Computational fluid dynamic modelling of particle deposition in human upper airways

This thesis documents a Doctoral research program whose objective was the study of particle deposition in the human lung, based upon a computational fluid dynamics (CFD) modelling. Research in this field started as early as 1960 with the general aim of understanding how various particle sizes can cause lung diseases and how the delivery of aerosolized drugs is more effectively deposited in the human lung. These early studies, however, focused heavily on experimental in vitro methods, at least until advancements in computing technology facilitated the use of CFD as a tool in engineering applications with high degree of accuracy. However, even after more than a half century of research, data on particle deposition in the human lung was less than ideal, and hence there was still a need to develop a holistic approach to this problem. The holistic approach was the main theme of this Doctoral research. The first part of this thesis presents background information about the human respiratory system; mechanisms of particle deposition and the governing equations. This information is then used for defining the model domain and the boundary initial conditions, as well as to provide support and physical interpretation of the numerical predictions. The second part of the thesis documents an in vitro experimental study whose objective was to determine the validity/veracity of the results. The experiments that were conducted used Laser Doppler Anemometry (LDA) to measure steady fluid flow under various operating conditions. Then a one to one simulation was created using a commercial CFD program (known as CFX), with the aid of Computer Aided Design (CAD). The LDA data obtained in vitro was used to validate the numerical prediction and build confidence in using CFX as a CFD tool for fluid flow and particle deposition simulation. A good agreement was obtained between the experimental and numerical predictions. The third section of this thesis is related to the fluid flow in a four generation bifurcation, in which the results were related to the upper section of the human lung. Flow fields along the plane and cross section planes were investigated. The findings from this part of the study showed a secondary flow downstream with imbalanced mass flow rate, upon using a zero relative pressure at the outlets. This section demonstrated how important the boundary condition was to obtaining realistic results. Moreover, an important aspect of CFD analysis was the quality of the mesh generation. Without a proper refinement of the mesh, the end results were of limited accuracy. The fourth section of the thesis consists of information on how to develop and create realistic human airway model, using the commercial package Solidworks, and how to refine the mesh using the advanced meshing tool ANSYS ICEM. A grid independence test was performed to ensure that the mesh generated can produce accurate results without compromising computational time. The final section of this thesis is related to the analysis of the particle depositions in human airways under various operating conditions. Particle deposition simulation was first studied on a symmetrical model and was later extended to asymmetrical airways and transient conditions. The findings showed that the particles which entered each of the five lung lobes were different and the deposition efficiency was found to be a proportional to the Stokes Number which, in turn, related to the size, density, and the velocity of the particle. Moreover, the results also showed how the entry position of a particle changed the location and possibility of deposition within the lung. The thesis concludes with a discussion of how the research findings contributed to research related to particle deposition in the human lung. Particular emphasis was also given to the simulation conditions that should be used in the analysis of particle deposition in the human lung, and how to generate meshes that could be used for accurately simulating bifurcating flow.