Influence of nozzle geometry on spray and flame formation in the context of flame spray pyrolysis

  • Einfluss der Düsengeometrie auf Sprühstrahl- und Flammenentwicklung im Kontext der Flammen Sprühstrahl Pyrolyse

Bieber, Malte; Kneer, Reinhold (Thesis advisor); Fritsching, Udo (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023

Abstract

Liquid atomization is assumed to significantly influence the synthesis of nanoparticles in spray-flames, i.e. Flame Spray Pyrolysis (FSP), since it determines the initial conditions for spray-, mixture- and flame formation. Toinvestigate mutual interdependencies between sub-processes the standardized SpraySyn nozzle is analyzed with respect to atomization, spray and flame formation. High-speed visualization of primary atomization, sprayphase and flame luminescence revealed significantly varying flame states, hence varying synthesis conditions. A pulsating flame combined with short droplet residence times in the flame, may lead to droplets being injected intoeither low or high flame states and varying final particle output. Timescales of spray density and flame fluctuations are quantified and compared. The latter are significantly larger and hence not correlated to spray density. Highspeed Schlieren imaging at nozzle exit suggests the formation of an annular dispersion gas barrier between spray phase and pilot flame. Thus, spray ignition depends on random large liquid lumps with high radial momentum to break through the dispersion gas for spray ignition. To improve spray-flame interaction the nozzle is modified, i.e. dispersion gas flow is angled towards the center axis. This way, radial shear and turbulence are expected to increase and comparison of experimental measurement data from both nozzles show an increased grade of atomization and improved flame stability. In cooperation with my project partners, particle characteristics are examined and compared via online and offline particle analytics with respect to particle structure evolution and product purity. The modified nozzle leads to smaller primary particle sizes and larger specific surface area of final agglomerates. In addition, a sensitivity of sintering dominance on the nozzle geometry is indicated. All results highlight, how each sub-process of FSP is highly influenced solely by nozzle geometry despite comparable operating conditions.

Institutions

  • Chair of Heat and Mass Transfer [412610]

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