Flow-induced vibration in two-phase flows requires the knowledge of flow regime and detailed flow characteristics. This paper outlines the development of fiber-optic probes to measure void fraction, local flow velocity and characteristic size (i.e., bubble diameter) of the two-phase mixture. The principle of operation of such probes is based on the difference in index of refraction between the liquid phase and the gas phase when in contact with a fiber-optic probe supplied with a laser light. The reflected signal levels for the gas phase and the liquid phase are very different thus providing a reliable measure of void fraction.
The paper describes the development of fiber-optic probes for measurements of internal two-phase flow in pipes and of external flow across a tube bundle. The use of double probes allows the measurements of local flow velocity and bubble size. Some detail measurements of flow in the gap between tubes in cross flow are presented. The fabrication of the very small and fragile probes required much development effort. The paper describes the difficulties and the solutions to assure good quality probes. Some data processing and data interpretation issues are also discussed.