Combustion Behavior of a Falling Sodium Droplet (Burning Rate-Constant and Drag Coefficient)

Abstract

Combustion behavior of a single sodium droplet has been studied experimentally, by use of a falling droplet. It is found that D2-law can hold for the sodium droplet combustion after the ignition, which can be observed to occur through an increase in the droplet temperature under the condition without a gaseous flame, suggesting that a surface reaction plays an important role in the ignition of sodium. It is also found that the burning rate-constant without forced convection has nearly the same value as those for conventional hydrocarbon droplets, although it is considered that the sodium combustion proceeds in an oxidizer-rich environment even in the air, which can be judged by comparing a temporal variation of the flame/droplet diameter ratio for the sodium droplet with that for the hydrocarbon droplet. A micro-explosion of the burning droplet is also observed when the oxygen concentration in the ambience exceeds 0.3 in mole fraction. As for the falling velocity and/or distance of the burning droplet, it has turned out that the use of the drag coefficient for solid sphere under isothermal condition is insufficient in obtaining their accurate values. It is found from another experiment that when Re>500, the drag coefficient of the falling droplet undergoing combustion is as high as 2 depending on combustion situation and/or droplet temperature, while that of the solid sphere under isothermal condition is 0.44.