Towards the Efficient and Time-accurate Simulations of Early Stages of Industrial Explosions

Combustion during a nuclear reactor accident can result in pressure loads that are potentially fatal for the structural integrity of the reactor containment or its safety equipment. Enabling efficient modelling of such safety-critical scenarios is the goal of ongoing work. In this paper, attention is given to capturing early phases of flame propagation. Transient simulations that are not prohibitively expensive for use at industrial scale are required, given that a typical flame propagation study takes a large number of simulation time steps to complete. An improved numerical method used in this work is based on explicit time integration by means of Strong Stability Preserving (SSP) Runge-Kutta schemes. These allow an increased time step size for a given level of accuracy—reducing the overall computational effort. Furthermore, a wide range of flow conditions is encountered in analysis of accelerating flames: from incompressible to potentially supersonic. In contrast, numerical schemes for spatial discretization would often prove lacking in either stability or accuracy outside the intended flow regime—with density-based schemes being traditionally designed and applied to compressible (Ma>0.3) flows. In the present work, a formulation of an all-speed, density-based, numerical flux scheme is used for simulation of slow flames, starting from ignition. Validation was carried out using experiments with spherical lean hydrogen flames at laboratory scale. Turbulence conditions in the experiments correspond to those that can arise in a nuclear reactor containment during an accident. Results show that the new numerical method has the potential to predict flame speed and pressure rise at a reduced computational effort.