A numerical study of high-velocity oxygen fuel thermal spraying process. Part I: Gas phase dynamics

Abstract

A mathematical model is formulated to simulate the effect of operational parameters on the gas dynamics that occur during high-velocity oxygen fuel (HVOF) thermal spraying. Computational fluid dynamic techniques are implemented to solve the Favre-averaged mass, momentum, and energy conservation equations. The renormalization group (RNG) κ-ɛ turbulence model is used to account for the effect of turbulence, and high-order interpolation schemes are employed to resolve compressibility effects in the supersonic jets. The calculated results show that the most sensitive parameters affecting the process are propylene flow rate, total flow rate of oxygen and propylene (oxyfuel flow rate), total inlet gas flow rate, and barrel length. The results show that increasing the total inlet gas flow rate has limited effect on the gas velocity and temperature inside the nozzle for the parameter range investigated in the present study. However, increasing the total inlet gas flow rate increases the total thermal inertia and momentum inertia; moreover, under these conditions the flame gas is retained at a high velocity and temperature for a longer distance. Increasing the oxyfuel flow rate significantly increases flame velocity and temperature, particularly after exiting the nozzle. The effect of propylene flow rate is significant and complex. In order to minimize the extent of the oxidation of the sprayed powder particles and to achieve a high flame temperature and velocity, the overall injected stream should be adjusted to be propylene-rich. The nitrogen flow rate significantly affects the gas flow inside the gun. On the basis of the calculated results, it is evident that, in order to obtain maximum gas velocity and temperature, the nitrogen flow rate should be kept to a minimum, provided that particles can be delivered to the gun in a smooth manner. By minimizing the entrainment of the surrounding air, a nozzle with a longer barrel length achieves a relatively high gas velocity and temperature for a longer distance than does a nozzle with a shorter barrel length. The calculated results are in good agreement with available experimental results.