The role of surface reactions in hypergolic ignition of liquid-solid systems

Abstract

The object of this paper is to demonstrate the fundamental role played by surface reactions in the process of hypergolic ignition, as revealed by a study of oxidizing liquid-solid fuel systems. The influence of the surface was initially substantiated by studying the effect of fuel particle diameter, d o, on ignition delay time τ. The results obtained showed that the law τ∞ d o 0.5 is universally applicable This can be explained by considering the surface reaction as the slow step of a series of consecutive reactions leading to combustion, thus constituting the rate-controlling factor in ignition lag. This slow step corresponds to a process of instantaneous nucleus formation accompanied by two-dimensional nuclei growth. The chemical reactivity of the system, which determines the ignition delay time, has been correlated with the chemical structure of the components of the hypergolic system. The order of time surface reaction with respect to the oxidizing agent was determined by varying the water content of the nitric acid solution. The results can be explained by considering that the reaction occurs through the intermediary of the adsorbed reactant. An attempt was made to determine whether the active component in nucleus formation was the solvent (N 2O 4, HNO 3), or the acid ion associated with the solvent (NO +, NO 2 +) through autosolvolysis. N 2 O 4 ⇋ N O + + N O 3 − 2 H N O 3 ⇋ N O 2 + + H 2 O + N O 3 − For this purpose, the NO 2 + ion concentration in the nitric acid was varied. The results obtained showed that (a) the NO 2 + ion is in a state of dissociation equilibrium with the nitric acid in the surface layer. (b_ the active component responsible for the hypergolle reaction with nitric acid is the NO 2 + ion, and the reaction occurs in the absorbed phase.