The Numerical Simulation Research Onairflow Patterns And Micro-particle Deposition In The Pulmonary Acinus With Rhythmic Wall Contraction/Expansion

During the long period of evolution, the human body has established a complexbifurcate pipeline system with the magnitude order of μm~mm for substancestransportation. It has a complex operational mechanism, thus ensuring its efficient masstransport function. Respiratory system for gas transport and blood circulation system forliquid transport meet in the pulmonary acinus. Here, airflow patterns are varied and themechanism is quite complex. Especially, a part of micro-particle carried by airflow canflow into respiratory system and deposite in the pulmonary acinus, and even penetrateinto blood circulation system through air-blood barrier. Then, these micro-particlescirculate in the whole body. Therefore, it is the pulmonary acinus that has a specialstatus in human body and significant research value.At this stage, the research of the pulmonary acinus is focused on its local property,only modeling a single alveolus or a single alveolated duct geometrically. However, thegeometric models in domestic studies which take into account both local property andglobal property hasn’t appeared up to now. After studying great amount of research athome and abroad, the whole pulmonary acinus2D and3D model are established basedon Weibel A model with reasonable assumptions and simplifications. The two modelsadopt the idea of flow assignment. After establishing the whole pulmonary acinus2Dand3D model, the multiphysics numerical simulation software COMSOL is used toanalyse and study airflow patterns under three unsteady breathing scenarios—sedentarytidal volume (TV) breathing, inspiratory capacity (IC) breathing, and high frequencyoscillation (HFO) breathing. Particle deposition rules of six sizes particles ranging from 0.01to3microns was observed. That alveolar rhythmic contraction/expansion is themain reason for air flow, so moving mesh is used to get alveolar walls moving, and thismakes the biological phenomenon realistic.The research on airflow patterns of pulmonary acinus uses both2D and3D model,and the simulated results obtained by the two models are basically the same. Bycomparing Reynolds number of three different breathing scenarios, the simulated resultsand the methods was proved to be reasonable and reliable in this thesis. By comparingstreamlines at each generation of the pulmonary acinus, the state was observed thatalveolar flows are dominated by recirculating flows in the proximal acinar generationsand radial flows in the deeper acinar generations. From the observation of alveolarstreamlines in specific acinar generation at representative time, the resulting flowpatterns remain unchanged throughout a breathing cycle except the transition stagebetween the inhalation and exhalation. By comparing streamlines at specific acinargeneration under three breathing scenarios, alveolar flows at specific acinar generationremain unchanged under three breathing scenarios. Based on the two points above, it isdeduced that alveolar streamlines depend on the alveolar topology only. By the analysisof pressure drop between the inlet and the outlet at each generation of the pulmonaryacinus, it can be verified that changes of pressure drop over time follow velocitychanges of alveolar wall contraction/expansion, and the analysis result conforms withthe macro explanation of breathing mechanism. At the same time, pressure dropdecreases with increasing of generations, and indicating pressure distribution is uniformin the deep pulmonary acinus.The research on micro-particle deposition rule in the pulmonary acinus usestwo-dimensional model only. Global deposition efficiency and single-generationdeposition efficiency isn’t a monotone function of particle size, and both of themdecreases firstly, and then increases. By focusing on two particle depositionmechanisms Brownian diffusion and gravity sedimentation, it is found that the dominantdeposition mechanism to particles in different size range is different. Gravitysedimentation is the dominant deposition mechanism for particles in the range of0.5to3microns, and Brownian diffusion is the dominant deposition mechanism for particlesin the range of0.01to0.5microns. At the same time, the effect of particle density on particle deposition efficiency is different for particles in different sizes. The effect ofparticle density is greater to larger particles. When lengthening the statistical time,global deposition efficiency is increasing except3micron particle. The trajectory ofparticles controlled by gravity is gravity-oriented, and the trajectory of particlescontrolled by Brownian force is random.The research on airflow patterns and micro-particle deposition rule in thepulmonary acinus provides a quantitative research method for assessment of the harm ofmicro-particles to human body.