CFD STUDY ON GAS EXCHANGE PHENOMENA INSIDE TRIPLE-BIFURCATION AIRWAYS IN HUMAN LUNGS UNDER ACTUAL BREATHING STATES

摘要

This paper presents numerical simulation of gas exchange phenomena inside triple bifurcation airways of the Weibel model in the human lungs, consisting of G3, G4, G5, and G6. The ICEM/CFD software is employed to construct a three-dimensional, symmetric network consisting of approximately 520,000 grids. The flow inlet is set at the entrance to the G3 airway subject to three different actual breathing conditions, namely at rest, light activity, and moderate exercise. The finite-volume scheme is used to numerically integrate the continuity and three momentum equations for a fully developed turbulent flow for which the LES turbulence model is implemented. Results are obtained for the velocity-vector, axial-velocity contour, and streamline distributions in the airways during the inhalation and exhalation cycles at various instants under actual breathing conditions. It is disclosed that during the inhalation process, fresh air, which is rich in oxygen, is sucked in through the central portion of the G3 airway and move along the walls of airways G4, G5, and G6 inward deep into the lung. During the exhalation process, however, the carbon oxide-rich air moves through about the central part of the branched airways G6, G5, and G4 into the G3 airway and is exhaled into the atmosphere. These kinds of fluid movement clearly indicate a distinct difference in transport routes between the fresh air being inhaled and the old air to be exhaled. Numerical simulation results also reveal that during the inhalation process, the secondary flow generated on the outer walls of the airways G4 and G5 captures the old air and releases it to be exhausted into tlw atmosphere in the subsequent exhalation process. In the exhalation process, however, the secondary flow produced at the bifurcation junction captures the fresh air and releases it to move inward into the lung in the successive exhalation process. In conclusion, through the "capture and release" mechanism caused by the formation and destruction of flow-separation regions, the fresh air can be transferred into the lungs and the old air can be exhaled from the lungs within few breathing cycles. This observation is in complete agreement with the existing experimental observation. Results obtained from the study sheds light on high-frequency ventilation (HFV) and inhalation-type drug delivery in the human body.