Experimental and numerical study of soot formation in counterflow diffusion flames of gasoline surrogate components

Soot formation is experimentally and numerically investigated in laminar counterflow diffusion flames burning ethylene and three typical gasoline surrogate components; n-heptane, iso-octane, and toluene. Laser-induced incandescence and a light extinction technique are employed to determine the soot volume fraction within the well-controlled region of the burner. The experiments are performed across a wide range of strain rates and stoichiometric mixture fractions. From the experimental data, sensitivities of soot formation on strain rate and stoichiometric mixture fraction are derived for each fuel. The fuels show sig- nificantly different sensitivities. For iso-octane and n-heptane, a higher sensitivity of soot production on the strain rate is observed as compared to ethylene and toluene. Moreover, the sensitivities of soot for- mation on the strain rate increase with increasing stoichiometric mixture fraction. One-dimensional sim- ulations of the flames investigated experimentally were performed using two different detailed chemical kinetic mechanisms, detailed chemical soot models, and the hybrid method of moments as well as a dis- crete sectional method to describe soot dynamics. The models are capable of predicting the soot volume fraction of the ethylene flames with remarkable accuracy, whereas for the gasoline surrogate components, the overall soot volume fractions are overpredicted for all tested models. In iso-octane flames, soot nu- cleation and PAH condensation rates are particularly enhanced. A reaction pathway analysis shows that in ethylene flames, the formation of benzene mostly originates from acetylene, while for iso-octane, large amounts of iso-butenyl form propyne, propargyl, and then benzene.Stephan Kruse, Achim Wick, Paul Medwell, Antonio Attili, Joachim Beeckmann, Heinz Pitsc

New experimental insights into acetylene oxidation through novel ignition delay times, laminar burning velocities and chemical kinetic modelling

The oxidation of acetylene is central to the oxidation of virtually all hydrocarbon fuels. It is also important for commercial industry, due to its wide range of applications such as flame photometry, atomic absorption, welding etc. In this study, ignition delay times (IDTs) for acetylene oxidation were measured at elevated pressures (10-30 bar) and temperatures (700-1300 K) in a high-pressure shock tube (HPST) and in a rapid compression machine (RCM). The range of pressures, temperatures and mixture compositions studied are at conditions never previously investigated in the literature. The new measurements highlight some major shortcomings in our understanding of the oxidation mechanism of acetylene. The importance of these findings is accentuated, considering the fundamental nature of acetylene chemistry in modelling larger hydrocarbons. These data are complemented by new laminar burning velocity (LBV) experiments, independently performed in two different laboratories. As commercial cylinders commonly contain acetylene gas dissolved in acetone, we have also tested the influence of acetone on acetylene reactivity. It was found that the measured LBVs in both laboratories decreased when acetylene dissolved in acetone was used versus when the pure acetylene was used. The IDTs displayed no such sensitivity. When compared to the literature data, the new LBVs for pure acetylene displayed a pronounced increase in the fuel-rich regime, and the peak flame speeds from TAMU and RWTH increased by about 21 and 14 cm/s, respectively. The kinetic models, with one exception, over-predict the measured IDTs by an order of magnitude at temperatures below similar to 1000 K. The reaction of acetylene with hydroperoxyl radicals is critical in accurately predicting the low-temperature, high-pressure IDT data. The experimental findings together with the interpreted models highlight the need for further work to better understand acetylene oxidation. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.The authors at NUI Galway recognize funding support from Science Foundation Ireland (SFI) via their Principal Investigator Program through project number 15/IA/3177. The work at PCFC, RWTH Aachen University was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) Project HE7599/2-1. The effort at TAMU was supported by a sub-award from the Pennsylvania State University through the University Coalition for Fossil Energy Research (UCFER) under U. S. Department of Energy grant DE-FE0026825.peer-reviewed2020-08-0