Conductivity wire-mesh sensors
Conductivity wire-mesh sensors have been developed for the investigation of fluid flows with and without gas fractions. The measurement principle is based on a local measurement of electrical conductivity of the fluid within the cross-section of a vessel or pipe by means of a mesh of crossing electrodes. The wire-mesh sensor can measure the conductivity distribution at high sampling rates and with a spatial resolution down to 2 mm. There are many application areas of these sensors. One main research field in our department today are basic experimental studies of gas/water and steam/water two-phase flows in safety relevant thermohydraulic components of nuclear power plants. Such studies are conducted for analysis of accident scenarios and validation and development of multi-phase thermohydraulic simulation codes such as CFX. As an example, wire-mesh sensors are a part of the instrumentation of departmental thermohydraulic test facilities, such as TOPFLOW, where wire-mesh sensors are used to obtain flow maps for vertical and horizontal pipeline flows. Through evaluation of velocity and gas fraction profiles generalized models for the behavior of two-phase flows under given geometric and thermodynamic boundary conditions are derived. Further areas of application for wire-mesh sensors are the investigation of substance mixing problems in chemical process plants and models of nuclear reactors, the investigation of cavitation and pressure shock phenomena in fluid pipelines, and water transport processes in soil.
The principle construction of a wire-mesh sensor for the examination of flows in a pipeline segment is shown in the right image. The sensor is essentially a mesh of wire or bar electrodes, one plane of electrodes being the current emitter electrodes and another plane arranged orthogonal to the emitter plane being the current receiver electrodes. Between the emitter and receiver electrodes there is a gap of a few millimeters distance where conductivity is measured in the crossing points of the electrodes. For that purpose the control electronics switches the emitter electrodes consecutively onto a defined electrical potential relative to the environment (ground potential of fluid and pipe metal material) and the current that flows to the receiver electrodes is measured at each electrode in parallel. In order to eliminate DC offsets and to protect the sensor from electrochemical potentials a bipolar emitter pulse scheme is used and then receiver electronics then measures the difference signal at each bipolar pulse. The small receiver current is amplified and converted to a voltage by means of transimpedance amplifier circuits and eventually converted to a digital signal that is transferred to the measurement PC by means of a fast digital signal processing electronics. Special care has been given to low cable capacitances and low input and output impedances of the transmitter and receiver circuits in order to minimize electrode cross-talk at high measurement frequencies. The dynamic range of the conductivity measurement is between 0.1 µS/cm (destilled water) and 1000 µS/cm (tap water). In case of gas-water or oil-water two-phase mixtures the comparatively high conductivity difference of the two phases can be measured directly with the sensor. For single-phase flows it is possible to use conductivity tracers, such as NaCl to label partial fluid volumes.
Wire-mesh sensors can be manufactured depending on application requirements in diversity of different cross-section geometry and operating parameters. Newest wire-mesh sensors can be employed in a environmental conditions range of up to 286 °C and 7 MPa. Associated electronics for signal generation and data acquisition achieves a maximum temporal resolution of 10,000 pictures/second.
For detailed information about wire-mesh sensors types.
The data of a wire-mesh sensor consists of a time sequence of digitally codes conductivity values for each mesh point. The first step of data processing is the determination of absolute conductivity values or alternatively an assignment of the relative conductivity values to the corresponding phase that is present in the flow. As a result we obtain the conductivity or phase distribution within the measurement plane at a contiguous sequence of temporal sampling points - thus a three-dimensional data volume. From data of gas-water two-phase flows it is possible to compute axial and radial gas fraction profiles and the integral gas fraction by proper integration of the gas fraction over certain cross-section areas. For the determination of gas bubble size distributions from the raw data special data analysis algorithms were developed that can identify single bubbles by means of a filling algorithm and compute volume and equivalent bubble diameters accordingly. Further, it is possible to measure the velocity distribution of the gas phase by placement of two wire-mesh sensors with a small axial spacing in the flow. Since the conductivity distribution reaches the second sensor with only minor spatial structure modifications with a time shift that is determined by the flow velocity after having passed the first sensor, we can obtain the local velocity values within the measurement cross-section from a computational cross-correlation analysis of the two sensor signal.
Dudlik, A.; Prasser, H.-M.; Apostolidis, A.; Bergant, A.
Water hammer induced by fast acting valves - experimental studies at Pilot Plant Pipework
Multiphase Science and Technology (2007) submitted
Pietruske, H.; Prasser, H.-M.
Wire-mesh sensors for high-resolving two-phase flow studies at high pressures and temperatures
Flow Measurement and Instrumentation 18(2007)2, 87-94
Manera, A.; Prasser, H.-M.; Lucas, D.; van der Hagen, T. H. J. J.
Three-dimensional flow pattern visualization and bubble size distributions in stationary and transient upward flashing flow
International Journal of Multiphase Flow 32(2006), 996-1016
Rohde, U.; Kliem, S.; Höhne, T.; Karlsson, R.; Hemström, B.; Lillington, J.; Toppila, T.; Elter, J.; Bezrukov, Y.
Fluid mixing and flow distribution in the reactor circuit, measurement data base
Nuclear Engineering and Design, 235(2005), 421-443
Prasser, H.-M.; Scholz, D.; Zippe, C.
Bubble size measurement using wire-mesh sensors
Flow Measurement and Instrumentation 12/4 (2001) 299-312
Prasser, H.-M.; Böttger, A.; Zschau, J.
A New Electrode-Mesh Tomograph for Gas-Liquid Flows
Flow Measurement and Instrumentation 9 (1998) 111-119