BRE DG511 PDF

INTRODUCTION

The widespread application of computer fire models has grown as a result of the freedom to innovate offered by performance-based regulation. Computer models enable designers to:

• test alternative fire-safety design solutions,

• quantify system performance specifications,and

• explore solutions to ‘what if?' design questions

These processes are assisted by new standards, such as BS 7974:2001 and BS ISO/TR 13387.

Many computer fire models are available, ranging from simple engineering correlations and zone models to more advanced field (CFD) models. These models simulate the heat- and mass-transfer processes associated with a compartment fire, the heart of any systematic approach to safe design. The essential difference between zone and field models is in the way they treat the movement of the products of combustion within the building envelope and their respective reliance on experimental information.

The two broad categories are illustrated in Figure 2, which shows the schematic representation of the same room fire-modelled by the two approaches. By solving the problem on a three-dimensional numerical grid (Figure 2b), not only are the predictions much more detailed, but an isosurface of the predicted temperature clearly shows that the fire plume becomes deflected by the air inflow at the doorway. This is important because it leads to higher entrainment rates of air into the fire and consequently higher smoke-production rates. The zone model does not account for this, unless it has been assumed a priori.