Experimental investigation of aluminum particle dust cloud combustion

An experimental investigation has been conducted to determine laminar flame speeds of micron- and nano-sized aluminum particle-laden aerosols using a Bunsen-burner type dust cloud apparatus under various well-controlled operating conditions. The aerosol compositions consisted of various percentages of different sized particles mixed with air (at a constant rate flow of 26.3 LPM) ranging from 100% micro-sized particles (5-8 μm) to mixtures with 30% by mass nanoparticles (100 nm). The dust concentration, β, was varied from 263 to 448 g/m³ (0.81≤ φ≤1.62). Measured laminar flame speeds of Al particle / air mixtures were found to be independent of equivalence ratio due in part to a small variation of flame temperature. The effect of oxidizer flow velocity was investigated to determine the influence of jet momentum on particle breakup and laminar flame speed. Flame speeds asymptotically increased with higher oxidizer shear velocity to finally reach a nearly constant rate. Therefore, the oxidizer inlet velocity was maintained beyond 200 m/s for maximum particle breakup. Hydrogen (ranging from 0% to 8% by vol) was introduced into the flow to provide high-temperature steam in the aerosol as well as to modify the thermal diffusivity and conductivity of the aerosol gas. The addition of H₂ caused the flame speed to increase due to its high thermal conductivity which is nearly seven times that of air. Thermophoretic sampling was performed to determine the transition of Al to Al₂O₃ through the flame zone. From the data analyzed, high-resolution micrographs showed the inner portion of the flame consisted of only aluminum and its outer flame region was comprised of smaller stoichiometric Al₂O₃ particles. Energy dispersion spectroscopy (EDS) results further verified that the core area of the flame was aluminum and the outer flame region consisted of elemental oxygen and aluminum at an approximate ratio of 3 to 2.