Fast Optical Tomography for Transient Process Diagnostics

E. Schleicher, U. Hampel, M. Da Silva and S. Thiele

Institute of Safety Research, Forschungszentrum Dresden-Rossendorf e.V., POB 51 01 19, 01314 Dresden, Germany, Email: e.schleicher@fz-rossendorf.de

ABSTRACT

Optical tomography is a potential technique for the visualisation of fast transient phenomena in multiphase flows. It can be applied for liquid-liquid, gas-liquid and gas-solid flows as long as the flow consists of a major transparent component/phase, such as a liquid or gas, and a minor component/phase that alters the intensity or light ray propagation path, such as solid particles or gas bubbles. The technique is also applicable in single phase flows with dye tracer marked fluid volumes. Recently we have built a fast optical tomography system that achieves frame rates up to 5 kfps and can be applied to study bubbly two-phase flows in pipes with an inner diameter of two inches. High frame rate at high spatial resolution in the range of 3 mm is obtained by application of a specially developed fully parallel 64 channel data logger with 1 MSample/s simultaneous sampling frequency. The tomography system works similar to an X-ray computed tomography system. 256 LEDs are used as radiation emitters and 32 avalanche photo diodes as detector elements. These are optically connected to the tomographic cross section by means of plastic optical fibres. For image reconstruction we use conventional CT algorithms. However, these algorithms suffer, to some degree, from the non-linear attenuation of the light at phase boundaries due to total reflection or refraction. Therefore, improved image reconstruction algorithms have been developed, which use a binary reconstruction scheme together with a-priori information on the gas phase. Different two and single phase flow phenomena have been investigated with the new fast optical tomograph and different image reconstruction algorithms have been compared concerning their quantitative accuracy. Reliable high resolution images of the cross section at very high temporal resolution are recoverable, especially for lower void fractions in bubble flow or lower particle numbers in liquids.

Keywords Optical tomography, multiphase flow sensors