Void fraction measurement using impedance method

Abstract

To investigate the relationship between void fraction and volume-averaged impedance in water-air mixtures, a Styrofoam simulator was designed and manufactured. Because the relative permittivity of Styrofoam is negligible compared to that of water, Styrofoam spheres immersed in water act like air bubbles. Three kinds of rectangular conductance electrode were examined to verify the performance of the Styrofoam simulator and to choose the optimum electrode shape. In addition, a water-air level swell facility was designed and constructed to verify the performance of recommended electrode shape developed using the Styrofoam simulator. Three circular conductance probes were designed and their impedance data in the water-air level swell facility were compared. Two-probe designs, characterized by probe-I and probe-II, were shown to be the best candidates for the measurement of volume-averaged void fraction. The impedances of the water-air mixtures with void fractions of 0.0-0.1 were similar to theoretical predictions, with a maximum error of 0.5%. Therefore, the Styrofoam simulator and circular conductance probes should prove useful for the measurement of volume-averaged void fraction in pool conditions. (C) 2003 Elsevier Ltd. All rights reserved.