Shock tube study of ignition delays and detonation of gaseous monomethylhydrazine/oxygen mixtures

The oxidation mechanism of gaseous monomethylhydrazine (MMH) above 1000 K consists of a simultaneous rapid decomposition and a slow oxidation followed by a rapid oxidation reaction. The ignition delays between the reflected shock arrival and the rapid oxidation were measured in a 38.4-mm-i.d. shock tube behind the reflected shock wave at 900-1440 K, 160-350 kPa, and for φ = 1-2.27, and mol.% Ar = 82-96, using MMH absorption at 220 nm. A simple relationship between ignition delay, shock conditions, and mixture composition is obtained. A chemical kinetic model based on MMH decomposition and on the oxidation of decomposition products has been developed, in order to calculate the ignition delays above 1000 K, and compare them to the experimental shock tube data. The temperature, pressure and concentration profiles, computed with a 300 reaction-model, reproduce well the pressure and absorption signals. Experimental and computed ignition delays agree in the ranges: 1070-1260 K and 190-290 kPa, within a 35% accuracy. If the ignition delay is sufficiently reduced, then a detonation wave can be initiated behind the incident shock. A study of detonation velocity and cellular structure for stoichiometric and rich mixtures, eventually diluted, has been carried out in the shock tube at low initial pressure. The critical conditions for the onset of detonation and the conditions for the propagation of a self-sustained detonation wave have been determined. The velocity deficits with regard to C-J values reach 20% at the onset of spin. Cell size versus pressure curves show that stoichiometric MMH/oxygen mixture is less detonable than both stoichiometric unsymmetrical dimethylhydrazine- and hydrazine/oxygen mixtures. No simple correlation exists between the detonation cell size and the induction distance for the explosive oxidation estimated from ignition delay data.