The reaction mechanism of catalytic decomposition of hydroxylammonium nitrate (HAN) aqueous solutions was investigated to understand the catalytic decomposition and to develop the thruster using a HAN-based monopropellant. It is important to understand the catalytic decomposition because the chemical reaction of the monopropellant is activated by a catalyst in the existing monopropellant thruster. Previously, several researchers investigated the thermal decomposition and the combustion characteristics of HAN-based solutions. However, catalytic decomposition of HAN-based monopropellant has not been well understood. In this study, in order to clarify the reaction mechanisms of catalytic and thermal decompositions, differential thermal analysis and thermogravimetric analysis of HAN aqueous solution and product gas analysis were conducted in the cases of catalytic and thermal decompositions. These analyses show that the Ir-based catalyst was effective to reduce the onset temperature of HAN decomposition and that the difference in product gas species was not obvious between catalytic and thermal decompositions. Based upon these results, a reaction mechanism of catalytic decomposition was discussed. It was found that the reaction mechanism of catalytic decomposition is almost the same as that of thermal decomposition. Furthermore, nitric acid (HNO3) was generated at lower temperature in the case of catalytic decomposition than that of thermal decomposition. Therefore, Ir-based catalyst cannot change the reaction mechanism but can activate the reaction path of HNO3 generation.
The effects of steam addition on the unstable behavior of hydrogen-air lean premixed flames under the adiabatic and non-adiabatic conditions were investigated using numerical calculations. We adopted the detailed chemical reaction mechanism for hydrogen-oxygen combustion, modeled with seventeen reversible reactions of eight reactive species and a diluent. Two-dimensional unsteady reactive flow was treated, based on the compressible Navier-Stokes equations. The burning velocity of a planar flame decreased as the steam addition and heat loss increased. A sufficiently small disturbance was superimposed on a planar flame to study the characteristics of intrinsic instability. We obtained the relation between the growth rate and wave number, i.e. the dispersion relation, and the linearly most unstable wavelength, i.e. the critical wavelength. As the steam addition increased, the unstable range became narrower, and the critical wavelength became longer. Taking account of heat loss, we obtained smaller growth rates and narrower unstable range. The superimposed disturbance developed owing to intrinsic instability, and then the cellular shape of flame fronts appeared. In cellular flames, compared with planar flames, high (low) concentration of active chemical species was found in downstream of convex (concave) fronts. This was caused by high (low) temperature at convex (concave) fronts due to the diffusive-thermal effects. The concentration of active chemical species became lower with increasing the steam addition and heat loss, which was because of the reduction of flame temperature. Moreover, the burning velocity of a cellular flame increased monotonically with an increase in the scale of premixed flames. The burning velocity of a cellular flame normalized by that of a planar flame increased as the steam addition and heat loss increased. The numerical results denoted that the steam addition and heat loss had a great influence on the unstable behavior of hydrogen-air lean premixed flames.
Binary hydroxyl ammonium nitrate (HAN) aqueous mixtures have been prepared. Four series of ternary mixtures have been synthesized with methanol and ethanol as fuels: two series with HAN excess and two other series with stoichiometric fuel contents. Thermal and catalytic decomposition of the prepared solutions have been analyzed. For binary HAN solutions, the thermal decomposition starts only once water has been fully vaporized and the oxidizer is in the liquid state. The influence of the fuel depends strongly on the oxidizer. Methanol and ethanol are vaporized before the decomposition, leading to results close to those observed for binary mixtures. HAN and HAN-fuel-based solutions display the highest catalytic effect with a temperature decrease of about 100° C.
