Yasuo Ose

Direct numerical simulation of turbulent channel flow under a uniform magnetic field for large-scale structures at high Reynolds number

A direct numerical simulation (DNS) of turbulent channel flow with high Reynolds number has been carried out to show the effects of the magnetic field. In this study, the Reynolds number for channel flow based on bulk velocity Ub, viscosity ν, and channel width 2δ was set to be constant; Reb=2δUb∕ν=45818. A uniform magnetic field was applied in the direction of the wall normal. The value of the Hartmann number, Ha were 32.5 and 65, where Ha=2δB0σ∕ρν. The turbulent quantities such as the mean flow, turbulent stress, and turbulent statistics were obtained by DNS. Although the influence of the magnetohydrodynamic dissipation terms in the turbulent kinetic energy budget was small, large-scale turbulent structures, e.g., vertical structures, low-speed streaks, ejection, and sweep, were found to decrease at the central region of the channel. Consequently, the difference between production and dissipation in the turbulent kinetic energy decreased with increasing Hartmann number at the central region and large-sc...

Numerical study on direct-contact condensation of vapor in cold water

Abstract Numerical analyses were carried out to predict quantitatively the condensation characteristics between the water and vapor inside a suppression tank in the International Thermonuclear Experimental Reactor (ITER). The suppression tank is one of the ITER safety devices. The pressure rise in an ITER vacuum vessel during an ingress-of-coolant event (ICE) is suppressed by the effect of condensation of vapor in the suppression tank. The water–vapor two-phase flow characteristics with condensation under the ITER pressure conditions were clarified numerically. Moreover, flow visualization experiments were carried out to understand the condensation behavior under low pressure and cold water. The predicted flow configurations agreed well with the experimental results.

Direct numerical simulation of carbon-dioxide gas absorption caused by turbulent free surface flow

Abstract The turbulent behavior of liquid and gas flows at a free surface is not very well understood in comparison with that of the motions at the solid boundary. In the present study, the DNS for the turbulent free surface flow with a shear wind has been carried out by means of a coupled gas–liquid flow solution procedure, i.e., multi-interface and advection and reconstruction solver (MARS) developed by one of the authors. Henrys law is applied to the evaluation of a saturated gas concentration at the free surface caused by the carbon-dioxide gas absorption. Resulting from the present DNS of turbulent free surface flow with carbon-dioxide gas transfer at the gas–water interface, the mean and root-mean-square distributions of velocity, pressure and concentration and their fluctuations throughout both the gas and liquid flow fields are obtained and compared with the experimental data. Finally, an exchange coefficient of carbon-dioxide gas at the turbulent free surface estimated in this study is in good agreement with the existing experimental and sea-measurement data.

Numerical Study on Subcooled Pool Boiling

*This study focuses on the clarification of the heat transfer characteristics of the subcooled pool boiling, the discussion on its mechanism, and the establishment of a boiling and condensation model for direct numerical simulation on the subcooled pool boiling phenomena. In this paper, three dimensional numerical simulations based on the MARS (Multi-interface Advection and Reconstruction Solver) with a boiling and condensation model which consisted of the improved phase-change model and the relaxation time based on the quasi-thermal equilibrium state have been conducted for the bubble growth process in the subcooled pool boiling. The numerical results regarding the bubble growth process of the subcooled pool boiling show in good agreement with the experimental observation results and the existing analytical equations among Rayleigh, Plesset and Zwick and Mikic et al. Therefore, it was found that the improved boiling and condensation model with the relaxation time consideration can predict the bubble growth process of the subcooled pool boiling phenomena.

Direct Numerical Simulation and Visualization of Subcooled Pool Boiling

A direct numerical simulation of the boiling phenomena is one of the promising approaches in order to clarify their heat transfer characteristics and discuss the mechanism. During these decades, many DNS procedures have been developed according to the recent high performance computers and computational technologies. In this paper, the state of the art of direct numerical simulation of the pool boiling phenomena during mostly two decades is briefly summarized at first, and then the nonempirical boiling and condensation model proposed by the authors is introduced into the MARS (MultiInterface Advection and Reconstruction Solver developed by the authors). On the other hand, in order to clarify the boiling bubble behaviors under the subcooled conditions, the subcooled pool boiling experiments are also performed by using a high speed and high spatial resolution camera with a highly magnified telescope. Resulting from the numerical simulations of the subcooled pool boiling phenomena, the numerical results obtained by the MARS are validated by being compared to the experimental ones and the existing analytical solutions. The numerical results regarding the time evolution of the boiling bubble departure process under the subcooled conditions show a very good agreement with the experimental results. In conclusion, it can be said that the proposed nonempirical boiling and condensation model combined with the MARS has been validated.

Large Scale Structures of Turbulent Shear Flow via DNS

A direct numerical simulation (DNS) of turbulent channel flow has been carried out to understand the effects of Reynolds number. In this study, the Reynolds number for channel flow based on a friction velocity and channel half width was set to be constant; Re τ = 1100. The number of computational grids used in this study was 1024 x 1024 x 768 in the x-, y- and z -directions, respectively. The turbulent quantities such as the mean flow, turbulent stresses and the turbulent statistics were obtained via present DNS. Large scale turbulence structures visualized by paralleled AVS/Express appear in whole region. The structures are merged by small scales structures.

Three-dimensional analysis of water-vapor void fraction in a fusion experimental reactor under water ingress

Abstract An integrated Ingress-of-Coolant Event (ICE) test facility was constructed to demonstrate that the International Thermonuclear Experimental Reactor (ITER) safety design approach and design parameters for the ICE are adequate. The integrated ICE test facility simulates the actual ITER components with a scaling factor of 1/1600. Before the integrated ICE experiments the water–vapor two-phase flow characteristics inside the test facility during the ICE were predicted by the three-dimensional numerical simulations using the modified Transient Reactor Analysis Code (TRAC). From the present study it was clarified numerically that the actual ITER safety approach during the ICE is adequate and the present numerical method is very effective to predict the water–vapor void fraction during the ICE.

Numerical Simulation on Subcooled Pool Boiling

In this paper, the numerical simulations based on the MARS (Multi-interface Advection and Reconstruction Solver) with a phase-change model including the bubble growth and the condensation processes are performed. As the results, it was found that the numerical results for both bubble growth and condensation rates were very slow compared with the experimental results and the existing analytical model. In order to solve this discrepancy, the original model is improved by introducing the following models based on the quasi-thermal equilibrium state: (1) the improved phase-change model including the large density change between water and vapor; (2) a relaxation time model derived by considering the unsteady heat conduction. Resulting from the numerical simulation with the present improved model, the numerical results in both bubble growth and condensation processes show in good agreement with the experimental results and the existing analytical model.

Numerical Study on Mass Transfer of a Vapor Bubble Rising in Very High Viscous Fluid

This study focused on a bubble rising behavior in a molten glass because it is important to improve the efficiency of removal of bubbles from the molten glass. On the other hand, it is expected that some gas species which exists in a bubble are transferred into the molten glass through the bubble interface, i.e., the mass transfer, subsequently, it may cause a bubble contraction in the molten glass. In this paper, in order to understand the bubble rising behavior with its contraction caused by the mass transfer through the bubble interface in the very high viscous fluid such as the molten glass, a bubble contraction model has been developed. The direct numerical simulations based on the MARS (Multi-interface Advection and Reconstruction Solver) coupled with the mass transfer equation and the bubble contraction model regarding the mass transfer from the rising bubble in very high viscous fluid have been performed. Here, the working fluids were water vapor as the gas species and the molten glass as the very...

Numerical Study on Bubble Behaviour and Heat Transfer Characteristics of Subcooled Pool Boiling Based on Non-empirical Boiling and Condensation Model

In this study, the transient three-dimensional numerical simulations based on the MARS (Multi-interface Advection and Reconstruction Solver) with the non-empirical boiling and condensation model have been conducted for isolated boiling bubble behaviour in a subcooled pool. The effects of the wettability on the heating surface for the subcooled bubble departure behaviour were investigated. The numerical results showed in very good agreement with the experimental results. Furthermore, resulting from the wall heat flux evaluation, it was found that the wall heat flux near the contact line at the bottom of the bubble just before the bubble departing from the heating surface increases with increases of the degree of subcooling.