Measurement of the velocity of the liquid phase in a bubble column by XPTV

Axel Seeger has won the “Award for Best Oral Presentation” at the PIV'01 Symposium in Göttingen, Germany, for the presentation “Assessment of flow structures in Bubble Columns by X-ray Based Particle Tracking Velocimetry”. Below you can find more information about the project and here you can find the paper.

For the investigation of flow structures in bubble columns and airlift reactors it is necessary to measure the local liquid velocity. Common optical methods such as Particle Image Velocimetry (PIV), Particle Tracking Velocimetry (PTV), and Laser Doppler Velocimetry (LDV) fail in bubble flows with large void fractions. The reason is the different refraction index of liquid phase and gas phase, which causes reflection and refraction. The new particle-tracking-based method described here (X-ray Particle Tracking Velocimetry, XPTV) solves the problem of light reflection and dispersion on phase boundaries by the use of X-rays instead of light. X-rays penetrate a gas/liquid flow in straight lines. XPTV allows measuring the three-dimensional velocity of the liquid phase. The particles used for this purpose are radio-opaque and buoyant. Every 40 ms a pair of X-ray images from different but fixed angles is taken. The three-dimensional position of the particles is calculated from each pair for each time step. From the motion of the particles the three-dimensional velocity of the liquid phase can be assessed. The method was applied to a test bubble column. This method is applicable also to opaque liquids, where optical methods fail.

The work is summarized in the dissertation of Axel Seeger: 'Entwicklung einer neuartigen Geschwindigkeitsmessmethode auf der Basis von Röntgenstrahlen für Blasensäulen mit hohem Gasgehalt'. Here is the quick link to the dissertation at shaker.de.

Figure 1: Measurement depth of optical PIV/PTV and the new XPTV in different media

Figure 2: Experimental setup

Two sources generate X-rays, which are directed through the bubble column onto the image intensifiers. A point P is mapped on the two image intensifiers I1 and I2 generating the points P1 and P2. The point P can be reconstructed from P1 and P2. By observing the motion of the particle, its velocity can be obtained by its displacement and the time difference. G1 and G2 are the straight lines between the points P1 and P2 on the image intensifiers and the X-ray sources S1 and S2.

Figure 3: Bubble column

Figure 4: Example of particle trajectories

Figure 5: Example of the flow

The trajectories are color coded. The colorbar at the right-hand bottom indicates, which color represents which velocity. The bounding box shows the investigated area. The superficial gas velocity was set to 1 mm/s and the used liquid was glycerine. The resulting void fraction was about 5 %.

The bubbles are moving through the holes at the bottom. The vectors identify the direction and the magnitude of the velocity of the liquid phase (color coded). The colorbar at the right-hand bottom indicates, which color represents which velocity. The bounding box shows the investigated area. The superficial gas velocity was set to 1 mm/s and the used liquid was glycerine. The resulting void fraction was about 5 %.

The visualization was performed with AMIRA .