Analyzing 3D fields of refractive index, emission and temperature in spray-flame nanoparticle synthesis via tomographic imaging using multi-simultaneous measurements (TIMes)

Abstract

Turbulent spray flames issuing from the SpraySyn 2.0 burner were investigated in the present work by means of tomographic imaging using multi-simultaneous measurements (TIMes). This small-scale standard burner is designed to study flame-spray synthesis of tailored nanoparticle systems. The applied measurement technique combined background-oriented schlieren tomography, computed tomography of light emission and two-color tomographic salt emission thermometry to reveal single-shot and time-averaged 3D fields of refractive index (linked to temperature, pressure and species concentration), CH* chemiluminescence (flame front indicator), SrOH emission from synthesized trace amount of Sr(NO3)2 dissolved in the liquid precursor solution (marker of the hot spray-flame products), and temperature of the hot spray-flame products, respectively. The salt emission thermometry technique was extended to tomographic measurements for the first time. Its implementation was based on the voxel-wise intensity ratio of reconstructed thermally-excited SrOH signals at two different regions in the visible spectrum that was converted to temperature using a single calibration curve based on multi-line OH planar laser-induced fluorescence measurements. The cost-effective setup for TIMes was composed of 43 machine-vision cameras equipped with bandpass spectrum filters, an innovative continuous semi-circular background pattern and LED light sources. The combined reconstructed fields provided information on the 3D flame structure in detail, showing the interaction between the flame front and spray stream, and the hot temperature regions. Spray-flame features were quantitatively analyzed, namely temperature, height, width, 3D tilt angle, and spreading half-angle of the flame plume. The temperature measurements proved to be robust against variations in the fuel/oxidizer equivalence ratio for the four investigated operating conditions. The spray flame issuing from the SpraySyn 2.0 burner at the standard operating condition was more stable than that in the previous burner generation, although more asymmetric near the nozzle exit and tilted, which might still cause heterogeneity of the synthesized nanoparticle system.