Impact of Particle-Size Distribution on Flow Properties of a Packed Column

A karst collapse column (KCC), a type of vertical structure that is common in the Carboniferous-Permian coal fields of North China, contains many graded broken rocks and often functions as a channel for groundwater inrush. This study used a custom-built apparatus to investigate the effects of the particle-size distribution on the seepage behavior of a sand particle mixture subjected to a high hydraulic gradient. Three different flow regimes were identified: (1) the Darcy regime, (2) the Forchheimer regime, and (3) the turbulent regime. When flow transition begins, the critical flow velocity, critical Reynolds number, and Forchheimer number increase as the nonuniformity coefficient Cu (or the coefficient of curvature Cc) increases when the porosity remains constant, and the critical hydraulic gradient decreases. The permeability increases linearly, and the inertia factor decreases exponentially as CuxCc increases. Using new experimental data, the validity of five widely used empirical formulas for permeability and the inertia factor were evaluated. The results indicated that the empirically corrected formulas for permeability and the inertia factor yielded high prediction accuracy and can be used to predict the permeability and inertia factor of KCCs in practical engineering.