Probing nonlocal tracer dispersion in flows through random porous media

Pulsed-field-gradient NMR is used to measure tracer dispersino flow through a porous medium. Data are presented for water flowing through packs of plastic beads at Peclet numbers 0 less than or equal to Pe less than or equal to 150, using strong, fast gradient pulses to measure pore-scale molecular displacements. The transition at Pe similar to 1 from tortuosity-reduced diffusion to dispersion is observed. The data are Fourier transformed to provide a measurement of the wave-number- and frequency-dependent nonlocal dispersion coefficient. The experimental results compare favorably with an approximate calculation of Koch and Brady [J. Fluid Mech. 180, 387 (1987); Chem. Eng. Sci. 42, 1377 (1987)].