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

Investigation of 3D effects in vertical two-phase flows

Vertical test section with moveable obstacle

Motivation and Background

In 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.

Moveable obstacle and driving mechanism
Fig. 2: Sketch of the movable obstacle
with driving mechanism
Fig. 1: Test section with obstacle and wire-mesh sensor

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.


Results

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.

Air/water flow JL = 1.6 m/s, JG = 0.09 m/s
Fig. 3: Air/water flow, JL = 1.6 m/s, JG = 0.09 m/s
Steam/water flow JL = 1.6 m/s, JG = 0.14 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).

References


Related links

CFD-calculations for two-phase flows in a vertical tube around a movable obstacle


Acknowledgement

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.


Contact

M. Beyer
Dr. D. Lucas


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