On fiber behavior in turbulent vertical channel flow

Abstract

In the present work, the dynamic behavior of inertial fibers suspended in a turbulent vertical channel flow has been investigated. The three-dimensional turbulent flow field was obtained from the Navier–Stokes equations by means of direct numerical simulation in an Eulerian reference frame. The fibers were modeled as prolate spheroidal particles in a Lagrangian frame and characterized by their inertia and shape. The translation and rotation of the individual fibers were governed by viscous forces and torques as well as by gravity and buoyancy according to Newton’s laws of motion. The test matrix comprised four different Stokes numbers (inertia) and three different aspect ratios (shape). The twelve different fiber types were suspended both in a downward and in an upward channel flow. Fiber orientation and velocity statistics were compared with channel flow results in absence of gravity.The results showed that gravity has a negligible effect for fibers with modest inertia, i.e. low Stokes numbers, whereas gravity turned out to have a major impact on the dynamics of highly inertial fibers. Irrespective of the bulk flow direction, a preferential alignment of the inertial fibers with the gravity force was found in the channel center where fibers have been known to orient randomly in absence of gravity. In the downward channel flow, the drift velocity of the fibers towards the walls was substantially higher for fibers than for spheres and also higher than when gravity was neglected. In the upward flow configuration, the modest drift velocity of inertial spheres was totally quenched for all fibers irrespective of shape. The suppressed drift velocity resulted in a more uniform fiber distribution throughout the channel as compared to the distinct near-wall accumulation in downward flow and in absence of gravity. This suggests that an upward flow configuration should be the preferred choice if a uniform fiber distribution is desired, as in a biomass combustion reactor.