Nuclear Magnetic Resonance Flow Imaging Research Articles

Counter-flow phenomena studied by nuclear magnetic resonance (NMR) velocimetry and flow simulations

Flow patterns including counter-flow and flow reversal effects have been studied by a combination of nuclear magnetic resonance flow imaging and numerical modeling using the finite volume method in the open-source computational fluid mechanics package OpenFOAM. Two cylindrical geometries have been used: In a concentric double-cylinder system the flow reversal under oscillatory rotation of the inner cylinder has been followed, and the time evolution of the flow reversal has been studied. We find extended periods of counter-rotating flow in the gap where fluid in the inner part of the gap follows the new direction of the rotor, while the outer part takes a longer time until the viscous forces transmit the reverted flow direction outwards. The radial position of the reversal of flow direction has been monitored as a function of the oscillation angle after the turning point. In the second cylindrical geometry, the rotating bob is placed off the center and a counter-rotating vortex is detected in the wider part of the gap. At constant viscosity and eccentricity, the position of the center of the vortex was found to depend on the rotation frequency of the bob. Qualitative and quantitative agreement between experiment and laminar (nonturbulent) flow simulations has been obtained for both steady-state flow using the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm and time-dependent flow using the Pressure Implicit with Splitting of Operators (PISO) algorithm.

Most water in the tomato truss is imported through the xylem, not the phloem: a nuclear magnetic resonance flow imaging study.

In this study, we demonstrate nuclear magnetic resonance flow imaging of xylem and phloem transport toward a developing tomato (Solanum lycopersicum) truss. During an 8-week period of growth, we measured phloem and xylem fluxes in the truss stalk, aiming to distinguish the contributions of the two transport tissues and draw up a balance between influx and efflux. It is commonly estimated that about 90% of the water reaches the fruit by the phloem and the remaining 10% by the xylem. The xylem is thought to become dysfunctional at an early stage of fruit development. However, our results do not corroborate these findings. On the contrary, we found that xylem transport into the truss remained functional throughout the 8 weeks of growth. During that time, at least 75% of the net influx into the fruit occurred through the external xylem and about 25% via the perimedullary region, which contains both phloem and xylem. About one-half of the net influx was lost due to evaporation. Halfway through truss development, a xylem backflow appeared. As the truss matured, the percentage of xylem water that circulated into the truss and out again increased in comparison with the net uptake, but no net loss of water from the truss was observed. The circulation of xylem water continued even after the fruits and pedicels were removed. This indicates that neither of them was involved in generating or conducting the circulation of sap. Only when the main axis of the peduncle was cut back did the circulation stop.

An investigation of concentration polarization phenomena in membrane filtration of colloidal silica suspensions by NMR micro-imaging

Nuclear magnetic resonance (NMR) micro-imaging has been used to investigate concentration polarization phenomena in membrane filtration of colloidal silica suspensions using a single tubular microfiltration membrane, with the feedstock fed to the inner lumen of the membrane and the filtrate removed from the (outer) shell side. 1 H NMR images, in which the signal intensity is weighted by the longitudinal relaxation time ( T 1) of the solvent (water) protons, clearly exhibit details of the formation and dissipation of the silica particle concentration polarization layers at the surface of the membrane in response to changes in trans-membrane pressure difference and feedstock crossflow rate. The images were used to map the spatial distribution of the silica polarization layer as a function of time, distance from the filter inlet, and applied trans-membrane pressure difference. In each case the polarization layer was observed to be highly asymmetric, being much thicker at the bottom of the module than at the top. The performance of the filter was compared for different orientations of the filter module. The permeate flux rate was shown to be highly dependent on the orientation of the filter axis with respect to the vertical. This is consistent with the fact that the observed asymmetry in the layer is caused by flow of the polarization layer over the surface of the membrane due to gravitational effects. Phase sensitive NMR flow imaging was used to map the 1D distribution of the feedstock crossflow on the lumen side of the membrane as well as measuring the axial flow profile within the concentration polarization layer itself. The axial component of flow of the polarization layers is driven, not by gravity, but by the shear induced by the feedstock crossflow. The flow profile of the polarization layer presented in this paper therefore provides direct experimental evidence for fluidity and motion of concentration polarization layers, an assumption which has been invoked for the development of some theoretical models but which has not previously been confirmed experimentally.

NMR flow imaging of aqueous polysaccharide solutions

Nuclear-magnetic-resonance (NMR) flow imaging with spatial resolution of the order of 200 μm is used to measure the velocity fields in aqueous solutions of 0.2% and 1% xanthan gum and 1% guar gum for steady flow in a 1.2 cm internal diameter cylindrical polymethylmethacrylate pipe. The velocity fields show little evidence of apparent wall slip and are differentiated to obtain relationships between shear stress and shear rate which span over four decades in shear rate for the xanthan solutions and approximately three decades for the guar solution; these data agree well with those obtained by cone-and-plate viscometry. The behavior of the xanthan solutions is well described by a power-law dependence of shear stress on shear rate, and the guar solution is better described by a Cross-type relationship. The implications of these studies for future NMR flow imaging studies of more complex systems are discussed.

NMR flow imaging of Newtonian liquids and a concentrated suspension through an axisymmetric sudden contraction

Using nuclear magnetic resonance (NMR) flow imaging to examine fluid motions at constant velocities or flows that change relatively slowly has been well-documented in the literature. Application of this technique to accelerative flows, on the other hand, has been limited. This study reports the use of an NMR flow imaging method, for which acceleration is not explicitly compensated in the NMR pulse sequence, to measure axial and radial fluid motions during flow through an axisymmetric sudden contraction. In this flow geometry, both velocity and acceleration are spatially dependent. The flow contraction ratio was 2:1. The method was first applied to examine Newtonian liquids at low and high Reynolds numbers under laminar flow conditions. The measured axial and radial velocity profiles, without accounting for acceleration effects in the data analysis, across the contraction are in excellent qualitative agreement with previous experimental data and theoretical calculations reported in the literature. Quantitative comparison of the axial and radial velocities with numerical results indicates that the maximum error from acceleration effects is about 10%. The method has also been used to examine the flow of a concentrated suspension (50% by volume of solid particles) through the contraction. The flow kinematics of the suspension at creeping flow conditions appear to mimic those of the Newtonian fluid with some slight differences. NMR images taken immediately following the cessation of flow suggest a slight degree of particle migration toward the center of the pipe downstream of the contraction.

Nuclear magnetic resonance in chemical engineering: Principles and applications

In recent years chemical engineers have shown an increasing interest in non-invasive measurement techniques; Nuclear Magnetic Resonance (NMR) is perhaps the ultimate technique of this kind. Over the past 10 years notable developments have been made in both spectrometer hadrdware our ability to understand and manipulate nuclear spin interactions, and it is now possible to address research areas in catalysis, materials science, mass transfer and flow visualisation which are of real interest to chemical engineers. This review is divided into two sections. Part I identifies three broad categories of magnetic resonance measurements: spectroscopy, diffusion measurement and imaging, and outlines the basic principles underlying these experiments. A summary of the various nuclear spin interactions and the chemical information they yield is given. In progressing to the introduction of diffusion measurements, the application of magnetic gradients is discussed and the basic Pulsed Gradient Spin Echo (PGSE) and related measurement techniques are presented. The principles of NMR imaging are then described in the context of the two popular experimental schemes: projection—reconstruction and spin-warp imaging. The principles and application of parameter-selective imaging experiments are outlined and limitations on attainable resolution are noted. Extension of NMR imaging to the study of flow phenomena is also discussed. Part II of the review reports a number of examples of NMR methods applied to problems of direct relevance to chemical engineers. This literature survey starts with an overview of applications of NMR spectroscopy in the fields of catalysis, adsorption, measurement of phase equilibria and the consideration of NMR as a quality control technique. The use of PGSE methods to study diffusion phenomena is discussed, with particular emphasis being placed on how theoretical models in combination with NMR experiments are being used to gain insight into transport processes occurring within porous media. Recent developments in NMR imaging and their application to the study of ceramics processing, polymers, porous media, catalysis, food processing, filtration processes, and transport within reactors and packed columns are also presented. Finally, the state-of-the-art in NMR flow imaging studies is discussed and the ability of NMR to study two-phase flow phenomena is highlighted.

Dean vortices with wall flux in a curved channel membrane system.: 6. Two dimensional magnetic resonance imaging of the velocity field in a curved impermeable slit

Nuclear magnetic resonance flow imaging (MRFI) was used to measure fluid flow noninvasively in a curved 180° slit with impermeable walls. The flow rates were kept constant at 0.5, 1.0, 1.75 and 2 times the critical Dean number ( De c). Above the critical Dean number, the laminar flow changed to unstable viscous flow resulting in rotating spiral vortices (called Dean vortices). The NMR results (wavenumber a=2.3) are in reasonable agreement with Finlay et al.'s theoretical predictions [J. Fluid Mech., 194 (1988) 417] in which the a varied from 2.4 to 2.7 for 1.5 De c to 2.0 De c. It is these vortices that can be used to depolarize the solute build-up near the membrane-solution interface. Also, we determined the distance the vortices persisted and how vortex strength decreased along the flat slit. It suggests the possibility that new membane modules can be designed with curved entrances (to produce vortices) and flat sections (to maximize packing considerations and reduce complexity).

Nuclear Magnetiic Resonance Imaging (NMRI) of Flow-Induced Microstructure of Concentratied Suspensions

AbstractThe application of nuclear magnetic resonance flow imaging (NMRI) to the study of Couette and falling-ball viscometry of solid/liquid suspensions is described. The suspension consisted of non-Brownian, monodisperse, neutrally-buoyant spheres of 50 vol%. NMRI data demonstrated flow-induced changes in the particle distribution in the suspension that strongly influence the accuracy in viscosity measurements using these flow geometries. In Couette flow, direct correlation between stress measurements and particle concentrations at various locations in the flow cell as a function of shear strain can be made. Our data directly confitrm the shearinduced particle migration theory proposed by Leighton and Acrivos.[1] In the falling-ball experiments, increased particle concentration at the leading edge and decreased concentration at the trailing edge of the ball was observed when the falling ball is big compared to the cylinder containing the suspension. This change in particle distribution can be directly related to the changes in fall velocity of the ball as a function of position in the cylinder.

NMR flow imaging of fluids and solid suspensions in Poiseuille flow

The application of nuclear magnetic resonance (NMR) flow imaging to the study of Poiseuille flows of single‐phase fluids and solid/liquid suspensions is demonstrated and investigated. Two fluids were studied: a mixture of a low‐molecular‐weight polyether oil and water which is Newtonian and an aqueous solution of a high‐molecular‐weight polymer which is non‐Newtonian. The velocity profiles of these fluids in Poiseuille flow measured by NMR are in excellent agreement with those predicted from viscometric data using a power law model. The suspensions investigated consisted of density‐matched compositions of solid poly(methyl methacrylate) beads of various concentrations up to 52% in a Newtonian, polyether‐based liquid. At particle loadings above 40% these suspensions exhibited rheological behavior akin to what has been reportedly observed for some solid‐rocket motor propellant compositions. NMR‐derived flow velocities of these suspensions at high particle loadings show clear deviations from the Newtonian parabolic behavior and some unusual intensity features which are not yet fully understood. The capabilities and limitations of NMR flow imaging as well as considerations for data interpretation in the special case of highly filled suspensions is discussed.

NMR Imaging of Industrial Flow Processes

ABSTRACTThe application of nuclear magnetic resonance flow imaging to the study of Poiseuille flows of solid/liquid suspensions is described. The basic imaging method is presented along with results on suspensions with solid particle loading levels up to 52 vol. % and viscosities up to 7kPoise. The most concentrated suspensions are non-Newtonian, and the NMR flow data show evidence for deviations from a simple parabolic velocity profile in straight pipes. Cross-sectional intensity images derived from the flow image data on concentrated suspensions are non-uniform. Possible explanations for these results are discussed in terms of current rheological understanding of suspension flow and relaxation weighting phenomena in the NMR imaging experiments.