Feasibility Analysis On Transient Speed Of Sound Investigations Using Laser-Induced Thermal Acoustics In Evaporating Droplets

The development of combustion processes to achieve higher efficiencies has led to a continuous increase in operating pressures that exceed the critical point of the injected fluids. Especially for supercritical atmospheric conditions with respect to the injected fluid, the fundamental physics of the occurring mixing and evaporation processes are not fully understood. In particular, quantitative data for the validation of numerical simulations as well as analytical models remain sparse. Laser-induced thermal acoustic (LITA) is a seedless, non-intrusive measurement technique capable of detecting speed of sound data within these mixing processes. Until now, the feasibility of time-resolved LITA measurements in complex fluid dynamic processes has not been proven. Therefore, a feasibility analysis on transient speed of sound investigations using laser-induced thermal acoustics in evaporating droplets has been conducted. LITA has hereby been applied in the wake of a free-falling acetone droplet evaporating in a nitrogen atmosphere at nearcritical conditions. To the authors’ best knowledge, this is the first-time transient LITA measurements investigating macroscopic fluid phenomena, such as fluid injection and evaporation processes, have been conducted. The evaporation of the droplet has been simultaneously visualised using shadowgraphy, whereas time-resolved speed of sound data have been directly detected by a high-speed LITA system with a high-repetition rate excitation laser source.