October 7, 2024 Dataset Restricted Access

FVV1455: CFD Simulation of Droplet Separators

Singh, Digvijay; Boden, Stephan; Schlegel, Fabian

Producer(s)

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.