Atmospheric ammonia scavenging mechanisms around a liquid droplet in convective flow

摘要

The purpose of this research is to study a single droplet scavenging mechanisms upon atmospheric ammonia by using the two-phase simulation method (TPSM). In the developed method, the continuity, Navier-Stoke, and species conservation equations of the gas phase and the liquid phase are fully solved numerically. The predicted results indicate that the gaseous ammonia is scavenged outward rapidly from the interface initially, on grounds of intrinsic solute-sink characterized by the droplet. When the exposure time is long to a certain extent, by virtue of increasing the solute concentration at the interface, the region affected by the uptake progressively moves back to the droplet. Transient variations of scavenging distances ahead of and behind the droplet are examined to recognize the extension and shrinkage characteristics of the scavenging process. Under the impact of the convective flow, the scavenging distance behind the droplet is always farther than that in front of the droplet. Considering the droplet internal motion, on account of drastic interaction between the primary vortex and the secondary vortex inside the droplet, the vortex bifurcation is exhibited. This further results in that the maximum value of the interfacial concentration tends to shift from the front stagnation point of the droplet to the aft one. In contrast, over the entire absorption process the mass flux is mainly contributed from the front portion of the droplet, resulting from the effect of the momentum boundary layer in the gas phase. Eventually, the predicted difference in absorption period between the TPSM and the rapid diffusion model (RDM) is evaluated. It reveals that the latter substantially underestimates the absorption time when a solute with larger mass diffusion number is regarded.