This article presents the lattice Boltzmann simulation of viscous fingering phenomenon in immiscible displacement of two fluids in porous media. Such phenomenon generally takes place when a less viscous fluid is used to displace a more viscous fluid, and it can be found in many industrial fields. Dimensionless quantities, such as capillary number, Bond number and viscosity ratio between displaced fluid and displacing fluid are introduced to illustrate the effects of capillary force, viscous force, and gravity on the fluid behaviour. The surface wettability, which has an impact on the finger pattern, is also considered in the simulation. The numerical procedure is validated against the experiment about viscous fingering in a Hele-Shaw cell. The displacement efficiency is investigated using the parameter, areal sweep efficiency. The present simulation shows an additional evidence to demonstrate that the lattice Boltzmann method is a useful method for simulating some multiphase flow problems in porous media.

The liquid droplet size distribution in gas-liquid vertical upward annular flow is investigated through a CFD (computational fluid dynamics)-PBM (population balance model) coupled model in this paper. Two-fluid Eulerian scheme is employed as the framework of this model and a population balance equation is used to obtain the dispersed liquid droplet diameter distribution, where three different coalescence and breakup kernels are investigated. The Sauter mean diameter d32 is used as a bridge between a two-fluid model and a PBM. The simulation results suggest that the original Luo–Luo kernel and the mixed kernel A (Luo’s coalescence kernel incorporated with Prince and Blanch’s breakup kernel) can only give reasonable predictions for large diameter droplets. Mixed kernel B (Saffman and Turner’s coalescence kernel incorporated with Lehr’s breakup kernel) can accurately capture the particle size distribution (PSD) of liquid droplets covering all droplet sizes, and is appropriate for the description of liquid droplet size distribution in gas-liquid annular flow.

Micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS) have become the focus of a great deal of attention in recent years. The flow and heat behaviors of micro-machines are usually different from that of macro-machines. In this paper, the heat and mass transfer characteristics of rarefied nitrogen gas flows in microchannels have been investigated using improved DSMC method. The influence of aspect ratio and wall temperature on the mass flowrate and wall heat flux in microchannels are studied parametrically. In order to examine the aspect ratio effect on heat and mass transfer, the wall temperature is set constant at 350 K, and the aspect ratio of the microchannel varies from 5 to 20. The results show that as the aspect ratio increases, the velocity of the flow decreases, and the mass flowrate also decreases. In a small aspect ratio channel, the heat transfer occurs throughout the microchannel, and as the aspect ratio of the microchannel increases, the region of thermal equilibrium becomes larger. To investigate the effects of wall temperature on heat and mass transfer, the aspect ratio of the microchannel is held constant at 10 and the wall temperature is changed from 300 K to 800 K. The results show that, when the wall temperature increases, the pressure increases slightly and the number density drops rapidly near the inlet. In addition, the Knudsen number increases as the wall temperature increases. It indicates that the increasing wall temperature of microchannel enhances the rarefaction of the gas flow. Moreover, as the wall temperature increases, the mass flowrate decreases, and the temperature will increase more rapidly near the inlet, with more heat transfer between the gas flow and the wall.

A lattice Boltzmann model for two-phase flow with large density ratio in body-fitted coordinates is proposed and validated through a series of comparison with analytical and published results with good agreement. A single bubble sliding over a curved surface and rising in a channel for different Eö tvos numbers is simulated. The results show that during its sliding process, the surface makes the bubble experience large deformation, with both horizontal and vertical velocities fluctuating. As the bubble continues to move, it departs from the surface and starts to rise freely in the channel. With Eö tvos number increases, the departure angle decreases.