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The gas-liquid two-phase flow widely exists in nuclear energy, chemical industry, petroleum engineering, and other industrial production fields. Two-phase flow will occur in pressurized water reactors during normal operation and accident conditions. The distribution of void fraction and interfacial area concentration in rod bundle channels has an important influence on the flow resistance, heat transfer, critical heat flux, and power distribution of the reactor. The complex geometric structure of rod bundle channels brings a great challenge to the local experimental measurement of two-phase flow. As a result, the existing database of the interfacial structure parameters in rod bundle channels is still insufficient, which greatly affects the accuracy of two-phase flow modeling. To clarify the transformation mechanism of the phase distribution in rod bundle subchannels and provide a database for future modeling, an experimental study of air-water two-phase flow in a 5×5 rod bundle channel was carried out in this paper. The four-sensor conductivity probe was used to measure the distribution of local two-phase flow parameters (void fraction, interfacial area concentration, bubble diameter, and bubble velocity) across the test section at 36.5 hydraulic diameters. The results show that with the increase of liquid phase velocity and the decrease of gas phase velocity, the “axial peak” gradually changes to the “wall peak” under the influence of radial force. The transition boundary of phase distribution is obtained and has a relatively accurate classification effect. The non-uniform void fraction distribution over the cross-section is observed, bubbles tend to gather in the central subchannels which are less affected by the wall and have a higher mass velocity. For the overall cross-section, the decrease of liquid phase velocity and the increase of gas phase velocity exacerbate the non-uniformity of bubble distribution. In the experimental conditions of this paper, the established correlation of void fraction and interfacial area concentration exhibits strong predictive capability. The average minimum relative error is ±18.2% and ±12.2% respectively. The drift flux model for the rod bundle channels should take into account various phase distribution types to enhance the modeling of distribution parameters. Furthermore, it is imperative to validate the model using comprehensive void fraction distribution databases to enhance the accuracy of prediction outcomes. Because of the small size of the bubble flow, the interfacial area concentration correlation established based on the vertical circular pipe database can also be used for rod bundle channels, but the modified correlation considering the influence of the bundle geometry on the bubble is more accurate. This study can provide a reference for the closure of the two-fluid model in the reactor thermal-hydraulic analysis program.

Reactor cavity cooling system (RCCS) is a competitive residual heat removal scheme for small modular reactors. In this paper, the RCCS has been simplified to a scenario that ascending air flow is got heated in vertical annular channels and driven by mixed convection. The mixed convection is a combination condition of forced convection (external-applied pressure difference) and natural convection (internal-generated buoyancy force). The scenario has been analyzed by numerical approach using SST (shear stress transport) k-ω turbulent model, which can obtain good accuracy at near-wall low Reynolds number region. Buoyancy number, which is defined as Gr/Re3, increases with heat loading, and it represents the relative intensity of natural convection and forced convection. The result shows that, heat transfer in ascending turbulent mixed convection is impaired with respect to forced convection at moderate heat loadings, and enhanced at high heat loadings. Thus, mixed convection can be divided into forced convection dominant region, mixed convection impairment region and natural convection enhancement region. According to different aspect ratios of vertical channels, the most serious impairment occurs when buoyancy number equals to 1×10-5. Additionally, the impairment in narrow channel is more serious than wide channel. The mixed convection heat transfer correlations are fitted based on Symolon’s correlation, which shows the best agreement with numerical analysis results in predicting the heat transfer. The mechanism of turbulent mixed convection phenomenon can be explained as turbulent laminarization and intensify, which is caused by non-uniformity of density and variation of gravitational body force. With increasing heat loading, non-uniform buoyancy effects cause marked distortion of the flow structure, with the velocity maximum moving from the core of flow to the near-wall region. Velocity profiles switch from inverted U-profile to M-profile, which is a typical profile of mixed convection. At the same time, Reynolds stress at near-wall region reduces, suppresses the production of turbulent kinetic energy, leads to laminarization and heat transfer reduction. The negative Reynolds stress generates from the velocity gradient of the M-shape profile in the core region, and increases with heat loading, provides extra turbulent kinetic energy. There are two peaks in the profile of turbulent kinetic energy, one is associated with the near-wall region and the other is associated with the inner flank of the M-profile, due to the negative turbulent shear stress. Turbulent kinetic energy and heat transfer coefficient are enhanced and increase with heat loading. Flow regime develop towards natural convection.

The shape and rising behavior of the horizontally arranged twin bubbles in a steady liquid are experimentally studied employing high-speed photography and digital image processing, and numerically studied by the Volume-Of-Fluid (VOF) method, in combination with a momentum equation coupled with a surface tension model. The movement trajectory and the velocity variation in horizontal and vertical directions of the horizontally arranged twin bubbles rising side by side, as observed in experiments, are described. According to the results, when two bubbles rise side by side, their horizontal velocity changes by the simple harmonic law; there is a cyclical process of two bubbles repeatedly attracted to and bounced against each other, rather than at constant distance between each other, and the bubbles swing up and down periodically in the water. The mathematical model and its numerical implementation are presented in detail. The validation of the model is confirmed by comparing the numerical and experimental results, which are in good agreement with each other; the numerical simulation can accurately reproduce the deformation, attraction, and repulsion of the bubble pairs. The phenomenon of attraction and repulsion is comprehensively analyzed from the viewpoint of a flow field. It is considered that the interaction between the bubbles is mainly influenced by the changes of the flow field due to vortex counteraction and wake merging effects.