Numerical investigation of multiphase feature in a polydisperse gas-solid flow: Effect of particle size, volume fraction and shape

The current study aims to elucidate the multiphase effect generated from the solid particle parameters like the size (10,50, and 100 mu m), volume fraction (10-4 to 10- 2) and shape (sphere, ellipsoid, cube, disk, and cylinder) on the background gas phase velocity in a dispersed gas-solid flow. A numerical investigation is performed using a compressible multiphase 2D CFD solver based on an Eulerian approach for both the gas and solid phases. The analysis considers the variation of particle size and volume fraction for spherical shaped particle and variation of particle shape and size at a fixed volume fraction of 5 x 10-3 with a configuration involving an under-expanded gas-solid jet injected into the still atmosphere at a pressure higher than the ambient. Compared to flow with negligible solid volume fraction, the gas phase vertical velocity is found to reduce significantly with the increase in volume fraction at a fixed particle size. However, the effect of the increase in volume fraction on the velocity is more significant with the decrease in particle size, particularly towards the lower range of the Stokes number, which corresponds to a finer particle size at a higher volume fraction. This suggests a combined effect of particle size and volume fraction on gas vertical velocity at a particular shape. Mathematical expressions as a function of particle loading ratio and solid volume fraction are proposed to describe the combined effect. Compared to flow with spherical-shaped particles, the gas vertical velocity is found to reduce with the decrease in sphericity of the particles. The effect of the non-spherical shape on the velocity becomes more pronounced with the increase in particle size. However, the trend is reversed for particle size of 10 mu m. Additionally, disk and cylinder-shaped particle, although having the same value of sphericity, results in different magnitudes of peak gas vertical velocity depending on the height-to-diameter ratio. This suggests that the effect on gas velocity cannot only be defined by the sphericity of the particle but a combined effect of the particle size and shape in the form of a shape descriptor has to be considered. The parametric analysis highlights that the interphasic drag force controlled by the solid particle size, shape, and volume fraction contributes to the multiphase features of the plume dynamics.