Takeo Kajishima

Large-eddy simulation of turbulent gas–particle flow in a vertical channel: effect of considering inter-particle collisions

The interaction between a turbulent gas flow and particle motion was investigated by numerical simulations of gas–particle turbulent downward flow in a vertical channel. In particular the effect of inter-particle collision on the two-phase flow field was investigated. The gas flow field was obtained by large-eddy simulation (LES). Particles were treated by a Lagrangian method, with inter-particle collisions calculated by a deterministic method. The spatial resolution for LES of gas–solid two-phase turbulent flow was examined and relations between grid resolution and Stokes number are presented. Profiles of particle mean velocity, particle wall-normal fluctuation velocity and number density are flattened as a result of inter-particle collisions and these results are in good agreement with experimental measurements. Calculated turbulence attenuation by particles agrees well with experimental measurements for small Stokes numbers, but not for large Stokes number particle. The shape and scale of particle concentrations calculated considering inter-particle collision are in good agreement with experimental observations.

Interaction between particle clusters and particle-induced turbulence

Abstract To investigate the two-way interaction between particles and fluid turbulence, a homogeneous flow field including solid particles was numerically simulated. Spherical particles are falling by gravity with the Reynolds number ranging from 50 to 400, based on slip velocity. Particular attention was focused on the clustering of particles, which might enhance turbulence by energy supply through larger scales in comparison with dispersed particles. In the higher Reynolds number case, particle clusters are formed due to the wake, and vortex shedding enhances them. But clusters cause fluid turbulence resulting in their break-up. The Reynolds-number dependence, the dynamics and the time scale of particle clusters are discussed in this paper.

Finite-difference immersed boundary method consistent with wall conditions for incompressible turbulent flow simulations

An immersed boundary method to achieve the consistency with a desired wall velocity was developed. Existing schemes of immersed boundary methods for incompressible flow violate the wall condition in the discrete equation system during time-advancement. This problem arises from the inconsistency of the pressure with the velocity interpolated to represent the solid wall, which does not coincide with the computational grid. The numerical discrepancy does not become evident in the laminar flow simulation but in the turbulent flow simulation. To eliminate this inconsistency, a modified pressure equation based on the interpolated pressure gradient was derived for the spatial second-order discrete equation system. The conservation of the wall condition, mass, momentum and energy in the present method was theoretically demonstrated. To verify the theory, large eddy simulations for a plane channel, circular pipe and nuclear rod bundle were successfully performed. Both these theoretical and numerical validations improve the reliability and the applicability of the immersed boundary method.

One-Equation Subgrid Scale Model Using Dynamic Procedure for the Energy Production

The transport equation of subgrid scale (SGS) kinetic energy, K SGS , is used for the large-eddy simulation (LES), considering its consistency with dynamic procedure. The dynamically determined parameter is suitable for describing the energy transfer from resolved turbulence to SGS portion. Thus the procedure is applied to the production term in the transport equation of K SGS , while the eddy viscosity in the filtered equation of motion is determined indirectly through K SGS . The statistically derived model for K SGS equation is adopted for the basis of our improvement. Computational examination has been conducted for fully developed turbulent flow in a plane channel. Agreement with DNS database was satisfactory. Moreover, in a channel on solid body rotation, our model reasonably reproduced the decay of SGS turbulence in the vicinity of the suction side.

Coupling between mass transfer from dissolving bubbles and formation of bubble-surface wave

The dynamic process of gas dissolution from a single CO 2 -containing bubble into a liquid is studied for bubbles with surface oscillations. Both experimental and theoretical analyses are conducted. In the experiment, the bubble behavior is continuously monitored by holding the bubble in the liquid flowing downward. For detailed observation of the bubble-surface motion, high-speed imaging is applied. The fluctuating surface, often associated with bubble deformation, is viewed as a surface with propagating waves. The dissolution rate, claimed to be largely influenced by the liquid-phase contamination level, is found to depend more inherently on the extent of wave motion. The persisting wave propagation is only observable for large bubbles with distinctive rim. The lower critical diameter is roughly 4 mm. The theoretical analysis lays its basis on modeling the dissolution process of a single bubble comprising two components of different solubility. The model demonstrates good predictive capability for the time variations in bubble diameter and gas-phase composition, the latter being measured via gas chromatography by sampling the gas from the suspended bubble. For pure CO 2 bubbles in the liquid pressurized using N 2 , in particular, the complete dissolution process is characterized by little change in the CO 2 mole fraction near unity followed by a transitory decrease down to zero.

Mass transfer and structure of bubbly flows in a system of CO2 disposal into the ocean by a gas-lift column

A new method for ocean sequestration of low-purity CO 2 gas emitted from fired power plant is developed. This is a gas-lift pump system, named progressive gas lift advanced dissolution (P-GLAD) system, to dissolve only CO 2 gas of combustion gas in seawater at shallow waters and to transport CO 2 -rich seawater to great depths. The system is an inverse-J pipeline set at the ocean at a depth between 200 and 3000 m and a releasing system of indissoluble gas. To improve the efficiency of the P-GLAD, one should elucidate bubbly flows accompanying gas phase dissolution formed in the gas-lift column of the system. In the present paper, first, the authors discuss mass transfer in bubbly flows of pure CO 2 gas and filtrated tap water along the pipe axis in laboratory-scale P-GLAD of 25 mm in diameter and 7.69 m in height. Second, mass transfer in bubbly flows of mixed gas (95% volume of CO 2 and 5% volume of pure air) and filtrated tap water in the same setup is discussed. The mass transfer coefficient of CO 2 in the later system has the values of 0.00031-0.000085. It is shown that the mass transfer coefficient is a function of the distance from the gas injection. Finally, the performance of the system is elucidated on the basis of the experimental and numerical investigations. The laboratory-scale P-GLAD dissolved over 98.5% of CO 2 injected in the liquid phase.

GLAD: A gas-lift method for CO2 disposal into the ocean

To mitigate global warming, we have proposed the GLAD (Gas-Lift Advanced Dissolution) system for CO2 release into deep seawater. It is an inverse-J pipeline set in the sea at a depth of 200–400m. CO2 bubbles injected into the pipe form a buoyant plume and dissolve into the seawater as they rise. This dense solution is released from the other side of the pipe. The feasibility of our method has been examined by the numerical simulation of gas-liquid, two-phase flow with a CO2 dissolution model. In the present paper, the performance and cost of the GLAD system are discussed based on a model plant.

A gas-lift system for CO2 release into shallow seawater

The GLAD system is proposed for the injection and dissolution of CO 2 gas into the ocean. It is a pipeline of inverse U shape settled in the shallow sea. CO 2 bubbles injected into the pipe form a buoyant plume, but they dissolve while rising. The dense solution is released from the other side of the pipe. In the present paper, possibility of our concept is confirmed by the numerical simulation of gas-liquid two-phase flow with a primitive model of CO 2 dissolution

Design factors in gas-lift advanced dissolution (GLAD) system for CO2 sequestration into the ocean

A new method for ocean sequestration of low purity CO 2 gas emitted from thermal power plants has been developed. The method utilizes a gas-lift pump system, named gas lift advanced dissolution (GLAD) system, that dissolves the CO 2 (which has been separated from combustion gas) into seawater at a relatively shallow depth of 200-300 m and then transports CO 2 -rich seawater to a depth greater than 1000 m. The CO 2 concentration of seawater after sequestration shall be limited to a certain value (e.g. 3 mol/m 3 ) so as to minimize the impact on marine life. This paper describes the numerical simulation model of GLADs two-phase flow with a CO 2 dissolution that was used to determine the optimal specification of the system. This numerical simulation resulted in the following conclusions: (1) The required length for full dissolution in the dissolution pipes is proportional to the diameter of the injection bubble, and increases linearly with the decrease in the purity of CO 2 gas. (2) The allowable injection rate of CO 2 gas, for a marine life, is proportional to the sectional area of dissolution pipe and increases with the increase in impurity (=1-purity) of the CO 2 gas and the diameter of the injection bubble. (3) Analysis of correlations among these factors will enable system optimization on the total cost basis including the costs for CO 2 separation from exhaust gas, transportation of CO 2 gas to the GLAD site and CO 2 sequestration.

The Effect of Mixing-vane Arrangements in a Subchannel Turbulent Flow

Large eddy simulation (LES) of developed turbulent flows in a rod bundle was carried out for four spacer designs. The mixing-vanes attached at the spacer were inclined at 30° or 20° they were arranged to promote the swirling or convective flow. These arrangements are possible elements to compose an actual rod bundle. Our LES technique with a consistent higher-order immersed boundary method and a one-equation dynamic sub-grid scale model contributed to an efficient treatment of the complex wall configurations of rods and spacers. The computational results reasonably reproduced experimental results for the drag coefficient and the decay rate of swirling flow. The profiles of the axial velocities and the turbulence intensities indicated reasonable trend for the turbulent flow in the rod bundle. The effect of mixing-vane arrangement on the lateral flows was successfully clarified: the cross flow took the longer way on the rod surface than the swirling flow and then was more significantly influenced by momentum diffusion at the no-slip wall. Therefore, the largely inclined mixing-vanes promoted the cross flow only in the neighborhood of the spacer; the swirling flow inside a subchannel could reach farther downstream than the cross flow.