Emission and laser absorption spectroscopy of flat flames in aluminum suspensions

Abstract

Imaging emission spectroscopy, spatially resolved laser-absorption spectroscopy, and particle image velocimetry (PIV) are applied to a flat flame stabilized in a suspension of micron-sized aluminum. The results from the combination of diagnostics are used to infer the combustion regime of the particles and to estimate the characteristic combustion time of the suspension. It is observed that the reaction zone of the flame in stoichiometric aluminumair suspensions exhibits strong self-reversal of the atomic aluminum emission lines. These lines also exhibit high optical depths in both emission and absorption spectroscopy. The strong self-reversal and high optical depths indicate high concentrations of aluminum vapor within the reaction zone of the flame at multiple temperatures. These features provide evidence of the formation of vapor-phase micro-diffusion flames around the individual particles in the suspension. In aluminum-methane-air flames, the lack of self-reversal and lower optical depths of the aluminum atomic lines indicate the absence of vapor-phase micro-diffusion flames, and point to a more heterogeneous, and likely kinetically-controlled, particle combustion regime. The reaction zone thickness is estimated from the spatially resolved profiles of aluminum resonance lines in both absorption and emission through the flame. The emission measurements yield a reaction zone thickness on the order of 1.7±0.3 mm in aluminum-air flames, and the absorption measurements yield a thickness on the order of 2.3±0.5. It is demonstrated that the combination of the combustion zone thickness measurement, flame temperatures determined from molecular AlO emission spectra, and particle velocity measurements from the PIV diagnostic permits an estimation of the burning time in the suspension. The burning time in stoichiometric aluminum-air suspensions using the suite of diagnostics is estimated to be on the order of 0.7 milliseconds.

Combustion diagnostics of metal particles: a review

“…In this section, the attempts to measure the spectra and the temperature of metal dust flame are summarized. [110]; and (b) emission spectroscopy with a scanning system [44].…”

“…Both absorption and emission spectroscopy have been extensively applied in combustion studies. The spectral feature acquired has been used to analyze the combustion regime [110], the width of the flame reaction zone [110], the thermal and concentration structures of flame [53], and the temperature of the condensed [44, 47-49, 53, 96, 97, 100, 106] or the gas phase [44,53,110]. Emission spectroscopy needs a spectrometer and some optical lenses.…”

“…These spectra were used to calculate the temperature of the condensed phase. To observe the atomic line or molecular spectra, spectroscopy usually needs a high spectral resolution, which is essential to resolve some special phenomena like selfreversal [110]. These spectra could also be used to derive the temperature of the gas phase around the metal particle by spectral fitting [44].…”

A high-resolution line list for AlO

“…The electronic spectrum of AlO is also important for various practical applications including studies of laser ablation of aluminium in air (Bol'shakov et al 2017;Ran et al 2017;Van Woerkom et al 2018), emission in laser-induced plasmas (Surmick & Parigger 2014), explosions (Kimblin et al 2017), flames (Soo et al 2017) and emissions from solid fuel rocket exhausts (Knecht et al 1996). Line lists were provided by Parigger & Hornkohl (2011) for such plasma studies and by Launila & Banerjee (2009) for much cooler mediums; both have the disadvantage from an astronomical perspective of only providing relative intensities.…”

Determination of aluminum-air burning velocities using PIV and Laser sheet tomography

“…The AlO(g) method integrates temporal instabilities of the flame position and artificially thickens the flame, which introduces inaccuracies in the determination of the burning velocity. Moreover, combustion of metal suspensions can be optically dense, causing multiple light scatter which makes the spatial determination of the emissions source at a specific wavelength unreliable[21]. The above-mentioned reasons might explain the data scatter from the burning velocities measurements conducted with the AlO(g) method.…”