RECIRCULATING FLOW IN AN EXPANDING ALVEOLAR MODEL: EXPERIMENTAL EVIDENCE OF FLOW-INDUCED MIXING OF AEROSOLS IN THE PULMONARY ACINUS

Mixing processes of aerosol particles in the pulmonary acinus by rhythmic expansion of alveolar walls have recently been simulated within a realistic geometric model of the alveolar duct (Tsuda et al., 1995). We studied the velocity fields in an expanding largescaled silicone copy of this alveolar model applying particle-image-velocimetry. Without wall expansion the flow fields are characterized by a large recirculation region within the alveolar sac and a separation streamline separating this region from the duct flow. With wall expansion, the topology of the flow entirely changed. A separation streamline at the alveolar opening disappeared. There was convective flow exchange between duct and alveolus. Large recirculation flows were often observed in the expanding alveolus, indicating the occurrence of stagnation saddle point in the alveolar flow field. Such singularity point is characteristic for the occurrence of chaotic flow mixing, which in turn leads to significant increase in deposition of fine aerosols in the pulmonary acinus.