Measurement of the Local Properties of Multiphase Flows

Abstract

Flows of mixed fluids in pipes are frequently encountered in several areas of engineering, such as chemical, petroleum and nuclear. Two key parameters characterising such flows are the local volume fraction distribution and the axial velocity distribution of the dispersed phase. In order to achieve a further understanding of the flow properties, vector velocities are important too. A common intrusive method that is used for acquiring these parameters is the local conductivity probe. The reason is that conductivity probes are more accurate than other measuring techniques, such as ERT (Electrical Resistance Tomography) systems, and are therefore used for the calibration and validation of ERT systems. Also the measurements from conductivity probes show a more representative distribution of volume fraction and velocity of the dispersed phase than other non intrusive methods. They are also useful for validating data produced by CFD (Computed Fluid Dynamics) simulations. In this thesis, research has been done on designing probes, and improving the related signal processing algorithms, and several experiments have been run in multiphase loops for measuring the local volume fraction and velocity of the dispersed phase in vertical and inclined pipes and in swirling flows. All these attempts have recognised an extra problem that is not negligible when using local conductance probes. This problem is the interaction between the probe and the bubble. It is known that local probes alter the true value of the bubble’s vector velocity due to the fact that bubbles are slowed down by the probe. A number of experiments were performed and a comparison between ERT and local conductivity probes was made. Both techniques gave velocity distributions of the dispersed phase which do not agree, showing that ERT is unable to accurately measure the gas velocity and volume fraction profiles. Furthermore the current thesis presents results from dual sensor and four sensor local conductivity probes in steady vertical and inclined air-water and oil-water flows and in steady swirling flows, and a proposed new design for fabricating a rotary index dual sensor probe with a new algorithm for the signal processing scheme is given. This new type of conductivity probe has a relatively small frontal area that reduces the bubble-probe interaction hence the probe’s effect on the dispersed phase is less that of other types of probe