*Mixing and dispersion of particle ropes in lean phase pneumatic conveyin

The objective of the present study was to investigate mixing mechanisms in lean phase pneumatic conveying, with the emphasis on techniques for dispersing of severe particle stratification caused by flow through a 90 degree elbow. The study consisted of a combined numerical and experimental study of the rope dispersion characteristics of various mixing devices which were installed immediately downstream of elbow. The laboratory experiments were conducted in a 0.154 m. I.D. vertical test section. Pulverized coal particles with a mean diameter of 59 micron were used as a test material. The numerical simulations were carried out using the CFX-4.2 code developed by AEA Technology. Local particle velocities and concentrations were measured using a reflective type fiber optic probe. Comparisons with the fiber optic probe measurements for time average particle concentration and velocities revealed that the CFD code accurately predicted most of the important flow behavior occurring in the particulate phase in the presence of the mixing devices.;The effect of secondary velocities on the rope dispersion characteristics was investigated by using a flow straightener installed after the elbow. Both numerical and experimental results revealed that the rope dispersion rate in axial and radial directions was significantly reduced in the absence of secondary velocities.;The effectiveness of several different types of flow mixers were investigated. These included, nozzles, air jet injection, swirl vanes, orifice plates, and deflector blocks. Although all mixing techniques were able to disperse the particle rope within nine pipe diameters from the bend exit plane, nozzles with beta ratios (beta) of 0.5 and 0.67, an orifice plate with a beta ratio (beta) of 0.7, and air jet injection from the inner wall caused the most more rapid rope dispersion. However, both the nozzles and orifice plate caused excessive pressure drop and the air jet injection technique increases the flow rate of conveying fluid carried by the pipe.;In addition to the time average particle concentration and velocity measurements, a detailed study was performed on the limits of the cross-correlation technique along with the window overlapped processing technique to measure instantaneous particle velocities using a fiber optic probe. It is shown that the cross-correlation technique is able to measure particle velocity fluctuations up to 75 Hz. More importantly, the window overlapping does not increase the velocity sampling frequency as claimed in the literature. However, this processing technique reduces the error in estimating rms particle velocities when the probe sampling frequency is approximately four times less than the highest frequency occurring in the flow.