An experimental study of the dispersion of powders into a pipe turbulent gas stream

Experiments have been conducted to study the phenomenon of powder injection into a turbulent gas stream flowing in a cylindrical pipe. The initial rad ial dispersion of powders effected by pipe Re-pipe and injector protrusion has been investigated by injecting 1 mu m fog droplets and 70 mu m silica p owders separately into the turbulent stream. The silica powder is injected at a feed rate to yield a solid to gas mass ratio ranging from 0 to 1.2. La ser Doppler velocimetry measurements have shown that 1 mu m particles follo w the gas stream quite well and that the turbulence intensity behind the in jector is found to increase 3% to 5% with injector protrusion 0.5 pipe diam eters into the turbulent stream. A semi-quantitative analysis of the forces acting on the particles injected into the turbulent stream indicates that the radial Reynolds shear force is the main mechanism of the dispersion of particles. In addition, the dispersion of particles is independent of pipe Re-pipe in the range from 5x10(4) to 1.1x10(5). However, it is strongly inf luenced by the geometrical position of the injector. Injectors that extend 0.75 and 0.5 pipe diameters into the flow yield the optimum powder dispersi on for the 1 mu m and 70 mu m particles respectively. This is a result of h igher Reynolds stress forces on the coarser particles than on the finer siz ed particles. Furthermore, these results imply that in reactors with very s hort particle residence times, the injection of smaller sized particles may result in less efficient heat and mass transfer as smaller sized particles take a longer time to disperse into the turbulent stream than larger parti cles.