Numerical Analysis of the Effects of Using Effervescent Atomization on Solution Precursor Thermal Spraying Process

Abstract

The solution precursor thermal spraying (SPTS) process is used to obtain nanosized dense coating layers. During the SPTS process, the in situ formation of nanoparticles is mainly dependent on combustion gas-temperature, gas-pressure, gas-velocity, torch design, fuel type, and oxygen-fuel (O/F) mixture ratios, precursor injection feeding ratio and flow rates, the properties of the fuel and precursor and its concentration, and the precursor droplets fragmentation. The focus of the present work is the numerical study of atomization of pure solvent droplet streams into a fine droplets' spray using an effervescent twin-fluid atomizer. For better droplet disintegration, appropriate atomization techniques can be used for injecting the precursor in the CH-2000 high-velocity oxygen fuel (HVOF) torch. The CFD computations of the SPTS process are essentially required because the internal flow physics of the HVOF process cannot be examined experimentally. In this research for the first time, an effervescent twin-fluid injection nozzle is designed to inject the solution precursor into the HVOF torch, and the effects on the HVOF flame dynamics are analyzed. The computational fluid dynamics (CFD) modeling is performed using a linearized instability sheet atomization (LISA) model and validated by the measured values of droplets size distribution at varied gas-to-liquid flow rate ratios (GLR). Different nozzle diameters with varied injection parameters are numerically tested, and results are compared to observe the effects on the droplet disintegration and evaporation. It is concluded that the effervescent atomization nozzle used in the CH-2000 HVOF torch can work efficiently even with bigger exit diameters and with higher values of viscosity and surface tension of the solution. It can generate smaller size precursor droplets (2 μm < d < 20 μm) that could help in the formation of fine nanostructured coatings.