Combustion of micron-sized aluminum particle, liquid water, and hydrogen peroxide mixtures

The combustion of aluminum particle, liquid water, and hydrogen peroxide (H₂O₂) mixtures is studied theoretically for a pressure range of 1-20MPa and particle sizes between 3 and 70μm. The oxidizer-to-fuel (O/F) weight ratio is varied in the range of 1.00-1.67, and four different H₂O₂ concentrations of 0%, 30%, 60%, and 90% are considered. A multi-zone flame model is developed to determine the burning behaviors and combustion-wave structures by solving the energy equation in each zone and enforcing the temperature and heat-flux continuities at the interfacial boundaries. The entrainment of particles is taken into account. Key parameters that dictate the burning properties of mixtures are found to be the thermal diffusivity, flame temperature, particle burning time, ignition temperature, and entrainment index of particles. When the pressure increases from 1 to 20MPa, the flame thickness decreases by a factor of two. The ensuing enhancement of conductive heat flux to the unburned mixture thus increases the burning rate, which exhibits a pressure dependence of the form r_b=ap^m. The exponent, m, depends on reaction kinetics and convective motion of particles. Transition from diffusion to kinetically-controlled conditions causes the pressure exponent to increase from 0.35 at 70μm to 1.04 at 3μm. The addition of hydrogen peroxide has a positive effect on the burning properties. The burning rate is nearly doubled when the concentration of hydrogen peroxide increases from 0 to 90%. For the conditions encountered in this study, the following correlation for the burning rate is developed: rb[cm/s]=4.97(p[MPa])0.37(dp[μm])-0.85(O/F)-0.54exp(0.0066CH2O2).