The Instrumentation Development

TOPFLOW will be equipped with advanced two-phase instrumentation mainly and adapted and developed in Rossendorf, such as wire-mesh sensors, needle-shaped conductivity probes with integrated thermocouple, gamma and X-ray tomography and passive ultrasonic droplet probes. Additionally, laser-doppler anemometry and a phase-doppler particle analyser are available. Two of these devices, the needle-shaped conductivity probes with integrated thermocouple and the wire-mesh sensors will be described in detail.

Advanced needle probes are equipped with a micro thermocouple substituting the traditional electrode. These probes combine a local phase indication with a fast temperature measurement, so that the temperature can be correctly related to the instantaneous phase state. This allows to distinguish between steam and non-condensable gas in the gas phase and can be used to measure the sub-cooling in the liquid phase and temperature gradients at the inter­phase boundary [5].

FZR has developed wire-mesh sensors for gas-liquid flows, which allow a fast visualisation of transient gas fraction distributions in a tube [6]. The function is based on the measurement of the local instantaneous conductivity of the two-phase mixture. The sensor consists of two electrode grids with 16 electrodes each, placed at a small axial distance behind each other. The conductivity is measured at the crossing points of the wires of the two grids. One plane of electrodes is used as transmitter, the other as receiver plane. During the measuring cycle, the transmitter electrodes are activated by supplying with voltage pulses in a successive order. The currents arriving at the receivers are recorded. This procedure is repeated for all transmitter electrodes.

Sensors of this kind were applied at FZR to an air-water flow test loop in a vertical pipeline (inner diameter D = 51.2mm) as well as to a cavitating flow behind a fast acting valve. The high resolution of the sensor allowed to obtain bubble size distributions and to study the evolution of the flow structure along the pipe [7]. The maximum time resolution available to perform these experiments was 1200 measurements per second. Recently, the measuring rate was increased to 10 000 frames per second with sensors of 16 x 16 measuring points. In the result it is possible to visualise and quantify individual bubbles or droplets at a much higher flow velocity, than before.

It is planned to apply this type of fast flow visualisation to different test sections of the TOPFLOW facility. In the first experiments, the flow pattern in a vertical pipeline of 200 mm diameter will be studied. A special developed sensor will allow to achieve a spatial resolution of 3 mm at a measuring rate of 2 500 frames per second. For this purpose, the sensor must consist of 64 transmitter and 64 receiver wires (64 x 64 measuring matrix).