Pulsating Flow of a Viscous Fluid over a Cavity Containing a Compressible Gas Bubble

A two-dimensional pulsating flow of a viscous fluid in a plane channel whose wall has rectangular microcavities partially or completely filled with a compressible gas is investigated. This problem formulation can clarify the friction reduction mechanism in a laminar sublayer of a turbulent viscous boundary layer flow over a textured stripped superhydrophobic surface containing periodically arranged rectangular micro-cavities filled with gas. It is assumed that the dimensions of the cavities are much smaller than the channel thickness. On the macroscale, the problem of one-dimensional unsteady viscous flow in a plane channel with no-slip conditions on the walls and a harmonic variation of the pressure difference is solved. The solution obtained in this way is used for formulating non-stationary in time and periodic in space boundary conditions for the flow on the scale of a chosen cavity (microscale), with the instantaneous volume of the gas bubble in the cavity depending on the instantaneous pressure over the cavity. The flow on the microscale near a cavity with a gas bubble occurs in the Stokes regime. The numerical solution is obtained using an original version of the boundary element method. A parametric numerical study of the flow field in a pulsating shear flow over a cavity with a compressible gas bubble is performed. The averaged parameters characterizing the effective ‘velocity slip’ of viscous fluid and the friction reduction in a pulsating flow over a stripped superhydrophobic surface are calculated.

CFD-DEM Simulations of Graphite Particle Collisions in Opposed Jet Mill

Graphite is widely used in nuclear reactors as moderator and structural material. Among present graphite preparation methods, air flow mill is considered to be qualified in the control of particle size and purity, and promising for future mass production. In this work, an opposed jet mill is designed to crush large graphite particles. The opposed jet mill accelerates the particles through two supersonic jet flows in opposite directions, and finally the particles collide in the crushing cavity. In order to estimate the performance of opposed jet mill, it is necessary to solve the coupling calculation of the compressible flow and the collision process of discrete particles. However, the research on calculating the compressible gas solid coupling problems is scarcely rare. In this paper, coupled CFD-DEM model is used to simulate the particle movement process with jet flows and accompanying jet in opposed jet mill. By comparing with experimental results, it is proved that these simulation results of the acceleration process of compressible gas through these nozzles and the collision process of the final two supersonic jet flows in the opposed-jet mill are accurate, with the accuracy model of the coupled CFD-DEM provided. The practice has proved that the contrastive flow mill has a broad application prospect in the production of graphite particles.