Stephen J. Gibbs

Partially restricted diffusion in a permeable sandstone : observations by stimulated echo PFG NMR

We demonstrate a variant, insensitive to eddy current effects, of an alternating pulsed field gradient technique designed to null the effect of background gradients in liquid-saturated porous media, using a 38 mm diameter sample of a natural sandstone. Measurements of the effective diffusivity confirm predictions of a decline as the square root of an effective diffusion time. A value of the ratio S/Vp for the dominant pores is extracted, yielding with T1 a value for the surface relaxivity. We test also a geometry-dependent data collapse recently suggested for a range of diffusion times and wavenumbers. The data agree with a sheet-like pore model for this granular sandstone, and fail to agree with a tube-like model; a pore length scale is also extracted.

Rheometry and detection of apparent wall slip for Poiseuille flow of polymer solutions and particulate dispersions by nuclear magnetic resonance velocimetry

A flexible and robust approach to nuclear magnetic resonance (NMR) based capillary rheometry has been developed. The precision of the technique has been explored, with particular regard to the capability for measurement of apparent wall slip velocities. The practical implications of alternative NMR rheometry protocols are discussed with regard to robustness and speed. NMR rheometry results are presented for a Newtonian fluid (aqueous 50.4% sucrose), a shear thinning solution (0.2% aqueous xanthan gum), and a particulate system composed of 5–50 μm irregular, soft agar gel particles. In all cases, fully developed Poiseuille flow was studied in a 4‐mm‐i.d., glass capillary. Radial velocity profiles were measured by NMR velocimetry; radial differentiation provides shear rate values, which have been scaled by the associated radial positions and measured pressure drops to determine viscosity as a function of shear rate. Agreement with cone‐and‐plate or parallel plate rheometry has been established for each syst...

Rapid MRI and velocimetry of cylindrical couette flow

A narrow-gap, temperature-controlled Couette flow rheometer has been developed to study fluid velocities within the annular gap between two concentric cylinders by nuclear magnetic resonance (NMR) imaging and velocimetry. Alternative pulsed-field-gradient-based nuclear magnetic resonance imaging strategies which may be used for measurement of velocity within the Couette flow device have been evaluated. These include two-dimensional (2-D) imaging techniques with acquisition times of several minutes and a one-dimensional (1-D) projection method which exploits the symmetry of the device to reduce overall measurement time to less than 1 min. Velocity measurements made using each technique are presented for a Newtonian fluid undergoing Couette flow at shear rates of approximately 20 and 60 s(-1).

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.

Comparisons of Magnetic Resonance Imaging Velocimetry With Computational Fluid Dynamics

The flow of Newtonian liquids through a pipe system comprising of series of abrupt expansions and contractions has been studied using several magnetic resonance imaging (MRI) techniques, and also by computational fluid dynamics. Agreement between those results validates the assumptions inherent to the computational calculation and gives confidence to extend the work to more complex geometries and more complex fluids, wherein the advantages of MRI (utility in opaque fluids and noninvasiveness) are unique. The fluid in the expansion-contraction system exhibits a broad distribution of velocities and, therefore, presents peculiar challenges to the measurement technique. The MRI protocols employed were a two-dimensional tagging technique, for rapid flow field visualisation, and three-dimensional echo-planar and gradient-echo techniques, for flow field quantification (velocimetry). The computational work was performed using the FIDAP package to solve the Navier-Stokes equations. The particular choice of parameters for both MRI and computational fluid dynamics, which affect the results and their agreement, have been addressed.

A concentric cylinder Couette flow system for use in magnetic resonance imaging experiments

The construction of a narrow-gap, concentric cylinder Couette flow system for use in magnetic resonance imaging (MRI) and velocimetry experiments is presented. The system can be operated over the range of temperatures 5-C and can produce shear rates up to , yet is compatible with the demands of high-spatial-resolution MRI . Details of the design and device construction are given and typical results for a distilled water sample at a shear rate of and a temperature of C are presented. The potential for further work is discussed.

Chemically resolved NMR velocimetry

Abstract Nuclear magnetic resonance imaging has been widely applied in industry as a noninvasive tool for the study of optically opaque fluids. The pulsed field gradient variant of this technique is sensitive to fluid displacement, allowing the measurement of fluid velocities in three spatial dimensions (velocimetry). It is also possible to include chemical sensitivity in the measurement. This article reviews the alternative protocols which might be implemented for chemically resolved nuclear magnetic resonance velocimetry, and illustrates two approaches. Results are presented which show separate velocity maps for oil and water in an oil-in-water emulsion flowing through a straight, rigid pipe and also through an abrupt expansion/contraction pipe system.

What roles are there for magnetic resonance imaging in process tomography

Currently pursued technologies and requirements for process tomography are briefly reviewed with emphasis on comparing existing methods and identifying roles for magnetic resonance techniques. It is concluded that fundamental studies of transport phenomena are among the beneficial applications of magnetic resonance techniques. After a brief review of the theory of magnetic resonance and a description of modern hardware for magnetic resonance imaging, specific examples of magnetic resonance investigations of mass and heat transfer are presented including studies of thermal processing, multiphase distributions, polymerization, and diffusion and flow. We conclude by speculating on future roles of NMR imaging for process developement and monitoring.