Plasma spraying using liquid precursors makes it possible to produce finely-structured coatings with a broad range of microstructures and properties. However, coating reproducibility and control of the deposition efficiency are critical to industrial acceptance of this process. With conventional d.c. plasma torches where the liquid feedstock is generally injected transversally into the plasma flow, they both depend on transient interactions between the liquid material and plasma jet. Realistic models for the liquid spray process may help to increase the understanding of the process. For most of the spray conditions, a key point of the model is its ability to predict the droplet fragmentation, as this mechanism conditions the trajectories of droplets and their subsequent treatment in the plasma flow. Various mathematical models for droplet fragmentation are proposed in the literature. However, they include parameters and constants that are determined from theoretical considerations and experimental results that still need to be validated for plasma spraying conditions.
This study deals with the simulation of liquid material injection and break-up in the plasma jet with the enhanced Taylor break-up analogy (ETAB) model. The model constants were adapted to plasma spray conditions thanks to the observation of the liquid in the plasma flow. The latter was carried out using a shadowgraphy system based on pulsed backlight illumination of the liquid material.
Summary: Plasma spraying using liquid precursors makes it possible to produce finely-structured coatings with a broad range of microstructures and, thus, properties. However, coating reproducibility and control of the deposition efficiency are critical to industrial acceptance of this process. With conventional d.c. plasma torches where the liquid feedstock is generally injected transversally into the plasma in the form or liquid jet or blobs, they both depend on transient interactions between the liquid material and plasma jet. Realistic models for the liquid spray process may help to increase the understanding of the process. For most of the spray conditions, a key point of the model will be its ability to predict the droplet fragmentation, as this mechanism will condition the trajectories of droplets and their subsequent treatment in the plasma flow. Various mathematical models for liquid jets and droplet fragmentation are proposed in the literature. However, they include parameters and constants that are determined from theoretical considerations and experimental results that still need to be validated for plasma spraying conditions.
This study deals with the simulation of liquid material injection and break-up in the plasma jet with the enhanced Taylor break-up analogy (ETAB) model that was implemented in the CFD Code_Saturne code. The ETAB model constants were adapted to plasma spray conditions thanks to the experimental observation of the liquid behaviour in the plasma flow. The latter was carried out using a shadowgraphy system based on pulsed backlight illumination of the liquid material using pulsed Nd-YAG lasers.
