Investigation of rotary-symmetric two-phase flows in vertical tubes
Motivation and Background
Gas/liquid flow in vertical pipes is studied to develop and validate geometry-independent closure relations for two-fluid CFD-models, particularly models for bubble forces, bubble coalescence and fragmentation. A small and a large vertical test section (stainless steel pipes), both 9 m long, with nominal diameters of 50 and 200 mm are equipped with wire-mesh sensors delivering sequences of gas fraction distributions over the entire cross-sections of the pipes with a resolution of 3 mm and a measurement rate between 2.5 and 10 kHz. The measurements in these two pipes are carried out in a broad range of superficial liquid and gas velocities (DN50: 0.1 – 4 m/s liquid, 0.01 – 7 m/s gas; DN200: 0.1 – 1.6 m/s liquid, 0.002 – 5 m/s gas) for air/water and steam/water flows.
Fig. 1 shows the parallel test sections with the water and gas injection and the positions of the wire-mesh sensors mounted at a distance between injection and measurement plane of about 7,600 mm. The upper end of the test sections is connected with a steam drum that serves as a separator vessel. From this tank, the test section pump feeds water into the pipes. Gas is injected from the bottom through various modules in order to investigate their influence on the flow patterns. For further information see the TOPFLOW facility web page.
Qualitative analysis of flow patterns
Fig. 2: Virtual side projections of the void fraction distribution (colour from red to yellow, water: blue) in the DN200 (left) and DN50 (right) test sections,
Jw = 1 m/s, Ja - parameter; relation vertical to horizontal scale = 1:1
An increase in the superficial gas velocity intensifies the process of bubble coalescence in both pipes. In the left part of figure 2, large bubbles appear in the flow and grow rapidly with increasing superficial gas velocity. These large bubbles are surrounded by small bubbles which adopt an irregular structure increasingly. The bubble density becomes also very inhomogeneous. At high gas flows, regular structures completely disappear and the flow pattern converts into a churn-turbulent regime.
However, the flow structure observed in the small DN50 pipe is quite different (Fig. 2 right side). In this case, the bubbly flow evolves into a slug flow with the characteristic Taylor bubbles at increasing gas flow rate.
Scale dependency of radial gas fraction, velocity and bubble distribution profiles
In addition to the qualitative analysis of the flow patterns, the measured data were also used to find quantitative differences of the two-phase flows in pipes with different diameters:
- The radially averaged gas fraction profiles for the small and large pipes show significant differences which are more pronounced for smaller superficial gas velocities. The profiles in the large pipe are more uniform than those in the small pipe.
- For the calculation of velocity profiles, the measurements were carried out with two wire-mesh sensors installed one above the other from those the data were cross-correlated. Compared to the gas fraction profiles, the velocity profiles are much less dependent on the pipe diameter, as well as on the relative inlet length within the investigated range. At higher superficial gas velocities, there is a clear tendency of higher gas velocities at the higher pipe diameter.
- The bubble size distributions show the appearance of a bimodal distribution at a superficial gas velocity of about 0.20 m/s. A further increase in the gas flow leads to a shift of the large bubble peak towards higher diameters, it becomes wider and the amplitude increases. The growing turbulence of the fluid by the increase in the gas flow does not lead to a dominance of the bubble break-up. The mean diameter of the large bubble fraction is much less in the small pipe compared to the large one at identical superficial velocities. Furthermore, it was observed that the peak is more narrow, and therefore significantly higher than in the large pipe. These characteristics are due to the Taylor bubbles that occupy almost the entire cross-section of the small pipe.
In addition to this investigation, the volume and shape of isolated bubbles were analysed. The bubble in Fig. 3 were recorded in a two-phase flow with the following superficial velocities, Jg: 1.3 m/s, Jw: 1 m/s. The picture contains the following visualisations (from left):
The virtual central cut through this bubble already confirms its churn-turbulent structure. But an analysis of the central cut of the gas/liquid interface reveals wide areas filled with water inside the bubble. This structure shows clearly no similarity with the regular shape of a Taylor bubble.
Fig. 3: Various kinds of visualisations
of a large bubble in the DN200 test section
- Prasser, H.-M.; Beyer, M.; Böttger, A.; Carl, H.; Lucas, D.; Schaffrath, A.; Schütz, P.; Weiß, F.-P.; Zschau, J.
Influence of the pipe diameter on the structure of the gas/liquid interface in a vertical two-phase pipe flow
Nuclear Technology 152(2005)Oktober, 3-22.
- Prasser, H.-M.; Gregor, S.
Evolution of interfacial area concentration in a vertical air/water flow measured by wire-mesh sensors
NURETH-11, 2005, Avignon, Frankreich, 2.-6. Oktober, 2005, paper: 398.
- Prasser, H.-M.; Beyer, M.; Carl, H.; Pietruske, H.; Schütz, P.
Steam-water experiments at high pressure to study the structure of the gas-liquid interface in a large vertical pipe
Annual Meeting on Nuclear Technology, Nuremberg, paper: 215.
- Prasser, H.-M.
Influence of the Gas Injection on the Void Fraction Profiles and Bubble Size Distributions of an Air/Water Flow in Vertical Pipes
5th International Conference on Multiphase Flow, ICMF’04, Yokohama, Japan, May 30–June 4, 2004, paper: 366.
- Prasser, H.-M.; Beyer, M.; Carl, H.; Schütz, P.
Scaling effects of the two-phase flow in vertical pipes - new results of TOPFLOW
Annual Report 2003 of the Institute of Safety Research, FZR-407.
- Prasser, H.-M.; Beyer, M.; Boettger, A.; Carl, H.; Lucas, D.; Schaffrath, A.; Schütz, P.; Weiß F.-P.; Zschau, J.
TOPFLOW tests on the structure of the gas-liquid interface in a large vertical pipe
Annual Report 2002 of the Institute of Safety Research, FZR-380.
Related linksCFD-calculations for two-phase flows in vertical tubes
This work is carried out in the frame of a completed research project funded by the German Federal Ministry of Economics and Technology, project number 150 1265.