OZONE TRANSPORT AND UPTAKE IN ANATOMICALLY-ACCURATE MODELS OF THE RESPIRATORY TRACT

Open Access
Author:
Keshavarzi, Banafsheh
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 07, 2011
Committee Members:
  • Ali Borhan, Dissertation Advisor
  • Ali Borhan, Committee Chair
  • James S Ultman, Committee Member
  • Michael John Janik, Committee Member
  • Rebecca Bascom, Committee Member
Keywords:
  • Respiratory fluid dynamics
  • Dosimetry modeling
  • Ozone
  • Computational fluid dynamics
Abstract:
Ozone (O3) is a highly reactive gas and a harmful air pollutant. A reproducible pattern of tissue injury induced by inhalation of O3 is believed to depend on the local dose delivered to the airway walls. To predict the local dose, we performed numerical simulations of ozone transport and uptake during inhalation in an anatomically-accurate geometrical model of the respiratory tract of a Rhesus monkey. The model geometry was created using three-dimensional reconstruction of MRI images of the respiratory tract, including the nasal passages, the larynx, and the first thirteen generations originating from the right bronchus. An unstructured mesh was generated for the resulting structure, and three-dimensional flow and concentration distributions were obtained through numerical solution of the Navier-Stokes, continuity, and species convection-diffusion equations. A quasi-steady diffusion-reaction model was used to account for the interaction between O3 and endogenous substrates in the respiratory tract lining fluid. The total rate of O3 uptake within each section of the respiratory tract was determined, and hot spots of O3 flux on the airway walls were identified. Hot spots of wall flux within the tracheobronchial tree are found to occur near the inlet of the trachea where the laryngeal jet impinges on the trachea wall, and at the bifurcations (especially the first bifurcation). The simulation results show that the structure of the upper airways has a significant effect on the distribution of ozone flux on the airway walls, by producing additional hot spots of ozone flux upstream of the trachea in the nasal vestibule, the dorsal and ventral parts of the middle turbinate, the medial part of the inferior turbinate, the ventral part of the inferior meatus, the medial part of the nasopharynx, and the larynx. In addition, the presence of the larynx leads to a more uniform wall flux distribution within the trachea, compared to the corresponding simulations in the same airway structure without the larynx. Results of the three-dimensional simulations for ozone uptake along a single asymmetrically-branched airway path were also compared to the predictions of an axisymmetric single-path model. The axisymmetric model consisted of a series of tubular airway branches of decreasing cross-sectional area connected through leakage zones that emulated the flow split at each bifurcation. The dimensions of this path were determined from three-dimensional reconstruction of the actual airway structure. Single-path simulations of gas uptake were found to be comparable to the predictions of the more realistic (but more computationally-intensive) three-dimensional simulations. The effect of different boundary conditions imposed at the outflow boundaries were also examined. It was found that a simpler and less costly truncated geometry of the tracheobronchial tree can be used in the simulations to accurately predict ozone uptake and wall flux, provided that the flow distribution at the outflow boundaries of the truncated geometry is based on the total cross-sectional area of all of their downstream outflows in the complete geometry of the tracheobronchial tree.