Combustion of ammonium dinitramide, Part 2: Combustion mechanism

Temperature profiles in the ammonium dinitramide (ADN) combustion wave were measured in the 0.04-10 MPa pressure interval using thin tungsten-rhenium thermocouples. The data obtained suggest that the ADN decomposition reaction in the condensed zone plays a dominant role in burning at low pressures. The heat feedback from the gas to the surface appeared to be negligibly small. It has been concluded that the reaction of AN dissociation to form NH3 and HNO3 controls the ADN burning surface temperature. The three-zone flame structure of the ADN combustion wave has been found, and the main chemical reactions occurring in the zones have been proposed. At low pressures (below 1 MPa), the burning of ADN can be satisfactorily described by a condensed-phase combustion model with the rate-controlling reaction being the ADN decomposition in the melt. A gas-phase model with the rate-controlling reaction being HNO3 decomposition in the first flame can be used at high pressures (above 10 MPa). In the middle pressure interval, ADN shows combustion instability caused by deficiency of heat produced in the condensed material. In this area the fast but energy-limited heat-release process in the condensed phase has to adapt itself to the slow but energy-rich heat-release process in the gas phase.