Ignition Delay and Limit of Ignitability for Sodium Pool

Abstract

Regarding sodium pool burning, ignition delay and ignitability are examined in order to obtain appropriate, comprehensive parameters that can correlate dominant system parameters influencing ignition. To this aim, theoretical work is first conducted, considering that the surface reaction exerts the main influence. 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 ignition delay, a comprehensive parameter, defined as the ignition delay time multiplied by oxygen concentration and divided by pool depth, is identified. For the limit of ignitability, another comprehensive parameter, defined as the product of oxygen concentration and representative length for the heat transfer, is derived. In order to evaluate the appropriateness of the theory, experimental comparisons are conduced, using 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 approximate magnitude are concerned, in spite of differences in experimental conditions and several assumptions made in the present analysis. Because of the simple forms of the comprehensive parameters, they are expected to be useful in evaluating the ignitability of sodium prior to pool burning in an oxidizing atmosphere. Furthermore, it is found that an increase in the volume-to-surface-area ratio of the sodium pool prolongs ignition delay, and that a decrease in pool diameter suppresses ignition, both of which can be attributed to the reduced contribution of the surface reaction that supplies heat during the induction period.