Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.
Pneumatic conveying, the process of transporting particulate materials through pipelines with gas as the carrier, is a commonly-used means of material transport in industry. In order to simplify the layout of the plant, the materials are frequently transported through ducts with numerous bends and pipe sections. For instance, coal-fired power plants operate on a continuous supply of pulverised coal which is transported from the mill through these ducts. The piping used in these transport systems commonly includes bends that have a significant effect on the gas-solid flow structure. As a result of centrifugal forces within these pipe bends, the gas and solid particles segregate and the particles form a dense structure known as a rope. This region has relatively high particle concentration. Additionally, deposition of particles occurs due to deceleration of particles. These particle behaviors lead to difficulties for plant operators in maintaining optimal conditions for combustion in furnaces as a result of irregularities in the pulverised fuel supply.
Experimental studies have been conducted in a 90-degree vertical-to-horizontal pipe bend. The solid particles used in this study are high density polyethylene (HDPE) beads and ground flaxseed, with diameters ranging from 686 to 1171 microns, and with a mean diameter of 871 microns. The particle densities are 0.86g/cc and 1.2g/cc respectively. Experiments have been performed for a range of gas velocities and solids loadings, and the results of these experiments show that the particles form a thick rope at high solids loadings. The rope formation conditions for the different particle size distributions were analysed and computational fluid dynamics (CFD) simulations are performed to supplement the experiments. Preliminary comparisons between the experimental and CFD simulation results show that both the flaxseed and HDPE particles exhibit a roping behaviour at solids loadings as low as 0.4; however, experimental results indicate that only the flaxseed exhibits roping at this loading. The reasons for this unexpected behaviour are being investigated.