Numerical study of monodispersed particle deposition rates in variable-section ducts with different expanding or contracting ratios

The Eulerian-Lagrangian approach based on Reynolds stress model (RSM) with turbulent fluctuation correction and discrete particle model (DPM) was adopted to investigate particle deposition behaviors in ducts. Particle deposition velocity profile in uniform-section duct was first predicted and validated well with the literature data. Then, particle deposition velocities, air flow field structures, particle trajectories and deposition mechanisms in variable-section ducts with different expanding and contracting ratios were investigated and analyzed in details. For expanding duct cases, particle deposition velocity first keeps constant, then greatly increases, finally decreases with the increase of particle size. The maximum particle deposition rate appears for 20–30 μm particles. As the growth of expanding ratio, the particle deposition velocities are significantly reduced for dp>5μm while almost not affected for dp<5μm. For contracting duct cases, particle deposition velocity keeps increasing when particle size increases. Moreover, particle deposition velocities are greatly increased for dp<30μm but very closed for dp>30μm, when the contracting ratio increases. The modification of deposition distance, the variation of air velocity along the streamwise direction as well as air flow structures are the main mechanisms to change the particle deposition characteristics, compared with uniform duct case. Besides, the “particle free zone” appears for large particles in expanding duct cases while it doesn't exist for contracting duct cases.