Investigation of Gas Bubble Velocities from Experimental Data of Ultrafast two-layer electron beam X-ray Tomography

While for measurements in diluted two-phase flows optical methods are frequently applied there is a clear demand for measurement systems for dense two-phase flows. Measurement systems which have the capability to measure flow condition in high accuracy and high detail is needed for understanding the physical mechanism of flow phenomena and also especially for improvement and validation of new two phase flow simulation code model.
At the Fluid Dynamics Institute at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) a unique ultrafast X-ray tomography named ROFEX (Rossendorf fast electron beam X-ray tomography) was developed. This non-intrusive measurement technique with high spatial and temporal resolution; enables to measure gas liquid phase distribution in detail and high accuracy. A frame rate up to 8000 Hz for simultaneous dual plane measurement and a spatial resolution of 2 mm can be reached. Large range of combination of gas-liquid velocities can be measured without any disturbance.
ROFEX works according to scanned electron beam principle. An electron beam is aimed to a circular metallic target using focusing and deflecting system. Hence rotating X-ray fan that radiates the flow in circular pattern is generated. A detector ring is employed to capture the attenuation value of the X-ray intensity. Further processing with filtered back-projection, reconstructed data result in the form of 3D gray value array. This array is processed with special bubble segmentation algorithm which results in bubble parameters such as bubble volume, detected position and detected time are able to be obtained at the end of this process.
Presently the gas bubble velocities are determined by a cross-correlation from dual measurement planes data which results in radial averaged gas velocity profiles. In this work a new improved method which has capability to derive velocities of single gas bubble inside the flow has been developed. The new method works by pairing the correct bubbles that are detected at both measurement planes. In order to acquire the correct bubbles pair, comparison of bubble parameters for instance volume, detected position and detected time are used. Therefore probability functions are defined for each parameter. If the correct bubbles pair is found, the difference of bubble time shift between both measurement planes can be determined. Therefore, gas bubble velocity is obtained by dividing the measurement plane distance with difference of bubble time detection.
In this paper, detailed explanation of the algorithm working principle is given. The algorithm was tested for wide range of flow characteristic and was validated using phantom measurement data. Radial average velocity obtained by this method was also compared with the result from cross-correlation. Velocity field result for wide range of flow structure was also created for further understanding of gas bubbles movement physical mechanism.
This work is carried out in the frame of a current research project funded by the German Federal Ministry of Economics and Technology, project number 150 1411.