Particle flight behavior in hypersonic flame spraying

Abstract

High velocity oxygen fuel (HVOF) coatings have high bonding strengths and compactness, which can improve the wear, corrosion, and fatigue resistance of an underlying matrix. These coatings are widely used in chemical industries, metallurgy, aerospace, and other fields. Here, we studied hypersonic flame spraying through simulating flame flow fields and particle flight processes using the computational fluid dynamics software Fluent. The HVOF system uses oxygen as a combustion-supporting gas and kerosene as fuel. The temperature, velocity, and pressure distributions of the flame flow in a spray gun before adding particles were studied. The dynamic flight behavior of spray particles was calculated using a discrete phase model, and the effects of particle size, injection velocity, and sphericity on particle trajectory, velocity, and temperature were investigated. The optimal particle size range was 30–50 μm. Particles that were too large collided with the inner walls of the spray gun, hindering the combination of the particles and matrix. Particles that were too small were liquid during flight, and readily reacted with oxygen, leading to a reduction in the amorphous content of the prepared coatings. In the optimal size range, particles were uniformly distributed in the center of the flame flow, and the particles were in a molten state, ideal for forming coatings with higher bonding strengths. A systematic study of injection velocities on spray particle dynamics, determined the optimal injection velocity for small, medium, and large particles as 10–15, 5–10, and 1–5 m·s−1, respectively. Compared with spherical particles, nonspherical particles had higher drag coefficients, greater acceleration in the flow field of the flame, and gained more kinetic energy and less heat during flight.

Keywords

• hvof thermal spraying
• numerical simulation
• fluid dynamics
• laval nozzle
• flight behavior