5310 Combustion Introduction Lecture 12

5310 Combustion Introduction Lecture 12 - MAE 5310:...

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MAE 5310: COMBUSTION FUNDAMENTALS Overview of Some Important Chemical Mechanisms October 13, 2011 Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk
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INTRODUCTION TO IMPORTANT CHEMICAL MECHANISMS Purpose Outline elementary steps involved in a number of chemical mechanisms of significant importance to combustion and combustion generated air pollution Fundamental ideas developed in chemical kinetics directly applicable to understanding complex real systems Precautionary note Complex mechanisms are evolutionary products of chemists’ thoughts and experiments, and may change with time as new insights are developed Therefore when we discuss a particular mechanism, we are not referring to the mechanism in the same sense that we might refer to the first law of thermodynamics
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THE H 2 -O 2 SYSTEM Hydrogen-oxygen system is important in rocket propulsion and also important subsystem in oxidation of hydrocarbons and carbon monoxide Depending on the temperature, pressure, and extent of reaction, reverse reactions may be possible In modeling the H 2 -O 2 system as many as 40 reactions can be taken into account involving 8 species: H 2 , O 2 , H 2 O, OH, O, H, HO 2 , and H 2 O 2 Consider detailed mechanism shown on the next slide Consider explosive behavior of H 2 -O 2 system Comments Understanding of detailed chemistry of a system is very useful in understanding experimental observations Such an understanding is essential to development of predictive models of combustion phenomena when chemical effects are important
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THE H 2 -O 2 SYSTEM
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H 2 -O 2 EXPLOSION CHARACTERISTICS: φ =1.0
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H 2 -O 2 EXPLOSION CHARACTERISTICS Follow a vertical line at, say 500 C 1 – 1.5 mm Hg there is no explosion Lack of explosion is a result of the free radicals produced in the chain initiation step (H.2) and chain sequence (H.3-H.6) being destroyed by reactions on the wall of the vessel Wall reactions break the chain, preventing build-up of radicals that lead to explosion Note that the wall reactions are not explicitly included in the mechanism since they are not strictly gas phase reactions. Symbolically: 1.5 – 50 mm Hg there is an explosion Direct result of gas-phase chain sequence H.3-H.6 prevailing over radical destruction at wall Remember that increasing the pressure increases the radical concentration linearly, while increasing the reaction rate geometrically 50 – 3,000 mm Hg there is no explosion The cessation of explosive behavior can be explained by the competition for H atoms between the chain branching reactions, H.3, and what is effectively a chain-terminating step at low temperatures, reaction H.11. Reaction H.11 is chain terminating because the hydroperoxy radical, HO
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This note was uploaded on 02/11/2012 for the course MAE 5310 taught by Professor Danielkirk during the Fall '09 term at FIT.

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5310 Combustion Introduction Lecture 12 - MAE 5310:...

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