Predicting Toxic Gas Concentrations Resulting from Enclosure Fires using the
Local Equivalence Ratio Concept Linked to Fire Field Models
Fire field modelling is based on the techniques of computational fluid
dynamics (CFD), which provide detailed variable solutions throughout the
computational domain quickly, repeatedly and cheaply compared with fire
experiments. A main component of fire effluent are toxic gases and one of the
main toxic gases responsible for a significant number of fire fatalities is
Carbon Monoxide (CO). Current engineering fire field models either ignore the
prediction of toxic gas generation or require the use of detailed chemistry to
predict toxic gas generation. These detailed chemistry models require
considerable data which is not generally available for most common building
materials. The main objective of the study presented in this thesis is to
develop practical and reasonable engineering fire field models to simulate the
production and movement of toxic gases in enclosure fires. The developed
toxicity models should be capable of working within the framework of the current
popular combustion models in fire safety engineering. In addition, the developed
models should not rely too heavily on hard to obtain experimental data.
The central idea behind the newly developed toxicity model is the use of the
Local Equivalence Ratio (LER). The species yields as functions of the Global
Equivalence Ratio (GER) and temperature are input parameters of this model.
Correlations for most building materials are available from small-scale fire
experiments. Similar approaches to this method are also developed using the
CO/CO2 and H2/H2O mole ratios. The LER methodology is further refined by an
approach which divides the computational domain for the calculation of toxic
gases into two parts, a control region in which the toxic gases are dependent on
the LER and temperature, and a transport region in which the toxic gas
concentrations are dependent on the mixing of hot gases with fresh air.
The toxicity model is then extended to two-fuel cases. In the two-fuel model,
the LER is a function of the two mixture fractions, which are used to represent
the mixture of the two different fuels, oxygen and combustion products. This
model is useful in simulating residential fires, in which wood lining of
sidewalls or ceilings is the second fuel.
Finally, the transportation of HCl within fire compartments is considered. HCl
generated from the combustion of materials containing chlorine is both a potent
sensory irritant and a strong pulmonary irritant. As HCl is transported in the
gaseous flows produced by fire, a considerable amount of HCl may be deposited on
enclosure walls and thus removed from the general transport flow. It is
therefore necessary to develop models which take into account the amount of HCl
deposited on the walls and removed from the flow. A mathematical model is
developed to simulate the exchange of HCl between gas boundary and wall surfaces
and the reaction of HCl with walls.
All the toxicity models developed in this study can be integrated into the
practical volumetric heat source approach and the Eddy Break-up (EBU) combustion
model typically used in practical engineering analysis. Finally, the integrated
fire models developed in this study are validated using a range of fire tests.