Chapter 16: Estimating an Enclosure Temperature During Solid Propellant Fires

In any industrial energetic facility, it is useful to predict the effect of an accidental fire. Solid propellants are designed to quickly transform into high temperature gases. It is therefore necessary to predict the pressure evolution during a confined fire. Some methods are semi-empirical and thus do not depend on many thermodynamic variables. Other methods are numerical and require that many variables be defined. In any safety application, choosing the right method will depend on the risks involved, possible consequences and means available. Lack of precise knowledge of these variables is often what discourages facilities and equipment designers from modelling the behaviour of their systems. One such variable is the temperature in an enclosure during a fire for which the pressure generation is the main concern. For events involving low charge densities, using the flame temperature is an over-approximation. Assuming instantaneous perfect mixing of the enclosure air and combustion gases results in an under-approximation. In this paper, these approximations will first be considered compared to an event sequence. Three temperature models will be reviewed. Firstly, a simple model will consider the geometric mean of the previously described extreme values. Secondly, the radiative heating of the enclosure air due to a localized fire source will be studied. Finally, a finite volume multidimensional numerical model which includes turbulence effects will be presented. These three methods are seen to be of increasing complexity and will be compared to the event timescale. This timescale will help indicate if the level of complexity is warranted.