Flow Recirculation in Cartilaginous Ring Cavities of Human Trachea Model

Despite the prevailing assumption of "smooth trachea walls" in respiratory fluid dynamics research, recent investigations have demonstrated that cartilaginous rings in the trachea and main bronchi have a significant effect on the flow behavior and in particle deposition. However, there is not enough detailed information about the underlying physics of the interaction between the cartilage rings and the flow.

This study presents an experimental observation of a simplified Weibel-based model of the human trachea and bronchi with cartilaginous rings. A transparent model and refractive index-matching methods were used to observe the flow, particularly near the wall. The flow was seeded with tracers to perform particle image velocimetry and particle tracking velocimetry to quantify the effect the rings have on the flow near the trachea and bronchi walls. The experiments were carried out with a flow rate comparable with a resting state (trachea-based Reynolds number of Re_D = 2650).

The results present a previously unknown phenomenon in the cavities between the cartilaginous rings: a small recirculation is observed in the upstream side of the cavities throughout the trachea. This recirculation is due to the adverse pressure gradient created by the expansion, which traps particles within the ring cavity, thus affecting the treatment of patients suffering from lung disease and other respiratory conditions.

The detection of recirculation zones in the cartilage ring cavities sheds light on the particle deposition mechanism and helps explain results from previous studies that have observed an enhancement of particle deposition in models with cartilage rings. These results bring to light the importance of including cartilage rings in experimental, numerical, and theoretical models to better understand particle deposition in the trachea and bronchi. In addition, the results provide scientists and medical staff with new insights for improving drug delivery.