Geometry Effect on Deposition and Residence Time of Polydisperse Fine Drops in Mini-Risers Under Developing Flow Condition

Mini-riser geometry effect on the transport, dispersion and deposition of fine water drops in the three-dimensional laminar developing flow was investigated numerically and experimentally. Circular, triangular and rectangular cross-section risers with a hydraulic diameter of 14mm were examined. Microscopic high-speed particle shadow velocimetry (PSV) technique was employed for planar velocity measurement of droplets. The experimental data were used to validate the simulation results obtained with an Eulerian-Lagrangian computational code. In addition, simulation results of the penetration were compared with the existing theoretical model, and a close agreement was found. In these simulations the polydispersed fine droplets were randomly distributed at the inlet of the risers in a size range of 0.01 to 10 mu m. For comparison of the effect of riser shapes, the airflow rate was assumed to be constant. Simulation results indicated significantly more droplets were deposited on the walls of the triangular and high aspect ratio rectangular risers. Brownian diffusion tends to increase the residence time especially for risers with a corner. Employing the riser shapes with sharper corners and higher aspect ratio was found to be more beneficial for increasing the droplet size moving in a supersaturated carrier gas, especially at high flow rates, due to higher residence time.