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Title: Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics

Abstract

A detailed chemical kinetic model for the autoignition of toluene reference fuels (TRF) is presented. The toluene submechanism added to the Lawrence Livermore Primary Reference Fuel (PRF) mechanism was developed using recent shock tube autoignition delay time data under conditions relevant to HCCI combustion. For two-component fuels the model was validated against recent high-pressure shock tube autoignition delay time data for a mixture consisting of 35% n-heptane and 65% toluene by liquid volume. Important features of the autoignition of the mixture proved to be cross-acceleration effects, where hydroperoxy radicals produced during n-heptane oxidation dramatically increased the oxidation rate of toluene compared to the case when toluene alone was oxidized. Rate constants for the reaction of benzyl and hydroperoxyl radicals previously used in the modeling of the oxidation of toluene alone were untenably high for modeling of the mixture. To model both systems it was found necessary to use a lower rate and introduce an additional branching route in the reaction between benzyl radicals and O{sub 2}. Good agreement between experiments and predictions was found when the model was validated against shock tube autoignition delay data for gasoline surrogate fuels consisting of mixtures of 63-69% isooctane, 14-20% toluene, and 17% n-heptanemore » by liquid volume. Cross reactions such as hydrogen abstractions between toluene and alkyl and alkylperoxy radicals and between the PRF were introduced for completion of chemical description. They were only of small importance for modeling autoignition delays from shock tube experiments, even at low temperatures. A single-zone engine model was used to evaluate how well the validated mechanism could capture autoignition behavior of toluene reference fuels in a homogeneous charge compression ignition (HCCI) engine. The model could qualitatively predict the experiments, except in the case with boosted intake pressure, where the initial temperature had to be increased significantly in order to predict the point of autoignition. (author)« less

Authors:
 [1];  [2];  [1]; ;  [3]
  1. Department of Chemical Engineering and Technology, Royal Institute of Technology, SE-100 44 Stockholm (Sweden)
  2. (United Kingdom)
  3. Shell Global Solutions, P.O. Box 1, Chester CH1 3SH (United Kingdom)
Publication Date:
OSTI Identifier:
20880637
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 149; Journal Issue: 1-2; Other Information: Elsevier Ltd. All rights reserved
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 33 ADVANCED PROPULSION SYSTEMS; TOLUENE; HEPTANE; PRESSURE RANGE MEGA PA 10-100; PRESSURE RANGE MEGA PA 01-10; MIXTURES; BENZYL RADICALS; COMBUSTION; HYDROGEN; SIMULATION; HYDROPEROXY RADICALS; OXYGEN; INTERNAL COMBUSTION ENGINES; CHEMICAL REACTION KINETICS; ACCELERATION; FORECASTING; COMPRESSION; AUTOIGNITION; HYDROCARBONS; TEMPERATURE RANGE 1000-4000 K; DIESEL ENGINES

Citation Formats

Andrae, J.C.G., Shell Global Solutions, P.O. Box 1, Chester CH1 3SH, Bjoernbom, P., Cracknell, R.F., and Kalghatgi, G.T. Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics. United States: N. p., 2007. Web. doi:10.1016/J.COMBUSTFLAME.2006.12.014.
Andrae, J.C.G., Shell Global Solutions, P.O. Box 1, Chester CH1 3SH, Bjoernbom, P., Cracknell, R.F., & Kalghatgi, G.T. Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics. United States. doi:10.1016/J.COMBUSTFLAME.2006.12.014.
Andrae, J.C.G., Shell Global Solutions, P.O. Box 1, Chester CH1 3SH, Bjoernbom, P., Cracknell, R.F., and Kalghatgi, G.T. Sun . "Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics". United States. doi:10.1016/J.COMBUSTFLAME.2006.12.014.
@article{osti_20880637,
title = {Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics},
author = {Andrae, J.C.G. and Shell Global Solutions, P.O. Box 1, Chester CH1 3SH and Bjoernbom, P. and Cracknell, R.F. and Kalghatgi, G.T.},
abstractNote = {A detailed chemical kinetic model for the autoignition of toluene reference fuels (TRF) is presented. The toluene submechanism added to the Lawrence Livermore Primary Reference Fuel (PRF) mechanism was developed using recent shock tube autoignition delay time data under conditions relevant to HCCI combustion. For two-component fuels the model was validated against recent high-pressure shock tube autoignition delay time data for a mixture consisting of 35% n-heptane and 65% toluene by liquid volume. Important features of the autoignition of the mixture proved to be cross-acceleration effects, where hydroperoxy radicals produced during n-heptane oxidation dramatically increased the oxidation rate of toluene compared to the case when toluene alone was oxidized. Rate constants for the reaction of benzyl and hydroperoxyl radicals previously used in the modeling of the oxidation of toluene alone were untenably high for modeling of the mixture. To model both systems it was found necessary to use a lower rate and introduce an additional branching route in the reaction between benzyl radicals and O{sub 2}. Good agreement between experiments and predictions was found when the model was validated against shock tube autoignition delay data for gasoline surrogate fuels consisting of mixtures of 63-69% isooctane, 14-20% toluene, and 17% n-heptane by liquid volume. Cross reactions such as hydrogen abstractions between toluene and alkyl and alkylperoxy radicals and between the PRF were introduced for completion of chemical description. They were only of small importance for modeling autoignition delays from shock tube experiments, even at low temperatures. A single-zone engine model was used to evaluate how well the validated mechanism could capture autoignition behavior of toluene reference fuels in a homogeneous charge compression ignition (HCCI) engine. The model could qualitatively predict the experiments, except in the case with boosted intake pressure, where the initial temperature had to be increased significantly in order to predict the point of autoignition. (author)},
doi = {10.1016/J.COMBUSTFLAME.2006.12.014},
journal = {Combustion and Flame},
number = 1-2,
volume = 149,
place = {United States},
year = {Sun Apr 15 00:00:00 EDT 2007},
month = {Sun Apr 15 00:00:00 EDT 2007}
}
  • A semidetailed mechanism (137 species and 633 reactions) and new experiments in a homogeneous charge compression ignition (HCCI) engine on the autoignition of toluene reference fuels are presented. Skeletal mechanisms for isooctane and n-heptane were added to a detailed toluene submechanism. The model shows generally good agreement with ignition delay times measured in a shock tube and a rapid compression machine and is sensitive to changes in temperature, pressure, and mixture strength. The addition of reactions involving the formation and destruction of benzylperoxide radical was crucial to modeling toluene shock tube data. Laminar burning velocities for benzene and toluene weremore » well predicted by the model after some revision of the high-temperature chemistry. Moreover, laminar burning velocities of a real gasoline at 353 and 500 K could be predicted by the model using a toluene reference fuel as a surrogate. The model also captures the experimentally observed differences in combustion phasing of toluene/n-heptane mixtures, compared to a primary reference fuel of the same research octane number, in HCCI engines as the intake pressure and temperature are changed. For high intake pressures and low intake temperatures, a sensitivity analysis at the moment of maximum heat release rate shows that the consumption of phenoxy radicals is rate-limiting when a toluene/n-heptane fuel is used, which makes this fuel more resistant to autoignition than the primary reference fuel. Typical CPU times encountered in zero-dimensional calculations were on the order of seconds and minutes in laminar flame speed calculations. Cross reactions between benzylperoxy radicals and n-heptane improved the model predictions of shock tube experiments for {phi}=1.0 and temperatures lower than 800 K for an n-heptane/toluene fuel mixture, but cross reactions had no influence on HCCI simulations. (author)« less
  • The ignition delay times of mixtures containing 35% n-heptane and 65% toluene by liquid volume at room temperature (i.e., 28% n-heptane/72% toluene by mole fraction) were determined in a high-pressure shock tube in the temperature range 620{<=} T{<=}1180 K at pressures of about 10, 30, and 50 bar and equivalence ratios, {phi}, of 0.3 and 1.0. The equation {tau}/{mu}s=9.8 x 10{sup -3} exp (15,680 K/T)(p/bar){sup -0.883} represents the data for {phi}=0.3 in the temperature range between 980 and 1200 K. At lower temperatures no ignition was found at 10 bar within the maximum test time of 15 ms, whereas formore » 50 bar, a reduced activation energy was observed. A pressure coefficient of -1.06 was found for the data with {phi}=1.0. No common equation for the data at {phi}=1.0 could be found analogous to that for {phi}=0.3 because the ignition delay times show no Arrhenius-like behavior. A comparison with ignition delay times of n-heptane/air and toluene/air for {phi}=1.0 and 30 bar shows that the values of the mixture of the two components are between the values of the single substances. Furthermore, the results confirm the negative temperature coefficient behavior found for the mixtures at 30 and 50 bar, similar to n-heptane/air. A comparison for the other pressure and equivalence ratio values of this study was not possible because of the lack of data for pure toluene. These experimental data have been used in the development of a chemical kinetics model for toluene/n-heptane mixtures as described in a companion paper. (author)« less
  • In this work, fluid flow and detailed chemical kinetic calculations were combined as a step along the way to understand the ignition process of natural gas (NG) in diesel environments. The kinetic scheme consisted of 31 species in 125 reactions. The flow calculations involved time-dependent, fully elliptic Navier-Stokes equations, and conservation equations of mixture mass, mass of 31 species, and energy. The simulations were done for a constant volume cylindrical chamber filled initially with air at high temperature (> 1,000 K) and pressure (25.2 atm) into which the cold natural gas was injected. The computations were confined to the nearmore » field of the jet assuming laminar, axisymmetric flow. Initial air temperature and chamber height were varied in this study. The computations revealed that the fluid dynamics affected the ignition delay and location of the ignition spot. Computed results from this work are consistent with experimental data on autoignition of NG in diesel environments.« less
  • Autoignition of toluene and benzene is investigated in a rapid compression machine at conditions relevant to HCCI (homogeneous charge compression ignition) combustion. Experiments are conducted for homogeneous mixtures over a range of equivalence ratios at compressed pressures from 25 to 45 bar and compressed temperatures from 920 to 1100 K. Experiments varying oxygen concentration while keeping the mole fraction of toluene constant reveal a strong influence of oxygen in promoting ignition. Additional experiments varying fuel mole fraction at a fixed equivalence ratio show that ignition delay becomes shorter with increasing fuel concentration. Moreover, autoignition of benzene shows significantly higher activationmore » energy than that of toluene. In addition, the experimental pressure traces for toluene show behavior of heat release significantly different from the results of Davidson et al. [D.F. Davidson, B.M. Gauthier, R.K. Hanson, Proc. Combust. Inst. 30 (2005) 1175-1182]. Predictability of various detailed kinetic mechanisms is also compared. Results demonstrate that the existing mechanisms for toluene and benzene fail to predict the experimental data with respect to ignition delay and heat release. Flux analysis is further conducted to identify the dominant reaction pathways and the reactions responsible for the mismatch of experimental and simulated data. (author)« less
  • A new technique of reduction of detailed mechanisms for autoignition, which is based on two analysis methods is described. An analysis of reaction rates is coupled to an analysis of reaction sensitivity for the detection of redundant reactions. Thresholds associated with the two analyses have a great influence on the size and efficiency of the reduced mechanism. Rules of selection of the thresholds are defined. The reduction technique has been successfully applied to detailed autoignition mechanisms of two reference hydrocarbons: n-heptane and iso-octane. The efficiency of the technique and the ability of the reduced mechanisms to reproduce well the resultsmore » generated by the full mechanism are discussed. A speedup of calculations by a factor of 5.9 for n-heptane mechanism and by a factor of 16.7 for iso-octane mechanism is obtained without losing accuracy of the prediction of autoignition delay times and concentrations of intermediate species.« less