Experiments on two-phase flow in a vertical tube with a moveable obstacle

• Main Authors: Prasser, H.-M., Beyer, M., Carl, H., Al Issa, S., Schütz, P., Pietruske, H.
• Other Authors: Forschungszentrum Dresden-Rossendorf, Institut für Sicherheitsforschung
• Format: Others
• Language: English
• Published: Forschungszentrum Dresden 2010
• Subjects: Two-Phase Flow; Gas bubbles; Gas volume fraction distribution; velocity fields
• Online Access: http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-28086; http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-28086; http://www.qucosa.de/fileadmin/data/qucosa/documents/2808/10476.pdf

A novel technique to study the two-phase flow field around an asymmetric diaphragm in a vertical pipe is presented, that enables producing data for CFD code validation in complex geometries. Main feature is a translocation of the diaphragm to scan the 3D void field with a stationary wire-mesh sensor. Besides the measurement of time-averaged void fraction fields, a novel data evaluation method was developed to extract estimated liquid velocity profiles from the wire-mesh sensor data. The flow around an obstacle of the chosen geometry has many topological similarities with complex flow situations in bends, T-junctions, valves, safety valves and other components of power plant equipment and flow phenomena like curved stream lines, which form significant angles with the gravity vector, flow separation at sharp edges and recirculation zones in their wake are present. In order to assess the quality of the CFD code and their underlying multiphase flow and turbulence models pre-test calculations by ANSYS CFX 10.0 were carried out. A comparison between the calculation results and the experimental data shows a good agreement in term of all significant qualitative details of the void fraction and liquid velocity distributions. Furthermore, the report contains a method to assess the lateral components of bubble velocities in the form of a basic theoretical description and visualisation examples. The plots show the deviation of the flow around the obstacle in term of vectors represented the average velocities of the instantaneous cross-sections of all bubbles in the time interval when they pass the measuring plane. A detailed uncertainty analyse of the velocity assessments concludes the presented report. It includes remarks about the comparison with a second method for calculating bubble velocity profiles - the cross-correlation. In addition, this chapter gives an overview about the influence of acceleration and deceleration effects on the velocity estimation.