INTERACTION OF GRAVITY AND ELECTROSTATIC EFFECTS IN PIPE FLOW OF A GAS-PARTICLE SUSPENSION

ABSTRACT

The interaction of gravity and electrostatic effects in isothermal, fully developed, horizontal turbulent pipe flow of dilute suspensions has been examined. Experimental study has validated the fact that, in the presence of gravity, increased space charge associated with increased local concentration due to gravity accentuates the asymmetry in mass flux and particles density distributions in the vertical plane passing through the pipe centerline. The space charge effect on the particle mass flux distribution near the pipe bottom is enhanced by increased particle density. Measurements were made with air suspensions of monodispensed particles of alumina and latex with non-uniform particle charge in pipes of 51 mm and 127 mm diameters.     

Solid-State Phase Relationships in the Calcia-Titania-Zirconia System at 1200°C

Phase relationships were investigated in the CaO–TiO₂–ZrO₂ system at 1200°C for compositions containing <50 mol% CaO using X-ray diffraction and electron probe microanalysis. The existence of two previously reported ternary phases, zirconolite (CaZrTi₂O₇) and calzirtite (Ca₂Zr₅Ti₂O₁₆), was confirmed. Each of these phases exhibited a significant range of homogeneity between TiO₂ and ZrO₂, while maintaining a nearly constant concentration of CaO. The ternary solubilities of the constituent binary phases were found to be small (typically <1 mol%), with the exceptions of the perovskites (CaTiO₃ and CaZrO₃). These latter phases displayed mutual solubilities of at least 22 mol% but exhibited significant variations in composition from grain to grain. Thermodynamic equilibrium was clearly not established in several samples, although most of the phase relationship information obtained was self-consistent.     

Carbide Coatings on Graphite Fibers by Liquid Metal Transfer Agent Method

Titanium and zirconium carbide layers were produced on the surface of graphite fibers. The process allows a controlled reaction of the fibers with the carbide-forming elements dissolved hi a molten tin bath, which acts as the transferring medium. The coatings were analyzed by optical and scanning electron microscopy, energy dispersive X-ray analysis, and X-ray diffraction. It was found that they consist of two distinct layers, one with the binary carbide composition in contact with the fiber core and an exterior one having a ternary carbide composition. The coatings are brittle, continuous, and uniform along and across the fibers and show good adherence to the carbon core. The carbide growth process is diffusion-controlled with an activation energy of 184 kJ/mol. The coated fibers are readily wetted by molten aluminum.     

MEASUREMENT AND CONTROL OF ELECTROSTATIC CHARGES ON PULVERIZED COAL IN A PNEUMATIC PIPELINE∗

Abstract

Pulverized coal particles flowing in a pneumatic pipeline are naturally charged to a detectable level due to collisions with pipe walls. Systematic charge measurements of Anthracite (AN) coals, Medium Volatile Bituminous (MVB) coals and Lignite A (LIGA) coals in a grounded copper pipe have been made with an upgraded charge measuring system. The net particle charges were found to be positive, although both negative and positive charges were detected in all experiments. Effects of air humidity and conveying velocity on particle charges were examined. The mean particle charge was found in the order of 10^?12 Coulomb and the charge-to-mass ratio in the order of 10^?5 C/kg. Charge elimination techniques by strict humidity control and by introducing neutralizing charge carriers, such as minus 1 μm activated charcoal fines, ammonia, and piezoelectric ionized gas were explored. Effective reduction of 70 % to 85 % of particle charges was achieved. A charge neutralization mechanism was proposed to interpretate the measured results.     

A NOVEL QUICK-CLOSING PROBE FOR MEASURING LOCAL PARTICULATE PHASE DENSITY IN GAS-PARTICLE SUSPENSIONS

ABSTRACT

An innovative quick-closing probe system was designed, built, and tested for measuring the local particulate phase density of gas-particle suspensions. Experiments with glass beads and powder coals were conducted in a horizontal channel with an aspect ratio of 11 to 1, and also in a 0·25 m I.D. bench-scale vortex chamber. Results of particle density distributions in these two systems indicated the ability of the probe to perform in highly turbulent, swirling, dilute and intermediate phase gas-particle suspension flows. The probe was connected with a microcomputer-assisted particle weighing system for data collection and reduction. Comparison of measured data by this probe with calculated results was assessed. Since this probe requires no calibration curve nor empirical equation, it serves as a primary standard for particle density measurements. The design features, working principle, component characteristics, overall performance, and test results are discussed in this paper.     

涡旋管内强旋湍流气固两相流的数值模拟

本文建立了基于一种合理考虑湍流-离心力相互作用的新的代数Peynolds应力模型和颗粒随机轨道模型的强旋湍流气固两相流动数学模型。应用该模型对新型涡旋管内的强度湍流气固两相流进行了数值模拟，揭示了涡旋管内气固两相湍流流动的基本特性以及改变切向出口气体流率对流动的影响。     

AN ELECTROSTATIC INDUCTION PROBE FOR MEASURING PARTICLE VELOCITY IN SUSPENSION FLOW

A probe for measuring particle velocity in pipe flow of a gas-solid suspension has been developed. A solid particle such as a 100 µm latex particle in suspension typically carries an electrostatic charge of say 10^-13 Coulomb via charge separation by surface contacts in normal handling. The tubular cylindrical probe makes use of the principle of electrostatic induction by a charged solid particle during its passage. This probe, which is insulated from ground by a teflon sleeve, converts the induced charge to voltage signals of sharp rise as the charged particle enters and a steep dip in voltage as it departs from the cylinder. Between the entering peak and the later dip at departure, the time of flight of the particle through the probe can be accurately measured and hence the particle velocity.     

MODELING OF SOLID FLOWS IN A FLUIDIZED BED WITH SECONDARY TANGENTIAL AIR INJECTION IN THE FREEBOARD

Abstract

The Vortexing Fluidized-Bed Combustion (VFBC) technique was recently developed for small- and medium-scale coal-burning boiler applications. Experimental observations showed that the general solid flows in the freeboard of a vortexing fluidized bed consisted of three successive stages: (1) spirally ascending motion before colliding the freeboard wall, (2) bouncing on the wall, and (3) sliding on the wall and exiting the freeboard. This study attempts to model these three stages of solid flows. The dimensionless governing equations for particle motion in the swirling field were presented taking into account the interactions of particle inertia, centrifugal force, viscous fluid drag, and gravity. Numerical solutions of particle velocities and trajectories were pursued, and effects of particle momentum transfer number, Froude number, and particle-wall restitution coefficient were delineated. The experimental validation of solid flows in the swirling freeboard was furnished with an 18 cm ID laboratory fluidized bed.     

Dynamic Grain Growth During Superplastic Deformation of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Both static and dynamic grain growth were studied during superplastic deformation of fine-grained yttria-stabilized tetragonal zirconia. It was found that significant grain growth does not take place below 1300°C. Both static and dynamic growth were found to obey a similar equation of the form D³−D³₀=kt, where D and D₀ are the instantaneous and initial grain sizes, respectively, t is the annealing time, and k is the kinetic constant for either static or dynamic grain growth. The activation energies were approximately 580 and 520 kJ/mol for static and dynamic grain growth, respectively.     

Role of Concurrent Cavitation in the Fracture of a Superplastic Zirconia—Alumina Composite

Superplastic zirconia-alumina composites exhibit very large elongations to failure of ≳500% under optimum conditions. A quantitative microstructural study reveals that the composite is susceptible to considerable internal cavitation, with the level of cavitation increasing with increasing strain rate. The observed inverse relationship between the level of cavitation and the elongation to failure suggests strongly that the tensile ductility is limited by the evolution of cavitation damage during superplastic deformation.