Numerical Modeling Of Hvof Flame And In-flight Particle

HVOF i s o ne of the very c omplicated process involving a large s eries of perplexing p hysical-chemical states, with lots of influence factors, many of which, furthermore, have interactive effect. Therefore, in order to improve the properties of the sprayed coatings, a deeper u nderstanding to the HVOF process mechanisms is strongly required except for a large amount of experimental tests. With numerical simulation approach, a more in-depth fundamental understanding of the momentum and heat transfer mechanisms and the thermophysical and fluid mechanical processes during the HVOF can be obtained. Which can serve as a theory direction to improve the properties of the sprayed coatings.In this study, a CFD software FLUENT was used to simulate the HVOF process, which ba sed o n the h ome-made s pray s ystem TJ-9000, oxygen and propane as combustion-support gas and fuel respectively, WC-12Co as feedstock powders. The simulation comprises two steps: the simulation of the continuous flame jet and the coupling simulation of the discrete particles with former continuous flame jet.When the total mass flow rate of The temperature of the flame jet can reach more than 3000K, the velocity is 1800m/s or so. As the cooling effect of the gun wall and the role of viscous resistance, the distribution of the aerodynamic parameters of the flame jet is not uniform, the values of the flame center are highest and decrease along the radial direction.The mass flow ratio of combustion support gas and fuel n has a significant impact t o t he flame t emperature a nd ve locity, presenting a k ind of pa rabolic relationship. So, for n there is a optimal value to the temperature and velocity of the flame jet. In this study, the op timal value n=3. The total flow rate of combustion support gas and fuel Qhas a clear influence to flame temperature, but little effect to flame temperature. Considering to raise the flame velocity, nand Qcan all be taken into a ccount, while w ant to raise o r l ower the f lame temperature, one f easible approach is to adjust the mass flow ratio of combustion support gas and fuel.The highest values of th e t emperature and velocity the particles can get will decrease w ith t he increase of p article size. When the particle is too small, the temperature and velocity have a strong following tendency to the flame jet, which shows a quick raise a nd a quick dr op. As the flame turbulence properties and the existence o f radial velocity, when the particle size is less than a certain range, the particles will fly a long t he radial direction a nd c an't i mpact onto t he substrate. Besides, when the particle size is too small, the particle had a nice melting before it strike the substrate, which led to the badly decomposition and decarburization of the WC. While the too large particle size will led to decreased bonding strength as well because of the low melting degree. In this study, the optimal particle size distribution is 20-40μm, with WC-12Co as feedstock powder.