Fibre Optical Sensors for Measuring Local Quantities in Gas Liquid Two-Phase Flows

For measuring the local void fraction, bubble frequency, bubble diameter and bubble velocity in two-phase flows, many types of fibre optical sensors are known. They use the difference of the refractive indices of the liquid and the gas phase as measuring principle. The power of the light emitting device has been reduced from 500 W to a few mW. The sensitive diameter of the fibre optical probe has been reduced from a few millimetres to a few micrometres. Single probes, double probes and arrangements of several probes up to bundles with 500 fibres have been developed. In the past the pure signals were displayed on a pen recorder or an oscilloscope. Nowadays high sophisticated signal processing units using transputers or FFT- analyzers give additional information about the two-phase flow pattern. Looking on their historical development an overview of the various types of fibre optical sensors will be given.