The influence of aspect ratio on distributions of settling velocities and orientations of long fibres

The influence of the aspect ratios of fibrous particles on their settling velocities and orientations is reported under super dilute conditions in which fibrous particles settle in air at Reynolds number of 3–70 based on fibre length and at aspect ratios of 35, 48 and 60. Measurements were performed using Particle Tracking Velocimetry (PTV) to calculate orientation and velocity based on the two end-points, following a method reported previously. With the mean volume fraction of 0.0005, the key findings are: 1) for a constant diameter of 20.1 μm, the absolute mean vertical settling velocity, V cx ¯ , is not independent of fibre length, the long fibre V cx ¯ is a little higher, while the settling velocity normalized by that of an equivalent sphere, Vcx/Veq-sph, decreases with an increase in fibre length over the range 700 μm to 1200 μm; 2) for a constant length of 700 μm, both V cx ¯ and Vcx/Veq-sph decrease with an increase in diameter over the range 14.5 μm to 20.1 μm; 3) for a constant aspect ratio but different length and diameter, V cx ¯ slightly increases with an increase in particle size, while Vcx/Veq-sph decreases with an increase in particle size; 4) angular velocities and their distributions for four types of fibrous particles were reported.