Effect of nozzle geometry on the gas dynamics and evaporation rates of Suspension High Velocity Oxy Fuel (SHVOF) thermal spray: A numerical investigation

Thermally sprayed coatings formed from sub-micron or nanoparticles feedstock provide enhanced coating properties and Suspension High Velocity Oxy Fuel (SHVOF) thermal spray is a process that allows the deposition of dense coatings from fine particulates. However, in SHVOF thermal spray, submicron and nanoparticles are significantly influenced by fluctuations in the gas velocity and temperature field because of the turbulence and disturbance of the gas fields from the breakup of the suspension due to the small inertia of the particles. In this study, an ethanol suspension containing widely used engineering ceramic particles, TiO2, with four different particle concentrations was considered 0, 10, 15 and 25 wt%. The aim of this paper was to model four nozzle geometries and to investigate the influence the nozzle geometry has on the gas flow and the evaporation rate at four particle concentrations. Comparing four nozzle geometries it is found that the gas exits the longer barrel nozzles at a high velocity; while the velocity in the free jet region beyond the shock diamonds is greater for the shorter nozzles. It is seen that the shorter the nozzle the higher the gas temperature in the free jet; which is due to the lower contact time of the gas with the cooled nozzle walls. This study also considered the effect the nozzle geometry has on the evaporation of the suspension. It was found that the longer the combustion chamber the lower the maximum evaporation rate. Moreover, higher particle concentrations result in evaporation commencing closer to the nozzle inlet due to the lower volatile fraction of the suspension.