Study On Airflow Movement Characteristics And Aerosol Deposition Rule In The Human Upper Respiratory Tract

Human health is threatened by air pollution aerosols and inhaled toxic aerosols through upper respiratory tract can result in respiratory diseases such as asthma, emphysema and bronchitis. The diseases, for example SARS and bird influenza, broke out because virus transmitted in the form of biological aerosols by human upper respiratory tract caused infection. In allusion to all kinds of respiratory diseases, inhaled pharmaceutical aerosols exhibit obvious advantage of preventing and treating the respiratory diseases. The deposition location and local concentration of toxic aerosols or drug aerosols, as well as the harmful degree of the toxic aerosols and the therapeutic efficiencies of drug aerosols are greatly determined by airflow field, aerosol properties, breathing patterns and geometric airway characteristics. Therefore, airflow movement characteristics in the upper respiratory tract were investigated and the deposition rule of toxic aerosols or drug aerosols in the human upper respiratory tract were discussed, which not only have significance of understanding the harm resulting from toxic aerosols, evaluating dosimetry-and-health-effect and exploring the mechanism for the diseases generation being attributed to toxic aerosols, but also play a very important role in improving therapeutic efficiencies of drug aerosols.Based on the present research, an entire human upper respiratory tract model with mouth, pharynx, larynx, trachea and triple bifurcation, combining the idealized mouth-throat model of ARLA(Aerosol Research Laboratory of Alberta) and the trachea-triple bifurcation of Weible's model, was presented. The methods of integrating numerical simulation of CFD with experimental research were used to study airflow movement characteristics and deposition rule of aerosols. The main conclusions of the present work are summarized as follows:The phenomenon of airflow separation appears near the outer wall of the pharynx and the trachea in the steady respiratory patterns. The high velocity zone is created near the inner wall of the trachea, which may result in the aerosol deposition. Two symmetry rotating secondary vortices are generated in the cross sections of the trachea, which lead to the increase of the shearing strength acting on the inner wall of the trachea and the decrease of the shearing strength acting on the outer wall of the trachea, while the axial velocity cause the distribution of the high shearing strength on the inner wall of the trachea, on the contrary, the shearing strength acting on the outer wall of the trachea is low. Secondary vortices lead to the aerosol deposition on the inside wall of the trachea. The airflow splits at the divider and a new boundary layer is generated at the inner wall of the downstream bifurcation with the high velocity near the inner wall of the trachea, the maximum velocity at the exterior of the boundary layer and low velocity near the outer wall of the trachea.The high velocity zone is created in pharynx, larynx and upper part of the trachea downstream of the glottis during the accelerating inhalation phase in the cyclic inhalation pattern, which may induce aerosol deposition due to inertial impaction. The airflow separates gradually near the outer wall of the pharynx and near the outer wall for the upper part of the trachea downstream of the glottis with the features of the separation zone appearing, which result in aerosol circulatory movement following the recirculation airflow in separation zones and the increase of detention time, ultimately lead to a few aerosol deposition on the outside wall of the pharynx and on the outside wall of the upper part of the trachea downstream of the glottis. During the accelerating inhalation phase, the parabolic shape velocity profiles appear in the middle plane of the bifurcation. The mainstream deflecting from the outer wall of the bifurcation flows into the inner wall of the bifurcation, which result in aerosol deposition easily on the inside wall of the bifurcation. The secondary vortices strength is enhanced gradually in the cross section of the bifurcation, which cause the increase of the probability of aerosol deposition on the inside wall of the bifurcation. The separation zone coming into being near the outer wall of the larynx extends to the inner wall of trachea and the phenomenon for airflow separation is presented near the inner wall of the pharynx at the same time during the accelerating inhalation phase. The separation zone near the outer wall of the trachea extends to the inner wall of the trachea and downstream of the trachea. The phenomenon for airflow separation comes into being near the inner wall of the first bifurcation and a small separation zone appears. No similar phenomenon is observed at the second and third bifurcation.Unlike the accelerating inhalation phase, the airflow velocity profiles in the middle plane of the trachea are uniform during the accelerating exhalation phase. Four secondary vortices are generated gradually in the trachea. The airflow flowing through the bifurcations make the velocity distribute uniformly, which especially make the maximum velocity around the axis disappear and the velocity profiles become uniform in the middle plane of the bifurcation. The typical parabolic velocity profiles appear in the middle plane of the bifurcation. The secondary flows in the bifurcation experience the change from two vortices to four vortices. During the decelerating inhalation phase, the airflow separation phenomenon is exhibited near the inner wall of the first and second bifurcation, finally the separation zone forms near the inner wall.The high axial velocity zone and secondary flow appear gradually in the pharynx, larynx, trachea and triple bifurcation, i.e., they are generated intermittently in the cyclic respiratory pattern. Therefore, the high shearing strength zone caused by the convergence of the airflow shearing strength acting on the airways wall form intermittently, only appear at the part time in the period. The direction of the shearing strength acting on the wall vary periodically, which not only result in the increase of the probability of the wall strain and tissue injury, but also lead to aerosol deposition easily in these areas at the same time, eventually may induce all kinds of respiratory diseases. The airflow movement, airflow shearing strength distribution and aerosol movement in the bifurcation during the exhalation phase are more complex than the inhalation phase.A replica of the human upper respiratory tract for experiment was constructed using stereolithography(SL). The experiment research was performed with the Particle Image Velocimetry(PIV) technique and inspiration flows were examined under steady flow conditions. The airflow movement characteristics in the human upper respiratory tract in the conditions of the low intensive respiratory patterns were discussed. The results show that the numerical simulation data are in reasonable agreement with experimental measurements, which verifies that the numerical simulation methods are accurate and reasonable.The Lagrangian method computed the trajectory of each aerosol in the human upper respiratory tract and the deposition characteristics of aerosols were investigated. Inertial impaction is the main mechanism of deposition for micro aerosols and inertial impaction parameter is an important parameter of evaluating particle deposition due to the inertial impaction. The DE(deposition efficiency) of aerosols in different zones of the upper respiratory tract increase with the increasing of inertial parameter, while turbulent dispersion, secondary flows and recirculation flows also influence aerosol deposition in the upper respiratory tract. The aerosol deposition patterns are little affected by the respiratory flow and aerosol properties. The high DE occurs in the larynx area due to the inertial impaction and turbulent dispersion. The DE is higher in trachea than in bifurcation, higher in cyclic inhalation pattern than in steady inhalation pattern and higher in cyclic inhalation pattern than in cyclic exhalation pattern.The test-bed for measuring aerosol deposition in upper respiratory tract was set up and the experiment for aerosol deposition was performed. The computed results agreed well with the experimental results. The average error is 11%. The numerical simulation method for aerosol deposition in the human upper respiratory tract can predict aerosol deposition patterns and the DE in different locations, and it is also an effective way of obtaining the deposition information for toxic aerosols or inhaled pharmaceutical aerosols.