## 3 Results and Discussion
### Comparisons of Deposition Efficiency Predictions Using DPM Approach
#### 3.2.3 Particle Diameter  
Figures 10 and 11 compare 1-5 \(\upmu\)m range one- and two-way particle deposition efficiency for 18-22 generations. In addition, Table 1 is prepared for easier comparing the values for the one and two coupling cases. Deposition efficiencies are displayed as bar diagrams giving total deposition. As expected, there is only a small difference in the deposition predicted for one- and two-way coupled particles. This can be explained by the differentiation among one-way particles coupling, which provides an acceptable approximation of the negligible influence between fluid flow and two-way particles coupling that causes the mutual influence of gas and particles is accounted to fluid momentum equations. Gravitational sedimentation, which causes particles to settle at the bottom of each alveolar cavity, is the main mechanism of deposition for 1-5 \(\upmu\)m diameter particles. As shown in Figure 6, the orientation of the gravitational force is perpendicular to the plan of the model. It is clear that the differing geometries of the alveolar has important role in particle deposition efficiency. The main airflow direction is 150 \(\upmu\)m parallel to the alveolar openings in the developed models. It would be reasonable for the total deposition efficiency to be greater in the model by increasing the distance travelled by particles. Under this condition, particles have more time for gravitational sedimentation to come closer to alveolar walls. Sedimentation is the primary mechanism of deposition in large  
Figure 9: Comparison between 1 and 5 \(\upmu\)m particles deposition by one- and two-way coupling system for 18–22 generations with conditions of gravity acting on [0, 0, \(-\)1] direction; (**a**) 18 st, (**b**) 19 st, (**c**) 20 st, (**d**) 21 st and (**e**) 22 st generations. particles with 5 \(\upmu\)m diameters, because of the very large sedimentation velocities, whereas particle inertia as the Brownian diffusion force dominates the transport of small particles. As expected, it is clear that deposition efficiency of one- and two-way DPM particles is increased by increasing the generation number. Such a phenomenon is due to reducing the Reynolds number and dominant viscous airflow effects in lower (larger number) generations. Moreover, one-way DPM provides higher deposition efficiency than two-way DEM in many generations of the developed model. According to the research of [PERSON] and [PERSON] [PERSON] and [PERSON] (2010), this result is probably due to the particle-fluid interactions, which re-suspend the sediment particles into the airway stream and hence passes from this generation. The two-way DPM method, despite its complexity, provides more extensive particle interaction and transport results than one-way DPM. Therefore, the results of the two-way DPM approach are more reliable than those of the other approach. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline  & **18** & \multicolumn{2}{c}{**19**} & \multicolumn{2}{c}{**20**} & \multicolumn{2}{c}{**21**} & \multicolumn{2}{c}{**22**} \\ \hline
**Disameter** & **One-Way** & **Two-Way** & **Disameter** & **One-Way** & **Two-Way** & **Disameter** & **One-Way** & **Two-Way** & **Disameter** & **One-Way** & **Two-Way** & **Disameter** & **One-Way** & **Two-Way** \\ \hline
1 & 1.17 & 0.3 & 1 & 2.32 & 0.94 & 1 & 6.33 & 4.32 & 1 & 15 & 13 & 1 & 30 & 28.4 \\
2 & 1.75 & 0.8 & 2 & 36095 & 1.41 & 2 & 14.23 & 12 & 2 & 23 & 22 & 2 & 43 & 47.1 \\
3 & 2.3 & 1.2 & 3 & 6.15 & 4.6 & 3 & 17.5 & 16.30 & 3 & 30 & 31.2 & 3 & 67.6 & 67.1 \\
4 & 3.975 & 3.33 & 4 & 12835 & 11.6 & 4 & 28.95 & 26 & 4 & 44.16 & 45.4 & 4 & 91.51 & 94.91 \\
5 & 8.49 & 7.73 & 5 & 22 & 20.75 & 5 & 41 & 39.33 & 5 & 67.6 & 74 & 5 & 94 & 98 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between 1 and 5 \(\upmu\)m range particles deposition for generations over 18–22 with gravity acting down in the direction of \(z\)-axis [0, 0, \(-\)1].