Summary

In this paper, silica particle formation in a turbulent flame was studied. Micron sized spray droplets from liquid tetra–ethyl–ortho–silicate were introduced into a turbulent hydrogen-oxygen flame with a patented Liquid Flame Spraying technique. In this technique, the spraying gas is one of the reactant gases, and in this particular study hydrogen was used. In the flame, the liquid precursor evaporates and reacts in gas phase. The chemical product is finally nucleated generating nanosized silica powder. The purpose of the study was to estimate the spatial distribution of the particle formation for improving the in-flame collection of nanoparticles in commercial applications, where subsequent particle agglomeration needs to be avoided. To achieve that, a simple but effective method for approximating the nucleation of silica vapour was utilised. Results show, that within the turbulent diffusion flame, there is a spatial zone of high temperature with under-saturated silica vapour. This high temperature zone is first following by a region where liquid nanoparticles are generated, then a region where solid silica particles are formed. In conventional laminar diffusion flames with lower temperatures, solid silica particles are directly generated from the silica gas. In our case, the liquid nucleation stage may be described with the classical nucleation theory, but the overall model fails to convert all the silica vapour into particulate form. Therefore, in a large scale it is insufficient and needs compensating modelling of full aerosol dynamics, including barrierless nucleation kinetics, condensation, coagulation, coalescence and particle agglomeration. Another approach is to use a simple equilibrium model based on a constant value for critical saturation ratio for particle forming vapour. However, even with this simpler tool, the on-set of particle formation was probed. The model showed that the particle formation begins before the actual flame region, is interrupted in the high temperature zone but subsequently continues after the hottest part of the flame. The result was verified in the experiments.