Aerosol deposition in 90-degree bends: An enhanced empirical equation and a design concept of reducing losses

Accurate theoretical calculations of particle deposition in a 90° bend rely on understanding its flow field characteristics. However, given the complexity of the flow patterns within the bend, most mathematical models utilize relatively simplified simulated flow fields. This study collected experimental data on aerosol particle losses in 90° bends, facilitating a clearer model selection. In addition, it also aimed to experimentally investigate the impact of expanding the cross-section of a bend and subsequently generalize these findings as design criteria. The results revealed that none of the models could accurately predict the experimental data concerning particles ranging from 1.4 to 10 μm in diameter and with 32 channels per decade sizing resolution. While the Reynolds number (Re) is a conventional metric for flow characterization, our study underscores the superior relevance of the Dean number (De) in bends due to centripetal force and secondary flow effects. Hence, a novel empirical equation was proposed after analyzing experimental data encompassing a De range of 1000 to 4200 and a Stokes number (Stk) range of 0.001 to 10. This equation solely relies on Stk to calculate the transport efficiency of particles in bends. In contrast, the model proposed by Pui et al. demonstrated a tendency to overestimate the transport efficiency of particles with Stk > 0.2 while underestimating the transport efficiency of particles with Stk < 0.2. Furthermore, extending the cross-section of the 90° bend by 0.2 times the bend diameter at the 45° position was shown to improve particles’ transport efficiency significantly.