Two-phase flow around obstacles
Experimental data with high temporal and spatial resolution are required for development and validation of numerical models for computational fluid dynamics. Besides pipe flows, two-phase flows around simple obstacles are suitable benchmark systems for code validation. This project deals with high-resolution visualization of co-current two-phase flows in upward vertical pipe flows with an orifice. Here, ultrafast X-ray tomography is applied as imaging technique. Furthermore, hot film anemometry technique is adapted and enhanced for this particular purpose.
The experimental studies are conducted at a vertical test section at the TOPFLOW facility. The actual test setup comprises an acrylic pipe of 5 m length and 53 mm inner diameter. Air and water run upward through the test section during the experiments. Therefore, air is injected at the bottom of the test section, using a specially designed injection module for generating a homogeneous bubble size distribution. A flow obstruction is installed halfway of the test section that causes a flow constriction. Ultrafast X-ray tomography scanner ROFEX is applied for imaging of the flow structure in several imaging planes up- and downstream of the obstruction. Thus, highly dynamic visualization of the phase distribution is achieved. Bubble velocities can be determined by scanning the flow in two planes simultaneously with a frame rate of 2.5 kHz each. The spatial resolution is approximately 1 mm. The tomography scanner can be moved freely along the vertical test section, which allows the characterization of the flow field at any desired axial position. Furthermore, a hot-film probe is applied for liquid velocity measurement. Here, gas distribution in the near vicinity of the probe tip is analyzed by simultaneously scanned X-ray cross-sectional images. Thus, implausible measurement values due to gas contact of the probe can be excluded for data analysis. Experiments are performed at 4 bar pressure and fluid temperature of 30 °C. Various flow conditions are investigated, ranging from disperse bubbly flow regime to slug flow regime, by setting appropriate gas and liquid superficial velocities. For flow constriction, a ring shaped and a half-moon shaped obstacle are installed, respectively. The present studies supplement earlier steam/water investigations that applied wire-mesh sensor technique for analyzing the flow around a movable obstacle.
The visualization(Video 9MByte) of the measurement data allows for a direct comparison of the flow structure with results of CFD simulations. The following figure shows an exemplary comparison of the central cut along the tube axis perpendicular to the straight edge of the half-moon shaped obstacle. The significantly uneven distributed phases can be clearly identified by the cross-sectional view. The very high temporal resolution of the measurement data allows for detailed characterization of the gas phase, e.g. bubble size distribution (diagram on right hand side).
For CFD simulation an Euler-Euler approach is applied that considers both, the gas and the liquid phase, as continuous phases. For modelling of mass, heat and momentum transfer across the gas-liquid interface, closure models are required for the simulations. For this study, inhomogeneous MUSIG model approach is applied. This model describes the gas phase as a number of bubble size fractions with according velocity groups. Thus, bubble breakup and coalescence effects, which are dominant in this study, can be taken into account. .
This work is funded by the German Federal Ministry of Economic Affairs and Energy (BMWi) with the grant number 1501481 on the basis of a decision by the German Bundestag.
- Neumann, M.; Hampel U.
Two-Phase Flow Studies in Complex Geometries.
48th Annular Meeting on Nuclear Technology, 16.05.-17.05.2017, Berlin, Deutschland