Effect of water vapor on ignition and combustion of boron lumps in an oxygen stream

Abstract

To clarify the effect of water vapor on the ignition and combustion of boron lumps, an experimental study was performed using the stagnation region of impinging oxygen with water vapor at room temperature, up to a 0.14 water vapor mole fraction. With increasing water vapor concentration, the critical ignition temperature decreased and the ignition delay time increased. The ignition delay time also increased with decreasing initial sample temperature in the presence of water vapor. During the ignition process, many bubbles made of H2 appeared on the boron surface covered with a liquid layer of B2O3. It was found that H2 produced by a heterogeneous reaction on the boron surface played an important role in the ignition process. The dependency of the critical ignition temperature and the ignition delay time on water vapor concentration was explained in terms of a balance between the B2O3 production rate and its removal rate: the former is due to reactions of boron with oxygen and water vapor at initial reactions, whereas the latter is due to the evaporation of the B2O3 layer, the reaction of water vapor with B2O3, and the bubbling of H2. The estimated mass burning rate was insensitive to water vapor concentration. The combustion process in the oxidizer with water vapor was observed to be basically the same as that without water vapor in the experimental range investigated.