[en] Background: Bubble generator is a key device in the off-gas removal system of the molten salt reactor (MSR) for breaking up the carrier gas into tiny bubbles. Purpose: The distribution rules of velocity, pressure and turbulent kinetic energy along the flow direction of the Venturi bubble generator were analyzed with the software FLUENT. Methods: Based on the experimental water loop with the target bubble generator, numerical simulation using FLUENT was performed. Both the multiphase model (Mixture), standard k-ε turbulent model were adopted and the SIMPLEC method was employed to get the coupled solutions. Results: The distribution of velocity along the flow direction shows that the air is entrained by the water flew in the throat section and the air flew close to the wall of the generator. The speed of air is reduced significantly in the diverging portion of the bubble generator. The velocity gradient creates great shear stress, making the bubbles break up. The distribution of pressure shows that the peak of the pressure gradient appears at the entrance of the diverging portion of the bubble generator. The numerical results are very close to the data in the experiment, which indicates that the quick recovery of pressure may accelerate the breakup of bubble. The turbulent kinetic energy peaks at the entrance of the diverging portion of the bubble generator, which suggests that strong energy exchange there, resulting in strong shear stress to break up the bubbles. Conclusion: The large velocity gradient and the peak of turbulent kinetic energy at the entrance of the diverging portion of the bubble generator cause great shear stress, which is the reason for the bubble breakup taking place intensively there. (authors)

[en] The filtered venting system has the functions of preventing the overpressure breakage of a primary containment vessel of a reactor and reducing the release amount of the radioactive materials to the environment. One of the filtered venting systems is a Venturi scrubber to remove small particles of the radioactive materials. However, the physics of the two-phase flow in the Venturi scrubber has not been clarified. In this study, the experimental results of the water-vapor two-phase flow in a Venturi tube are reported. As the gas flow rate increased, the supply amount of scrubbing water decreased and eventually stopped because the pressure at the throat increased with increasing the gas density. This phenomenon was caused by the gas compressibility. By visualizing the two-phase flow in the Venturi tube, we cannot measure the spraying from the water supply port, which is considered to affect decontamination of the Venturi scrubber. However, it was confirmed that the water film on the wall at the diverging part of the Venturi tube, and the dispersed water droplets were formed intermittently from the water film. In addition, generation of the liquid at the divergent area was observed even though scrubbing water-supply stopped. (author)

[en] Highlights: • Self-priming occur because of pressure balance between inside and outside of throat is confirmed. • VS has similar flow with a Venturi tube except of disturbance and burble flow is considered. • Some of atomization simulated are validated qualitatively by comparison with previous studies. - Abstract: From the viewpoint of protecting a containment vessel of light water reactor and suppressing the diffusion of radioactive materials from a light water reactor, it is important to develop the device which allows a filtered venting of contaminated high pressure gas. In the filtered venting system that used in European reactors, so called Multi Venturi scrubbers System is used to realize filtered venting without any power supply. This system is able to define to be composed of Venturi scrubbers (VS) and a bubble column. In the VS, scrubbing of contaminated gas is promoted by both gas releases through the submerged VS and gas-liquid contact with splay flow formed by liquid suctioned through a hole provided by the pressure difference between inner and outer regions of a throat part of the VS. However, the scrubbing mechanism of the self-priming VS including effects of gas mass flow rate and shape of the VS are understood insufficiently in the previous studies. Therefore, we started numerical and experimental study to understand the detailed two-phase flow behavior in the VS. In this paper, to understand the VS operation characteristics for the filtered venting, we performed numerical simulations of two-phase flow behavior in the VS. In the first step of this study, we perform numerical simulations of supersonic flow by the TPFIT to validate the applicability of the TPFIT for high velocity flow like flow in the VS. In the second step, numerical simulation of two-phase flow behavior in the VS including self-priming phenomena. As the results, dispersed flow in the VS was reproduced in the numerical simulation, as same as the visualization experiments.

[en] A cavitating venturi in the auxiliary feed water system of a nuclear power plant is a flow control device that limits the maximum flow rate in critical conditions. In this research, a numerical study was conducted to predict the characteristics of the cavitating flow in a venturi with various geometrical configurations. Assuming axisymmetric geometry and a steady state, equations of the flow field were solved numerically. The mixture model for the multi-phase flow and the standard k-ε turbulence model were adopted. The primary diffuser angle was varied to perform detailed parametric study. The flow and pressure variations, shape of the cavitation bubble, and corresponding flow control characteristics were summarized. Finally, the calculation results were analyzed to provide a physical basis for the design limits proposed in a previous patent. It is expected that the results of this study will be useful information for the future design of an optimized venturi

[en] The working principle of Venturi purification device and its purification of aerosol have been described. Then, taking the gaseous iodine as an example, the absorption process of insoluble gas pollutants is discussed, the calculation methods of the gas-liquid contact area, mass transfer rate and efficiency of mass transfer are educed, and the factors that affect the efficiency of mass transfer are analyzed. (authors)

[en] Super oxygenated water (SOW) is an effective technology for remediation of hydrocarbon contaminants in groundwater. Two methods, both practical for application during field trials and full scale implementation across a site, have been developed for generating SOW and delivering SOW in-situ to a target treatment area. One method utilises a custom built SOW tower which has been designed and commissioned by Golder and a second method utilises venturi. Performance testing of the SOW tower and venturi was conducted to: Verify the efficiency of the SOW tower and the venturi in generating SOW. Compare the dissolved oxygen levels in water delivered by the SOW tower and the venturi. Assess and compare oxygen concentrations in atmosphere above the air-water interface. (author)

[en] As one of filtered venting systems which should be installed in light water reactors from the viewpoint of protecting a containment vessel and suppressing the diffusion of radioactive materials, there is a system composed of venturi scrubbers. The radioactive materials in the contaminated gas are collected into liquid. By forming dispersed flow in the venturi scrubber, interfacial area between liquid and gas is enhanced, finally, large decontamination factor is realized. In evaluation for the decontamination performance of the venturi scrubber, interface characteristics of droplets and liquid film are important. In this study, as a part of evaluation method of the interfacial area, the liquid film thickness in the venturi scrubber was measured. And evaluate the results of investigation experimentally for each ruffling average thickness and liquid film in a fluidized condition. The cross section area of a venturi scrubber is a rectangular one manufactured a transparent acrylic for visualization. In the venturi scrubber, a pressure drop occurs in the throat part by the inflow of air from the compressor. Water flows from the tank by a pressure difference between a suctioned hole with head pressure and a throat part. An annular spray flow is then formed in the venturi scrubber. (author)

[en] In the wake of Fukushima Daiichi nuclear disaster, reviews of the safety of nuclear facilities have been conducted in the world beginning with Japan. Countermeasures against severe accidents in nuclear power plants are an urgent need. In particular, from the viewpoint of protecting containment and suppressing diffusion of the radioactive materials, it is important to install filtered venting devices to release high pressure pollutant gas to the atmosphere with elimination radioactive materials in the gas. One of the devices for the filtered venting is a Multi venturi scrubber system (MVSS), which is used to realize filtered venting without any power supply in European reactors. The MVSS is composed of a “venturi Scrubbers” part, in which there are hundreds of the venturi scrubbers, and a “bubble column” part. In the MVSS, all of the venturi scrubbers is branched off from a vent line which connect between the containment and the MVSS. In an operation mode of the MVSS, the radioactive materials are eliminated through the gas-liquid interface from the pollutant gas to the liquid phase of a dispersed flow in the venturi scrubber and a bubbly flow in the bubble column part. The dispersed flow is formed from the liquid, which is suctioned from around the venturi scrubber through the hole for suction (called self-priming). In previous studies, an evaluation method for the scrubbing performance of the venturi scrubber was developed. However, actual hydraulic behavior in it is too complicated, the previous evaluation was not validated the hydraulic behavior and studied the effect of differences between the simulated hydraulic behavior and an actual one on the performance of the venturi scrubber. To develop a validated evaluation method for the scrubbing performance, it is important to develop detailed evaluation method for the hydraulic behavior in the venturi scrubber. To simulate the complicated hydraulic behavior, we consider to use analysis code TPFIT. Then, the objective is to validate the hydraulic behavior simulated by TPFIT. As approaches, numerical analysis by TPFIT under air-water conditions was conducted and its hydraulic behaviors were compared with the previous studies qualitatively. (author)

[en] Highlights: • A computational model to predict the efficiency of venturi scrubber is developed. • Efficiency increased with increase in gas velocity and liquid injection velocity. • Dimensionless Performance Parameter is found to increase with increasing efficiency. • Improved efficiency for multi-stage and variable injection diameter system is presented. - Abstract: To understand the performance of a venturi scrubber operating in self-priming mode, the present study develops a computational model to simulate the interacting three phase flow occurring in the venturi scrubber. The computational model is based on an Euler- Lagrangian framework where parcels of droplets and aerosols are tracked throughout the flow domain. The model incorporates the effect of droplet collision and increased momentum source in the regions of high liquid concentration. Additionally, the droplet inertial effects on the shear induced multi-jet breakup is included to predict the evolution of the droplet size distribution. The predicted local liquid loading, pressure drop and the collection efficiency were found to be in good match with the cited experimental values. The jet penetration and the droplet mean diameter is combined to form a unique dimensionless number, called the Performance Parameter, to quantify the multi-jet characteristics in the throat section of the scrubber. Higher values of Performance Parameter corresponded to higher efficiencies of the scrubber, hence serves as an analytical tool to assess the performance of the scrubber. Based on the parametric study for different throat gas velocities, liquid injection velocities, changing throat length and changing orifice diameter, the applicability of such a scrubber to self-priming mode is discussed. Further, for transients incurring lower gas velocities and lower operating height, the advantages of multi-stage injection and variable injection diameter are presented.

[en] Highlights: •Static pressure distributions at nozzle exit are studied under different gas velocity. •Static pressure distributions at liquid channel cross section are studied under no injection and injection conditions. •The influence of static pressure distributions on injection performance is investigated. •Feasible structure design to further improve injection flow rate is proposed. -- Abstract: The venturi scrubber working in self-priming mode is one of the most efficient gas cleaning devices to remove the radioactive particles and gases from gaseous stream during severe accident in nuclear power plant. This paper focus on improving injection performance in a split type self-priming venturi scrubber, the static pressure distributions at gas and liquid channels in the scrubber are studied emphatically, the experimental results indicate that pressure at the center of nozzle exit is lower than at the wall, and the variation laws in radial are different with increasing gas velocity. When the average gas velocity at throat U_avg = 64.3 m/s, the static pressure difference between center and wall is 2.1 KPa; with the increase of gas velocity, pressure at centre and wall reduce gradually and the pressure difference become significantly, when the average gas velocity U_avg = 225.8 m/s, pressure at the center is lower 23.2 KPa than at the wall, relative deviation is about 45.6%. However, when U_avg ≥ 230 m/s, the static pressure is not continue to decrease but reverse to recover with increasing gas velocity, and recovery rate at the wall is greater than at the center. The condition before the transition point (U_avg U_avg ≥ 230 m/s) is defined as resistance dominate area, while pressure in this area will reverse to recover, the venturi scrubber design should be ensured in velocity dominate area. The injection performance of self-priming venturi scrubber is closely relate to pressure distribution characteristics at nozzle exit, in condition of no injection or injecting air, pressure at liquid channel is consistent with the pressure at the wall of nozzle exit, which is higher than the average static pressure; when injecting water, an additional pressure increment will generate at liquid channel duo to the momentum exchange between gas and liquid, and lead to the effective pressure difference for injection reduce further. On this occasion, the influence of liquid channel area and resistance coefficient on injection performance become important, increase liquid channel area is effective for improving injection flow rate.