Extinction and instability mechanisms of premixed turbulent flames

University Of Southern California, Los Angeles CA

Investigators

Abstract

Experiments will be conducted to develop a more comprehensive understanding of the burning properties of fuel-air mixtures highly diluted with excess air or exhaust gas and thereby obtain knowledge that could be applied to improve the efficiency and reduce harmful emissions from internal combustion engines and other energy conversion devices. Both hydrocarbon-air and hydrogen-air mixtures will be tested; hydrogen is entirely different from other fuels because of its lower minimum flame temperatures supporting combustion, low ratio of mixture thermal diffusivity to fuel mass diffusivity and lower heat losses, which lead to unique flame properties that are difficult to quantify. Three sets of experiments will be performed. First, to study the effects of turbulence, experiments in a jet-stirred combustion chamber will be conducted. Burning rates, conditions for flame extinguishment and emissions will be examined for varying mixtures and turbulence conditions. Second, to assess the effects of flame stretching, the burning rates and shapes of flame edges at the boundary between burning and extinguished regions of the flames will be studied in a turbulence-free apparatus. Third, to study the effects thermal expansion without hydrodynamic strain (other than that generated by self-induced flow), the propagation rates and stability properties of flames propagating in narrow channels (i.e. Hele-Shaw cells) will be studied. This is considered especially relevant since most laboratory experiments are conducted in open apparatuses where thermal expansion is relaxed in the directions parallel to the flame plane, whereas combustion in most practical devices (e.g., IC engines) occurs in very confined geometries where this relaxation cannot occur. The use of fuel-air mixtures (particularly hydrogen-air mixtures) highly diluted with excess air or exhaust gas is an environmentally friendly way of operating internal combustion engines but the ability to employ these mixtures in engines is hindered by the lack of knowledge of the rates at which these flames burn, the flame shapes they produce, and if/when these flames will extinguish. While many hydrogen fueled vehicles use fuel cells, it is still likely that for many applications, internal combustion engines will be the best power system due to the vastly superior power to weight ratio of engines compared to fuel cells even though the engine efficiencies are slightly lower than fuel cells. Moreover, most of harmful emissions concerns of hydrocarbon-fueled engines are practically eliminated using hydrogen fuel. The information obtained in this work will provide a better understanding of combustion of hydrogen, hydrocarbon and hydrogen/hydrocarbon mixtures. This data will be beneficial not only for engine design, but also in reducing fire and explosion hazards associated with fuel manufacturing, transportation and storage. The proposed work will support a graduate student and two undergraduates, the support for one of which is reserved for a member of an under-represented group.