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 H_2 appeared on the boron surface covered with a liquid layer of B_2O_3. It was found that H_2 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 B_2O_3 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 B_2O_3 layer, the reaction of water vapor with B_2O_3, and the bubbling of H_2. 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.
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