Abstract
In spray flame synthesis the processes of spray formation and evaporation of the single droplets greatly affect the morphology and size of particles formed. An in situ measurement of these parameters is thus essential for process control and development. In this work, wide-angle light scattering (WALS) is applied to measure droplet sizes in a spray flame. The scattering data of the spherical droplets are evaluated by applying Mie-theory. For droplet sizing, the number of characteristic maxima in the scattering pattern and the measured scattering intensities are evaluated. Droplet size distributions and their parameters were determined by repetitive exposures in various heights above the nozzle outlet for two solvents: pure ethanol and a mixture of ethanol and 2-ethylhexanoic acid at a volume ratio of 35/65. While for ethanol the median droplet size decreases with increasing height, it decreases less for the mixture, which in general exhibits increased droplet sizes for all heights compared to pure ethanol. Furthermore, we could show that using air instead of nitrogen as a co-flow barely affects droplet evaporation in the flame.
Highlights
For a wide range of materials such as fumed silica, titania or indium tin oxide, gas-phase synthesis is a well-established process for the production of nanoparticles on an industrial scale [1,2,3]
Laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA) in particular are widely used in fluid dynamic research and industrial applications for spray-characterization, as these methods allow to measure velocity, size and concentration of spherical objects like droplets and bubbles simultaneously at a high repetition rate [15,16,17,18,19,20]
As multiple droplet scattering cannot be avoided at small heights above the burner surface (HAB), a manual verification of all fit results was performed resulting in a median value of 17.3 μm, i.e. higher by 25%, from 584 evaluable sphere sizes within 562 images (28.2% of all frames)
Summary
For a wide range of materials such as fumed silica, titania or indium tin oxide, gas-phase synthesis is a well-established process for the production of nanoparticles on an industrial scale [1,2,3]. Among various production processes in the gas phase, spray flame synthesis (SFS), often termed flame-spray pyrolysis (FSP), is a promising approach for a continuous and costeffective production of a variety of functional semiconductor- and metal-based inorganic nanoparticles like titania, zirconia, iron oxides and many others at industrial scales [4,5,6,7,8,9,10,11]. Laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA) in particular are widely used in fluid dynamic research and industrial applications for spray-characterization, as these methods allow to measure velocity, size and concentration of spherical objects like droplets and bubbles simultaneously at a high repetition rate [15,16,17,18,19,20]. PDA requires sufficiently low droplet concentrations as problems arise if several droplets are simultaneously present in the measurement volume
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