Numerical Simulation Of The Effects Of Airway Deformation And Alveolar Pores On Distal Lung Flow And Mass Transfer

The intrapulmonary environment is extremely complex,especially in the distal lung region.The sputum in the distal lung of patients with pneumonia usually blocks the bronchus,causing physiological reactions such as cough to promote sputum excretion,and usually needs to use drug targeted transport to treat respiratory diseases such as bronchial obstruction.Therefore,understanding the hydrodynamics mechanism of sputum excretion in the distal lung and the deposition mechanism of particulate matter is of great significance for the prevention and treatment of respiratory diseases.The distal lung(13^th-23^rd generation)is located at the distal end of the lung,which has complex anatomical and biomechanical characteristics.The basic characteristics are that the space of bronchus and alveoli is completely filled,and all alveoli and bronchus synchronously undergo large-scale deformation（contraction/expansion）.Due to the constraints of experimental conditions,the current research on intrapulmonary flow is mostly numerical research based on computational fluid dynamics（CFD）,which can better describe the temporal and spatial changes of flow parameters,such as velocity and pressure.Based on respiratory physiology,anatomy and Weibel-A model,this paper makes reasonable assumptions and simplification of the distal lung(13^th-23^rd generation,including trachea,bronchus and acini).The tessellational acinar model（including bronchus and alveoli）and distal lung model with alveolar pores were constructed by means of plane regular hexagon tessellational subdivision,fusion and coordinate transformation.The bronchial inlet was set as a constant pressure boundary.All walls were set as periodic moving boundaries,so that the alveoli and all bronchus contract and expand in the same phase.The airflow field of distal lung and acinus was studied by Euler method;and Euler-Lagrange method were used to study the particles deposition in the distal lung;the level set model was used to study the movement of sputum.The governing equations include continuity equation and N-S equation for controlling air flow,Newton’s second law equation for controlling particle movement,and level set function for controlling sputum movement.The air flow,particle deposition and sputum movement were studied by COMSOL Multiphysics software.The effects of bronchial obstruction,bronchial deformation,respiratory cycle and alveolar pore size on the flow field,particle deposition efficiency and net displacement of sputum movement were studied.（1）The study of air flow in acini shows that:（1）when the bronchus is not blocked,the alveolar pore has no effect on air flow;（2）when the bronchus is blocked,the alveolar pore will establish a bypass ventilation downstream of the blocked area to maintain airflow.With the increase of alveolar pore size,the pressure difference between the blocked side and the non-blocked side decreases.（3）The study of airflow in the distal lung shows that when considering the deformation of the upstream bronchus of the distal lung,the calculated velocity and flow of the distal intrapulmonary bronchus and acini will be reduced.Compared with not considering the deformation,the flow velocity can be reduced by 80%.（2）The study of particle deposition in the distal lung shows that:（1）the high-frequency respiration of the distal lung and the deformation of the upstream bronchus of the distal lung increase the particle deposition efficiency of the upstream bronchus and weaken the deposition efficiency of the downstream acinar region.（2）When the bronchus is blocked,the alveolar pore provide a pathway for particles to enter the downstream of the blocked area.（3）With the increase of respiratory frequency,the deposition efficiency of particles downstream of the blockage area increases.（3）The study of sputum movement in the distal lung shows that:（1）the alveolar pore is the premise for the net displacement of sputum,and there is an optimal alveolar pore size.（2）The increase of respiratory intensity and the decrease of expiratory cycle will break the symmetry of sputum movement under low Reynolds number in the distal lung and realize the net displacement of sputum movement.（3）The greater the respiratory intensity and the smaller the expiratory cycle,the greater the net displacement of sputum.The study of the flow field characteristics of acini and distal lung,the deposition law of particles in distal lung and the movement law of sputum provide a quantitative research method for evaluating the harm of particles to human body and treating respiratory diseases and alleviating sputum blocking bronchus.At the same time,it is of great significance to study the deposition law of particles and the mechanical mechanism of sputum excretion in the distal lung for the prevention and treatment of respiratory diseases and self-relief of sputum blocking the respiratory tract.