The autoignition of practical fuels at HCCI conditions: High-pressure shock tube experiments and phenomenological modeling (original) (raw)
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Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics
Combustion and Flame, 2007
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 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.
Combustion and Flame, 2012
Ignition delay times of surrogate biodiesel fuels were measured in a high-pressure shock tube over a wide range of experimental conditions (pressures of 20 and 40 bar, equivalence ratios in the range 0.5-1.5, and temperatures ranging from 700 to 1200 K). A detailed chemical kinetic mechanism developed for the oxidation of a biodiesel fuel and a B30 biodiesel surrogate (49% n-decane, 21% 1-methylnaphthalene, and 30% methyloctanoate in mol%) was used to simulate the present experiments. Cross reactions between radicals from the three fuel components and reactions of methylnaphthalene oxidation recently proposed in the literature were introduced into the model in order to improve ignition delay time predictions at low temperatures. The new scheme (7865 reversible reactions and 1975 species) yields improved model predictions of concentration profiles measured earlier in a jet-stirred reactor, and also represents fairly well the present experimental data over the entire range of conditions of this study. Sensitivity analyses and reaction path analyses were used to rationalize the results.
Autoignition of gasoline surrogates mixtures at intermediate temperatures and high pressures
Combustion and Flame, 2008
Ignition times were determined in high-pressure shock-tube experiments for various stoichiometric mixtures of two multicomponent model fuels in air for the validation of ignition delay simulations based on chemical kinetic models. The fuel blends were n-heptane (18%)/isooctane (62%)/ethanol (20%) by liquid volume (14.5%/44.5%/41% by mole fraction) and n-heptane (20%)/toluene (45%)/isooctane (25%)/diisobutylene (10%) by liquid volume (17.5%/55%/19.5%/8.0% by mole fraction). These fuels have octane numbers comparable to a standard European gasoline of 95 RON and 85 MON. The experimental conditions cover temperatures from 690 to 1200 K and pressures at 10, 30, and 50 bar. The obtained ignition time data are scaled with respect to pressure and compared to previous results reported in the literature.
Proceedings of the Combustion Institute, 2009
Ignition-delay times were measured in shock-heated gases for a surrogate gasoline fuel comprised of ethanol/iso-octane/n-heptane/toluene at a composition of 40%/37.8%/10.2%/12% by liquid volume with a calculated octane number of 98.8. The experiments were carried out in stoichiometric mixtures in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: Temperatures of 690-1200 K, and pressures of 10, 30 and 50 bar, respectively. Ignition delay times were determined from CH * chemiluminescence at 431.5 nm measured at a sidewall location. The experimental results are compared to simulated ignition delay times based on detailed chemical kinetic mechanisms. The main mechanism is based on the primary reference fuels (PRF) model, and sub-mechanisms were incorporated to account for the effect of ethanol and/or toluene. The simulations are also compared to experimental ignition-delay data from the literature for ethanol/iso-octane/n-heptane (20%/62%/18% by liquid volume) and iso-octane/n-heptane/toluene (69%/17%/14% by liquid volume) surrogate fuels. The relative behavior of the ignition delay times of the different surrogates was well predicted, but the simulations overestimate the ignition delay, mostly at low temperatures.
A high-pressure shock tube characterization and auto-ignition delay investigations
A high pressure shock tube "HPST" has been designed for the purpose of chemical kinetics studies at elevated pressures and temperatures. The present HPST is designed as a versatile tool and includes the features of a fast compression, optical accessibility, and capability for specie measurement. Characterization experiments establish the suitability of the tube for chemical kinetic studies and show that highly repeatable experimental conditions up to 40 bar and temperatures between 1300 and 2000 K can be obtained. As well, tailoring gas mixture in the driver section, used to obtain a longer test time, is studied in the characterization experiments. Using this facility, autoignition investigations are conducted for methane mixture (1% CH 4 ; 4% O 2 ; 95% Ar ; f=0,5) at 10, 20 and 40 bar pressures and temperatures from 1300 K up to 2000 K.