[en] Highlights: • The microfluidic holding and coalescence behaviors of droplets subjected to an optical trap within a channel flow were studied numerically. • The LBM multiphase model was used to obtain a two-phase flow field. • A sufficiently high laser power was required to capture an incoming droplet and induce coalescence with the subsequent droplet. • The effects of various flow and optical parameters on the lagging distance were investigated. - Abstract: The microfluidic holding and coalescence behaviors of droplets subjected to an optical trap within a channel flow were numerically studied by using the lattice Boltzmann method and the dynamic ray tracing. A tightly focused Gaussian laser beam positioned laterally with respect to the channel flow direction was used as the optical trap. In such a system, seriate droplet coalescence was observed between an optically trapped droplet and the subsequent droplet. The numerically predicted droplet coalescence behavior agreed well with the experimental results. A sufficiently high laser power was required to capture an incoming droplet and induce coalescence with the subsequent droplet. The effects of various flow and optical parameters on the coalescence behavior were investigated.

[en] Highlights: • Performance analysis of a multi-stage safety injection tank by a numerical method. • MSIT can inject coolant from a single tank in multiple stages during a LOCA. • Effects of break size, heights, and pressure-loss coefficients of injection lines. • MSIT can provide continuous coolant injection without problems of delays. A performance analysis of a multi-stage safety injection tank (MSIT) was carried out using a numerical method. A MSIT is capable of injecting coolant from a single tank in multiple stages in the event of a loss of coolant accident (LOCA) at a nuclear power plant. The performance and consequences of two- and three-stage MSITs were compared. The effects of break size, installation heights of the injection lines, and pressure-loss coefficients of each injection line were scrutinized. By selecting pertinent design parameters, the MSIT can satisfy its design requirements, by providing continuous coolant injection without problems of delays or overlaps for a minimum of 72 h after a LOCA.

[en] The behavior of a microdroplet in a uniform flow and subjected to a vertical optical force applied by a loosely focused Gaussian laser beam was studied numerically. The lattice Boltzmann method was applied to obtain the two-phase flow field, and the dynamic ray tracing method was adopted to calculate the optical force. The optical forces acting on the spherical droplets agreed well with the analytical values. The numerically predicted droplet migration distances agreed well with the experimentally obtained values. Simulations of the various flow and optical parameters showed that the droplet migration distance nondimensionalized by the droplet radius is proportional to the S number (z_d/r_p = 0.377S), which is the ratio of the optical force to the viscous drag. The effect of the surface tension was also examined. These results indicated that the surface tension influenced the droplet migration distance to a lesser degree than the flow and optical parameters. The results of the present work hold for the refractive indices of the mean fluid and the droplet being 1.33 and 1.59, respectively

[en] Highlights: • Flow in fibrous porous media is studied using lattice Boltzmann method. • Multiple-relaxation-time (MRT) LBM scheme is used. • The structures of overlapping fibers yield 2.5 times larger permeability. • Fiber arrangement affects the permeability only when slip flow occurs. -- Abstract: The permeabilities of microscale fibrous porous media were calculated using the multiple-relaxation-time (MRT) lattice Boltzmann method (LBM). Two models of the microscale fibrous porous media were constructed based on overlapping fibers (simple cubic, body-centered cubic). Arranging the fibers in skew positions yielded two additional models comprising non-overlapping fibers (skewed simple cubic, skewed body-centered cubic). As the fiber diameter increased, the fibers acted as granular inclusions. The effects of the overlapping fibers on the media permeability were investigated. The overlapping fibers yielded permeability values that were a factor of 2.5 larger than those obtained from non-overlapping fibers, but the effects of the fiber arrangement were negligible. Two correlations were obtained for the overlapping and non-overlapping fiber models, respectively. The effects of the rarefaction and slip flow are also discussed. As the Knudsen number increased, the dimensionless permeability increased; however, the increase differed depending on the fiber arrangement. In the slip flow regime, the fiber arrangement inside the porous media became an important factor