Mixing and dispersion of particle ropes in lean phase pneumatic conveying

The objective of this study was to investigate mixing mechanisms in lean phase pneumatic conveying, with the emphasis on techniques for dispersing the severe particle stratification caused by flow through a 90° elbow. This type of stratification is referred to as a particle rope. The paper describes a combined numerical and experimental study of the rope dispersion characteristics of various mixing devices that were installed immediately downstream of the elbow. The laboratory experiments were conducted in a 0.154 m I.D. vertical test section. Local particle velocities and concentrations were measured using a reflective type fiber optic probe. The numerical simulations were carried out using the CFX-4.2 code developed by AEA Technology. fect of secondary velocities on rope dispersion was investigated by using a flow straightener installed after the elbow. The results show that the rope dispersion rate in both the axial and radial directions was significantly reduced in the absence of secondary velocities. pes of flow mixers investigated included nozzles, air jet injection, and swirl vanes. Although all mixing techniques were able to disperse the particle rope within nine pipe diameters from the bend exit plane, nozzles with beta ratios of 0.5 and 0.67 and air jet injection from the inner wall caused the most rapid rope dispersion. However, the nozzles cause excessive pressure drop and the air jet injection technique increases the flow rate of conveying fluid carried by the pipe.