Suppressing Viscous Fingering in Porous Media with Wetting Gradient.

The viscous fingering phenomenon often occurs when a low-viscosity fluid displaces a high-viscosity fluid in a homogeneous porous media, which is an undesirable displacement process in many engineering applications. The influence of wetting gradient on this process has been studied over a wide range of capillary numbers (7.5 × 10-6 to 1.8 × 10-4), viscosity ratios (0.0025 to 0.04), and porosities (0.48 to 0.68), employing the lattice Boltzmann method. Our results demonstrate that the flow front stability can be improved by the gradual increase in wettability of the porous media. When the capillary number is less than 3.5 × 10-5, the viscous fingering can be successfully suppressed and the transition from unstable to stable displacement can be achieved by the wetting gradient. Moreover, under the conditions of high viscosity ratio (M > 0.01) and large porosity (Φ > 0.58), wetting gradient improves the stability of the flow front more significantly.

Turbulent CFD Simulation of Two Rotor-Stator Agitators for High Homogeneity and Liquid Level Stability in Stirred Tank.

Good solid-liquid mixing homogeneity and liquid level stability are necessary conditions for the preparation of high-quality composite materials. In this study, two rotor-stator agitators were utilized, including the cross-structure rotor-stator (CSRS) agitator and the half-cross structure rotor-stator (HCSRS) agitator. The performances of the two types of rotor-stator agitators and the conventional A200 (an axial-flow agitator) and Rushton (a radial-flow agitator) in the solid-liquid mixing operations were compared through CFD modeling, including the homogeneity, power consumption and liquid level stability. The Eulerian-Eulerian multi-fluid model coupling with the RNG k-ε turbulence model were used to simulate the granular flow and the turbulence effects. When the optimum solid-liquid mixing homogeneity was achieved in both conventional agitators, further increasing stirring speed would worsen the homogeneity significantly, while the two rotor-stator agitators still achieving good mixing homogeneity at the stirring speed of 600 rpm. The CSRS agitator attained the minimum standard deviation of particle concentration σ of 0.15, which was 42% smaller than that achieved by the A200 agitators. Moreover, the average liquid level velocity corresponding to the minimum σ obtained by the CSRS agitator was 0.31 m/s, which was less than half of those of the other three mixers.

Molecular dynamics simulations of shock loading of nearly fully dense granular Ni-Al composites.

We used molecular dynamics simulations to study the shock propagation, inhomogeneous deformation, and initiation of the chemical reaction characteristics of nearly fully dense reactive Ni-Al composites. For shocks with piston velocities Up ≤ 2.0 km s-1, particle velocity dispersion was observed at the shock front, which increased on increasing the shock strength. Plastic deformation mainly occurred at the grain boundaries or grain junction during the shock rise and was accompanied by the generation of a potential hot spot in the region where severe plasticity happens. The composite exhibited higher strength and lower reactivity than the mixtures with certain porosity. In addition, the shock-induced premature melting of Al led to the expansion of particle velocity dispersion from the wavefront to the shocked zone and the formation of a heterogeneous velocity field for stronger shocks beyond critical Up (2.5 km s-1). The velocity heterogeneity in the shocked region led to localized shear, strong erosion of Ni, and occurrence of ultrafast chemical reactions. Therefore, the shock-induced premature melting of Al led to the mechanochemical effect and played a role in the shock-induced chemical reaction in the reactive metal system.

Effect of particle packing and density on shock response in ordered arrays of Ni + Al nanoparticles.

We investigate the shock response of Ni + Al reactive nanoparticle systems through molecular dynamics simulations. The powder configurations with varying arrangements and densities are constructed by stacking equal-sized Ni and Al particles based on five typical crystal structures, i.e., zinc-blende, NaCl, CsCl, AuCu and the close-packed. The effects of configuration and shock strength on mechanochemical and diffusion processes in the shock-induced chemical reactions are characterized. A reaction kinetic model is developed to describe these behaviors, assess the extent of mechanochemical effect, and explain the occurrence of ultra-fast reaction. Significant dependence of shock wave velocity, plastic deformation, temperature response, chemistry and microstructure change on particle packing and density is observed under shock loading at the same piston velocity, but we see a relatively weak dependency on the stacking mode with the same density. Our results indicate the important role of particle coordination number and density in shock response of energetic powder materials.

Investigation of the shock-induced chemical reaction (SICR) in Ni + Al nanoparticle mixtures.

Molecular dynamics (MD) simulations are used to investigate the shock-compression response of Ni + Al spherical nanoparticles arranged in a NaCl-like structure. The deformation and reaction characteristics are studied from the particle level to the atomic scale at various piston velocities. Shock-induced chemical reactions (SICRs) occur during non-equilibrium processes, accompanied by a sharp rise in temperature and rapid mixing of atoms. The preferentially deformed Al particles form a high-speed mass flow relative to the Ni at the shock front, which impinges on the Ni particles, and mixing of Ni and Al atoms occurs immediately at the interface. The particle velocity dispersion (PVD) that appears at the shock front has important implications for the initiation of shock-induced chemical reactions. We show that dislocations are mainly generated at the beginning of particle deformation or at the shock front, and do not directly affect the occurrence of SICRs. The intimate contact of the molten Al and the amorphous Ni is found to be critical to the subsequent reactions for the extensive mixing of Ni and Al. We conclude that the mechanisms of SICRs involve mechanochemical processes near the shock front and subsequent interdiffusion processes.