Abstract
Dry deposition of airborne particles in duct air flow over a backward-facing step (BFS) is commonly encountered in built environments and energy engineering. However, the understanding of particle deposition characteristics in BFS flow remains insufficient. Thus, this study investigated particle deposition behaviors and efficiency in BFS flow by using the Reynolds stress model and the discrete particle model. The influences of flow velocities, particle diameters, and duct expansion ratios on particle deposition characteristics were examined and analyzed. After numerical validation, particle deposition velocities, deposition efficiency, and deposition mechanisms in BFS duct flow were investigated in detail. The results showed that deposition velocity in BFS duct flow monotonically increases when particle diameter increases. Moreover, deposition velocity falls with increasing expansion ratio but rises with increasing air velocity. Deposition efficiency, the ratio of deposition velocity, and flow drag in a BFS duct is higher for small particles but lower for large particles as compared with a uniform duct. A higher particle deposition efficiency can be achieved by BFS with a smaller expansion ratio. The peak deposition efficiency can reach 33.6 times higher for 1-μm particles when the BFS expansion ratio is 4:3. Moreover, the “particle free zone” occurs for 50-μm particles in the BFS duct and is enlarged when the duct expansion ratio increases.
Highlights
Dry deposition of aerosol particles in duct air flow over a backward-facing step (BFS) is of significant relevance in built environments and energy engineering, such as for particle removal devices, building ventilation systems, and pulverized coal burners [1,2,3,4]
As a BFS can greatly modify flow structure and turbulent kinetic energy (TKE) distribution, particle dispersion and deposition in BFS flow are quite different from the case of uniform duct flow [5]
Fluid velocity fluctuation produced by a Gaussian random distribution as well as turbulent vortex were used in the discrete random walk (DRW) model [32]
Summary
Dry deposition of aerosol particles in duct air flow over a backward-facing step (BFS) is of significant relevance in built environments and energy engineering, such as for particle removal devices, building ventilation systems, and pulverized coal burners [1,2,3,4]. For the nonuniform duct case, Haber [22] and Lee [23] predicted the deposition behaviors of particles in expanding and contracting alveolus by solving the creeping flow equations, as the flow velocities in their cases were quite slow. This method can capture the basic flow eddies. The influencing factors on particle deposition characteristics, including different air flow velocities, particle sizes, and expansion ratios of the BFS duct, were considered in the study. The deposition mechanisms of BFS flow were studied and compared with a uniform duct case
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