Fire research abstracts and reviews Volume 13, 1971 (1971) / Chapter Skim
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Pages 35-51
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From page 35... ...
THE ROLE OF CHEMISTRY IN FIRE PROBLEMS: GAS-PHASE COMBUSTION KINETICS RAYMOND F R I E D M A N Factory Mutual Research Corporation Introduction The hterature on combustion kinetics is extensive. A book entitled "Atomic and Free Radical Reactions,''^ pubhshed m 1954, contams over 2000 references A later, more selective book entitled "Chemical Kmetics of Gas Reactions,'"" contams 1478 references Another book, entitled "Chemistry of Combustion Reactions,'" contams 922 references A fourth book, of bmited scope, entitled "Chemistry m Premixed Flames,"*
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From page 36... ...
188 FIBE BESEABCH in which chemical kinetics plays a role (ignition, flammabihty limits, extmguishment, flame spread through premixed gas, transition to detonation, etc.) , it is necessary to Imow the sequence of chemical reactions occumng which transform reactants to products (the mechanism)
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From page 37... ...
ABSTRACTS AND REVIEWS 189 Experiments to Produce a Compilation of Reaction Rates As an alternate to a general theory yieldmg reaction rates, one might systematically measure thousands of reaction rates for unimolecular, bunolecular, and termolecular reactions of possible mterest m fire problems, over an appropriate range of temperatures, and tabulate these Mechamsms could then be worked out. The difficulty is that virtually all reactions of mterest mvolve free radicals and unstable species, and do not occur m isolation; i e at least one of the products of the reaction to be studied may react with one of the reactants, for example, to give other products, etc.
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From page 38... ...
190 FIKE HESBARCH Also, in the same book, NewhalF provides a tabulation of 33 reactions involving the same species, with the exception of HCO, HO2, and O3, and the addition of N2O and NO2. In comparing the two hsts, one sees that values differ moderately on almost every reaction To compare ratios, one must note that most of the reactions are temperature-dependent, so that when different temperature dependencies are chosen for a given reaction by the two groups of authors, the plots of rate vs.
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From page 39... ...
ABSTBACTS AND REVIEWS 191 experimentally with fast gas reactions The onginal pioneering work demonstrating the cntical importance of radicals and cham reactions was done m the 1920's and early 1930's by Semenov, F .
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From page 40... ...
192 FIBB BESEABCH these cases of slow oxidation of organic solids at low temperature, biological oxidations are known to be involved, and sterilization will prevent ignition. In spite of the inabihty to treat real cases quantitatively, the availabihty of the theory certainly broadens our conceptual understanding of the phenomenon.
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From page 41... ...
ABSTRACTS AND REVIEWS 193 still exists because sufficiently accurate experimental measurements of conditions at the igmtmg surface have not been achieved, and separate decomposition studies of the individual mgredients (e g , ammomum perchlorate) have revealed tremendous complexity of behavior Smce fires are characterized by lower temperatures, less steep gradients, and slower rates than sohd propellants, we may hope that experimental probing of igmtmg or bummg wood or plastics will be more readily handled The problem is not easy.
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From page 42... ...
194 FERE RESEAUCH which depends on the fundamental laminar bummg velocity of the mixture existing at any pomt (which is relevant to the foregomg research) , but also on the turbulence present, and particularly on the additional turbulence generated by the event itself as it occurs, mfluenced strongly by any turbulence-generating objects m the path of the flame.
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From page 43... ...
ABSTRACTS AND REVIEWS 195 The theory assumes that each successive element of the surface must be brought up to a critical pjTolysis temperature characteristic of the material before it can Igmte This is obviously a chemical kinetic parameter, but it can m pnnciple be measured in separate expenments and the results are not highly sensitive to the exact value used for the parameter. No gas phase kmetic parameters appear m the theory; i.e., the gas phase reactions are assumed to be mfinitely fast, once fuel vapors and oxygen have diffusively mixed.
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From page 44... ...
196 TIKE RESEARCH any material, is found to depend considerably on sample size and onentation. A large area of ignorance in regard to the kmd of propagation under discussion is the ability to predict the magnitude of heat flux produced by a fire of given size at a given location near the fire Of course, the emissivity of a flame depends m part on the soot content, which may be lonetically controlled.
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From page 45... ...
ABSTRACTS AND REVIEWS 197 Let US consider some simple examples of gas-phase combustion inhibition. First, we know that an inert additive such as mtrogen or a nearby cold surface (quenching)
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From page 46... ...
198 FIRE RESEARCH countercurrent flows meet. Thus, for this geometry, additive mtroduced into the air has a better chance to pass through the flame than additive in the fuel.
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From page 47... ...
ABSTRACTS AND REVIEWS 199 to the sohd. As previously noted, there is httle available but speculation m this area.
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From page 48... ...
200 FERE RESEARCH easily be estabhshed because of synergistic mteractions between components. To put the vanous toxic hazards m perspective, relative to carbon monoxide, we hst the threshold hmits for an eight-hour workday (Federal Safety and Health Standards, 1960)
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From page 49... ...
ABSTRACTS AND REVIEWS 201 grams: Aff"VTF % CO 10-* 13 10-' 7 10-2 2 10-» 0.2 The size of the chamber, the size of the fuel elements, and the degree of insulation of the chamber influenced the results.
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From page 50... ...
202 FIRE RESEARCH mixed combustion parameters, such as flammabihty limit, mmimum spark igmtion energy, bummg velocity, and quenchmg distance, have a degree of relevance to fires. The aforementioned parameters are each dependent m part on chemical kmetic mechamsms.
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From page 51... ...
ABSTRACTS AND REVIEWS 203 21 VON ELBE, G Eighth Symposium {Inlemational) on Combustion, pp 41-43, Williams and WUkina, 1961 22.
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