An Experimental Investigation into the Rheology of Fine Powders for Modelling Fluidized Dense-Phase Pneumatic Conveying

Abstract

Fluidized dense-phase pneumatic conveying systems are being used in several industries to convey fine powders in an energy efficient and environment friendly manner. Modelling of the dense-phase turbulent flow of powders is not a trivial task due to the complex nature of the fluidized dunes. For accurate modelling of important flow parameters, such as minimum transport limits and the effects of particle size distribution on the transport capacity, it is of utmost importance to study the rheology of the powders. In this work, an experimental facility was developed in which yield stress and viscosity was measured under fluidized and unfluidized conditions. Tests were conducted with three different depths of immersions and three rotational speeds of the spindle. Yield stress has been measured for six different fine powders, such as fly ash and cement (d50: 19-139 μm; ρs: 1950-2910 kg/m3; ρbl: 660-1080 kg/m3). Viscosity measurements were done on three different fly ash samples from different field ESP from a power plant. It was concluded from the experimental results that with fluidization, the yield stress and viscosity decreased considerably. It was also found that in unfluidized state, yield stress increases with an increase in the mean particle size of the powders. Yield stress and viscosity values also increased with the increase in depth of spindle immersion, but decreased with increase in rotational speed of the spindle. Froude numbers were calculated for minimum transport boundary points obtained from the pneumatic conveying characteristics for fly ash and cement conveyed in two pipelines (65 mm I.D x 254 m long, 80/100 I.D x 407 m long). These values were related to the yield stress values of these powders at a standard depth and rotational speed of the spindle. It was found out that as the yield stress of powders increased, the Froude number requirement for minimum transport also increased.