No abstract

No abstract

In this paper, we present a simple and accurate lattice Boltzmann (LB) model for immiscible two-phase flows, which is able to deal with large density contrasts. This model utilizes two LB equations, one of which is used to solve the conservative Allen-Cahn equation, and the other is adopted to solve the incompressible Navier-Stokes equations. A forcing distribution function is elaborately designed in the LB equation for the Navier-Stokes equations, which make it much simpler than the existing LB models. In addition, the proposed model can achieve superior numerical accuracy compared with previous Allen-Cahn type of LB models. Several benchmark two-phase problems, including static droplet, layered Poiseuille flow, and spinodal decomposition are simulated to validate the present LB model. It is found that the present model can achieve relatively small spurious velocity in the LB community, and the obtained numerical results also show good agreement with the analytical solutions or some available results. Lastly, we use the present model to investigate the droplet impact on a thin liquid film with a large density ratio of 1000 and the Reynolds number ranging from 20 to 500. The fascinating phenomena of droplet splashing is successfully reproduced by the present model and the numerically predicted spreading radius exhibits to obey the power law reported in the literature.

No abstract

No abstract

Perovskite-based LaFeO 3 /Al 2 O 3 −Kaolin oxygen carriers, prepared by a wet-mixing−kneading method, have been examined for chemical-looping reforming over a microfixed bed reactor and a circulating fluidized bed reactor. An oxygen carrier containing 60% LaFeO 3 over 15Al 2 O 3 -25Kaolin exhibits higher reactivity, and can converted methane to syngas with high selectivity. The addition of Al 2 O 3 −Kaolin improves the oxygen migration rate from bulk to surface, and increases the amount of the very reactive oxygen species for CO 2 formation. The CH 4 conversion and CO selectivity rely heavily on reaction temperature and bed height. Increasing the fuel reactor temperature and bed height is beneficial for the reforming application with natural gas as fuel over a bubbling fluidized bed reactor. Preliminary results on the circulating fluidized bed reactor show that the CH 4 conversion is about 25%, and CO selectivity is about 70% at an oxygen carrier-fuel molar ratio (1.32) and 900 °C. It is helpful to improve the redesign of the experimental facility and optimization of process parameters in further research. There is still a high potential for further improvement in mechanical strength and attrition resistance of perovskite-based oxygen carrier by optimization of binder and support.

No abstract

In this paper, a novel lattice Boltzmann (LB) model based on the Allen-Cahn phase-field theory is proposed for simulating axisymmetric multiphase flows. The most striking feature of the model is that it enables to handle multiphase flows with large density ratio, which are unavailable in all previous axisymmetric LB models. The present model utilizes two LB evolution equations, one of which is used to solve fluid interface, and another is adopted to solve hydrodynamic properties. To simulate axisymmetric multiphase flows effectively, the appropriate source term and equilibrium distribution function are introduced into the LB equation for interface tracking, and simultaneously, a simple and efficient forcing distribution function is also delicately designed in the LB equation for hydrodynamic properties. Unlike many existing LB models, the source and forcing terms of the model arising from the axisymmetric effect include no additional gradients, and consequently, the present model contains only one non-local phase field variable, which in * Corresponding author. this regard is much simpler. In addition, to enhance the model's numerical stability, an advanced multiple-relaxation-time (MRT) model is also applied for the collision operator. We further conducted the Chapman-Enskog analysis to demonstrate the consistencies of our present MRT-LB model with the axisymmetric Allen-Cahn equation and hydrodynamic equations. A series of numerical examples, including static droplet, oscillation of a viscous droplet, breakup of a liquid thread, and bubble rising in a continuous phase, are used to test the performance of the proposed model. It is found that the present model can generate relatively small spurious velocities and can capture interfacial dynamics with higher accuracy than the previously improved axisymmetric LB model. Besides, it is also found that our present numerical results show excellent agreement with analytical solutions or available experimental data for a wide range of density ratios, which highlights the strengths of the proposed model.