Computational fluid dynamics investigation of human aspiration in low-velocity air: orientation effects on mouth-breathing simulations

Abstract

Computational fluid dynamics was used to investigate particle aspiration efficiency in low-moving air typical of occupational settings (0.1-0.4 m s(-1)). Fluid flow surrounding an inhaling humanoid form and particle trajectories traveling into the mouth were simulated for seven discrete orientations relative to the oncoming wind (0°, 15°, 30°, 60°, 90°, 135° and 180°). Three continuous inhalation velocities (1.81, 4.33, and 12.11 m s(-1)), representing the mean inhalation velocity associated with sinusoidal at-rest, moderate, and heavy breathing (7.5, 20.8, and 50.3 l min(-1), respectively) were simulated. These simulations identified a decrease in aspiration efficiency below the inhalable particulate mass (IPM) criterion of 0.5 for large particles, with no aspiration of particles 100 µm and larger for at-rest breathing and no aspiration of particles 116 µm for moderate breathing, over all freestream velocities and orientations relative to the wind. For particles smaller than 100 µm, orientation-averaged aspiration efficiency exceeded the IPM criterion, with increased aspiration efficiency as freestream velocity decreased. Variability in aspiration efficiencies between velocities was low for small (<22 µm) particles, but increased with increasing particle size over the range of conditions studied. Orientation-averaged simulation estimates of aspiration efficiency agree with the linear form of the proposed linear low-velocity inhalable convention through 100 µm, based on laboratory studies using human mannequins.

Keywords: CFD inhalability; aspiration efficiency; computational fluid dynamics; continuous inhalation; inhalable particulate mass; mouth breathing; orientation averaged; particle aspiration; particle transport; ultralow velocity.

Fig. 1.

Simulation geometry examples for (a) humanoid at 60° to oncoming wind in simulated wind tunnel and (b) humanoid at 15° to oncoming wind. Both images are oriented looking into the domain entrance, with freestream velocity directed into the page. In (a), the dark surface is the floor, the light gray rectangle in the center is the outflow of the domain, and the medium gray planes are the side and ceiling walls.

Fig. 2.

Comparison of simulated velocity profiles from simulations (0.2 m s⁻¹ freestream, 4.33 m s⁻¹ suction velocity) to flow visualization provided by D.K. Sleeth (0.24 m s⁻¹ freestream, 20 l min⁻¹ breathing).

Fig. 3.

Velocity contours near the inhaling mouth for 0.2 m s⁻¹ freestream with (a) 1.81 m s⁻¹, (b) 4.33 m s⁻¹, and (c) 12.11 m s⁻¹ suction velocities and (d) 0.4 m s⁻¹ freestream with 4.33 m s⁻¹ suction velocities. Legends indicate velocity in meter per second.

Fig. 4.

Example particle trajectories for 0.4 m s⁻¹ freestream velocity and moderate inhalation simulations at 15° orientation. Each image shows 20 particles released upstream at 0.01 m to the right of the mouth center (Y), with the top particle 0.02 m higher than the bottom one.

Fig. 5.

Example particle trajectories for 0.4 m s⁻¹ freestream velocity and moderate inhalation simulations for 7 µm particles at orientations (a) 90°, (b) 135°, and (c) 180° relative to the oncoming wind. Each image shows 20 particles released upstream with the top particle 0.02 m higher than the bottom one.

Fig. 6.

Example particle trajectories for 0.4 m s⁻¹ freestream velocity and moderate inhalation simulations for 82 µm particles at orientations (a) 90°, (b) 135°, and (c) 180° relative to the oncoming wind. Each image shows 20 particles released upstream with the top particle 0.02 m higher than the bottom one.

Fig. 7.

Upstream critical areas, within which all particles in the freestream will be inhaled, for (a) 7 µm and (b) 82 µm aerodynamic diameter particles injected into the freestream of 0.4 m s⁻¹ with mouth inhalation velocity equivalent to moderate breathing (4.33 m s⁻¹).

Fig. 8.

Critical areas for 7 µm particles at 15° orientation, by velocity conditions of simulation. The number corresponds to the freestream velocity, in meter per second, and the letter indicates the inhalation velocity at the mouth (R = at-rest at 1.81 m s⁻¹, M = moderate at 4.33 m s⁻¹, and H = heavy at 12.11 m s⁻¹).

Fig. 9.

Mean aspiration efficiency (fraction) over all simulation conditions.

Fig. 10.

Freestream-averaged aspiration efficiency estimates for mouth breathing from simulations (solid lines and filled markers) and from Sleeth and Vincent (2011) experimental studies (dashed lines and open markers) for what we describe in this paper as (a) at-rest breathing and (b) moderate breathing. The solid gray line without data markers is the current ACGIH IPM curve.

Fig. 11.

Aspiration efficiency from simulations (solid lines) compared to current IPM criterion and proposed low-velocity inhalable criterion (I = 1 − 0.0038d _ae) from Aitken et al. (1999) (dashed lines).