Instantaneous temperature and OH concentration imaging by two-line OH-LIF in spray-flame nanoparticle synthesis

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The temperature distribution determines the product in nanoparticle synthesis in flames, as precursor reactions, flame dynamics, particle nucleation, and growth depend on the temperature history of the aerosol. Fields of gas-phase temperature and OH concentrations are measured by two-line OH laser-induced fluorescence (LIF) in the SpraySyn burner during iron oxide nanoparticle formation. A correction of the LIF images based on the Beer–Lambert law was applied to compensate for spatial laser beam attenuation within the flame. This correction enables quantitative evaluation of the OH-LIF signals, allowing the determination of OH concentrations together with instantaneous temperature fields by LIF-intensity ratioing of two selected transitions excited in the A 2 Σ–X 2 Π (1,0) electronic band system. We investigate the effect of varying oxygen dispersion gas flow rates on the temperature distribution in the spray flame operated with pure ethanol and with mixtures of ethanol and 2-ethylhexanoic acid. In the latter case, the effect of iron(III) nitrate nonahydrate (INN) addition as a precursor for iron-oxide nanoparticle synthesis is examined for 0.01 and 0.05 mol/l. Increasing the dispersion gas flows reduces the overall temperatures and narrows the high-temperature zone. INN addition causes a slight temperature increase (by 20 K) at 0.05 mol/l. Temperatures determined from instantaneous measurements are compared with measurements from multi-line OH-LIF thermometry (Karaminejad et al., Appl. Energy Combust. Sci. 2023) and differences are discussed. The measured temperature and OH datasets provide detailed insight into the thermo-chemical structure of SpraySyn flames and serve as valuable input for the development and validation of CFD models for particle formation. Novelty and significance statement This study demonstrates the first application of two-line OH laser-induced fluorescence (LIF) for single-shot gas-phase temperature imaging in the particle-laden environment of the SpraySyn burner used for nanoparticle synthesis. Unlike the previous multi-line LIF approach that provided only time-averaged data, this method resolves instantaneous temperature fields together with OH concentration (number density) distributions. A key innovation is the implementation of a spatially resolved correction for laser beam attenuation using the measured laser light transmission and Beer–Lambert analysis. This correction enables compensation of laser attenuation within the flame and the determination of OH concentration fields simultaneously with instantaneous temperature measurements. Consequently, accurate temperature retrievals and quantitative OH diagnostics become possible, enabling direct comparison between instantaneous and averaged data for CFD validation in turbulent nanoparticle-forming flames.

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