Drop Dynamics in Heterogeneous Spray Flames for Nanoparticle Synthesis

Abstract

In flame spray pyrolysis (FSP), the understanding of the atomization and spray formation of the coaxial atomizer is a key step to control the mixing of reactants, and precursor release, as well as nanoparticle formation and growth mechanisms. The drop size and drop dynamics of fuel/precursor mixtures are relevant parameters to control the particle synthesis via gas-to-particle conversion for tailoring the nanoparticle. The investigation of the multiphase flow structures, especially in the dense spray region using sophisticated nonintrusive laser diagnostics, such as phase Doppler anemometry (PDA), enables a characterization of the burner with multiscale spatial and temporal resolution. In the present work, the evolution of drop size and velocity under varying iron nitrate (FNT) concentration of the precursor is investigated allowing for an estimation of precursor release and evaporation via detailed PDA measurements. The drop size distributions of pure ethanol, and ethanol + 2-ethylhexanoic acid, as well as for FNT-based spray flames, were studied in order to reveal the influence of metal precursors on the combustion. Drop size/velocity correlations were determined in order to resolve the size class averaged evolution of drop velocities. The evolution of the drop size distribution along the axial centerline revealed a variation of the evaporation rate in space and increased evaporation rates for FNT containing spray flames. Furthermore, bimodal drop size distributions were observed for heights above the burner larger than 40 mm in FNT precursor spray flames, indicating the occurrence of possible µ-explosions of drops, that have been reported in the literature for similar systems but only in single-droplet experiments so far.

Combustion of Multi-Component Drops during Flame Spray Pyrolysis

“…For the spray flame formation, the standardized SpraySyn burner, which is a coaxial atomizer unit equipped with a concentrical pilot flame, is utilized applying the standard operation conditions described in the work of Schneider et al [13]. For a detailed description of the operation conditions and nozzle geometry please revise [4,13]. The single drop combustion was conducted using a piezoelectric drop generator ejecting uniform single droplets (≈ 112 µm) in a cuvette (10 x 10 x 45 mm) with a frequency of 10 Hz.…”

“…However, little is known about the influence of the process conditions (d D , u D ,u G , T D , T G ) on the occurence of µ-explosions and about their occurence within spray flames. In our prior work [4] we observed in high-speed recordings of the iron flame luminosity bright sparks in the optical accessible spray flame boundary that are linked to large µ-explosions. Detailed phase Doppler anemometry (PDA) measurements revealed the formation of bimodal droplet size distributions (DSDs) that are suspected to be the result of µ-explosions.…”

“…where Re l , Re D , Re G and We aero are the liquid Reynolds number, the drop Reynolds number, the jet Reynolds number and the aerodynamic droplet Weber number, respectively. The quantities ofū G ,ū D and d D,10 are obtained via detailed PDA measurements with the setup according to our prior work [4].…”

“…The countless materials that can be produced from a broad spectrum of possible FSP precursors available provide a large potential on the production of functional nanoparticles at laboratory and industrial scale. The liquid atomization of such precursors in a coaxial atomizer is expected to be a key process in FSP since short breakup length and time scales set the initial conditions for the flame reaction and a fast precursor release [3,4,5] for the particle nucleation that take place at a few miliseconds. In order to ensure a fast precursor release, high-enthalpy solvents with boiling points (T b ) above 373 K are attractive.…”

Theoretical investigation of global rainbow refractometry in spray combustion of polydisperse microdroplets

“…The theoretical rainbow scattering is calculated using light scattering theory (Lorenz-Mie [23][24][25]). The DSDs used within the following simulations are based on experimental DSDs from FSP that were determined via PDA experiments in prior work [26]. The obtained theoretical results are the first step towards the application of GRR in the field of nanomaterial synthesis for a wide range of possible precursor systems.…”