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Innovative sensor technology for industrial applications

08.10.2007
In many industrial application areas so-called multiphase flows determine efficiency and safety issues of processes and plants. Everyone might know such flows from carbonated soft drinks, where after opening a bottle carbon dioxide bubbles rise in the liquid.

In the industry multiphase flows occur, for instance, in chemical reactors, power plants and turbo machinery. At the Forschungszentrum Dresden-Rossendorf a new sensor was recently developed which enables for the first time measurement and visualization of complex flows of arbitrary substance mixtures with high spatial and temporal resolution.

The analysis and modeling of industrial processes with methods of the experimental and theoretical thermal fluid dynamics is one of the tasks of the Institute of Safety Research at the Forschungszentrum Dresden-Rossendorf (FZD). Within the context of this work the new capacitance wire-mesh sensor was developed. This new sensor is a successor of the well known conductivity wire-mesh sensor, which was also developed at FZD and is currently being employed around the world. The conductivity wire mesh-sensor is based on the measurement of electrical conductivity in a flow cross-section and therefore only suited for the investigation of flow mixtures with an electrically conductive component, for example water-steam flows. The new capacitance wire-mesh sensor is now able to measure arbitrary substances, such as oil or other organic fluids. In this way, this sensor is applicable in a number of new fields, for example in process engineering and oil industry.

As the conductivity wire-mesh sensor the capacitance wire-mesh sensor consists of a set of wire electrodes stretched across a vessel or pipe in two slightly separated planes. Within one plane electrodes run in parallel whereas electrodes of different planes are perpendicular to each other. Thus, a grid of electrodes is formed in the cross-section (see figure). An associated electronics measures the electrical capacitance in all crossing points, which in turn is a measure of the dielectric constant of the substance surrounding each crossing point (the dielectric constant represents the extent to which a material concentrates the electrical field). The electronics is optimized to measure the tiny electrical capacitances of the crossing points which are in the range of only few Femtofarad (10-15 F).This can be done at very high time resolution of up to 10000 frames per second. Since different substances have different dielectric constant values, the sensor can discriminate phases or components. As an example the figure below shows a slug flow of air with a dielectric constant of 1 and silicone oil with a value of 2.7 in a pipe. Although the dielectric constants values of the two substances are rather close to each other, they are precisely distinguished by the capacitance wire-mesh sensor.

The capacitance wire-mesh sensor from the Forschungszentrum Dresden-Rossendorf may be employed in industrial applications where complex flow conditions are to be investigated. It does not require optical access to the flow as other measurement techniques do. The results of this work were recently published in the journal “Measurement Science and Technology”.

Christine Bohnet | alfa
Further information:
http://www.fzd.de
http://www.fz-rossendorf.de

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