Computational fluid dynamic modeling of gas flow characteristics in a high-velocity oxy-fuel thermal spray system

A computational fluid dynamics (CFD) model is developed to predict gas dyna mic behavior in a high-velocity oxy-fuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a 12 mm combustion chamber linke d to a parallel-sided nozzle. The CFD analysis is applied to investigate ax isymmetric, steady-state, turbulent, compressible, and chemically combustin g flow both within the gun and in a free jet region between the gun and the substrate to be coated. The combustion of oxygen and propylene is modeled using a single-step, finite-rate chemistry model that also allows for disso ciation of the reaction products. Results are presented to show the effect of (1) fuel-to-oxygen gas ratio and (2) total gas flow rate on the gas dyna mic behavior. Along the centerline, the maximum temperature reached is inse nsitive to the gas ratio but depends on the total flow. However, the value attained (similar to 2500 K) is significantly lower than the maximum temper ature (similar to 3200 K) of the annular flame in the combustion chamber. B y contrast, the centerline gas velocity depends on both total flow and gas ratio, the highest axial gas velocity being attained with the higher flow a nd most fuel-rich mixture. The gas Mach number increases through the gun an d reaches a maximum value of approximately 1.6 around 5 mm downstream from the nozzle exit. The numerical calculations also show that the residual oxy gen level is principally dependent on the fuel-to-oxygen ratio and decrease s by approximately fivefold as the ratio is varied from 90 to 69% of the st oichiometric requirement. The CFD model is also used to investigate the eff ect of changes in combustion chamber size and geometry on gas dynamics, and the results are compared with the nominal 12 mm chamber baseline calculati ons.