Transient Evolution Mechanism and Sensitivity Analysis of Multiphase Flow in HVOF Thermal Spraying
High-velocity oxygen fuel(HVOF)thermal spraying is an emerging technology for surface modification with a low cost and short cycle for preparing coatings that can deposit metal and ceramic powders on different substrate surfaces.The use of liquid kerosene can save fuel under high-power outputs.Before entering the combustion chamber,kerosene fuel was atomized using a coaxial stabilizer.The degree of atomization directly affected the combustion efficiency,which in turn affected the particle temperature and velocity.The transient solution of HVOF spraying visually presents the fragmentation,evaporation,and combustion processes of fuel droplets and more accurately reflects the change in the flame flow characteristics.Quantitatively demonstrating the transient evolution characteristics of thermal spraying is important for optimizing the spraying process.Presently,most studies have been carried out on the steady state in HVOF thermal spraying,which does not demonstrate the transient evolution of the multiphase flow.In this study,a numerical model of the transient evolution of HVOF thermal spraying was established using a JP5000 gun.The effects of the fragmentation and gasification processes of the kerosene droplets and flame flow on the flight characteristics of the particles were comprehensively considered.The continuous and discrete phases were tracked by a two-way coupled Eulerian-Lagrangian method,where the flame flow was considered as the continuous phase and tracked by the Eulerian method,whereas the WC-12Co particles and kerosene droplets were regarded as the discrete phase and tracked by the discrete phase.The realizable k-ε turbulence model and eddy dissipation model(EDM)with the one-step reaction were used to express the flame flow characteristics.The KHRT(Helmholtz Rayleigh)Taylor fragmentation model was used to describe the fragmentation process of aviation kerosene droplets,revealing the interaction between the evaporation and fragmentation of droplets,flame flow,and particle swarm during spraying.The effects of the kerosene droplet diameter,oxygen/fuel ratio,and reactant mass flow rate on the flight characteristics(temperature and velocity)of the particles were evaluated based on the reliability theory.The results showed that HVOF thermal spraying has the characteristics of highly nonlinear and transient evolution.The flame flow shows irregular turbulent behavior from ignition to combustion and obvious regularity as the combustion proceeds.As the particle size decreased,the particle flight trajectory became more concentrated,which was conducive to the heating and acceleration of the particles.The flight temperature and velocity of the particles decreased with an increase in the particle size.The temperature and velocity of the flame flow were high in the barrel,the particle residence time was short,and the heating and acceleration efficiencies were limited.Therefore,the temperature and velocity of particles were low in the barrel and rapidly increased in the air domain,which was because the Mach cone at the gun outlet played a vital role in heating and acceleration of the particles.Particles impacting the substrate in the molten state produce significant melt deformation,which is critical for the formation of high-quality coatings.During impact,the velocity of the particles should be as high as possible at a suitable temperature.This causes the particles to undergo significant deformation at the moment of impacting the substrate,improving the bonding strength of the coating and substrate and reducing the porosity of the coating.Owing to the effect of the high pressure in the combustion chamber,kerosene droplets aggregate to form large droplets,which gather near the fuel inlet in the form of a suspension.The surface of the suspension droplets underwent fragmentation and evaporation,and evaporation occurred in the same region as fragmentation.The influences of the reactant mass flow rate,oxygen/fuel ratio,and fuel droplet diameter on the particle temperature and velocity were demonstrated by sensitivity calculations.The combination of numerical modeling and sensitivity analysis is the most cost-effective means of optimizing the thermal spraying process and provides a theoretical reference for parametric optimization.
万方数据
维普
