Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

2 Publications

Evolution of Interfacial Area Concentration in a Vertical Air-Water Flow Measured by Wire-Mesh Sensors

Prasser, H.-M.; Gregor, S.

Conservation equations for one or more classes of the interfacial area concentration is one of the concepts to take into account the evolution of the flow structure in a gas-liquid two-phase flow in thermal hydraulic or, respectively, CFD modeling. The measurement of the interfacial area concentration is therefore of large importance for the development and validation of those models. The paper presents the results of an application of wire-mesh sensors to obtain the interfacial area concentration in vertical pipes of 52.3 and 194.1 mm inner diameter as an alternative to the widely used multiple-tip optical fiber probes. The measuring data of a mesh sensor is represented by a threedimensional matrix of local instantaneous gas fractions recorded as a time sequence (sampling frequency 2.5 - 10 kHz) at the crossing points of the wires (resolution in x-y direction: 3 mm).
Velocity information necessary to convert the time axis in an Eulerian z-axis was obtained by means of cross-correlation calculated between the signals of two identical mesh sensors located at a small distance behind each other. Three methods are used to deduce the interfacial area concentration from the wire-mesh sensor data:
• Calculation of the sum of the interfacial area represented by bubbles, which are identified as regions of connected gas filled elements in the data array, where the superficial area of an individual bubble is calculated under the assumption of a ellipsoid bubble shape. This method is restricted to low gas fractions, where bubble shapes are still close to the assumed geometry.
• A second method consists in the calculation of the sum of the contribution of the gas fraction in each local control volume of the sensor formed by two crossing wires. This control volume is approximated by a sphere with a volume-equivalent diameter. Furthermore, it is assumed, that the gas-liquid interface is a plane that cuts this sphere in a part filled with gas and another part that is filled with liquid. In this way, a correlation between the local interfacial area and the local instantaneous gas fraction can be found, in which the local interfacial area concentration is at its maximum at 50 % volumetric gas fraction and decreases to zero at both 0 % and 100 % local instantaneous gas fraction. This method can be applied to large bubbles with an arbitrary shape.
• In order to decrease the measuring error arising from the fact that gas fractions in measuring points inside large bubble as well as inside completely liquid-filled regions are not exactly equal to 100 % or, respectively, 0 %, and which therefore may significantly contribute to an overestimation of the interfacial area, a third method was applied, which is based on the identification of points that belong to the surface of the bubbles.
All three methods were applied to a vertical air-water flow. Since the distance between the sensors and the gas injection was varied, it was possible to characterize the change of the interfacial area concentration along the pipe. By combining the method with bubble size measurements, a decomposition of the interfacial area concentration into more then one bubble size classes was performed as well. The obtained results were compared to the findings reported in literature. The use of mesh sensors proved to be very efficient, since a bubble-size decomposed interfacial area concentration measurement in an entire pipe cross-section can be performed in a few seconds, which is very efficient compared to the use of local probes.

Keywords: gas-liquid flow; wire-mesh sensor; interfacial area concentration; vertical flow; two-phase flow

  • Lecture (Conference)
    Eleventh International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 11), 02.-06.10.2005, Avignon, France
  • Contribution to proceedings
    Eleventh International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH 11), 02.-06.10.2005, Avignon, France. paper 398.

Permalink: https://www.hzdr.de/publications/Publ-6378
Publ.-Id: 6378