Simulating particle settling in inclined narrow channels with the unresolved CFD-DEM method

Acceleration of particle settling in inclined containers, known as the Boycott effect, is a common natural phenomenon, which can significantly affect the particle conveying process. Particularly for proppant transport constrained in narrow channels, physical mechanisms related to this effect remain undetermined. In this study, an Eulerian-Lagrangian method, i.e., the unresolved computational fluid dynamics-discrete element method, has been adopted to simulate the particle settling process in inclined narrow channels, with an aim to deepen the understanding of the Boycott effect on proppant settling. The simulation results illustrate that two types of granular-induced instabilities dominate the sediment settling process in inclined fractures, which are analogous to Kelvin-Helmholtz instability and Rayleigh-Taylor instability, respectively. Due to the instabilities, spatial inhomogeneity of the particle phase, i.e., particle clustering, is clearly observed. Based on parametric studies, it is proven that there exists a critical inclination angle where the acceleration ratio reaches a maximum, which ranges from 2 to 6 in this work. Moreover, the critical angle can shift between 60 degrees and 75 degrees as physical properties change, such as fluid viscosity, particle size, and particle density.