Direct measurement of entrainment during nanoparticle synthesis in spray flames

Using phase Doppler anemometry the scale of turbulence and the spray dilution by air entrainment are investigated during nanoparticle synthesis by flame spray pyrolysis (FSP). Ethanol and a solution of 0.5 M zirconium propoxide in ethanol are dispersed and combusted using an external-mixing, gas-assisted atomizer resulting in product ZrO₂ particles of about 11 nm in diameter as determined by nitrogen adsorption at a production rate of 100 g/h. Solution droplet size distributions of the solid cone spray are measured and related to standard correlations of spray atomization. Air entrainment and the radial spread of the expanding jet are determined from the gas velocities in horizontal planes across the spray cone at different heights above the nozzle. The isotropy of the turbulence is investigated using measured axial and radial velocity fluctuations. The turbulent flow is characterized by the integral time and Kolmogorov scales as well as the average shear rates acting on droplets and particles. The flow structure of these spray flames is of major importance as it determines product particle size, polydispersity, morphology, and crystallinity.