Ignition Delay and Limit of Ignitability for Sodium Pool (Theory and Experimental Comparisons)

Abstract

Relevant to sodium pool burning, ignition delay and ignitability are exmined in order to obtain appropriate parameters that can fairly correlate dominant system parameters influencing the ignition. For this aim, a theoretical work has first been conducted, being considered that the surface reaction can mainly exert influences. It is found that both the ignition delay time and the limit of ignitability are closely related to the pool size and oxygen concentration, as well as the initial sodium temperature. Specifically, for the ignition delay, a comprehensive parameter defined as the ignition delay time multiplied by oxygen concentration and divided by pool depth has been identified. For the limit of ignitability, it is found another comprehensive parameter defined as a product of oxgen concentration and representative length for the heat transfer. In order to evaluate appropriateness of the theory, experimental comparisons have been conducted, by use of experimental data in the literature. It is demonstrated that there exists a fair degree of agreement between the experimental and analytical results, as far as the trend and/or approximate magnitude are concemed, in spite of differences in experimental conditions and several assumptions done in the present analysis. Because of simple forms of the comprehensive parameters, they are expected to be useful in evaluating ignitability of sodium prior to pool burning in an oxidizing atmosphere. Furthemore, it is found that the increase in volume-to-surface ratio of the sodium pool prolongs the ignition delay, and that the decrease in pool diameter suppresses the ignition, both of which can be attributed to the reduced contribution of the surface reaction that can supply heat in the induction period.