Investigation of 3D effects in vertical two-phase flows
Motivation and BackgroundIn these experiments, the large test section with a nominal diameter of DN200 was used to study the flow field around an asymmetric obstacle. This is an interesting test case for CFD code validation, since the obstacle creates a pronounced three-dimensional two-phase flow field. Curved stream lines, which form significant angles with the gravity vector, a recirculation zone in the wake and a flow separation at the edge of an obstacle are interesting effects, which are common in industrial components and installations.
Because it is not possible to design a movable wire-mesh sensor, another way was applied: the sensor remains stationary and the obstacle - a half-moon diaphragm - is moved up and down in the DN200 test section (Fig. 1).
Since the installation shown in Fig. 2 can either be flanged from below or from above the wire-mesh sensor, flow fields can be measured both upstream and downstream of the diaphragm. The distance between diaphragm and wire-mesh sensor was varied as follows: ± 520 mm, ± 250 mm, ± 160 mm, ± 80 mm, ± 40 mm, ± 20 mm, ± 15 mm, ± 10 mm.
Fig. 2: Sketch of the movable obstacle
with driving mechanism
Runs were performed with air/water flows under ambient conditions (about 30 °C and 1 bar), and with steam/water mixtures at a saturation pressure of 1, 2, 4 and 6.5 MPa in a range of superficial velocities for gas 0.04 – 0.8 m/s and water 0.1 – 1.6 m/s. The following web pages contain detailed information about the boundary conditions of air/water or steam/water flows.
All measurements were carried out with one high temperature DN200 wire-mesh sensor. The evaluated data were used for studies on void fraction, and velocity, as well as bubble size distributions.
Figs. 3 and 4 show the void fraction and velocity distributions in the mid-plane along the pipe axis, perpendicular to the linear edge of the half-moon shaped diaphragm. Furthermore, 16 two-dimensional distributions corresponding to the measuring planes are given.
Fig. 3: Air/water flow, JL = 1.6 m/s, JG = 0.09 m/s
Fig.4: Steam/water flow at 6.5 MPa,
JL = 1.6 m/s, JG = 0.14 m/s
- Started at an axial position of 80 mm upstream of the diaphragm, the velocity profiles become asymmetric and a pronounced maximum forms on the unobstructed side of the pipe.
- On the front side of the diaphragm, a stagnation point is clearly visible in the velocity plot. This region also shows a local minimum of the void fraction.
- A high velocity jet is formed downstream of the obstacle and is not released beyond the end of the measuring domain.
- The equilibrium profile found 520 mm upstream of the obstacle is not re-established at the distance of 520 mm downstream.
- A strong void maximum is formed upstream of the diaphragm that nearly follows the linear edge of the half-moon plate.
- Furthermore, a gas fraction maximum was measured straight above the obstacle corresponding to an oval structure in the velocity distributions which indicates a recirculation area.
- Only for the air/water run a quite complicated two-dimensional gas fraction distribution was found downstream of the obstacle (20 – 160 mm) which finally evolves to a hole in the void fraction distribution 520 mm above the obstacle (Fig. 3).
- Despite of this hole, the steam/water and air/water flows show generally similar structures of void fraction and velocity distributions.
- The overall void fraction for steam/water flows is significantly lower than for air/water ones due to steam condensation taking place in the test section. For this reason, the steam/water flow presented in Fig. 4 was chosen with a higher steam superficial velocity to make air/water and steam/water flows comparable.
- An analysis of the steam/water runs with various pressures shows that the steam condensation is lower during the experiments performed at a high pressure (6.5 MPa).
- Prasser, H.-M.; Beyer, M.; Al Issa, S.; Carl, H.; Pietruske, H.; Schütz, P.
Gas-liquid flow around an obstacle in a vertical pipe
Nuclear Engineering and Design, (2007)submitted.
- Frank, T.; Prasser, H.-M.; Beyer, M.; Al Issa, S.
Gas-liquid flow around an obstacle in a vertical pipe – CFD simulation & comparison to experimental data
6th International Conference on Multiphase Flow, ICMF 2007, 09.-13.07.2007, Leipzig, Germany, paper: S6_Thu_B_50.
- Ruyer, P.; Seiler, N.; Beyer, M.; Weiss, F.-P.
Bubble size distribution modelling for the numerical simulation of bubbly flows
6th International Conference on Multiphase Flow, ICMF 2007, 09.-13.07.2007, Leipzig, Germany, paper: S6_Thu_A_48.
- Al Issa S.; Beyer M.; Prasser H.-M.; Frank Th.
Reconstruction of the 3D velocity field of the two-phase bubbly flow around a half moon obstacle using wire-mesh sensor data
6th International Conference on Multiphase Flow, ICMF 2007, 09.-13.07.2007, Leipzig, Germany, paper: S6_Thu_D_60.
- Krepper, E.; Beyer, M.; Frank, T.; Lucas, D.; Prasser, H.-M.
Application of a population balance approach for polydispersed bubbly flows
6th International Conference on Multiphase Flow, ICMF 2007, 09.-13.07.2007, Leipzig, Germany, poster: PS6_6.
- Krepper, E.; Lucas, D.; Prasser, H.-M.; Beyer, M.; Frank, T.
CFD simulation of the two phase flow around an obstacle applying an imhhomogeneous multiple bubble size class approach
Annual Report 2006 of the Institute of Safety Research, FZR-465.
- Prasser, H.-M.; Frank, T.; Beyer, M.; Carl, H.; Al-Issa, S.; Pietruske, H.; Schütz, P.
Gas-liquid flow around an obstacle in a vertical pipe - experiment and computational fliuid dynamics simulation
Annual Report 2005 of the Institute of Safety Research, FZR-457.
This work was carried out in the frame of a completed research project funded by the German Federal Ministry of Economics and Technology, project number 150 1265.
Electronic equipment for the wire-mesh sensors was developed in co-operation with TELETRONIC GmbH.