Consequences of an asymmetrical confinement in the transfer phenomena for a cylinder at low Reynolds numbers

In most of the systems where heat or mass transfer occur geometrical symmetries are often present. In this paper, we wish to know if the global flux transferred to cylindrical particles in motion (sedimentation, etc.) could be influenced when introducing some geometrical disturbance breaking the general symmetry of the system. To answer this question, we numerically investigate the simple configuration of a single cylindrical particle moving at constant speed between two parallel walls when the particle is off the symmetry plane and when the thermal boundary conditions are of the Dirichlet or Neumann type. When the geometrical disturbance of the system increases, the results show that the backflow that appears in such confined situations yields a non-intuitive evolution of the transfer in convective regimes. In this case a minimum of the flux appears off the symmetry plane. However, in purely diffusive regimes, we find a monotonical evolution of the transfer.     

CFD simulation of dilute phase gas-solid riser reactors: Part I—a new solution method and flow model validation

A three-dimensional simulation of a dilute phase riser reactor (solid mass flux: ) is performed using a novel density based solution algorithm. The model equations consisting of continuity, momentum, energy and species balances for both phases, are formulated following the Eulerian-Eulerian approach. The kinetic theory of granular flow is applied. The gas phase turbulence is accounted for via a k-ε model. An extra transport equation describes the correlation between the gas and solid phase fluctuating motion. The solution algorithm allows a simultaneous integration of all the model equations in contrast to the sequential multi-loop solution in the conventional pressure based algorithms, used so far in riser simulations. The simulations show an unsteady behaviour of the flow, but a core-annulus flow pattern emerges on a time-averaged basis. The abrupt nature of the T type outlets causes a significant recirculation of gas and solid from the top of the riser. The flow near the outlets is highly non-symmetric and has a three-dimensional character. A significant decrease of the gas phase turbulence and particle granular temperature across the riser length is attributed to the presence of small particles, which is qualitatively consistent with the experimental data from literature.