Influences of pressure on reduced-gravity combustion of HAN-methanol-water droplets in air

Reduced-gravity experiments were performed to investigate combustion characteristics of individual monopropellant droplets composed of hydroxylammonium nitrate (HAN), methanol, and water. The experiments were conducted using the 2.2 Second Drop Tower at the NASA John Glenn Research Center at Lewis Field in Cleveland, OH. Gravitational levels were about 10⁻⁴ times normal gravity and experiments were performed in air at about 25 °C and with pressures from 0.1 to 1 MPa. Droplets initially in the 1-mm size range were supported on quartz fibers and ignited using a hot wire that heated the gas phase. Initial droplet compositions varied from zero (initially pure methanol droplets) to a stoichiometric mixture of 69.4% HAN, 15.2% water, and 15.4% methanol by mass. Results indicated that increasing the pressure increased burning rates, delayed extinction, and promoted easier ignition of droplets. Decreasing the initial mass fraction of methanol reduced burning rates, increased the difficulty of ignition, and promoted gas-phase flame extinction. Internal bubbling was observed at certain initial droplet compositions. Aerosol formation was observed for higher HAN loadings at elevated pressures after the visible gas-phase flame had extinguished, which may be indicative of condensed-phase HAN reactions.