Numerical assessment of air conditioning and aerosol deposition characteristics in human respiratory system
Creators: name: MA, Jiawei
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401204
subject: Computational methods in fluid flow, heat and mass transfer (incl. computational fluid dynamics)
classificationCode: 401706
subject: Numerical modelling and mechanical characterisation

description: This thesis presents a numerical investigation of the air flow dynamics and aerosol particle transport mechanism in human respiratory system. Utilising the cutting-edge Computational Fluid Dynamics (CFD) methodology, several subjects with distinct regional morphological phenotypes were comprehensively studied by evaluating the nasal cavity air conditioning performance and aerosol particle deposition characteristics.

Despite the fact that advances in numerical modelling have led to increased understanding of inhaled airflow in human respiratory airways, study on the inhaled airflow dynamics across different human subjects with variable airway morphologies remains limited. Instead of using traditional way to classify the subjects with gender, age or race/ethnicity, this study classifies the subjects' airway according to specific regional morphological difference. In the human respiratory airway study, six human nasal cavity models were included for the comparison. In the meantime, a monkey nasal cavity model was also investigated, as animals have been widely used as surrogates for evaluating the health effects of inhaled airborne particulate matter.

This thesis is composed of eight chapters:

Chapter 1 to Chapter 3 demonstrate the research background, literature review and the research methodology. Chapter 4 to Chapter 6 focus on the numerical study on human nasal cavity region. Chapter 7 shows the results of  the CFPD simulation on typical surrogate animal models.

In Chapter 4, six reconstructed human nasal cavity models from Computed Tomography (CT) scan were studied. Scope of investigation includes  the airflow patterns, heat and mass transfer, and nasal resistance along the nasal passages in each model. For the mass transfer, a Two-Film Theory has been  applied to model the water vapour transport between the inhaled air and nasal mucosa. In Chapter 5, three representative human nasal cavity models were selected to perform the nanoparticle exposure analysis using Computational Fluid-Particle Dynamics (CFPD) approach. The study compares the total and regional deposition patterns of naturally inhaled nanoparticles among subjects with different vestibule phenotypes. In Chapter 6, an advanced drug delivery concept named Bidirectional Nasal Delivery was simulated and analysed in the selected nasal cavity models. Utilizing three nasal models, the nanoparticles and microparticles were released to simulate the Bidirectional Nasal Delivery method. In Chapter 7, a monkey nasal cavity model was applied to identify the air flow pattern and  to evaluate the potential risk of inhaled toxicants. 

Chapter 8 summarises the results and main findings of the entire thesis. The main findings of the studies in this thesis are listed below:

 - Human nasal vestibule notches significantly alter the respiratory airflow patterns with severe velocity acceleration at the narrowed region containing the nasal valve, which eventually reduces nanoparticle deposition rate in olfactory regions. 

 - Bi-directional drug delivery approach in human nasal cavities study showed that nanoparticles within 1 nm diameters can evenly distribute in nasal cavity region, and deposition 'hot spots' mostly occurred around the anterior portion of the middle meatus where the bulk flow was intercepted by the convoluted nasal passage. With the increase of particle size, the deposition inhomogeneity became even worse. For 10 µm particles, deposited particles were highly packed at the anterior middle meatus and the local deposition density was 70 - 80 folds greater than the overall averaged level.

 - In the monkey nasal cavity study, majority of inhaled nanoparticles propagated via the inferior nasal passage, and obvious particle concentrating preference was found in the left side chamber due to the slightly deviated septum. Being in the most upper part of the nasal cavity, the olfactory region hardly received nanoparticles from inhalation ex-posure.