Introduction In the past few years

Introduction
In the past few years, Computational Fluid Dynamics (CFD) has been applied to describe the fluid flow in a wide range of applications including medical and engineering applications because of its ability to allow people to investigate different fluid flow variables as well as fluid forces. One way of using CFD simulations is to characterize the fluid flow in human airways models. The air flow can enter human bodies through breathing and reach the lungs, which mainly responsible for delivering oxygen and removing carbon dioxide. Air goes to the human body through the mouth and the nose into the trachea. The trachea then divided into two main primary bronchi (left and right) and each bronchus consists of smaller secondary bronchi. This reducing behavior in the diameter will continue until the air reaching to the alveoli. The air flow contains different particle sizes in both micro and nano-size particles, which affect the human health if they deposited in the undesirable place of the body and resulting in unwanted side effects. As a result, aerosol drug delivery devices are very important and proper design method required to deliver the particles into the lungs and then to the bloodstream without deposition in the respiratory tract. Two main factors that influence the particle deposition, which are particle size and human’s flow manners during inhalation. There are two main approaches in the literature to treat the particles in the air flow through the respiratory system which is Euler-Lagrange and Euler-Euler methods. The main difference between them is Euler-Lagrange works properly for low volume fractions and single phase flow while the Euler-Euler method can be used to solve two-phase flow with high phase volume fractions.
Summary of the main paper
In this study, CFD simulation based on realistic geometry model of the human airways was rebuild from computed tomography (CT) scan images to study the effect of the different breathing conditions on airflow behavior and particle transport and deposition, while almost all models available in the literature were based on a simplified geometry models and didn’t consider the three-dimensional precipitation of each branch in the bronchial tree. The air flow behavior and particle deposition fraction were investigated in several breathing conditions like light (15L/min), normal (30L/min) and heavy (60L/min) breathing conditions, different mass flow rate (m ?=4.2097*?10?^(-11) kg/s,m ?=5.2622*?10?^(-9) kg/s,and m ?=4.2097*?10?^(-8) kg/s, spherical particle density ?_p=1000 kg/m^3 , and with various particle diameters 1, 5, and 10?m.
The realistic geometry of the human respiratory tract was obtained from a CT scan of a healthy 63 years old, non-smoking male. The CT-scan consisted of 373 images were used to build the model which starts from the inlet of the trachea until the end of the secondary bronchi (G0 – G2). These slices were saved in DICOM files, then imported in the 3D-DOCTOR software which is a powerful software in the field of image processing. Subsequently, all images disconnected and exported the constructed model to Standard Tessellation Language format (STL). The STL format file is prepared for importing in CATIA-V5 software for converting the.STL format to CATPART format then has been converted to IGES format. Finally, ANSYS-Workbench 15 was used to create the face, volume, mesh and extension tubes at inlet and outlets and a mesh file was produced and then read into ANSYS FLUENT 15.
In the meshing process, the unstructured mesh was created because the realistic human respiratory tract are irregular and they noticed that the solution changed when the grid refined further, but beyond 1,800,000 the solution becomes independent of the grid size. Then ANSYS FLUENT 15 was used to solve the governing equations (continuity, momentum, and k – ? SST turbulence model) with finite volume method (FVM) and with the suitable assumptions such as the flow is steady state, incompressible, and Newtonian fluid while the boundary conditions applied in this simulations were mass flow rate for the inlet, pressure for the outlet, and no slip boundary conditions for the walls. Also, the discrete phase model (DPM) was used to compute the particle trajectory by applying the balance of forces equation acting on the particle.
Their results showed that the maximum deposition occurred for particle diameter d_p=10?m which independent of the flow rate. However, for m ?=30 L/min, when d_p=5 ?m and 10 ?m and for flow rate=15 L/min, when? d?_p=1 ?m, the particle precipitation fraction have an extreme amount in zone number 1 and 3. Moreover, the pressure distribution has its maximum value when the flow rate 60 L/min that it is reasonable. At last, at the flow rate = 15 L/min, 30 L/minand 60 L/min with d_p=5 ?m and at 0.4 s after injecting the particles, about 4.3%, 8.5% and 5.2%, respectively of the particles trapped to the walls. The main reason for choosing this study was it is one of the first researches that investigate the particle transport and deposition on a realistic airway model and gives a better understanding of the air-particle mechanisms in the realistic airway and therefore would be helpful in the medical industry especially for drug delivery scientists

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