Nuclear Magnetic Resonance Studies of Granular Flows – Current Status

Nuclear magnetic resonance (NMR) is a non-intrusive method that can characterize not only the particulate density but also velocity and velocity fluctuation parameters. A survey of all the known NMR measurements of granular flow will be followed by a brief description of NMR as it applies to granular flow. Two new experiments, both involving flows in partially filled rotating horizontal cylinders, will be described. First, the effect of a stationary blade to suppress the azimuthal velocity of particles being brought up and deposited into the flowing layer on flow-velocity profiles will be studied. Suppressing the azimuthal velocity reduces the deviation of the velocity profile from a quadratic dependence on the height above the rigid layer. Second, a new NMR scheme will be presented that yields spatial distributions of collisional correlation times for macroscopic particles undergoing granular flow. It is based on Pulsed-Gradient Spin-Echo strategy that is commonly used to measure molecular diffusion in liquids. The scheme will be demonstrated with an example from shear flow in a partially-filled horizontal cylinder. Spatially resolved collisional correlation times and velocity fluctuation intensities are derived from the measurements and have values of ∼1 ms and ∼10 -3 m 2 /s 2 respectively, at the center of the free surface for 2 mm particles in a 70 mm diameter cylinder rotating at 2.36 rad/s.