Capacitance-Type Void Meter

For studies of the boiling-water nuclear reactor (BWR) and two-phase flow, it is very important to measure the void-fraction, the volume fraction of steam in two-phase flow. Many methods for the void-fraction measurement have been developed hitherto. They are, however, insufficient in respect of their accuracy, response, handling and so forth. This paper reports a method for void-fraction measurement which is based on changes of the equivalent dielectric constant of the two-phase flow. Both the instantaneous and the local void-fraction can be measured with a fast response by this method. Preliminary experiments were carried out with both simulated voids in the forms of glass tubes and real voids (air bubbles) in the air-water mixture to investigate the characteristics of the device for the void-fraction measurement (capacitance-type void meter). They showed that the linearity of the device is good. The basic principle of this void-fraction measuring method is discussed, comparing real voids with equivalent circuits consisting of capacitances and resistances. It has been ascertained that this method is principally founded on the equivalent dielectric constant changes of the two-phase flow caused by the changes of the void-fraction. Experimental results of this void meter applied to a thermodynamic-hydrodynamic loop which simulates the BWR are shown and it is verified that the local void-fraction can be measured with a fast response. As a void detector is used for this device, its shape and dimensions must be investigated before applying the device to the BWR, because the core area into which the detector is to be inserted is limited. Several types of void detectors have been made and examined. The results show that a probe-type electrode has the best characteristics for the purpose. (author)