Experimental investigation of stratified two-phase flows
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)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.
Fig. 2: The HAWAC inlet device
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.
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.
The hydraulic jump in a closed channel
Fig. 5: Time sequence of slug generation
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.
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.
- C. Vallée, D. Lucas, M. Beyer, H. Pietruske, P. Schütz, H. Carl
Experimental CFD grade data for stratified two-phase flows
Nuclear Engineering and Design, Vol. 240/9 (2010), pp. 2347-2356 (doi:10.1016/j.nucengdes.2009.11.011)
- Y. Bartosiewicz, J.-M. Seynhaeve, C. Vallée, T. Höhne, J.-M. Laviéville
Modeling free surface flows relevant to a PTS scenario: Comparison between experimental data and three RANS based CFD-codes. Comments on the CFD-experiment integration and best practice guideline
Nuclear Engineering and Design, Vol. 240/9 (2010), pp. 2375-2381 (doi:10.1016/j.nucengdes.2010.04.032)
- C. Vallée, T. Höhne, H.-M. Prasser & T. Sühnel.
Experimental investigation and CFD simulation of horizontal stratified two-phase flow phenomena
Nuclear Engineering and Design, Vol. 238/3 (2008), pp. 637-646 (doi:10.1016/j.nucengdes.2008.08.003)
- C. Vallée.
Hydraulic jump in a closed horizontal two-phase flow channel
Proceedings of the ICMF 2007, Leipzig, Germany, July 9–13 2007, Paper No. S5_Fri_A_63
- C. Vallée & T. Höhne.
CFD validation of stratified two-phase flows in a horizontal channel
Annual Report 2006 of the Institute of Safety Research, FZR-465, 2007
- T. Höhne & C. Vallée.
Numerical prediction of horizontal stratified flows
Proceedings of the CFD2008 conference, Trondheim, Norway, June 10-12 2008, Paper No. 08-12
- CFD simulations for stratified flows
- Experiments in the hot leg model of the TOPFLOW facility
- Picture galery of the "Schaufenster der Wissenschaft 2006" (Altmarktgalerie Dresden, 02.10.2006)
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