The generation of carbon monoxide in compartment fires
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For the purpose of fire analysis and fire safety engineering, the development of empirical correlations for major species yields in compartment fires has become an important priority due to the inability to calculate these quantities from first principles. Studies of simplified upper layer environments have shown that major species production rates can be correlated with the equivalence ratio in what is known as the Global Equivalence Ratio concept (GER). Due to the simplification in these past experiments, it was not known if the GER concept was valid for compartment fires. Therefore, there was a need to determine if correlations existed between major species yields and the equivalence ratio for actual compartment fires. Since the flow of toxic gases from a room poses a hazard to building occupants, it was also important to determine if correlations for CO yield outside of a compartment on fire exist, particularly when external burning occurs.
A 2.2 m3 test compartment was used to investigate the burning of four fuels (hexane, PMMA, spruce and flexible polyurethane foam) in compartment fires. The test compartment was specially designed with a two-ventilation path system which allowed the direct measurement of the plume equivalence ratio (the ratio of the fuel volatilization rate to the air entrainment rate normalized by the stoichiometric fuel-to-air ratio). Empirical correlations between the upper layer yield of major species and the plume equivalence ratio were shown to exist The results reveal that the production of CO is primarily dependent on the compartment flow dynamics (i.e., the equivalence ratio) and upper layer temperature. A chemical kinetics study indicated that increased compartment temperature affects upper layer species yields in two ways 1) the generation of species in the plume is changed and 2} oxidation of post-flame gases in the layer is affected. The correlations developed in the compartment fifes were qualitatively similar to those developed by Beyler for simplified upper layer environments. However, quantitative differences existed and are explained by the temperature effect.
The species yields downstream of hexane compartment flues were investigated and compared to upper layer yields. Results showed that downstream CO yields can be correlated to the plume equivalence ratio when taking into account the occurrence of external burning. When sustained external buming occurred for equivalence ratios greater than 1.7, downstream CO yields were reduced to 10 to 25 percent of the upper layer value. Results are very encouraging in indicating that an ignition criterion based on lean flammability limits is useful in predicting the flammability of upper layer gases in compartment fires. An ignition index value of 1.3 indicated the occurrence of sustained external burning and, thus, a reduction of CO, for the hexane fires studied.
- Doctoral Dissertations