ON THE ROLE OF CARBON DIOXIDE IN THE COMBUSTION OF ALUMINUM DROPLETS

ABSTRACT

In this paper, the influence of carbon dioxide on the combustion of aluminum droplets is investigated. Millimeter-sized droplets were heated and ignited by a laser in an aerodynamic levitation system in several CO₂ containing atmospheres (H₂O/CO₂, H₂O/CO₂/N₂) with a large range of compositions (wet – x_H2O < 3%, 80/20, 50/50, 12.5/87.5, 50/25/25, 20/40/40). The combustion processes were observed with a high-speed CCD camera, and the droplet radiation was recorded by two optical pyrometers. The ignition occurs with the oxide coating breakdown, which liquefies to form an initial cap moving on the droplet surface. Aluminum vaporizes and burns with the oxidizers as a detached diffusion flame. The droplet regression rate, i.e., the burning rate, strongly depends on the oxidizing atmosphere, from β = 0.58 mm²/s in wet 50% CO₂/50% N₂ to β = 2.45 mm²/s in 80% H₂O/20% CO₂. It is shown that CO₂ is the worst oxidizer with a smaller “oxidizer efficiency” compared to H₂O and O₂ . The burning droplet temperature in wet CO₂ and in the H₂O/CO₂ mixtures is around T ≈ 2600 K, and is smaller in H₂O/CO₂/N₂ (T ≈ 2450 K). The non-correlation between the burning rates and the droplet temperatures confirms that the combustion processes are limited by molecular diffusion, and highlights the influence of H₂ in the gas-phase transport. An estimation of the exponent n of the “d ⁿ law” shows that n decreases with the increase of CO₂ from n = 1.7 to n = 0.6. The stagnant burning rates are evaluated as being 7 to 9 times smaller than the measured ones with convective effects. Furthermore, during the droplet regression, the oxide cap dimensions also regress, and it is generally completely removed. Oxide cap regression rates are estimated and are slower than their respective burning rates. Nevertheless, there is a good correlation between the oxide cap regression rate and the droplet temperature that shows that the oxide cap regression results from the chemical decomposition of Al₂O₃ by the liquid Al droplet producing gaseous Al_xO_y. For CO₂ concentration higher than 40%, a solid phase suddenly appears on the liquid Al surface which entirely covers the droplet leading to the end of the gas-phase combustion. This phenomenon is the consequence of the massive dissolution of carbon in the droplet during burning. Analyses of the unburnt residues showed amounts of dissolved carbon up to 20–23% molar, which is near the saturation concentration limit. Thus, the solid coating corresponds to the ejection of carbon from the droplet because of its continuing regression and is expected to be also present for smaller particles. Therefore, carbon dioxide plays a double role. First, it participates in the gas-phase combustion, but it is the worse oxidizer with smaller burning rates. Second, CO₂ causes the carbon dissolution in the Al droplet and finally stops the gas-phase burning. A further implication could be that carbon dioxide may promote the appearance of a combustion regime with surface reactions only.

Keywords:

• aluminum
• droplet
• combustion
• carbon dioxide
• dissolution
• convection

Notes

*For wet atmospheres, x_H2O < 3%.

**With laser on.

*With the assumption of a mass fraction of oxygen: 0 < Y_O < 0.1.