| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 91 === The interactions between turbulence flow and particle motion are investigated using Large eddy simulation in a vertical, fully-developed channel flow at two Reynolds numbers 180 and 644 which are based on the half-channel width and friction velocity. The flows are loaded separately with three classes of spherical, heavy particles with diameters smaller than the Kolmogorov length scale. The gas-phase flow field is obtained by the large eddy simulation (LES) incorporated with the dynamic mixed model. Each individual particle is tracked by a Lagrangian method. Inter-particle collisions are taken into consideration by using a deterministic collision detection method and the hard sphere model. The objective of this work is to investigate the influences of turbulence structure on particle dispersion, the effect of inter-particle collisions on the gas phase, mechanisms responsible for turbulence modulation, and the effect of particle-wall interactions on both phases.
Particles with small Stokes numbers tend to transport toward the wall by the sweep events and be trapped in the ejection-like environments. This process accumulates particle along the low speed streaks in the near-wall region. The inter-particle collisions cannot be ignored even in particle-dilute cases. Indeed, consideration of the inter-particle collisions in the modeling can enhance the particle transverse mixing and increase the attenuation of fluid turbulence. Attenuation of turbulence kinetic energy is due to either the particle drag effect or the modification on the transport mechanism of turbulence kinetic energy. The suppression of Reynolds stresses with increasing mass loading ratio decreases the production rate of turbulent kinetic energy. As a result, the energy contented in the fluid decreases. However, the effect of particles on gas-phase turbulence is not uniform for all scales. Presence of particles suppresses turbulence at large scales, while it increases the energy contained at small scales. The turbulence spatial structure is modified by which the energy flux to dissipative scales is increased. The model constant predicted by the dynamic mixed model becomes smaller and the Kolmogorov length scale becomes larger with increasing mass loading ratios. In addition, in the case of lower wall roughness, the effect of wall roughness enhances turbulence in most regions of the channel except at the channel centre where turbulence intensity is attenuated. In contrast, for a channel with high wall roughness, strong turbulence attenuation across most of the channel is observed
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