Laminar-to-turbulent fluid-particle flows in a human airway model

As in many biomedical and industrial applications, gas–solid two-phase flow fields in a curved tube with local area constrictions may be laminar, transitional and/or turbulent depending upon the inlet flow rate and tube geometry. Assuming steady incompressible air flow and non-interacting spherical micron-particles, the laminar-to-turbulent suspension flow problem was solved for a human airway model using a commercial software with user-supplied pre- and post-processing programs. All flow regimes (500<Relocal<104) were captured with an low-Reynolds-number k–ω turbulence model. Considering different steady inspiratory flow rates (15⩽Q⩽60 l/min) and Stokes numbers, the three-dimensional simulation results show the following: (i) The onset of turbulence after the constriction in the larynx can be clearly observed when the inspiratory flow rate changes from low-level breathing (Qin=15 l/min) to high-level breathing (Qin=60 l/min). The flow reattachment length in the trachea becomes shorter, the axial velocity profile becomes more blunt, and the secondary flow decays faster with the occurrence of transition to turbulence. (ii) Particles follow the basic relationship between airflow and particle motion very well at the lower inspiratory flow rate (Qin=15 l/min); however, particle motion seems to be random and disperse, i.e., influenced by flow fluctuations in case of high inspiratory flow (Qin=60 l/min). (iii) Turbulence can enhance particle deposition in the trachea near the larynx to some extent, but it is more likely to affect the deposition of smaller particles (say, St<0.06) throughout the airway at relatively high flow rates (Qin=30 and 60 l/min) due to turbulent dispersion. However, the particle size and inhalation flow rate (i.e., Stokes number) are still the main factors influencing particle deposition when compared with turbulent dispersion alone. The methodology outlined can be readily applied to other two-phase flows undergoing changing flow regimes in complex tubular systems.