Stephen A. Altobelli

Experimental observations of particle migration in concentrated suspensions: Couette flow

Nuclear magnetic resonance (NMR) imaging was used to observe the evolution of radial concentration and velocity profiles of initially well‐mixed concentrated suspensions of spheres in viscous Newtonian liquids undergoing flow between rotating concentric cylinders (wide‐gap, annular Couette flow). In Couette flow, particles migrate from the high shear‐rate region near the inner rotating cylinder to the low shear‐rate region at the outer wall. The particle concentration near the outer wall approaches maximum packing for randomly distributed spheres at steady state, and velocity profiles reveal that the suspension is almost stagnant in these regions. For unimodal suspensions of spheres, the shear‐induced migration of large particles results in concentric two‐dimensional, circular sheets of particles arranged in hexagonal close‐packed arrangements extending inward from the outer wall. This paper examines the functional dependence of particle migration in concentrated suspensions undergoing shear flow in a wid...

Non-invasive measurements of granular flows by magnetic resonance imaging

Magnetic Resonance Imaging (MRI) was used to non-invasively measure velocity and concentration of granular flows in a partially filled, steadily rotating, long, horizontal cylinder. First, rigid body motion of a cylinder filled with granular material was studied to confirm the validity of this method. Then, the density variation and the depth of the flowing layer, where particles collide and dilate, and the flow velocity profile were obtained as a function of the cylinder rotation rate.

Velocity and concentration measurements of suspensions by nuclear magnetic resonance imaging

Nonuniform velocity and concentration distributions have been measured for suspensions of small, negatively buoyant particles flowing in horizontal tubes. All measurements were performed during statistically steady flow conditions with average fluid velocities up to 25 cm/s. The mean particle concentration φs ranged between 0.0≤φs≤0.39. Time‐averaged measurements were recorded noninvasively by using a three‐dimensional, flow‐compensating, nuclear magnetic resonance technique. This method spatially resolves both the fluid velocity and particle concentration distributions from a single imaging experiment and permits detailed analysis of the flow‐induced structure of nondilute suspensions.

Steady particulate flows in a horizontal rotating cylinder

Results of discrete element method (DEM) simulation and magnetic resonance imaging (MRI) experiments are compared for monodisperse granular materials flowing in a half-filled horizontal rotating cylinder. Because opacity is not a problem for MRI, a long cylinder with an aspect ratio ∼7 was used and the flow in a thin transverse slice near the center was studied. The particles were mustard seeds and the ratio of cylinder diameter to particle diameter was approximately 50. The parameters compared were dynamic angle of repose, velocity field in a plane perpendicular to the cylinder axis, and velocity fluctuations at rotation rates up to 30 rpm. The agreement between DEM and MRI was good when the friction coefficient and nonsphericity were adjusted in the simulation for the best fit.

Note: NMR imaging of shear‐induced diffusion and structure in concentrated suspensions undergoing Couette flow

This note describes nuclear magnetic resonance (NMR) imaging of two concentrated suspensions undergoing flow between rotating concentric cylinders (wide‐gap, annular Couette flow). Suspensions of both monodisperse (50% by volume) and bidisperse (60% by volume) spheres are studied. We find that particles migrate from the higher shear rate regions near the rotating inner cylinder to the lower shear rate regions near the stationary outer wall, establishing large concentration gradients after only a short time. In addition, the large particles in the bimodal suspension form concentric cylindrical sheets, parallel to the axis of the Couette device, which rotate relative to each other. In the Couette devices used in these studies, no significant axial migration of the particles is observed: the dispersion of particles is almost entirely in the radial direction. This particle migration and structure formation is believed responsible for torque reductions and other anomalous behavior witnessed during rheological ...

Nuclear magnetic resonance imaging of particle migration in suspensions undergoing extrusion

Nuclear magnetic resonance imaging was used to measure fluid velocity and fluid fraction in suspensions flowing into an abrupt four-to-one contraction in pipe diameter, through a section of smaller diameter pipe, and out of an abrupt expansion back to the original pipe size. Suspensions of 50% by volume of particles in a Newtonian liquid were forced to flow by a plunger moving at a constant, slow velocity. Two sizes (100 and 675 μm diameter) of suspended spheres were studied. Conditions were such that buoyant, inertial, Brownian, and surface forces could be assumed to be negligibly small. Little change in particle concentration was seen in the region of the contraction until the plunger was within about one pipe diameter of the contraction. The particles in the small diameter section of pipe migrated toward the pipe axis, the region of lowest shear rate. Particle concentration varied downstream of the pipe expansion, especially in a suspension of the larger particles. Over time, particles were partially s...

NMRI study: asial migration of radially segregated core of granular mixtures in a horizontal rotating cylinder

In this study, we use NMRI (nuclear magnetic resonance imaging) to non-invasively study the three-dimensional evolution of a segregation phenomenon which occurs in a simple geometry, a long horizontal cylindrical drum or a kiln. We show, for the first time, in a binary size mixture of spheres, the development of a radial core of small particles along the axis of the cylinder which evolves to axially segregated bands.

Eddy current compensation by direct field detection and digital gradient modification

Abstract We demonstrate an empirical magnetic field gradient waveform optimization technique which compensates for eddy current distortions and nonideal gradient amplifier performance. The desired gradient response is used as a trial function, and a pickup coil in the magnet bore detects the actual pulsed gradient. The temporal behavior of the gradient is digitized and recorded with the existing NMR spectrometer hardware and is used for the calculation of a new trial function table. The method and some typical results as well as some practical considerations are described.

Liquid and gas flow and related phenomena in monolithic catalysts studied by 1H NMR microimaging

NMR is employed to study liquid and gas flow in a tube and in the channels of shaped catalysts, mass transport during drying of water saturated porous monoliths, and gas adsorption by porous alumina pellets. NMR flow imaging and pulsed field gradient NMR are shown to yield the same average propagators for liquid and gas flow in a straight circular tube. The complicated patterns of gas or liquid flow in shaped reactors are characterized successfully by NMR flow imaging through the detection of all three flow velocity components.

Numerical analysis of liquid-solids suspension velocities and concentrations obtained by NMR imaging.

Abstract Analyses of some of the steady-state, fully developed, and isothermal carrier fluid velocity and solids concentration data of Altobelli et al. [1] and