| Nuclear Magnetic Resonance (NMR) and NMR Imaging (NMRI) have been used for measurement and characterization of various complex flows. First an NMRI tubular rheometer for polymer melt systems is described. This technique allows the viscosity to be determined over a range of shear rates from a single measurement. It relies upon obtaining steady state velocity profiles in tubular pipes using NMRI and simultaneously measuring the pressure drop. In these experiments, the shear viscosity of a low density polyethylene melt was measured at shear rates between 1.8 to 12.1 s−1 and was found to be approximately 117.7 Pa.s. The viscosity was also determined using a parallel plate rheometer and was found to compare favorably with the NMRI determined value.; Next, the effects of periodic velocity fluctuations (0 to 65 Hz) on NMR flow images are investigated experimentally and theoretically. In the experiments, laminar pipe flow of water was examined. The flow was driven by a constant pressure head with a superimposed sinusoidal component. The velocity profiles obtained from the experimental results compare well with the theoretical calculations. Both theory and experiments show that flow fluctuations produce artifacts in the form of “ghosts” of the primary image, which are spaced at equal intervals in the phase encoding (flow) direction. The amplitudes of the ghosts diminish at the frequencies higher than 30 Hz.; Finally, an NMR method to study flows involving random fluctuations is described. The technique is based on the application of modulated magnetic field gradients at different modulation frequencies to probe the velocity autocorrelation of the random fluctuation component at the corresponding gradient modulation frequencies. This method was employed to measure the spectra of velocity autocorrelations in turbulent tube flow of water in the frequency range of 5 to 125 Hz. Experimental results reveal that, as flow rate increases, the peak of the turbulent diffusion coefficient spectra becomes narrower and higher compared to those at lower Reynolds numbers, indicating the size range of eddies that contribute to the turbulent diffusion becomes narrower as Reynolds number is increased. |
