Contact

Matthias Beyer
Experimental Thermal Fluid Dynamics
m.beyerAthzdr.de
Phone: +49 351 260 - 3465, 2865
Fax: 13465, 2818

Dr. Dirk Lucas
Head Computational Fluid Dynamics
d.lucasAthzdr.de
Phone: +49 351 260 - 2047
Fax: +49 351 260 - 12047

Experimental investigation of stratified two-phase flows

Motivation

During the transport of multi-phase mixtures in horizontal or slightly inclined conduits, mainly stratified flow regimes are generated, as in chemical industrial facilities, in oil production installations or in power plants. These multi-phase flows are difficult to control and can strongly influence the operation or efficiency of the system they flow through. A slug flow may for example affect the mechanical integrity of pipelines or valves because of the high pressure surges they induce.

In order to simulate these complex flow conditions, the so-called computational fluid dynamics codes (CFD) are currently under development for two-phase flow applications. For this purpose, new models are implemented in CFD that must be checked against experiments. The aim of our experimental investigations of stratified two-phase flows is mainly to deliver high resolution data that is needed for the validation of CFD codes.

The Horizontal Air/Water Channel (HAWAC)

The horizontal air/water channel HAWAC Fig. 1: Schematic view of the Horizontal Air/Water Channel (HAWAC)

The Horizontal Air/Water Channel (Fig. 1) is devoted to co-current flow experiments. The 8 m long test-section has a rectangular cross-section of 100 x 30 mm² (height x width), leading to a length-to-height L/h = 80. A special inlet device (Fig. 2) was designed to provide defined boundary conditions at the channel inlet. Therefore, air and water are injected separately into the test-section: the air flows through the upper part and the water through the lower part of the inlet device. In order to provide homogenous velocity profiles at the test-section inlet, 4 wire cloth filters are mounted in each part of the inlet device. Air and water come in contact at the edge of a 500 mm long blade that divides both phases downstream of the filter segment. The free inlet cross-section for each phase can be controlled by inclining this blade up and down. In this way, the water level at the test-section inlet can be controled. Both, filters and the inclinable blade, provide well-defined inlet boundary conditions for generic investigations of stratified two-phase flows and therefore offer very good code validation possibilities.

The HAWAC inlet device
Fig. 2: The HAWAC inlet device

Measurement techniques

The channel is designed for the application of optical measurement techniques, which deliver the high resolution required for CDF validation. Therefore, the acrylic glass test-section with rectangular cross-section provides good observation possibilities.

Videometry

High-speed video observation was applied from the side of the channel. An algorithm was developed to recognise the interface in the images. This allows the extraction of quantitative values from the images (e.g. the water level in any cross-section). Further statistical treatments of the data show the interface structure and are used for comparisons between experiments and CFD.

Further possibilities

It is planned to use the PIV (particle image velocimetry) system of the intitute for velocity measurements in the water flow (Fig. 3). Furthermore, pressure transducers should be installed for the investigation of slug flow.

Flow field in a slug measured with PIV
Fig. 3: Example of the secondary flow field in a slug

Results

Flow pattern map of the HAWAC

The maximum superficial velocities achieved in the test-section are 2 m/s for the water and 8 m/s for the air. A flow pattern map (Fig. 4) was established on the basis of visual observations of the flow structure at different combinations of the gas and liquid superficial velocities. The observed flow patterns are: stratified flow, wavy flow, elongated bubble flow and slug flow.
Flow pattern map of the HAWAC
Fig. 4: Flow pattern map of the HAWAC

Slug flow experiments

High-speed video observation was applied during slug flow. The camera images show the generation of slug flow from the inlet of the test-section (Fig. 5). Immediately after the inlet, a stratified flow is observed with slight waves generated by the high air velocity. One of these waves grows rapidly and develops into a slug, which travels along the channel. At the slug front, an important droplet entrainment is visible which is driven by the air flow through the gap on top of the slug.

Time sequence of slug generation
Fig. 5: Time sequence of slug generation

The hydraulic jump in a closed channel

At high water flow rates, especially when the inlet blade is inclined down, a hydraulic jump can be realised in the test-section. The hydraulic jump is the discontinuous transition between super- and subcritical flow and is characterised by a steep rising of the water surface. From high-speed video observations and the interface capture algorithm, the probability distribution of the water levels was calculated in each vertical cross-section and was represented in a picture of the test-section (Fig. 6). This shows both the structure and the dynamics of the interface.

probability distribution in a hydraulic jump
Fig. 6: Representation of the probability distribution of the water level measured in a hydraulic jump

Moreover, experiments were performed to point out the influence of the air flow rate on the hydraulic jump in a closed channel. These experimental data obtained on a stationary phenomenon involving high turbulence by variation of the momentum exchange between the phases makes the hydraulic jump in a closed channel to a sensitive code validation case.

Code benchmark

The Horizontal Air/Water Channel is:

  • OECD-NEA Benchmark test facility,
  • reference test facility for the German CFD-network program.
In the frame of these code benchmark programs, the experimental results obtained at the HAWAC facility are made available for partners.

Selected references

Related links

Acknowledgements

This work is carried out in the frame of a current research project funded by the German Federal Ministry of Economics and Technology, project number 150 1329


Contact

Matthias Beyer
Experimental Thermal Fluid Dynamics
m.beyerAthzdr.de
Phone: +49 351 260 - 3465, 2865
Fax: 13465, 2818

Dr. Dirk Lucas
Head Computational Fluid Dynamics
d.lucasAthzdr.de
Phone: +49 351 260 - 2047
Fax: +49 351 260 - 12047