Laser Absorption Measurements of Hydroperoxy-Related Reactions in Shock Tubes at Intermediate Temperatures

Stanford University, Stanford CA

Investigators

Abstract

Laser Absorption Measurements of Hydroperoxyl-Related Reactions in Shock Tubes at Intermediate Temperatures R. K. Hanson, Stanford University Public Abstract Hydrogen/oxygen oxidation is one of the most important systems in combustion today. This has led to the development of an extremely accurate description of H2/O2 chemistry at high temperatures (above 1200 K). However, at intermediate temperatures (from about 800 to 1200 K) the rate coefficients of many important chemical reactions are uncertain, particularly those involving HO2, the hydroperoxyl radical, and H2O2, hydrogen peroxide. H2O2 kinetics can dominate the chemistry in hydrogen-powered engines, such as homogenous charge compression ignition engines (HCCI), and are critical to describing engine knock and diesel ignition. An understanding of H2O2 chemistry is also needed as a basis for attacking the larger problem of peroxy-radical (RO2) chemistry important in oxidation at intermediate temperatures of hydrocarbons. New measurements with smaller uncertainties of the rate coefficients in the H2O2 system at intermediate temperatures are urgently needed to improve chemical models and computer simulations of these processes. Alkanes are important components of all hydrocarbon fuels such as gasoline, kerosene and diesel. During combustion, these alkanes (such as n-heptane and iso-octane) decompose into alkyl radicals. Subsequently these alkyl radicals are either oxidized or further decompose. These two alkyl pathways of oxidation and decomposition, and their relative rates, dictate the subsequent behavior of the combustion system. The properties of these reactions cause a change in the oxidation mechanism in the intermediate temperature regime which results in the "negative temperature coefficient" (NTC) behavior in hydrocarbon oxidation. Complete understanding of the alkyl oxidation mechanism has implications in combustion chemistry, auto-ignition, engine knock, atmospheric chemistry and radical reaction chemistry. Quantitative measurements of transient radicals, including OH, HO2 and alkyl-peroxy-radicals, will be made in these systems using UV laser absorption in shock tubes. From these measurements of time histories for species concentrations, reaction rates will be determined for several critical hydrocarbon combustion reactions. Specific reactions of interest include: H2O2 OH + OH; H + H2O2 HO2 + H2; C2H5 +O2 HO2 + C2H4; C3H7 +O2 HO2 + C3H6. These reactions have been identified as major contributors to the uncertainty in the modeling of intermediate temperature (800-1200 K) combustion. Key contributions will be the measurements themselves, falling in a key temperature region and measured at pressures from 1 to 100 atm, and a new 215nm deep-UV laser-absorption diagnostic method. This research will provide an opportunity to train a generation of engineering scientists in modern combustion kinetics and state-of-the-art laser diagnostic and shock tube techniques. Public dissemination of this data through the web using current databases will provide the public, students, teachers and government workers with the information needed to critically understand combustion processes. With one of the important problems facing Americans today being generation of greenhouse gases from combustion processes and the subsequent effect of global warming, research on combustion fundamentals is critically needed.