The formation of (Al2O3)n clusters as a probable mechanism of aluminum oxide nucleation during the combustion of aluminized fuels: Numerical analysis

Imaging Aluminum Particles in Solid-Propellant Flames Using 5 kHz LIF of Al Atoms

“…Moreover, the LIF signal is a good marker for phase transformation of the Al particle from its solid phase to its liquid phase (melting point of 930 K) and to evaporating Al (boiling point of 2745 K), which is burning. The flame is located in the droplet area since evaporating Al atoms react with the ambient gases [21,22] to form new species including alumina (Al 2 O 3 ). Our imaging technique enables Al transformation inside the combustion to be monitored.…”

“…Al 2 O 3 is assumed to be formed and to be molten at first (melting point at 2300 K). According to a new theoretical combustion prediction [21,22] a homogeneous alumina nucleation process is taking place inside the hot flow around the aluminum droplets (which can be very small) by oxidation of the Al gas. Chemical phase transformation ends when temperature decreases so that species condensate.…”

“…Although not exactly identical because of the time delay, it has very similar behavior in the three images, showing that nearly no gaseous aluminum is present in this region, and providing a sketch of its evolution. Each image from Figure 2b shows that smoke emission and fluorescence are dragging upward, since both Al vapor and fine Al 2 O 3 nanoparticles flow inside the flame gases [21,22]. Nevertheless, the velocity is moderate (Section 3.1.2.).…”

“…The location of the maximum probably corresponds to a region in which temperature is higher, on average, since dilution by surrounding N 2 in the combustion chamber reduces hot-gas temperature further away from the solid-propellant burning surface. The signal is persistent until the aluminum is totally consumed, the lack of fuel lowering the reaction rate [21,22]; thus, very few LIF droplets can be seen above 10 mm from the burning surface (reached after about 10 ms spent in the hot-gas flow). This should indicate that the combustion of the droplets is over at this height.…”

“…Aluminum signal is divided by 100 and disappears at 3 radii on the lower side, whereas it lasts for 6.5 radii on the upper side. It would be useful to compare such extents to numerical simulations of single-droplet combustion from [7,21,27]. However, it is important to note that simulations are performed in a simplified environment with oxidation at 1 bar.…”