Modeling of quasi-steady sodium droplet combustion in convective environment

Abstract

In the event of accidental leakage of sodium from the systems of a Liquid Metal Fast Breeder Reactor (LMFBR), a spray of liquid sodium droplets may be formed which will burn by reacting with the surrounding atmospheric oxygen. In order to understand the burning characteristics of the complete spray, combustion of an individual sodium droplet forms the basis and this has been investigated in the present study. A comprehensive numerical model has been developed to analyze the isolated sodium droplet combustion in a mixed convective environment. The governing equations for mass, momentum, species and energy conservation have been solved in axisymmetric cylindrical coordinates using the Finite Volume Method (FVM). Finite rate kinetic mechanisms have been incorporated to simulate droplet burning, using available kinetics data for basic sodium oxidation reactions. Salient features of the numerical model include a global single-step reaction for sodium oxidation in air and the incorporation of property variations with temperature and concentration. An equilibrium mixture of sodium peroxide, sodium monoxide and sodium vapor is considered as the final reaction product, taking into account the effects of dissociation reactions. The numerical model has been validated with experimental results available in literature. Results for the fuel mass burning rates and flame shapes are presented for different sizes of droplets burning under different free-stream conditions. The model predicts the occurrence of envelope flame around a sodium droplet even at fairly high free-stream velocities.