Effects of blockage ratio and Reynolds number on particle deposition in duct airflow over a forward-facing step
Deposition processes of particles in duct airflow over a forward-facing step (FFS) were numerically studied by computational fluid dynamics method. Reynolds stress model with velocity fluctuation modification and discrete particle model with turbulent particle dispersion were developed to predict airflow fields and particle motions respectively. After grid independence study and numerical validation, the effects of different Reynolds numbers and blockage ratios on particle deposition characteristics were investigated. Particle deposition velocities, deposition efficiency and deposition mechanisms in duct with a forward-facing step were analyzed and discussed. The results showed that particle deposition velocity in FFS duct keeps monotonically increasing as particle diameter increases. Moreover, particle deposition profile in FFS duct increases as blockage ratio and Reynolds number increases. Deposition efficiency is higher for small particles (dp < 10 µm) but lower for large particles (dp > 10 µm) than uniform duct case. When air velocity is fixed as 5.5 m/s, the maximum deposition efficiency of FFS duct can reach 84 when λ = 0.25 and particle size is 1 µm. Moreover, when air velocity increases, the deposition efficiency will increase. Reduction of deposition distance, production of high TKE region and interception of windward surface by FFS are dominant mechanisms to increase particle deposition rates. Moreover, the “particle free zone” appears when particle diameter is more than 10 um and blockage ratio is 0.5.