Study of inverse H2O2-droplet combustion

Abstract

The fundamentals of inverse droplet combustion for a single droplet of 90 wt.% of hydrogen peroxide (H2O2) suspended in a quiescent nitrogen-balanced methane atmosphere were studied under various ambient mole fractions of methane from 0.55 to 1 at standard conditions (298 K and 0.1 MPa in absolute). Burning behavior of the droplet was recorded with a camera and thermocouple to capture and carefully analyze the flame, droplet, and temperature characteristics during the entire burning event. After spontaneous ignition, a spherical flame was successfully established even under a normal gravity environment, and the overall droplet burning behavior was found to be similar to that observed for normal (fuel) droplet combustion, following the classical d-square law. Compared to normal droplet combustion, the burning rate constant for inverse droplet combustion was faster, and the time-averaged standoff ratio (flame-to-droplet diameter ratio) became smaller, irrespective of the ambient mole fraction of methane. This was mainly attributed to the location of the diffusion flame, as defined by oxidizer-fuel stoichiometry, and the ideal rests closer to the droplet surface for inverse droplet combustion. The potential 1-D flame structure was numerically investigated with support of specialized software (CHEMKIN) to help with calculating a suitable transfer number (B) for inverse droplet combustion. The calculated transfer number for inverse droplet combustion at an ambient methane mole fraction of 0.21, approximating the ambient mole fraction of oxygen under equivalent normal droplet combustion conditions, yielded B = 11.8.