Lift Force Model Research Articles

Numerical Simulation of Boiling Two-Phase Flow in Tight-Lattice Rod Bundle by 3-Dimensional Two-Fluid Model Code ACE-3D

Two-fluid model can simulate two-phase flow by computational cost less than detailed two-phase flow simulation method such as interface tracking method or particle interaction method. Therefore, two-fluid model is useful for thermal hydraulic analysis in large-scale domain such as a rod bundle. Japan Atomic Energy Agency (JAEA) develops three dimensional two-fluid model analysis code ACE-3D that adopts boundary fitted coordinate system in order to simulate complex shape flow channel. In this paper, boiling two-phase flow analysis in a tight-lattice rod bundle was performed by ACE-3D code. The parallel computation using 126 CPUs was applied to this analysis. In the results, the void fraction, which distributes in outermost region of rod bundle, is lower than that in center region of rod bundle. The tendency of void fraction distribution agreed with the measurement results by neutron radiography qualitatively. To evaluate effects of two-phase flow model used in ACE-3D code, numerical simulation of boiling two-phase in tight-lattice rod bundle with no lift force model was also performed. From the comparison of calculated results, it was concluded that the effects of lift force model were not so large for overall void fraction distribution of tight-lattice rod bundle. However, the lift force model is important for local void fraction distribution of fuel bundles.

Lift force in bubbly flow systems

From the significance of three-dimensional simulation of dispersed flow systems in many engineering fields, extensive study was conducted for lift force in a single particle system as well as a multiparticle system. In this study, the lift force in a single particle system was modeled by considering the effect of bubble deformation on the lift force. The model was finalized based on existing data obtained in the range of particle Reynolds number from 3.68 to 78.8, viscous number from 0.0435 to 0.203 and Eötvös number from 1.40 to 5.83. The viscous number is defined by μ f / [ ρ f σ σ / ( g Δ ρ ) ] 1 / 2 where μ f , ρ f , σ , g and Δ ρ are, respectively, fluid viscosity, fluid density, surface tension, gravitational acceleration and density difference between phases. The applicability of the model to higher particle Reynolds number system such as an air–water system was qualitatively examined. The lift force model developed in a single particle system was extended to a multiparticle system. The applicability of the extended lift force model was qualitatively examined. The similarity between drag and lift forces were also discussed.

Quasi-direct numerical simulation of lift force-induced particle separation in a curved microchannel by use of a macroscopic particle model

The macroscopic particle model (MPM) based on the finite volume method is employed to validate a mechanism of lift force-induced particle separation in a curved microchannel. According to the particle velocity at each time step in the unsteady simulation, the MPM gives momentum to those fluid cells touching the particle physical boundary. The summation of the given momentum with the reversed sign is divided by the time step to obtain the hydrodynamic force acting on the particle. Namely, the existence and motion of the particle causes fluid flow around the particle, while the flow field caused by the particle determines the particle motion by means of the hydrodynamic force. Therefore, the MPM can be regarded as implementing a quasi-direct numerical simulation over the static computational cells. The lift force acting on a spherical particle in a shear flow is a purely hydrodynamic force caused by the flow field around the particle. It is expected, therefore, that the MPM could predict the lift force effect without any additional model. At first, it is shown that the MPM is capable of predicting particle migration away from the wall of a straight microchannel due to the lift force. In a curved microchannel, subsequently, the particle trajectories from representative release points predicted by the MPM are compared to those predicted by a common particle tracking method without any lift force model. The MPM predicted that the particle trajectories are confined in the outer region of the channel cross-section. On the other hand, the circulating trajectories predicted by the tracking method tend to expand due to centrifugal force caused by the Dean vortices. It is concluded, therefore, that the lift force due to the steep shear rate is a significant factor to cause particle separation in a curved microchannel.

A Numerical Study of the Influence of Particle Density on Lift Force-Induced Separation in a Micro-Separator/Classifier by a Macroscopic Particle Model

A Macroscopic Particle Model (MPM), which can be regarded as a quasi direct numerical simulation and hence does not need any drag and lift force models, is applied to examine the effect of particle density on lift force-induced separation in a micro-separator/classifier. The computational domain is a 30-degrees arc channel with a radius of 20 mm connecting to 5-mm straight channels at the both ends. In this study the modeled is an upper-half of the channel with a width of 200 μm and a depth of 75 μm. The particle diameter was chosen to be 20 μm, which is well separable in experiments and is suitable for MPM in which a particle should contain several fluid cells. The density was examined in the range of slightly lighter to denser, compared with water. The particle trajectories from representative points are predicted by MPM and also by a traditional particle tracking method (DPM) for comparison. The DPM without a lift force model predicted that the trajectories expanded over the cross-sectional plane with an increase of particle density since a secondary flow pattern called Dean vortices caused strong centrifugal force acting outwards from its center. On the other hand, the trajectories predicted by MPM were almost confined in the outer-half of the plane due to the lift force acting inwards to the vortex center. It seems that encountering lift force due to a steep shear and centrifugal force due to the Dean vortices are balanced regardless of the particle density since both forces likely increase with particle inertia. It is notable that particles can be hydraulically separable regardless of the density ranging slightly lighter to denser compared to the medium.

On the influence of near-wall forces in particle-laden channel flows

Computation of a turbulent dilute gas–solid channel flow has been undertaken to study the influence of using wall-corrected drag coefficients and of the lift force on the dispersed phase characteristics. The incompressible Navier–Stokes equations governing the carrier flow were solved by using a direct numerical simulation approach and coupled with a Lagrangian particle tracking. Calculations were performed at Reynolds number based on the wall-shear velocity and channel half-width, Re τ ≈ 184, and for three different sets of solid particles. For each particle set, two cases were examined, in the first one the particle motion was governed by both drag and lift wall-corrected forces, whereas in the other one, the standard drag force (not corrected) was solely acting. The lift force model used represents the most accurate available expression since it accounts for weak and strong shear as well as for wall effects. For this study, we considered elastic collisions for particles contacting the walls and that no external forces were acting. Present results indicate that the use of the lift force and of the drag corrections does not lead to significant changes in the statistical properties of the solid phase, at the exception of some statistics for the high inertia particles.

Influence of the lift force in direct numerical simulation of upward/downward turbulent channel flow laden with surfactant contaminated microbubbles

In this work, we employ direct numerical simulation of turbulence one-way coupled to Lagrangian tracking to investigate microbubble distribution in upward and downward channel flow. We consider a closed channel flow at Re τ = 150 and a dispersion of microbubbles characterized by a diameter of 220 μ m . Bubbles are assumed contaminated by surfactants (i.e., no-slip condition at bubble surface) and are subject to drag, gravity, pressure gradient forces, Basset history force and aerodynamic lift. Our results confirm previous findings and show that microbubble dispersion in the wall region is dominated by the action of gravity combined with the lift force. Specifically, in upward flow, bubble rising velocity in the wall region generates a lift force which pushes bubbles to the wall. In downward flow, bubble rising velocity against the fluid generates a lift force which prevents microbubbles from reaching the viscous sublayer. In the wall region, we observe bubble preferential segregation in high-speed regions in the downflow case, and non-preferential distribution in the upflow case. This phenomenon is related to the effect of the lift force. Compared to experiments, the current lift force model produces larger consequences, this effect being overemphasized in the upflow case in which a large number of bubbles is segregated near the wall. In this case, the resulting bubble wall-peak of concentration outranges experimental results. These results, so deeply related to the lift force, underline the crucial role of current understanding of the fluid forces acting on bubbles and help to formulate questions about available force models, bubble–bubble interactions and two-way coupling which can be crucial for accurate predictions in the region very near the wall.

Controlling bubbles

In this short overview we report on our ongoing work on the dynamics of bubblesin various flows. Three different situations are explored: the competition betweenacoustic and hydrodynamics forces in a vertical pipe (Rensen J, Bosman D,Magnaudet J, Ohl C D, Prosperetti A, Tögel R, Versluis M and Lohse D 2001Phys. Rev. Lett. 86 4819), a rising bubble on which shape oscillations have beeninduced (de Vries J, Luther S and Lohse D 2002 Eur. J. Phys. B 29 503),and a bubble in a rotating horizontal cylinder. Whereas for the first twosituations the standard bubble force models (Magnaudet J and Eames I 2000Annu. Rev. Fluid Mech. 32 659) are consistent with our measurements,modifications for the lift force model seem to be required in the last case.

Estimation of wind loads on VLFS of semisubmersible type

This paper presents the results of investigation of wind lift force on VLFS of semisubmersible type. In the present study, wind–wave tank experiments on the scale model were also conducted in regular progressive waves. Lift forces and wind pressure were measured as a function of column diameter, unit number, superstructure and wave condition, and divided into time-averaged mean and fluctuating components. The importance of lift force, especially at the leading edge of deck, was verified in the experimental results. The results showed that fluctuation in lift force which was caused by interaction between the wind and wave was significantly large in typhoon conditions. Measured lift forces were linearly related to fluctuation of wind velocity. A lift force model is further proposed for the estimation of lift forces on VLFS of semisubmersible type and the effectiveness of this model was confirmed by the present experimental data.

Effets de parois sur la distribution de taux de vide dans les écoulements à bulles

On the basis of numerical simulations of bubbly flows, we evaluated the role of turbulence and interfacial forces in the void fraction distribution near the wall. The simulations of pipe flow in micro-gravity situations show that the turbulent contributions issued from the added mass force play an important role in the phase distribution phenomenon. In normal gravity, the simulations of the boundary layer and pipe bubbly flows allow us to adjust the lift force model in which we take into account the wall interaction effect.