Reliable prediction of two-phase flow characteristics is important for safety and economy improvements of BWR plants. We have been developing two-phase flow measurement tools and techniques for BWR thermal hydraulic conditions, such as a 3D time-averaged X-ray CT system, an ultrasonic liquid film sensor and a wire-mesh sensor. We applied the developed items in experiments using the multi-purpose steam-water test facility known as HUSTLE, which can simulate two-phase thermal-hydraulic conditions in a BWR reactor pressure vessel, and we constructed a detailed instrumentation database. We validated a 3D two-phase flow simulator using the database and developed the reactor internal two-phase flow analysis system.

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In this study, a numerical analysis method applicable to estimation of the boiling heat transfer has been developed. Currently, the experimental correlations or the empirical laws have been applied to evaluate the boiling heat transfer. Therefore, it is difficult to predict the effects of the change of the heated surface geometry, thermal-hydraulic conditions, the surface activation or modification, because out of the application range of these correlations. The purpose of this work is to construct the boiling two-phase analysis method for thermo-fluid phenomena, and to realize “Design-by-Analysis” independent on the experiments and empirical laws. For this purpose, it is important to predict steam-water interface structure characteristics of the two-phase flow directly. Until now, for evaluating the boiling phenomena, Diffusive Interface Model for the bubble interface tracking was applied. In this model, the steam-water interface is diffuse with a finite width, and values of the thermodynamic properties change between water and steam smoothly within the interface region. For evaluating the wettability of heated surface, the surface energy is estimated by using the phase-field model. The wetting phenomena during boiling are able to be analyzed directly with this model. We present the numerical results of nucleate pool boiling phenomenon by using the developed analysis method. We succeeded in simulating the boiling process, vapor bubbles nucleation, growth, and departure behavior on a heated surface. By present analysis method, it was confirmed that the boiling heat transfer coefficient could be evaluated quantitatively without the experimental correlations.

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Nanofluid is the liquid containing colloidal dispersion of nanometer-sized solid particles. Anomalously high thermal conductivity of nanofluids compared to the base fluid was reported in 1995, and then a number of studies have been conducted so far for the nanofluids from various aspects. This paper presents a review of the researches on the critical heat flux and boiling heat transfer of nanofluids.

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This report presents a state of art of measurement technique of interfacial area concentration in a gas-liquid two-phase flow. Based on the conventional 4-sensor probe method, a simple algorithm was derived to measure bubble velocity assuming bubble shape. Measurement error in the bubble velocity and shape is caused by the interface curvature, which directly affects the measurement error of the interfacial area concentration. Correction method of interface curvature is shown to improve measurement accuracy of the interfacial are concentration.

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This paper presents a micro-multiphase flow measurement technique using ‘multicolour confocal micro-particle image velocimetry (PIV)’ by wavelength separation technique. The present system can measure the dynamic interaction between flows in two different phases, such as liquid-liquid or solid-liquid, simultaneously and separately. The system has high temporal resolution up to 2000 frames per second, and high spatial resolution up to 0.116 μm/pixel in plane and 0.58μm out of plane, which is enough to resolve cell size measurement target.
　In this paper, three measurement demonstrations are shown. There are flow inside and outside of moving microdroplet, droplet formation at T-shaped junction and red blood cells (RBCs) dispersed flow. The droplet is studied as a liquid-liquid multiphase flow using immiscible two liquids of oil and water. This study clarifies not only three-dimensional flow structure using continuity equation-assisted method, but also shear force between its interface. These informations are necessary for estimation of mixing effect using circulation in the droplet and discussion on droplet formation mechanism.
　The measurement of RBCs mixed flow requires more due to its unsteadiness and randomness derives from characteristics of live cells. Our developed ‘Target-tracking system’ can measure one individual cell for long time under the high-magnification measurement condition. Finally the tank-treading motion of RBC and the surrounding flow structure simultaneously for the investigation of specific behavior of RBC.

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The purpose of this study is to improve the critical power prediction accuracy of a film dry-out model at a high mass flux. The factor that overestimated the critical power in a high mass flux region was analyzed, and it was found from analysis results that entrainment due to boiling in the liquid film was the main factor. Both film dry-out models with and without the boiling effect was compared with a wide range of critical power data including BWR operating conditions. The average and standard deviations of prediction accuracy (prediction/measurement) without the boiling effect were 1.44 and 0.25, respectively. The prediction accuracies with the boiling effect were 0.98 and 0.097. Thus, the prediction accuracy was improved. It was clarified that the boiling in the thin liquid film was more likely to occur, using Chen’s forced convection correlation, Bergles-Rohsenow‘s onset boiling correlation and Tsukuda’s boiling curve.

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The heat removal capability of systems using natural circulation depends on natural circulation flow rate. Therefore, accurate prediction of natural circulation flow rate is important for safety design. The purpose of this study is to obtain the natural circulation data and develop the evaluation method of natural circulation flow rate under two phase conditions. Natural circulation experiments are conducted at atmospheric pressure, using air-water mixtures in both single channel and multi-channel. Major conclusions are as follows; (1) Experimental data for verifying the evaluation method of natural circulation flow rate can be obtained. (2) Void fraction model and two phase multiplier model are selected by comparing test results. Evaluation method of two phase natural circulation flow rate is developed. (3) In single channel and multi-channel, evaluations of natural circulation flow rate and pressure distribution along test loop are in good agreement.

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The present paper was devoted to improving the understanding and modeling of upward turbulent bubbly flow (TBF) in the large noncircular duct. Recent experiments of the upward TBF in a large vertical square duct showed a significant peak of the axial liquid phase velocity (W_l) near the corner and wall except the pronounced corner and wall peak of the void fraction [1]. However, few observations of such significant velocity peak were reported near the wall in the currently existed circular-pipe database except slightly peak. The paper presents the analysis on the formation of the typical shape of W_l by considering an index to show the shape of velocity profile and the formation condition of the typical M-shape. The index to show the shape of the velocity profile was discussed. It indicates the flow with smaller wall resistance, larger and steeper profile of the void fraction is easier to form M-shaped velocity profile. Besides, it was obtained that for the upward TBF in the large square duct, there may exist two layers with different flow types between the wall and center.

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In a companion paper (Zhang, H. N., Sun, H. M., Yokomine, T., Kunugi, T., Japanese J. Multiphase Flow, submitted), for upward turbulent bubbly flow (TBF) in the large square duct, the observation of the typical M-shaped velocity profile were described. The present paper further discussed its formation by considering the duct geometry effect on the ability of gathering bubbles to near the wall region and the wall shear resistance. It indicated that a large square duct owns not only stronger bubble gathering ability to the wall but also the smaller wall shear resistance, which was responsible for the formation of the typical M-shaped velocity profile. In addition, the flow characteristics were discussed based on the normalization of liquid phase velocity, which suggested the two-layer model according to the different flow types therein.

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