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Research Papers

Effects of Oral Airway Geometry Characteristics on the Diffusional Deposition of Inhaled Nanoparticles

[+] Author and Article Information
Jinxiang Xi

Department of Mechanical Engineering, Virginia Commonwealth University, 601 West Main Street, P.O. Box 843015, Richmond, VA 23284

P. Worth Longest1

Department of Mechanical Engineering and Department of Pharmaceutics, Virginia Commonwealth University, 601 West Main Street, P.O. Box 843015, Richmond, VA 23284pwlongest@vcu.edu

1

Corresponding author.

J Biomech Eng 130(1), 011008 (Feb 07, 2008) (16 pages) doi:10.1115/1.2838039 History: Received December 04, 2006; Revised May 14, 2007; Published February 07, 2008

The deposition of ultrafine aerosols in the respiratory tract presents a significant health risk due to the increased cellular-level response that these particles may invoke. However, the effects of geometric simplifications on local and regional nanoparticle depositions remain unknown for the oral airway and throughout the respiratory tract. The objective of this study is to assess the effects of geometric simplifications on diffusional transport and deposition characteristics of inhaled ultrafine aerosols in models of the extrathoracic oral airway. A realistic model of the oral airway with the nasopharynx (NP) included has been constructed based on computed tomography scans of a healthy adult in conjunction with measurements reported in the literature. Three other geometries with descending degrees of physical realism were then constructed with successive geometric simplifications of the realistic model. A validated low Reynolds number k-ω turbulence model was employed to simulate laminar, transitional, and fully turbulent flow regimes for the transport of 1–200 nm particles. Results of this study indicate that the geometric simplifications considered did not significantly affect the total deposition efficiency or maximum local deposition enhancement of nanoparticles. However, particle transport dynamics and the underlying flow characteristics such as separation, turbulence intensity, and secondary motions did show an observable sensitivity to the geometric complexity. The orientation of the upper trachea was shown to be a major factor determining local deposition downstream of the glottis and should be retained in future models of the respiratory tract. In contrast, retaining the NP produced negligible variations in airway dynamics and could be excluded for predominantly oral breathing conditions. Results of this study corroborate the use of existing diffusion correlations based on a circular oral airway model. In comparison to previous studies, an improved correlation for the deposition of nanoparticles was developed based on a wider range of particle sizes and flow rates, which captures the dependence of the Sherwood number on both Reynolds and Schmidt numbers.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Geometric surface models of the extrathoracic oral airway including the (a) realistic with NP, (b) realistic without NP, (c) elliptic, and (d) circular models

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Figure 2

Computational meshes of the extrathoracic airway models with different meshing styles: (a) unstructured tetrahedral mesh with a refined prism mesh in the near-wall region for the realistic model with NP and (b) structured hexahedral grid for the elliptic model. The circular model is also meshed with a multiblock structured hexahedral grid.

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Figure 3

Predicted aerosol DE versus particle diameter in the four models considered with a comparison to experimental data for inspiration flow rates of (a) Qin=4l∕min and (b) Qin=10l∕min

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Figure 4

Midplane velocity vectors, contours of velocity magnitude, and in-plane streamlines of secondary motion for the realistic model with the NP under light activity conditions (Qin=30l∕min). Slice 4 corresponds physiologically to the glottic aperture.

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Figure 5

Comparisons of velocity profiles in the midpharynx (Slice 3 in Fig. 4) between realistic models with and without the NP under light activity conditions (Qin=30l∕min). No significant discrepancy is evident in the velocity magnitude contours and secondary motion streamlines between the (a) realistic model with the NP and a 10% nasal-oral FP, (b) realistic model with NP but no FP, and (c) realistic model without the NP. (d) The axial velocity profiles from the dorsal to the ventral part of the midpharynx in these three models are also very similar.

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Figure 6

Midplane and cross-sectional concentration contours of 5nm aerosols in the (a) realistic with NP and 10% nasal-oral FP, (b) realistic without NP, (c) elliptic, and (d) circular airway models under light activity conditions (Qin=30l∕min). Slice 4 in each model represents the minimum diameter glottis. The slices are not to scale.

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Figure 7

Predicted ultrafine particle DE versus particle diameter in the four models considered with different inspiration flow rates: (a) Qin=15l∕min, (b) Qin=30l∕min, and (c) Qin=60l∕min

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Figure 8

DEFs for 5nm aerosols under light activity conditions (Qin=30l∕min) for (a) realistic with NP, (b) realistic without NP, (c) elliptic, and (d) circular models

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Figure 9

Outlet particle profiles for 5nm aerosols under light activity conditions (Qin=30l∕min) in (a) realistic with NP and a 10% nasal-oral FP, (b) realistic without NP, (c) elliptic, and (d) circular airway models

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Figure 10

Variation of Sherwood number (Sh) as a function of Reynolds (Re) and Schmidt (Sc) numbers in the four airway models considered (a) in comparison to the empirical correlation of Cheng (24) (b) An updated correlation is proposed, which captures an expanded dependence of Sh on Re and Sc. (c) 3D surface plot of the newly developed correlation.

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