Measurement of Two-Phase Flow Structure in a Narrow Rectangular Channel

Abstract

The flow channel of the fluids in the nuclear reactor core and the heat exchanger is relatively small. In order to understand the two-phase flow phenomena in the narrow channel, many interesting studies have been carried out, for example, mini capillaries (Wong, et al., 1995, Han & Shikazono, 2009), rectangular channels with small gap (Mishima, et al., 1993, Xu, et al., 1999, Hibiki & Mishima, 2001) and rod bundles with tight-lattice geometry (Tamai et al., 2006, Sadatomi et al., 2007, Kawahara et al., 2008). Especially, a triangle tight-lattice rod bundle has been adopted as a fuel rod configuration in high conversion boiling water reactor (Iwamura et al., 2006, Uchikawa et al., 2007, Fukaya et al., 2009). This has a narrow gap of about 1mm in the coolant channel. Therefore, two-phase flow in the tight-lattice bundle or narrow channel should be clarified for thermal-hydraulic analysis of high conversion reactor. In the past several years, the acquisition of experimental data and the modelling of the flow in tight-lattice rod bundle have been done. Tamai et al. (2006) evaluated the effect of the gap width and the power profile from the pressure drop measured in tight-lattice 37 rod bundles. Sadatomi et al. (2007) studied the void fraction characteristics in double subchannels with tight-lattice array and the data was compared with some correlations and subchannel codes. Furthermore, their group estimated the wall and the interfacial friction forces from the measured void fraction and pressure drop using the same subchannel (Kawahara et al., 2008). However, the advanced measurement techniques of the spatio-temporal phase distribution and velocity field are required for the high accurate analysis of the flow. For the void fraction distribution measurement, the radiation methods have been developed to evaluate the flow in the bundle. Kureta (2007a, 2007b) developed a neutron radiography method for three-dimensional tomographic imaging of two-phase flow. The threedimensional flow structures in the tight-lattice rod bundle were visualized. Although the spatial void fraction distribution can be obtained by the neutron imaging method, it is difficult to measure the time variation with high temporal resolution. In addition, there are some limitations of the uses. Thus, the authors focused on two-phase flow measurement using electrical conductance. Wire-mesh sensor (WMS), which uses the difference of electrical conductance between gas and liquid phases, has received attention as crosssectional void fraction distribution measurement method (Prasser et al., 1998). To apply the WMS measurements to the flow in the rod bundle, a lot of electrode wires have to beinstalled over the small cross section and there are several intrusive effects to the flow (Ito et al., 2011a). Therefore, a novel void fraction measurement method with the electrodes on the wall of the flow channel was developed for two-phase flow in the narrow channel (Ito et al. 2010b). The wire electrodes were fixed on the opposing walls of the narrow channel and the conductance in the narrow gap was measured. As a result, three-dimensional void fraction distributions can be obtained. On the other hand, a liquid film sensor based on the electrical conductance measurements was developed (Damsohn & Prasser, 2009). This can estimate the liquid film thickness distribution on the wall in a flow channel. In addition, this sensor was applied to the annular flow measurement in the double subchannels of a square-lattice bundle. The liquid film behavior in the rod bundle was well visualized (Damsohn & Prasser, 2010). Thus, the novel measurement method for two-phase flow in the narrow channel has been developed by combining the void fraction detection and liquid film sensor (Ito et al., 2011c). The instantaneous distributions of the liquid film thickness on a wall and the void fraction in the gap were measured simultaneously by this method. In this chapter, the measurement principle of the novel technique is described, and the measurements in a narrow rectangular channel with a gap width of 1.5mm are carried out by using two liquid film sensors. The liquid film thicknesses on two channel walls and the void fraction in the gap are estimated from the measured electrical conductance. In addition, the void fraction is recalculated by considering the film thickness and interfacial area concentration is obtained by reconstructing the gas-liquid interfacial structure. Furthermore, the individual bubble parameters are obtained by time-revolved reconstruction of the flow.