Characterization of the thermal structure of six clustered microflames seeded with TaN particles through emission spectroscopy

The temperatures of TaN seed particles and electronically excited CH radicals (i.e. CH∗) emanating from a methane-air microflame burner were characterized using VIS-NIR emission spectra while varying fuel-air flow rates. The burner consisted of six micro-nozzles arranged in a circle surrounding a central nozzle through which air and TaN seed particles with sizes between 0.3 and 3 μm were injected. The temperature profiles of the particles at various heights above the base of the central nozzle, obtained by their VIS-NIR continuum emission, showed a well-defined constant temperature region that extended well beyond the actual flame front and changed as fuel and oxidizer flow rates were varied. High spectral resolution measurements were conducted to obtain CH∗ A2Δ-X2Π emission spectra which were then compared with theoretical CH∗ emission spectra from Lifbase simulations to determine rotational and vibrational temperatures. The CH∗ vibrational temperatures were clearly greater than the rotational temperatures, the difference of which was attributed to equilibration of the rotational temperature with the gas dynamic temperature and the excitation of vibrational states through chemical reactions. Analyses of the TaN particles before and after seeding using scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy with energy dispersive X-ray analysis (XPS/EDX) showed surface oxidation occurred in the flame. Comparing the shapes of the spectral emission with theoretical Planck emission corrected for TaN emissivity and Ta2O5 transmittance, the oxide layer was found to be clearly thinner than 100 nm in the high temperature regions. Our results show an ability to control the duration to which seed particles are subjected to high temperature reactions by adjusting fuel and oxidizer flow rates.