INTRODUCTION

In cooperation with the fire protection engineering community, a numerical fire model, Fire Dynamics Simulator (FDS), is being developed at NIST to study fire behavior and to evaluate the performance of fire protection systems in buildings. Version 1 of FDS was publicly released in February 2000 ^[1], ^[2]. To date, about half of the applications of the model have been for design of smoke handling systems and sprinkler/detector activation studies. The other half consist of residential and industrial fire reconstructions. Throughout its development, FDS has been aimed primarily at the first set of applications, but it is now clear that some improvements to the fundamental algorithms are needed to address the second set of applications. The two most obvious needs are for better combustion and radiation models to handle large fires in relatively small spaces, like scenarios involving flashover.

Version 1 of FDS contains a relatively simple combustion model that utilizes "thermal elements," massless particles that are convected with the flow and release heat at a specified rate. While this model is easy to implement and relatively cheap computationally, it lacks the necessary physics to accommodate underventilated fires. A more comprehensive method that handles oxygen consumption more naturally solves an equation for a conserved scalar quantity, known as the mixture fraction, which is the fraction of gas at a given point...