Multi-scale modelling and risk assessment for inhaled fibers

As the primary organs of human being, the respiratory airway is in direct contact with atmosphere and therefore vulnerable to a range of illnesses. In particular, exposure to airborne fibers is dangerous and lacking in relative knowledge. This thesis presents a numerical investigation of the dynamics of inhaled fiber to uncover their transport and deposition mechanisms in human respiratory airway by utilising Computational Fluid Dynamics (CFD) methodology. Instead of using a single linear motion of equation to simulate the particle transport and deposition, this project adopts coupled translational and rotational motion of equations to investigate the spherical and fibrous particles transport and deposition in 3D human respiratory airways, utilizing a fiber code developed in the ANSYS FLUENTTM UDF (user Defined Function). Five different comparative respiratory airway models are included in this project, and the shear-induced lift and rotation on microfiber transport and deposition in low Reynolds number flows are studied to fill the gap on significance of the lateral migration of particles in the deposition process. This thesis is composed of eight chapters: Chapter 1 presents the motivation of the study and illustrates the potential risk of inhaled fibers to human respiratory airway in context of the fast development of carbon nanotube and carbon fiber industry. The research objectives and outlines are described. Chapter 2 conducts the literature review focusing on the spherical and fibrous particle transport and deposition in human respiratory airway. The lift and rotation toward microfiber transport and deposition in low Reynolds number flows, which is overlooked in the past, however, could be of significant importance to practical applications are also presented. Chapter 3 introduces the numerical simulation methodology utilized in the study, including the image processing, geometry modelling, meshing, governing equations, turbulence modelling, Lagrangian particle tracking method, equivalent sphere method and deposition criteria. In chapter 4, two respiratory tract models, including a parametrically controlled approximate airway model and a CT-based patient specific airway are used to demonstrate the air-particle flows. Air-flow patterns and spherical particles transport and deposition characteristics in these two airway models are evaluated and compared. Chapter 5 investigates the transport and deposition of fibrous particles in the human nasal cavity, resolving the coupled translational and rotational motion to fill the gap. A detailed single fiber trajectory in the 3D nasal chamber, accounting for the coupled translational and rotational movement, is visually presented for the first time. In chapter 6, transport and deposition of non-neutrally buoyant ellipsoidal fibers in Poiseuille flow in horizontal and vertical channels are systematically examined, and various lateral migration scenarios are carefully investigated. In chapter 7, an extended complete realistic respiratory airway from nasal cavity to the distal bronchial tracts up to the 15th generation, covering the lower respiratory airway, is adopted. Airflow, deposition efficiencies and deposition patterns of the spherical and fibrous particles in the complete respiratory airway are investigated. Chapter 8 summarises the main findings of this thesis, which are listed below: • The CFD study of comparative air-particle flows in the upper tracheobronchial trees identifies that the approximate airway model developed by Kitaoka (KG model) could reproduce reliable airflow and predict accurate spherical particle deposition characteristics when the airway surface and bending irregularity are ignored. • The study of spherical particle and fiber deposition in the nasal cavities indicates that fibers' dimensional characteristics (diameter and length) affect the fibers' transport and deposition in the nasal airway. Aerodynamic diameter is insufficient to measure the fiber deposition efficiencies in some special circumstances. Though trajectory paths of fibrous particles with the same aerodynamic diameter are highly analogous, subtle differences in rotation in time and space due to fiber lengths could lead to different deposition outcomes in the human nasal cavity. In contrast to earlier observations, the current computational simulation indicates neither fiber nor spherical particles have a consistent higher deposition rate than the other in the human nasal cavity. • In the study of the shear-induced lift and rotation on microfiber transport and deposition in low Reynolds number flows, the fibers drift across the flow streamlines due to the shear-induced lift and rotation in the vertical channel. While the lateral migration of fiber is negligible in a horizontal channel where the gravity toward the lower wall is dominant. The difference should be further investigated and accounted for in practical applications when predicting fiber deposition in wall bounded flows. • According to the study of fibers deposition in the complete respiratory airway, the fiber deposition characteristics are significant different in nasal cavities, laryngeal airway and lung due to the effects of geometry and impaction from upstream. It is important to account for upstream impact on fiber deposition, especially for high-inertia fibers with large aspect ratios and aerodynamic diameters.