Fast visualization of transient two-phase flows and mixing processes in single phase flows by means of electrode-mesh sensors

In the Institute of Safety Research of the Forschungszentrum Rossendorf, Germany, electrode-mesh sensors were developed, which allow the measurement of the electrical conductivity distribution in a flow duct with a rate of up to 10 000 frames per second. This can be used either for the detection of the gaseous phase in a gas-liquid flow or for mixing studies in single phase flow, when the components have different electric conductivities. Beside a description of the working principle, this lecture focuses on different applications of the method, which will be illustrated by digital image sequences obtained by electrode-mesh sensors.
The sensor consists of two planes of wire grids placed into the flow in a short distance behind each other. The angle between the wires of both grids is 90 deg. The wires of the first plane (transmitter plane) are supplied with pulses of a driving voltage. During the measuring cycle, they are activated by a multiplex circuit in a successive order. After an analogue/digital conversion the receiver signals are recorded by a data acquisition computer connected to AD converters and stored for each receiver electrode separately. This procedure is repeated for all transmitter electrodes. In this way the distribution of the electrical conductivity over the cross section occupied by the sensor is obtained row by row. This data is used for slow-motion visualization of transient flows and for quantitative measurements of gas fractions and their profiles, bubble sizes and velocities.
Special attention was given to achieve a DC free excitation of the electrodes in order to suppress the imaginary part of the measured impedance. Measures were taken to suppress cross-talk between parallel electrodes. The first generation device allowed measurements in a grid of 16 x 16 measuring points distributed over the cross section of the flow duct with a rate of 1200 frames per second.
The high spatial and time resolution of the wire-mesh sensor allows to calculate bubble size distributions from the sequence of frames. Due to the high measuring rate each bubble is mapped in several successive instantaneous frames. After assigning the local instantaneous gas fractions (pixels) to bubbles by a bubble recognition algorithm, the volume of each identified bubble is obtained by adding the local values belonging to the selected bubble. Capabilities and accuracy of this method were investigated using a sensor, which was built into a transparent perspex channel. Combined visualizations using the frames of a high-speed video camera together with the measuring sequences of the wire-mesh sensor have shown that the sensor acts as a bubble fragmenting obstacle. It could nevertheless be proven, that the sensor signal represents the bubble geometry present in the upstream flow, i.e. before the disturbance, with a good accuracy.
In the first application the sensor was used to study the evolution of the flow pattern in an upwards air-water flow. The visualization shows the result of an air injection through wall orifices of 4 mm diameter (pipe diameter 51.2 mm) and the evolution of the flow pattern from L/D = 0.6 to L/D = 60. Large primary bubbles are observed, which are both coalescing and fragmenting in the direction of the flow. Large bubbles tend to travel towards the center of the pipe. At the end of the test section a slug flow is established. Another wire-mesh sensor was used by the University of Munich, Germany, to visualize the effect of a globe valve to the flow regime in a horizontal pipe. The image sequence shows the initial slug flow through the open globe valve and the transition of the flow pattern during closing of the glob valve.
Beside air-water flows the sensor can also be applied to measure the void fraction in steam-water mixtures. The sensor is used to study natural circulation instabilities in a boiling water reactors. For this purpose, two wire-mesh sensors are installed at the CIRCUS test facility of the U...