Numerical modeling of radiation heat transfer from combusting droplets for a sodium fire analysis

Abstract Heat radiation is one of dominant heat transfer processes during a sodium fire event which is a concern in sodium-cooled fast reactor plants. The multi-dimensional sodium fire analysis code AQUA-SF employs the 6-flux gas radiation model to simulate radiation heat transfer in atmospheric gas and to structural walls with the consideration of the radiation effects of suspended sodium by-products as aerosols. Here, this original model does not calculate radiation heat transfer from combusting droplets of sprayed sodium. All the combustion heat transports to atmospheric gas surrounding the combusting droplet except for the heat distributed to the droplet in this model. However, some of combusting heat should transport to structural walls directly by heat radiation in actuality. Due to this modeling, atmospheric temperature and pressure are overestimated and on the other hand, temperature of structural walls is underestimated. Therefore, this study aims to model radiation heat transfer from combusting droplets numerically. In the modeling, radiation energy transport on the combusting area around a sodium droplet is formulated considering emission, absorption and scattering as interaction with surrounding gas radiation. Radiation energy from the combusting droplets is added to the source term of the radiation transport equation in the 6-flux gas radiation model. Direct radiation heat transfer from combusting droplets can be simulated by this improved model. The improved model is tested through the verification analyses of single droplet combustion and the benchmark analysis on the upward sodium spray combustion experiment. The results of the test analyses show increase in heat transfer to the walls due to the droplet radiation. As the result, the sodium fire analysis becomes more reasonably by the improved model.