Wire-mesh sensors for high-resolution gas-liquid multiphase flow visualization

Wire-mesh sensors are flow imaging devices and allow the investigation of multiphase flows with high spatial and temporal resolution. This type of sensor was introduced about ten years ago at Research Center Dresden-Rossendorf, Germany and since then it has been successfully employed to investigate several single phase and two-phase flow phenomena. The sensor can be seen as a hybrid solution in between intrusive local measurement of phase fraction and tomographic cross-sectional imaging. It comprises of two set of wires stretched over the cross-section of a vessel or pipe. The planes of wires are perpendicular to each other with a small axial separation between them, thus forming a grid of electrodes. The associated electronics measures an electrical property of the flowing media at each crossing point in a very fast and multiplexed manner. The wire mesh subdivides the flow channel cross section into a number of independent sub-regions, whereas each crossing point represents one sub-region. Each of the measured signals reflects the constitution of the flow within its associated sub-region, i.e. each crossing point acts as local phase indicator. Hence, the set of data obtained from the sensor directly represents the phase distribution over the cross-section and no reconstruction procedure, e.g. by solving an inverse problem as usual in tomography, is needed in order to determine cross-sectional phase distributions. The first generation of wire-mesh sensors is based on conductivity measurements, thus being able to investigate electrically conducting fluids only. Typically air-water and steam-water flows have been investigated. Recently, the capacitance wire-mesh sensor has been developed and tested, which allow the investigation of non-conducting fluids such as oil or organic liquids. The newest systems are able to produce up to 10,000 images per second. Sensors can be constructed to operate under temperatures up to 286 °C and pressures up to 7 MPa. First this article reviews the measuring principle of wire-mesh sensors. In addition, measurement results of the application of a new-developed capacitance wire-mesh sensor to investigate two-phase gas-oil flow are presented. Furthermore, the use of a wire-mesh sensor for the investigation of a simulated three-phase flow in a laboratory setup is described and discussed. Thus, the wire-mesh sensor can considered as a simpler and inexpensive alternative to investigate either two-phase gas-liquid or three-phase gas-liquid-liquid flows. The good accuracy achieved in the permittivity measurement allows the wire-mesh system to securely distinguish even each of the three phases of a gas-oil-water flow.