Flow-induced Mixing at Low Reynolds Number in an Expanding and Contracting Alveolus Model

A recent prospective cohort study (Dockery, 1993) has shown that particulate pollutants below a few micrometers pose the greatest health hazard. Aerosols of this size range penetrate deep into the distal region of the lung and are likely to deposit in the lung periphery. The mechanisms involved in mixing and deposition of these particles in the alveolar region, however, are still not fully understood. In prior studies it has been assumed that convective (flow-induced) mixing between inhaled particles and acinar residual gas is insignificant since the nature of acinar flow is fully viscous and reversible (Davis, 1972, Ultman, 1978). Reversibility implies that the expired gas retraces the motion of inspired gas. However, experimental data contradict this widely-accepted assertion (e.g. Heyder, et al., 1988, Anderson, et al., 1989). In studies on single breath bolus dispersion in human lung, for example, Heyder, et al., (1988) showed appreciable bolus dispersion in the lung periphery which could not be accounted for solely by the particles’ intrinsic motion (Brownian motion and gravitational sedimentation). This suggested the existence of convective mixing at the level of acinus. Although several mechanisms (Cinkotai, 1974, Yu, et al., 1977, Taulbee, et al., 1978, Otani et al., 1990, 1991, Rosental, 1992, Miki, et al., 1993) have been postulated to resolve this question, none of these mechanisms has been tested fully.