Edward K. Levy

Formation and dispersion of ropes in pneumatic conveying

Abstract In this study, the solid flow nonuniformities which develop in lean phase upward flow in a vertical pneumatic conveying line following a horizontal-to-vertical elbow were investigated. Laboratory experiments were conducted in 154 and 203 mm I.D. test sections using pulverized-coal particles (90% less than 75 μm) for two different 90° circular elbows having pipe bend radius to pipe diameter ratios of 1.5 and 3.0. The experiments covered a range of conveying air velocities and solids mass loadings. Experimental measurements of time-average local particle velocities, concentrations, and mass fluxes were obtained using a fiber-optic probe which was traversed over the cross-section of the pipe. The measurements indicate a continuous rope-like structure forms within the elbow. The rope maintains its continuous structure until it disintegrates into large discontinuous clusters at downstream locations. Comparisons of the results of CFD simulations of turbulent gas-particle flow and time-average experimental data were used to explain rope formation and dispersion. The CFD simulations, based on the Lagrangian particle-source-in-cell method, predict a denser particle rope as the nondimensional radius of curvature (R/D) is increased, agreeing with trends in experimental data. The individual effects of secondary flows and turbulence on axial dispersion of the rope were studied computationally and the results show both mechanisms are important.

Gas-solid flow behavior in a horizontal pipe after a 90° vertical-to-horizontal elbow

Abstract The characteristics of the particle flow in a horizontal pipe following a 90° vertical-to-horizontal elbow were investigated both numerically and experimentally. Laboratory experiments were conducted in a 0.154 m ID test section. The effects of air velocity, the ratio of air-to-solids mass flow rate, geometry of the elbow and inlet conditions on gas–solid flow patterns were investigated experimentally. Pulverized coal with a mean particle diameter of 50 μm was used as the solid material. Experiments were performed with conveying air velocities ranging from 15 to 30 m/s and air-to-solids mass flow rate ratios of 1 and 3, with elbows having bend radius to pipe diameter ratios of 1.5 and 3. Measurements of particle concentration and particle velocity were performed at various locations along the horizontal pipe using a fiber-optic probe which was traversed over the pipe cross-section of the pipe. It was observed that the strong rope created by the elbow disintegrates within an axial distance of 10 pipe diameters. Fully developed concentration and velocity profiles were obtained within approximately 30 pipe diameters from the elbow exit plane. The rope behavior was different for the two elbows studied ( R / D =1.5 and 3). The shapes of the fully developed profiles were found to be independent of inlet conditions. CFD simulations of gas–solid flow through 90° circular elbows were performed using the Lagrangian approach. The simulations were used to predict the location of the rope and its dispersion rate along the horizontal pipe after the elbow exit plane.

Roping phenomena in pulverized coal conveying lines

Abstract Laboratory experiments were performed to measure the characteristics of a particle rope in a vertical pipe following a horizontal to vertical elbow. The experiments were performed with pulverized coal and air in a 0.154 m diameter pipe, with average velocities of 29 m/s and air-to-solids mass flow rate ratios from 1 to 3. A fiber optic probe was used to determine the transverse variations of particle velocity and concentration at different axial locations downstream of the elbow. Measurements were also made with an orifice plate at the elbow exit and these show that the flow patterns created by the orifice lead to accelerated rates of axial dispersion of the rope.

Mixing and dispersion of particle ropes in lean phase pneumatic conveying

Abstract 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. The effect 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. The types 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.

Effect of an acoustic field on bubbling in a gas fluidized bed

Very fine particles in the Geldart C range, are difficult to fluidize because of interparticle forces. When a bed of fine particles is fluidized in the presence of an acoustic field, the sound waves agitate the bed material, helping to disrupt the large clusters of particles formed by interparticle forces, thus promoting more uniform fluidization and bed expansion. The present paper reports on the combined effects of gas velocity and frequency and intensity of the sound waves on bubbling behavior. Data on minimum bubbling velocity, bubbling frequency and bed expansion were obtained in a shallow batch fluidized bed with fine fly ash particles at room temperature.

Effect of a combination of two elbows on particle roping in pneumatic conveying

As gas and particles flow through an elbow in a lean phase pneumatic conveying system, the particles stratify into a relatively small portion of the pipe cross-section, forming a dense rope-like structure. Laboratory experiments and numerical simulations were carried out to determine the effect of two closely spaced elbows on roping behavior in a vertical pipe downstream of the second elbow. The results show that the combination of elbows results in a stationary rope which spirals around the inside of the vertical pipe, adjacent to the pipe wall. The angular position of the rope, peak particle concentration, and particle velocity in the rope were found to depend strongly on the length of the pipe connecting the two elbows. The results also show that the particle velocities in the rope are significantly lower in the case of two elbows than they are with a single elbow.

USE OF COAL DRYING TO REDUCE WATER CONSUMED IN PULVERIZED COAL POWER PLANTS

This is the first Quarterly Report for this project. The background and technical justification for the project are described, including potential benefits of reducing fuel moisture, prior to firing in a pulverized coal boiler. A description is given of the equipment and instrumentation being used for the fluidized bed drying experiments. Results of fluidization and drying tests performed with North Dakota lignite, having a 6.35 mm (1/4 inch) top size, are presented. The experiments were performed with a 381 mm (15 inch) settled bed depth, with inlet air and in-bed heater surface temperatures of 44.3 C (110 F), and with the superficial air velocity ranging from 0.2 m/s to 1.4 m/s. Drying rate is shown to be a strong function of air velocity, increasing seven-fold from 0.2 m/s to 1.4 m/s. Increases in velocity from 0.75 m/s (minimum fluidization velocity) to 1.4 m/s resulted in a doubling of the drying rate.

Bubbling characteristics of sound-assisted fluidized beds

Abstract Low frequency sound has been used to promote fluidization of fine cohesive powders in a 0.15-m I.D. column. The bed was fluidized with air at conditions both near and above minimum bubbling. Experiments were performed with powders of density ranging from 1.1 g/cm3 to 3.95 g/cm3 and particle size ranging from 11 to 80 μm. A loudspeaker positioned at the top of the column generated the acoustic field. The sound pressure at the distributor plate was used to characterize the pressure distribution throughout the bed. It was found that high intensity sound disrupted the cohesive nature of the powders, permitting both homogeneous and bubbling fluidization. The sound expanded the bed up to the onset of bubbling. The data show the minimum bubbling velocity is affected by the sound pressure level, particle density and particle size. In addition, the data show the sound pressure level also affects bubble size. The bubble frequency depends most strongly on the excess gas velocity and the basic bubbling mechanism remains similar to that of larger particles without sound.

Prediction of pneumatic conveying flow phenomena using commercial CFD software

Turbulent two-phase flow calculations were performed with a commercial CFD computer code referred to as FLOW3D to determine how well its predictions compare to available experimental data. Predicted results obtained for pressure drop and velocity variations in fully developed vertical upflows in circular pipes are in good qualitative agreement and are frequently in quantitative agreement with published data. Results obtained for vertical flow through orifices show the effect of orifice size and conveying air velocity on the rate of dispersion of particle ropes.

Solids exchange between the bubble wake and the emulsion phase in a two-dimensional gas-fluidized bed

Abstract Information on the rates of solids exchange between the bubble wake and the emulsion phase in a gas-fluidized bed is needed in modeling solids mixing and segregation patterns. Experimental data are presented for two-dimensional bubbles, giving wake exchange rate as a function of minimum fluidization velocity and bubble diameter. Comparisons are made with recent theoretical models for wake exchange.