Investigation of particle size effect on flame velocity in the combustion of nano/micron-sized aluminum particles in air

It is observed that a diffusion-controlled mechanism applied to the burning of micron-sized particles is not applicable to the combustion of nano-sized particles burning under kinetically controlled conditions. Furthermore, when heat transfer occurs between micron-sized particles and air, Nusselt number can be assumed to be constant and equal to 2, while this number is a function of Knudsen number when heat transfer occurs between nano-sized particles and air. Ignition temperatures of micron- and nano-sized particles are also different. In this article, mass and energy conservation equations for both particle and gas phases are solved. By doing so, flame velocity is obtained. Afterwards, with respect to different combustion characteristics of micron-and nano-sized particles such as ignition temperature, burning time, and Nusselt number, the effect of particle size on the flame velocity of aluminum particles combustion in air is studied and compared with experimental and numerical results. At the equivalence ratio of 0.85, it is shown that flame velocity is proportional to d(-0.94) and d(-0.56) for micron- and nano-sized aluminum particles, respectively.