Analysis of Lagrangian dynamics of ammonia spray using computational singular perturbation*

Computational singular perturbation (CSP) is an advanced mathematical tool employed to analyze the different time scales in a physical process. Given a set of five state variables, Lagrangian droplet dynamics is described by a set of five ordinary differential equations (ODEs). Similarly to the existing CSP framework for reacting flows, the modal analysis of the ODE system provides an indication of the active modes and the relative time scales with the definition of a tangential stretching rate (TSR). In addition, by defining all the processes that contributes to the droplet evolution, it is possible to compute time and amplitude participation indices of a single process to the active modes and to the state variables evolution.  In this work, the CSP-spray approach is first introduced and applied to single ammonia and acetone droplets to analyze the characteristics of the modes and, subsequently, it is applied to a set of direct numerical simulations of  diluted ammonia spray jets, revealing key processes affecting evaporation and clustering of ammonia droplets.