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International Journal of Energy for a Clean Environment
SJR: 0.151 SNIP: 0.224 CiteScore™: 0.21

ISSN Print: 2150-3621
ISSN Online: 2150-363X

International Journal of Energy for a Clean Environment

Formerly Known as Clean Air: International Journal on Energy for a Clean Environment

DOI: 10.1615/InterJEnerCleanEnv.v4.i2.40
72 pages


G. M. Faeth
Department of Aerospace Engineering, the University of Michigan, Ann Arbor, Michigan 48109-2140, USA
C. H. Kim
Department of Aerospace Engineering, The University of Michigan, Ann Arbor, MI
O. C. Kwon
Department of Aerospace Engineering, The University of Michigan, Ann Arbor, MI


In spite of their effectiveness as flame suppressants, the manufacture of Halons was stopped in 1994 due to their large stratospheric ozone depletion capabilities, in order to comply with the Montreal Protocol. The resulting need for alternative flame suppressants has prompted extensive efforts to better understand flame suppression mechanisms and to develop environmentally acceptable Halon replacements. These efforts are reviewed in the present article, limited to flames involving either hydrogen or hydrocarbon fuels reacting with oxygen due to their relative tractability for detailed numerical simulations. The article begins with consideration of ozone chemistry in the stratosphere in order to highlight the transport and chemical reaction properties of flame suppressants that are responsible for ozone depletion. Recent experimental and computational studies of laminar premixed hydrogen-fueled flames are then considered in order to establish capabilities to predict the fundamental properties of these flames, e.g., their unstretched laminar burning velocities and their flame response to stretch as characterized by Markstein numbers. These results are then extended to consider effects of flame suppressants on the properties of laminar premixed hydrogen-fueled flames. The flame suppressant results show that they mainly function by reducing the concentrations of chain-carrying radicals (H and OH among others) in the reaction zone of flames with corresponding reduced reaction rates causing reduced laminar burning velocities and enhanced effects of extinction. An interesting corollary of this effect, however, is that reduced radical concentrations in the reaction zone also promotes preferential-diffusion/stretch instabilities that enhance flame reaction rates and tend to resist the action of flame suppressants. The new understanding of effects of flame suppressants on radical concentrations in flames is then exploited to gain a better understanding of the mechanism of flame suppression for both chemically-active and chemically-passive flame suppressants in both premixed and nonpremixed flames. These considerations help to identify the potential of various substances to act as effective flame suppressants but with negligible stratospheric ozone depletion potentials. Among these, water mist exhibits flame suppression capabilities potentially comparable to Halon 1301 but with little potential impact on the environment, including ozone depletion; therefore, additional study and development of methods of flame suppression by water mist clearly are merited.