Flow patterns and convective dispersion in the conducting airways of the human lung

Transport of gas and particles within the human lung is of fundamental importance to everyday life. Dispersion of particles in the lung is relevant to three areas: design of therapeutic aerosols and their delivery systems, risk assessment of exposure to toxic particles, and clinical diagnosis of lung function via the inhaled bolus technique. Gas dispersion is critical to optimizing alternative modes of ventilation, such as high-frequency ventilation. Understanding respiratory mechanics is hindered by the complexity inherent to a physiological system such as the human lung.; Convective mixing axially disperses mass in the human respiratory tract. A host of mechanisms have been offered to explain convective dispersion in the lung. Three of these mechanisms, simple axial streaming, augmented dispersion, and steady streaming, are the focus of this effort. Simple streaming is the transport of solute into the airway by a radially or azimuthally varying primary velocity component. Redistribution of mass by lateral mixing from secondary currents is termed augmented dispersion. Steady streaming is the non-zero drift of species in an oscillating flow.; Eulerian (particle image velocimetry) and Lagrangian (laser-induced fluorescence) measurements in a multi-generation model of the airways were conducted in the present work. Reynolds number (Re), flow direction (inspiration and expiration), generation, branch, and Womersley number were the main independent variables in this study. Modeling and simulation were also employed to support the experiments. A summary of the data produced in this effort consisted of secondary flow fields, flow visualizations, effective diffusivities, and transport profiles. This information shows that, while secondary flows increase with Re, secondary currents do not appear to greatly influence the dispersion of passive tracer. Simple streaming may dominate augmented dispersion in the conducting airways for the parameter space investigated since the axial diffusion coefficient does not vary from Re=10 to 100. The data also suggest an alternative explanation for the steady streaming mechanism. Steady streaming in the airways may not be exclusively due to velocity profile differences, since the transport profile is comparable on inspiration and expiration. Rather, the branching nature of lung may contribute to steady streaming.