FVV1455: CFD Simulation of Droplet Separators

For industrial applications, the two-fluid model is preferred due to its <br>
efficient modelling of small-scale interfaces. Whereas, a thin film model, <br>
based on a long wave approximation, is used for the unresolved interfaces to <br>
obtain the film features by solving the 2D Navier-Stokes equations for wall <br>
films. Within the project, the target is to develop an experimentally validated <br>
3D-CFD model to investigate the separation efficiency of droplet separators for <br>
fuel cell systems. A hybrid model is developed, which couples the two-fluid <br>
model with a thin film model via mass transfer terms for droplet deposition, <br>
droplet entrainment and film separation. A two-way coupling between droplets <br>
and the thin film is established using mass and momentum source terms, derived <br>
analytical and from available experiments. The droplet separator is an essential <br>
component of an automotive fuel cell system that segregates a significant amount <br>
of liquid fractions from the air-water mixture. The flow dynamics inside a <br>
droplet separator consist of a dispersed gas and liquid with a wall adhered <br>
thin liquid film. The modelling is divided into the following stages due to the <br>
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 <br>
the characteristics of films and the separation efficiencies of droplet <br>
separators, high-quality experimental data must be carefully acquired. For the <br>
experimental investigations an air-water two-phase flow loop was set up. The <br>
flow loop is extensively instrumented in order to provide precise data on the <br>
respective operating conditions such as mass low and pressure drop. The <br>
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 <br>
conditions at a total of 27 measurement points covering a wide range of mostly <br>
wavy and annular inlet flow conditions. The resulting comprehensive set of <br>
experimental data provides an excellent basis for the development and validation<br>
of numerical design tools required by the industry.