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New insights into the breathing physiology from transient respiratory nasal simulation

We investigated the flow characteristics and heat transfer during nasal breathing in the complete human upper airway through the respiratory cycle using transient numerical simulations. We postulate the complete airway from the nasal cavity to the trachea most accurately represents dynamic airflow patterns during inhalation and exhalation as they are likely to be affected by downstream anatomical structures. We constructed a 3-Dimensional model from a healthy adult computed tomography (CT) scan. Computational Fluid Dynamics simulations were performed with Ansys Fluent software using the Stress-Blended Eddy Simulation (SBES) turbulence model, looking at airflow patterns, velocity, mucosal temperature and humidity (H2O fraction). We simulated one-and-a-half breathing cycles (5.65 seconds) and discarded the first inhalation cycle to avoid start-up effects. The results demonstrated that airway geometry structures, including the turbinates, the soft palate and the glottic region, affect the flow patterns differently during inspiration and expiration. It also demonstrated phenomena not seen in steady flow simulations or those without the lower respiratory tract geometry, including the nasopharyngeal temperature imprint during inhalation, the nasopharyngeal jet during exhalation and the flow structures of the larynx and laryngeal jet. The inclusion of the exhalation phase demonstrates airflow pre-conditioning before inhalation, which we postulate contributes to achieving alveolar conditions. Alveolar temperature and humidity conditions are not achieved by the nasal cavity alone, and we demonstrate the contribution of the nasopharynx and larynx to air conditioning. Including the complete airway with realistic anatomy and using transient airflow modelling provided new insights into the physiology of the respiratory cycle.