Four-way coupled simulations of the gas–particle flow in a diffuser

• Particles can induce reattachment in the air flow in a conical diffuser.
• High wall roughness increases collisions, which homogenizes particle distribution.
• The mechanism for flow reattachment is the particle distribution uniformization.
• Interparticle collisions are quantitatively important at moderate mass loadings.

In this work, a numerical investigation on the gas–particle flow in a vertical diffuser is carried out. This study was motivated by the experimental work of by Kale and Eaton (1984), who noticed that the fully attached flow in a diffuser in the freeboard region of a particle bed would become detached if no particles were present. It was concluded at the time that this effect was not caused by the high inlet turbulence levels, but rather by the particles. With the goal to better understand the interactions between the particles and the fluid in a diffuser, simulations of the particle-laden gas flow in a vertical conical diffuser are run using the Euler/Lagrange approach. The model, which includes particle-to-particle collisions, the particle influence on the gas phase and wall roughness effects, is first validated based on experimental results from a horizontal channel and a vertical diffuser for both the continuous and dispersed phases at different mass loadings. Investigations on the effects of particles at different mass loadings and wall roughness on the diffuser flow are then carried out. It has been found that, even at moderate mass loadings, particles can significantly affect the diffuser flow pattern, and actually reattach the otherwise separated flow under some conditions. It has also been found that wall roughness plays a very important role in homogenizing the particle distribution at the diffuser section. The resulting more uniform concentration and velocity profiles can then reenergize the separated boundary layer and reattach it to the wall. The mechanism for the flow reattachment owing to the high wall roughness is investigated and an explanation is proposed.

Particle distribution in the diffuser body with smooth (a) and rough walls (b).Figure optionsDownload as PowerPoint slide