Novel Insights into Liquid Behavior in Atmospheric Plasma Jets

Liquid injection plasma spraying is of increasing interest, because it permits the use of very fine and also nano-powders for thin and finely structured coatings (suspension plasma spraying, SPS) and “in-flame” synthesis of complex material compositions (solution precursor plasma spraying, SPPS). For both processes a detailed understanding of liquid behavior in the plasma jet is of crucial importance for process and coating optimization. In this paper we present new approaches to study liquid injection diagnostics by observing micrometric sized droplets in plasma jet. A high speed camera with 93.000 fps and 260 ns exposure time at high magnification in shadowgraphy setup allowed the observation of the different stages of single drop fragmentation and atomization in hot atmospheric plasma. It was observed that though drops enter the high temperature core of atmospheric plasma, their evaporation is delayed at least till fragmentation. The main mechanism of droplet break-up is fragmentation due to high velocity of plasma as it was observed in earlier work for drops in cold fast streams. Moreover, it was recorded that in the fast plasma core, the single spherical drops break-up mechanism undergo a transition during in-flight. It starts as sheet stripping upon entering the atmospheric plasma jet periphery at the injection and changes to wave crest stripping mode upon after when reaching close to the core of plasma. In the former case, drops are flattened to lenticular discs which are fragmented at the rim into finer droplets. It is expected that the vaporization does happen at the surface but this probably forms a buffer layer and hinder heat transfer from plasma to drop. This knowledge enabled a new simplified modeling approach to predict drop trajectories with the drop atomization cone model and allowed to calculate average plasma momentum in these experiments to be around 9.3·10⁴ kg s^-2 m^-1