Fast optical tomography for transient process diagnostics

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 1MSample simultaneous sampling frequency. The tomography system works similar to an X-ray computed tomography system. As radiation emitters we use 256 LEDs and as detector elements 32 avalanche photo diodes which are optically connected to the tomographic cross section be means of plastic optical fibres. For image reconstruction we use conventional CT algorithms. These algorithms, however, suffer to some degree from the non-linear attenuation of the light at phase boundaries due to total reflection or refraction. Therefore, we currently develop improved image reconstruction algorithms which use a binary reconstruction scheme together with a-priori information on the gas phase. With the new fast optical tomograph we have investigated different two and single phase flow phenomena and compared different image reconstruction algorithms concerning their quantitative accuracy. Especially for lower void fractions in bubble flow or lower particle numbers in liquids we get reliable high resolution images of the cross section at very high temporal resolution.