  conditions at a total of 27 measurement points covering a wide range of mostly  wavy and annular inlet flow conditions. The resulting comprehensive set of  experimental data provides an excellent basis for the development and validation of numerical design tools required by the industry.

The research project was self-financed (FVV funding no. 1455) by the FVV e.V.