Formation of dark zone and its temperature plateau in solid-propellant flames: A review

A comprehensive review on the formation of dark-zone temperature plateaus in solid-propellant flames has been conducted. Various key physiochemical processes involved in the formation of a dark zone were investigated for a broad range of propellants, including nitramine, ammonium dinitramide, and double-base propellants. The effects of propellant formulation, surface condition, burning rate, and preconditioned temperature, as well as ambient pressure and external stimuli, were investigated systematically. Dominant chemical pathways responsible for the formation of a dark zone were identified. Major species in the dark zone were tabulated for all the solid propellants of concern. Apart from NO, important species in a dark zone are CO for double-base, HCN for nitramine, and N₂O for ADN propellants. These chemical species typically undergo reactions with high activation energies and their chemical-induction and flow-residence times are instrumental in explaining the existence of a dark-zone temperature plateau. The transport velocity must be sufficiently high to allow for chemical species to travel farther from the propellant surface before they overcome the activation energies to react, thereby giving rise to a temperature plateau. Conversely, if the transport velocity is low, then reactions take place much closer to the propellant surface, producing a very short or even no dark zone. The flame stand-off distance decreases as the pressure increases because of the increase in the reactant concentrations and gas densities, which in turn increase the rate of reactions and reduce the transport velocity.