Estimation of Void Fraction in Dispersed Bubbly Flow With a Constant Electric Current Method

In several void fraction measurement methods, electric void sensors are online measurement method, simple construction and lower cost. In electric measurement, we research on a constant electric current method which is one of conductance methods. By using this method, we can measure volumetric void fraction with higher temporal resolution although the method cannot measure 2D and 3D distribution of void fractions. Besides, multiple measuring electrodes can be installed at a short distance. And then, flow is not obstructed by measuring electrodes. However, the constant electrical current method has been applied in annular flow in previous studies. Void fraction is estimated by cross-sectional ratio of gas and liquid phases in this method. For this reason, dispersed bubbly flow is not applied because cross-section ratio is not continuous in a flow direction. In the present study, Maxwell’s theory, Bruggemann’s treatment and polarization method are applied in order to measure void fraction of dispersed bubbly flow more accurately. Maxwell’s theory is an estimation of a resistance of a mixture with two difference resistivity by calculating electric potential in the mixture. Bruggemann’s treatment is based on Maxwell’s theory but it implies the assumption of a large size-range of particles in surrounding medium. In polarization method, bubbles are assumed to be dielectric bodies. Therefore if voltage is applied to gas-liquid two-phase flow, electrical charges in bubbles are polarized, and polarization electrical field generates. A difference of voltages in bubbly flow and liquid single phase flow assumes to be caused by polarization fields. Void fraction in vertical flow is measured experimentally by the previous method, Bruggemann’s treatment, Maxwell’s theory and polarization method in order to investigate the accuracy of these estimations. Working fluid is air and tap water. The accuracy is measured by comparing with a quick shut valve method and observations. Besides, we investigate effects of flow structure and bubble shape to measurement accuracy. Flow structure is changed by changing gas and liquid volume flow rate. In the experiment for bubble shape, a rising bubble by buoyancy is measured. The bubble shape observed by a high speed video camera is compared with the electrical signal measured by the constant electric current method. From experimental results, it is confirmed that void fraction in bubbly flow and froth flow is estimated more accurately by Maxwell’s theory, Bruggemann’s treatment and polarization method, and change of bubble shape correlates with fluctuation of void fraction measured by the constant electric current method.