Understanding radioactive aerosol behavior is important during severe accidents in nuclear power plants. In particular, pool scrubbing at a suppression pool is a key factor for estimating the leakage of radioactive aerosol into the environment. Therefore, we have to elucidate the mechanism of the aerosol transfer from the gas phase to the liquid phase during bubbly flow. Currently, models to evaluate the amount of aerosol removal by pool scrubbing have been incorporated into integrated severe accident analysis codes such as MELCOR. However, the models for bubbles and aerosols behavior have not been validated enough by experiment. Therefore, the measurement of bubbles and aerosols behavior is required for validation and reconstruction pool scrubbing model. However, the quantitative measurement of bubbly flow is very difficult by complicated bubble shape and overlapping of bubbles in visualization. The wire-mesh sensor (WMS) is one of the most effective measuring methods for complicated bubbly flow. The reconstruction method of bubbly flow by WMS measurement is suggested by Prasser et al. This method can measure the bubble surface, meanwhile, individual bubble velocity and volume are not one-to-one correspondence because bubble volume is calculated by using radial average velocity. Kanai et al. proposed a method for acquisition bubble three-dimensional velocity method by bubble tracking. In this method, bubble velocity and volume are one-to-one correspondence. Thus, we suggested an algorithm of bubble surface measurement by applying a bubble tracking method. The bubble three-dimensional velocity and surface area is obtained by this algorithm. As a result, bubble rising velocity in bubbly flow is higher than bubble terminal velocity according to causing liquid phase velocity. In addition, the gas-liquid total interfacial area is overestimated in MELCOR predict by overestimating bubble breakup. This result suggests that the overestimating of DF at downstream is caused by overestimating bubble breakup in MELCOR model.

Accurate three-dimensional CFD simulation is of great importance to the flow and heat transfer of flow boiling in the steam generator in nuclear power plant, in which bubbly flow is the main flow pattern. One of the biggest challenges for the CFD simulation is the coalescence and breakup model for bubbles dispersed in the continuous fluid. Recently, several new bubble coalescence and breakup models have been developed combined with the inhomogeneous Multi-Size-Group (MUSIG) model in the Eulerian framework in addition to the standard closure model (Luo and Svendsen (1996), and Prince and Blanch (1990)), which has been widely adopted in the CFD simulations in bubbly flow using the Eulerian method. For example, Liao had developed a comprehensive closure model (2015), in which five collision mechanisms and four breakup mechanisms have been included in the model. However, proper coefficients for the Liao model must be determined to properly address the proportion for various mechanism in the coalescence and breakup models. In this paper, we compared the predictions using the standard closure model and the Liao model (2015) combined with the iMUSIG model for the case of adiabatic upward bubbly flow in vertical pipe, as well as the corresponding experimental results reported by Lucas et al (2008) which were obtained at TOPFLOW test facilities. Six simulated working conditions are chosen. The significance of various mechanisms on the bubble coalescence and breakup were analyzed based on the bubble size distributions in various heights obtained in the CFD predictions and experiments. Finally, a set of proper coefficients for various mechanisms in the coalescence and breakup models of Liao (2015) was developed for the simulated conditions.

Gas-liquid two-phase flow has high potential in heat transfer and mixing capabilities, and therefore it is utilized in various technologies such as nuclear reactor and chemical plants. There are several flow regimes since the gas-liquid interface transforms constantly. For the sake of safety and optimization in operating plants, it is crucial to understand the behavior of the gas-liquid interface. We have focused on extracting the bubble features in the bubbly flow by filming the bubbly flow with a high-speed camera and training convolutional neural network (CNN) for feature extraction. The assumption made was bubbles in the bubbly flow being ellipsoids. Since void fraction and interfacial area concentration are one of the geometric parameters in the two-phase flow models, like two-fluid model, it becomes possible to evaluate the flow field of the two-phase flow quickly and quantitively by calculating these parameters from the extracted features. We have compared two-phase flow parameters with the conventional object detection method using bounding boxes, and the new ellipse fitting method to identify the best region proposal shape. As a result, the conventional method showed higher accuracy in extracting bubble features under our flow conditions.

The interfacial area transport equation is a more accurate and stable way to compute the interfacial area concentration than the traditional empirical correlation in the two-phase two-fluid model. And among the parameters in the two-group interfacial area transport equation, the interfacial area concentration weighted area-averaged gas velocity is an important parameter to close the two-group area-averaged interfacial area transport equation in the system analysis code. However, there has been no theory model to compute the interfacial area concentration weighted area-averaged gas velocity until now. So this study established the theory model for two-group interfacial area concentration weighted area-averaged gas velocity based on the drift-flux model for the two-phase dispersed bubble flow. The experimental data were selected from the published literature, which include the detailed two-phase interfacial structure experimental data for the slug bubble flow. The interfacial area concentration weighted area-averaged gas velocity model predicted the selected experimental data well, which validated the developed model. Moreover, the difference between the interfacial area concentration weighted area-averaged gas velocity and the void weighted area-averaged gas velocity is clarified quantitatively for the first time. The theory model developed in this study can be improved and then be used to compute the interfacial area weighted area-averaged gas velocity because it includes the empirical parameter of conventional drift-flux model.

Bubbly flow or foam flow could prevail in the subcooled flow boiling of seawater in an annulus. Compared with the bubbly flow of de-ionized water in the subcooled flow boiling, the bubble size of seawater is smaller. Further, bubbly flow with small bubbles or foam flow in a subcooled flow boiling of seawater would be affected by different heat fluxes, mass fluxes and inlet subcooling. The change in the boiling behaviors due to different conditions can be reflected in terms of variations of bubble characteristics. However, details of the characteristics of bubbles in seawater generated by the two-phase subcooled flow boiling are still limited. In the present study, bubble characteristics are measured and detailed by an image analytic method in the software Image J. The subcooled flow boiling experiments with different heat fluxes are conducted in a vertical upward annulus with artificial seawater at the inlet temperature of 90°C and mass flux of 1200 kg/m 2 s. The two-phase flow patterns are recorded by the high-speed camera in 6000 frame/s. To conduct measurements of bubble characteristics using Image J, images are selected as one set of measurement from the videos recording under different heat fluxes. After preprocessing of images, measurements of bubbles and fitting procedures with ellipses are conducted. Averaged values of descriptors of bubbles are obtained, including areas, perimeters, lengths of major axes, lengths of minor axes, circularities, angles, solidities, and aspect ratios, etc. It provides a possible way to investigate the details of bubbly flow or foam flow in the subcooled flow boiling of seawater.

The radioactive aerosol removal equipment is used as one of the safety systems of nuclear reactors. In this equipment, microparticles of aerosol are removed through gas-liquid interfaces of two-phase flow. The mechanism related to the removal of microparticles through the gas-liquid interface is not precise; a numerical evaluation method of performance of aerosol removal equipment is not realized. Then, we have started to construct a numerical simulation method to simulate the removal of microparticles through gas-liquid interfaces. In this simulation method, a detailed two-phase flow simulation code TPFIT is used as the basis of this method. TPFIT adopts an advanced interface tracking method and can simulate interface movement and deformation directly. Also, to simulate the movement of particles, the Lagrangian particle tracking method is incorporated. By combining the interface tracking method, and the Lagrangian particle tracking method, the interaction between interfaces and microparticles can be simulated in detail. To solve the Lagrangian equations of particles, fluid properties and fluid velocity surrounding aerosol particles are evaluated by considering the relative position of particles and gas-liquid interface, to simulate particle movement near the interface. In this paper, we show an outline and preliminary results of this simulation method.

To accurately simulate the bubble motions in the turbulent flow liquids, two way coupled Euler–Lagrange method is adopted in this work. The continuous phase is solved with the Navier–Stokes equations based on the Euler grid, while the individual bubble is tracked by using the Lagrange frame of reference. Two–way coupling is realized by transferring the interaction forces to the momentum equation of the continuous phase at each fluid flow timestep and in turn influencing the bubble motion. The interaction forces including the drag force, lift force, wall lubrication force and virtual mass force are carefully selected. Turbulent models are of significance to capture the fluid flow conditions. Comparing the different cases treated by the k-ε, PANS and LES models with the experimental data, the results of the PANS with RWM and BIT, and LES can dynamically display the oscillation of the bubble plume and the calculated time averaged axial liquid velocity reasonably agrees with the measured data. In the case of bubble injective flow conditions in large containers, the bubble induced turbulent effect can be almost neglected. However, the RWM describes the fluctuating velocity should be considered applied with the PANS model. In addition, the saturated vapor–liquid system under high pressure of 6.9 MPa is numerically simulated by implementing the PANS with R WM and BIT.

In relation to the modeling of the one-dimensional interfacial area transport equation for an adiabatic bubbly flow in a vertical rod bundle, some existing models of sink and source terms were reviewed and evaluated. Based on the reviewed interaction mechanisms of bubbles and turbulent eddies, a new interfacial area transport model has been proposed. Two important impacts on bubble interaction have been taken into account in the new model: the effects of spacer grids with mixing vanes and the impacts of geometry structure. The spacer grids breakup large bubbles into small bubbles resulting in enhanced bubble random collision at the downstream of the spacer grids. Void transport is the main contribution between spacer grids. The new interfacial area transport model has been evaluated against the obtained experimental data in 17 bubbly flow conditions. The results indicate that the new model can predict the interfacial area concentration with the relative error of 19.9%. It is recognized that the proposed model is promising for predicting the interfacial area concentration for a bubbly flow in a rod bundle.

The gas-liquid two-phase flow behaviors are always associated with its dynamic void fraction, such as flow resistance, heat transfer coefficient, phase distribution, critical heat flux etc. As regard to the commercial PWR and BWR, rod bundles are the typical geometry, which contains many sub-channels for coolant flowing. In present study, the sub-channel void fraction was measured in 5 × 5 rod bundles with the sub-channel impedance void meter consisting of 12 strip electrodes. Based on the measured void fraction in different sub-channels, the void fraction dynamics, PDF (probability distribution function) and CDF (cumulative distribution function) curves were analyzed to make clear the effect of superficial gas and liquid velocity, flow development and casing tube. The empirical correlation for PDF of dynamic sub-channel void fraction has been developed, which showed good fitness with PDF and CDF curves and satisfying accuracy of averaged void fraction.

Due to the increase of computing efficiency and power, full-resolution two-phase flow simulations have become a practical research tool for model development and analysis of reactor flows. The expansion of state-of-the-art high performance computing (HPC) facilities allows for the use of direct numerical simulation (DNS) coupled with Interface Tracking Methods (ITM) to perform full resolution simulations. Given adequate spatial and temporal resolution, DNS can resolve all relevant turbulent scales, allowing for the extraction of high quality and detailed turbulent and two-phase flow numerical data for use in model development. While larger scale bubbly flow DNS are becoming ever more affordable, it is still computationally expensive due to the requirements of the spatial discretization. This presents the largest obstacle for future applications of DNS. For this reason, mesh adaptation techniques are sought after to reduce the computational expense of bubbly flow simulations in complex geometries. By fully resolving only the areas of specific interest, the computational costs of DNS can be reduced. Grid refinement can be based on the location of the interface between the two phases, area of greatest turbulent intensity, averaged bulk fluid velocity data, or the prediction of bubble movement. Coupled with an advanced bubble tracking algorithm, the path of individual bubbles moving through the computational domain can be predicted, and the computational mesh refined within the path area. This refinement can create tracks of greater resolution for the bubbles to move through in the domain, while keeping the bulk resolution of the mesh coarser. Through these means, the overall cost of the simulation is reduced, while high quality numerical data is still obtainable. This work outlines the enhancement of existing mesh adaptation algorithms to implement the bubble tracking refinement, and its practical applications to full resolution two-phase flow simulations.

In order to decrease the leakage and diffusion of Fission Products (FPs) in a severe accident (SA) of nuclear power plants, BWR have a suppression chamber filled with water to decontaminate the polluted air from the reactor vessel when emergency ventilation is done. It’s called pool scrubbing having a function of decontamination effect that transfer particles from gas to liquid phase by blowing gas containing FP into water. decontamination factor (DF) which is an index of decontamination performance evaluation of pool scrubbing is predicted and calculated by using a flow model in the existing analysis model such as MELCOR code. Evaluation of the DF by pool scrubbing is important in analyzing SA progress of nuclear power plants. For this reason, validation of flow model and decontamination model is important. It is necessary to validate whether the analysis result of each model matches the actual phenomenon. Despite of these needs, generic experimental data on gas-liquid two phase flow including aerosol is insufficient in confirming the validity of existing model. Therefore, the aim of this study is to investigate aerosol behavior in air jet and its relation to bubbly flow structure during pool scrubbing. This is to obtain the flow structure when air - aerosol released to the pool part and are obtained. Transition behavior of particles from the gas phase into the liquid phase was observed by high-speed camera. In addition, the effect of aerosol on void fraction in the pool was measured by wire mesh sensor from the nozzle exit to the water surface ware also observed. Ejecting air-aerosol from the nozzle to the pool part at various flow rates, visualization of bubble diameter, bubble aspect ratio, void fraction and the gas phase velocity were done by using image processing and a wire-mesh sensor. From these results, we compared the parameters by the presence or absence of aerosol. DF is also compared with the model used in the MELCOR code.

Mechanical non-homogeneous and thermal non-equilibrium phenomenon exists in two-phase critical flow compared with single phase flow. A one-dimensional two-fluid critical flow model is developed for initially subcooled water flowing in pipe or orifices. The model accounts for thermal nonequilibrium between the liquid and vapor bubbles and for interphase relative motion. In this model, an improved correlation to calculate flashing inception location and surperheat is proposed. The model consists of six conservation equations as well as a seventh equation representing bubble growth in bubbly flow. Closure of the set of governing equations is performed with constitutive relationships which determine the interfacial momentum terms due to mass exchange, wall to liquid and wall to vapour frictional forces, liquid to gas interfacial force and interfacial heat transfer rate. The model considers the development of three flow regimes, namely, bubbly, churn and annular flow regimes. Model predictions compare favorably with experimental data over a wide range of pressures and pipe diameters and lengths.

Aerosols as the main component of radioactive products in migration performance, which is an important factor that a unclear reactor accident present strong diffusion and affects the distributions of source and dose level in reactor containment, and they are therefore expected to be deposited in liquid phase such as in suspension pool and filtered containment venting device. In this paper, the deposition characteristics of micro-nano aerosols in rising bubble under pool scrubbing condition is studied with experiment, the aerosols size in the research range from 20 nm to 600 nm, and the bubble morphology mainly concern homogeneous bubbly flow. The results show that the deposition efficiency and mechanism of aerosol closely relate to gas flow rate, liquid level, particle size and bubbles size and so on. The aerosol deposition near 85nm is proved most difficult because of the convert of deposition mechanisms. In a high liquid level condition, micro-nano aerosol filtration efficiency is enhanced but gradually gradual. Under different gas flow rate, air bubble residence time and the bubble size distributions affect the filtration efficiency of aerosols.

Clarifying thermal-hydraulic characteristics in a nuclear reactor core is important in particular to enhance the thermo-fluid safety of nuclear reactors. Spacers installed in subchannels of fuel assemblies have the role of keeping the interval between adjacent fuel rods constantly. Similarly, in case of PWR the spacer has also the role as the turbulence promoter. When the transient event occurs, two-phase flow is generated by boiling of water due to heating of fuel rods. Therefore, it is important to confirm the two-phase flow behavior around the spacer. So, the effect of the spacer affecting the two-phase flow was investigated experimentally at forced convective flow condition. Furthermore, in order to improve the thermal safety of current light water reactors, it is necessary to clarify the two-phase flow behavior in the subchannels at the stagnant flow condition. So, the bubbly flow data around a simulated fuel rod were obtained experimentally at the stagnant flow condition. A wire-mesh sensor was used to obtain a detailed two-dimensional void fraction distribution around the simulated spacer and fuel rod. As a result of this research, the bubbly behavior around the simulated spacer and fuel rod was qualitatively revealed and also bubble dynamics in the sub-channels at the conditions of forced convective and stagnant flows were evaluated. The present experimental data are very useful for verifying the detailed three-dimensional two-phase flow analysis codes.

Traditionally, the flow regime in two phase flow in rod bundles are considered in a global sense by the visualization. However, a sub-channel flow regime is required to understand and model the two phase flow structures in rod bundles. In this work, a sub-channel impedance meter was designed to get the dynamic feature in the sub-channels, which was applied to identify the sub-channel flow regime by the fast search and finding peaks of the cumulative probability distribution functions (CPDFs) objectively. In the present study, five flow regimes, namely bubbly flow, quasi-cap bubbly flow, quasi-slug flow, cap-turbulent flow and churn-turbulent flow were defined and recognized. The sub-channel flow regimes at the same cross section were compared to each other, which show similar feature with the local flow regimes in pipe. It is possible to identify nine different global flow regime configurations by combining the corner, side and inner sub-channel flow regime at the same cross section, which was drawn in the 2D sub-channel flow regime.

An open natural circulation system has the characteristics of a simple structure, superior safety performance and strong heat removal capability. However, during long-term operation, the flow instability may occur due to the reduction of the driving force, which will have adverse effects on the heat removal capability and safe operation of the system. Thus, injecting air into the riser is designed in this paper to improve the driving force of the circulation flow, reduce the possibility of flow instability, and increase the heat removal capability. In order to investigate the influence of air injection on the evolution of flow pattern, resistance characteristics and circulation flow rate, the method of visual observation and data analysis is used based on different pore sizes porous media, air injection rate and submergence ratios. The ratio of the driving pressure head to the resistance pressure drop is proposed as the basis for assessing the effect of air injection on the ability of natural circulation. It is found that the driving force of natural circulation increases with the increase of air injection rate, and the circulation flow rate increases obviously when the bubbly flow appears in the riser. However, when the transition from bubbly flow to churn flow appears, the growth of the circulation flow rate slows down because the resistance increases faster than the driving force. Therefore, it can be known that the best performance is obtained when bubbly-churn flow appears in the top of the riser. What’s more, the capacity of lifting water will be reduced and churn flow will appear prematurely when the submergence ratio decreases. This means that in the process of open natural circulation system design, the submergence ratio of the system should be increased as much as possible. Finally, in this paper, it is found that the bubble pump with PS = 0.2 μm has better performance.

In order to investigate the seismic vibration effect on two-phase flow structure, experiments were performed for upwards bubbly flow in an annulus channel with and without externally-induced vibration. The inner and outer diameters of the annulus are 19.1 mm and 38.1 mm respectively, and the total height of the test section is 2.32 m. To simulate seismic vibrations, the test section is attached to an eccentric cam vibration module with an eccentricity of 9.5 mm. The eccentric cam rotation speed can reach up to 210 rpm. Local two-phase flow parameters were measured along radial direction within the annulus gap using miniaturized four-sensor conductivity probe at axial location of z / D h = 77. The semi-instantaneous local parameters at different vibration angles were analyzed by tracing the quasi-sinusoidal acceleration signal under vibration conditions. The results showed that the seismic vibration can significantly affect the local parameters in bubbly flow regime. Void fraction can increase by 10% compared with non-vibration condition. During the vibration cycle, the void fraction also changed greatly, especially in the near wall region. The interfacial area concentration (IAC) and Sauter mean diameter displayed the same behavior as the respective void fraction profiles.

An experimental study of the process of bubble detachment from a single capillary was performed. Glycerin was used as a test liquid. To study the effect of physical properties of liquid on the process of bubble formation the experiments were conducted at different temperatures. The dependencies of mean bubble diameter on gas flow rate, liquid temperature and liquid velocity along with the size of capillary are presented. The data on the most typical regimes of bubbly mixture formation are obtained. The bubble coalescence near the capillary is shown to be the process which determines the shape of bubble size distribution in the flow.

Earthquake is one of the most serious phenomena for safety of a nuclear reactor in Japan. Structural safety of nuclear reactors has been studied and nuclear reactors were contracted with structural safety for a big earthquake. However, it is not enough for safe operation of nuclear reactors because thermal-fluid safety is not confirmed under the earthquake. For instance, behavior of gas-liquid two-phase flow is unknown under the earthquake conditions. Especially, fluctuation of void fraction is an important factor for the safe operation of the nuclear reactor. In the previous work, fluctuation of void fraction in bubbly flow was studied experimentally and numerically. In case of the earthquake, the fluctuation is not only the flow rate, but also body force on the two-phase flow and shear stress through the pipe wall. Interactions of gas and liquid through their interface also act on the behavior of the two-phase flow. The fluctuation of the void fraction is not clear for such complicated situation under the earthquake. Our study has investigated the behavior of gas-liquid two-phase flow experimentally and numerically. In this paper effects of vibration on bubbly flow in the components and construct experimental database for validation and performs visualization experiments of a rising single bubble in a rectangular water tank on which sinusoidal vibration was applied. In this paper, results of visualized experiment evaluated by the visualization techniques, including positions of a bubble, a shape of the bubble and the bubble tilt angle were shown. In the results, bubble behavior were affected by the table oscillation. The bubble tilt angle is also almost same value of the bubble movement angle. It is implied that higher table oscillation frequency than 20 Hz quite weakly affects on fluctuation of bubble tilt angle frequency.

An earthquake is one of the most serious phenomena for the safety of a nuclear reactor in Japan. Therefore, structural safety of nuclear reactors has been studied and nuclear reactors ware contracted with structural safety for a big earthquake. However, it is not enough for safety operation of nuclear reactors because thermal-fluid safety is not confirmed under the earthquake. For instance, behavior of gas-liquid two-phase flow is unknown under the earthquake conditions. Especially, fluctuation of void fraction is an important factor for the safety operation of the nuclear reactor. In the previous work, fluctuation of void faction in bubbly flow was studied experimentally and theoretically, to investigate the stability of the bubbly flow. In such studies, flow rate or void fraction fluctuations were given to the steady bubbly flow. In the case of the earthquake, the fluctuation is not only the flow rate, but also a body force on the two-phase flow and a shear force through a pipe wall. Interactions of gas and liquid through their interface also act on the behavior of the two-phase flow. The fluctuation of the void fraction is not clear for such complicated situation under the earthquake. Therefore, in this research project, the behavior of gas-liquid two-phase flow is investigated experimentally and numerically in the series of study. In this study, to investigate the effects of vibration on bubbly flow in the components and construct an experimental database for validation, we performed visualization experiments of vertical bubbly flow in a rectangular water tank on which a sine wave vibration was applied. In this paper, results of visualized experiment evaluated by the visualization techniques, including positions of bubbles, shapes of bubbles and liquid velocity distributions around bubbles, were shown. And liquid velocity distribution around bubbles by the PIV measurement was also shown. In the results, bubble behaviors were affected by oscillation. And the cycle of the bubble tilt angle was almost same as the cycle of oscillation table velocity.