Modelling of ignition and fire in vented enclosures.
Fire development in a vented enclosure can proceed in an explosive and
disastrous manner called flashover. This thesis examines when, why and how flashover
occurs and gives the answers in terms of a few determining dimensionless parameters.
The mechanism of flashover considered in this thesis is an enhancement of the burning
rate because of thermal radiation from a layer of hot smoke, produced in the course of
the fire, to the fire bed.
A model, which is proposed for the problem, describes the development fro~ the
moment of ignition incorporating the traditional two-zone approach. During early fIre
development the density and temperature of the lower zone are reasonably assumed to be
close to their initial value. Flashover itself is assumed to occur early in the fIre
development, within the fuel controlled combustion regime.
The model is analysed using the techniques of classical thermal explosion theory.
Explicit criteria are found analytically and graphically to determine if the fIre will achieve
flashover or not. The temperature-time characteristics of the fIre development are
obtained explicitly for the fIrst time. It is shown that the thermal inertia of the
compartment walls can have a significant effect upon the development.
The effect of geometrically scaling the compartment is considered. Nondimensional
analysis makes such study effective and leads to a square root relationship
for the temperature/time characteristics of the fire development.The correlation between the model, four prevIOUS models and small scale
experiments is examined. Under reasonable assumptions all models are shown to be
described by the same mathematical problem. This means that the criterion for flashover
and the development characteristics can be used for any of the modified models
observed. Results are illustrated for an experimental fire box used in many experiments.