Spatiotemporally resolved surface temperature measurement of aluminum ignition and combustion in steam and oxygenated environments

We report micrometric imaging measurements of the ignition of micron-sized aluminum (Al) wires and the consequent combustion processes of single micron-sized Al droplets in hot steam and steam with oxygen. The specially designed flames were anchored on a modified burner where a small central H2/O2 diffusion flame in the center jet provided a hot flow (>2330 K) for Al ignition, and the diffusion flame together with a laminar co-flow of premixed H2/N2/O2 flame provided the vitiated flow of desired combustion environment. As soon as the horizontally arranged Al wire was moved to the center hot jet, a high-speed color camera was triggered to record the transient event of the ignition and combustion process. The high imaging quality and spatial resolution allow direct observation of the structure of the burning particles, i.e., the liquid core, the bright flame front encapsulating the center droplet, and the coalescent alumina agglomerates downstream above the burning Al droplets. A distinct thin condense sheet was observed defining the flame front in all symmetric and asymmetric combustion modes, indicating that homogeneous oxidation dominates the heat-release reactions as a diffusion flame. A color ratio RGB pyrometry was calibrated to provide temperature distribution over the burning particles. In the steam condition, the center droplet temperature in the symmetric phase was analyzed to be ∼2650 K and the temperature of the flame front ∼3400 K, while in the oxygenated environments, the droplet temperature was evaluated to be ∼2790 K, and the temperature of the flame front ∼4150 K. The transient temperature evolution of different parts of the burning elements was recorded and analyzed. To the authors’ awareness, the current study represents the first structure-resolved temperature measurement of the combustion of single aluminum particles in high-temperature steam. These measurements can provide unprecedented spatiotemporally resolved information on Al combustion for improved insight into the governing mechanism.