Airflows In Human Airways Based On Computational Fluid Dynamics

Airway is prerequisite channel to change gas between human body and the outer environment, understanding the function of airway on the physiological and pathological conditions has important scientific and clinical significance. At present, using MDCT (Multi-detector computed tomography), a breath hold scanning can approximately obtain the continuous and (or) high resolution and isotropic overlapped layer structure images of the whole lung. However, the data of airway structure obtained by MDCT cannot clearly reflect related advanced physiological (ventilation) function information. CFD (Computational fluid dynamics) is a method of using computer and numerical discretization to solve the fluid mechanics problems, which can be used to study the airflow in the human airways.We investigated the airflow in the human airways in this paper by the method of CFD. Firstly, constructed an ideal, three-dimensional, two bifurcation, symmetrical airway tree model by using 3D software named SolidWorks, researched the velocity and pressure distribution in the model and the effects of similarity ratio (generation i+1 and generation I airway, airway diameter ratio, length ratio), bifurcation angle and bronchial plane turning angle on the airflow. Then, studied the 4 left pulmonary artery sling (Left pulmonary artery sling, LPAS) patients and 1 healthy control subjects (normal control, NC), extracting of trachea and main bronchus to establish the structural model from CT images, conforming the personalized entrance velocity according to the weight and the entrance area. The distribution of Air velocity, wall stress on the surface, wall shear stress (WSS) on trachea and bronchi in the patients with LPAS were calculated by ANSYS. In addition, using one-way coupling (Uni-directional coupling fluid structure interaction, UCFSI) method, the wall deformation and stress distribution of LPAS patients under the influence of airway was investigated. Finally, the structure model with multistage airways was extracted from CT images (128 outlets), researched not only the airflow on the static and transient conditions, but also the distribution and sedimentation of PM2.5 in the model by liquid-solid two-phase flow model.The main results include:(1) the distribution of velocity and pressure in the ideal airway tree model is symmetrical generally, but they become to be uneven from the fourth grade bronchus; the velocity in endobronchial is less than that of the outer bronchial; pressure drops with the similarity ratio and reduce, but also increased the volume of the airway; pressure drop will be increased when the angles of bifurcation and bronchial are changed. (2) For patients with LPAS, the pressure drop is 78.9-914.5 Pa, far higher than the normal control subjects (0.7 Pa), due to the trachea and (or) main bronchial stenosis; left main bronchus mass flow ratio is not related to outlet area, It is suggested that the C-shaped trachea plays an important role on facilitating the air flow into the left bronchus with the inertia force.; the distribution of velocity, wall pressure and WSS in the airways for LPAS patients is more uneven compared with NC, high velocity, low wall pressure and high WSS can be observed in the stenosis. (3) Wall deformation and equivalent stress for LPAS patients airway is 50-900 and 90-100 times of NC, the moving direction of trachea and (or) main bronchial have relation with the shape, on the contrary, with the net reaction force, the velocity and wall thickness impact on stress and deformation is nonlinear. (4) For the normal subject, there is no significant eddy and streamlines are quite smooth in every phrase. The efficiency of ventilation is very high. The maximal pressure is 22.85 Pa in inspiratory phase, the maximum pressure drop is 7.59 Pa in expiratory phase; the wall pressure distribution is relatively uniform, but they obey different rules; wall shear stress is very small, the peak in the inspiratory and expiratory is the same about 3.02 Pa. The modes of gas mixing inspiratory and expiratory phase are different; there is little difference between the flow percent between inspiratory phase and expiratory phase, proportional to each lobe’s volume.It is obviously that personalized entrance boundary conditions and the airway structure model based on CT images, CFD can provide more quantitative information about the airflow in the airway, including velocity, pressure, wall pressure, wall shear stress, airway wall deformation, PM2.5 sedimentation. The quantitative information will help people to understand the ventilation function associated with the structure information on the physiological and pathological conditions, so as to reveal the different pathogenesis of respiratory diseases.