Shock tube measurements of OH concentration time-histories in benzene, toluene, ethylbenzene and xylene oxidation

Abstract

Microsecond-resolved measurements of OH concentrations time-histories were performed during the oxidation of four common monocyclic aromatic hydrocarbons, namely benzene, toluene, ethylbenzene and o-xylene, behind reflected shock waves. Ar-diluted mixtures of 200 ppm aromatics in stoichiometric O2 were shock-heated to temperatures of 1513–1877 K and pressures of about 1.2 atm, where quantitative and time-resolved concentration time-histories of OH were measured by narrow-linewidth laser absorption using a novel solid-state laser near 308.6 nm, at the peak of the Q1(5) transition of the OH A-X (0,0) rovibronic band. The use of the Q1(5) transition improved the detection sensitivity of OH by a factor of 2.2, as compared to the R1(5) transition utilized in most previous OH absorption measurements. The resulting OH time-history data accurately resolved the fine details in multiple stages of oxidation of these aromatics, e.g. the initial decomposition of fuel, the subsequent H-abstraction and radical scavenging, and the final radical surge and onset of ignition, thereby providing multiple valuable kinetics targets for validation and improvement of modern reaction models. Three comprehensive reaction models of aromatics combustion from recent studies were evaluated based on the current experimental results. Key reactions governing the OH time-histories were identified, and further studies on selected reaction rate constants were suggested to improve these models. To our knowledge, the current work presents the first quantitative measurements of OH time-histories in the oxidation of benzene, ethylbenzene and o-xylene, and hence provides a critical and unique contribution to the database needed for development and optimization of modern reaction kinetic models regarding aromatics combustion.