Shock tube measurements of branched alkane ignition times and OH concentration time histories

The ignition of methylcyclohexane (MCH)/air and ethylcyclohexane (ECH)/air mixtures has been studied in a shock tube at temperatures and pressures ranging from 881 to 1319 K and 10.8 to 69.5 atm, respectively, for equivalence ratios of 0.25, 0.5, and 1.0. Endwall OH* emission and sidewall pressure measurements were used to determine ignition delay times. The influence of temperature, pressure, and equivalence ratio on ignition has been characterized. Negative temperature coefficient behavior was observed for temperatures below 1000 K. These measurements greatly extend the database of kinetic targets for MCH and provide, to our knowledge, the first ignition measurements for ECH. The combination of the MCH measurements with previous shock tube and rapid compression machine measurements provides kinetic targets over a large temperature range, 680-1650 K, for the validation of kinetic mechanisms. C 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: [82][83][84][85][86][87][88][89][90][91] 2009

Section: Results Modeling and Discussionsupporting

confidence: 76%

Ignition time measurements for methylcylcohexane‐ and ethylcyclohexane‐air mixtures at elevated pressures

Vanderover¹,

Oehlschlaeger²

2008

“…An activation energy of ≈31 kcal mol −1 (Eq. (1)) for high-temperature MF ignition data is considerably lower than reported values for alkanes [61,67] as well as other oxygenates, including alcohols [68,69], ethers [71], and esters [15] (typically 35-45 kcal mol −1 ) at similar high-temperature conditions. The higher global activation energy barriers for these fuels are a result of the typical hydrogen abstraction-alkyl radical β-scission radical chain reaction oxidation mechanisms.…”

Section: Shock Tubecontrasting

confidence: 60%

Methyl formate oxidation: Speciation data, laminar burning velocities, ignition delay times, and a validated chemical kinetic model

Dooley

Burke

Chaos

et al. 2010

150

204

The oxidation of methyl formate (CH 3 OCHO) has been studied in three experimental environments over a range of applied combustion relevant conditions: 1. A variable-pressure flow reactor has been used to quantify reactant, major intermediate and product species as a function of residence time at 3 atm and 0.5% fuel concentration for oxygen/fuel stoichiometries of 0.5, 1.0, and 1.5 at 900 K, and for pyrolysis at 975 K. 2. Shock tube ignition delays have been determined for CH 3 OCHO/O 2 /Ar mixtures at pressures of ≈ 2.7, 5.4, and 9.2 atm and temperatures of 1275-1935 K for mixture compositions of 0.5% fuel (at equivalence ratios of 1.0, 2.0, and 0.5) and 2.5% fuel (at an equivalence ratio of 1.0). 3. Laminar burning velocities of outwardly propagating spherical CH 3 OCHO/air flames have been determined for stoichiometries ranging from 0.8-1.6, at atmospheric pressure using a pressure-release-type high-pressure chamber.A detailed chemical kinetic model has been constructed, validated against, and used to interpret these experimental data. The kinetic model shows that methyl formate oxidation proceeds through concerted elimination reactions, principally forming methanol and carbon monoxide as well as through bimolecular hydrogen abstraction reactions. The relative importance of elimination versus abstraction was found to depend on the particular environment. In general, methyl formate is consumed exclusively through molecular decomposition in shock tube environments, while at flow reactor and freely propagating premixed flame conditions, there is significant competition between hydrogen abstraction and concerted elimination channels. It is suspected that in diffusion flame configurations the elimination channels contribute more significantly than in premixed environments. C

“…Reflected shock conditions were determined using ideal shock relations. Further details of the shock tube setup can be found elsewhere [17,18].…”

Section: Methodsmentioning

confidence: 99%

Direct measurements of the reaction OH + CH₂O → HCO + H₂O at high temperatures

Vasudevan

Davidson

Hanson

2004

The reaction of hydroxyl [OH] radicals with formaldehyde [CH 2 O] was studied at temperatures ranging from 934 K to 1670 K behind reflected shock waves at an average total pressure of 1.6 atm. OH radicals were produced by shock-heating tert-butyl hydroperoxide [(CH 3 ) 3 CO OH], while 1,3,5-trioxane [(CH 2 O) 3 ] was used in the preshock mixtures to generate reproducible levels of CH 2 O. OH concentration time-histories were inferred from laser absorption using the well-characterized R 1 (5) line of the OH A-X (0, 0) band near 306.7 nm. Detailed error analyses, taking into account both experimental and mechanism-induced contributions, yielded uncertainty estimates of ±25% at 1595 K and ±15% at 1229 K for the rate of the reaction between CH 2 O and OH. These uncertainties are substantially lower than the factor of two uncertainty currently used for this reaction at high temperatures. The rate constants were fit with the recent low-temperature measurements of Sivakumaran et al. (Phys Chem Chem Phys 2003,5,4821-4827) to the three-parameter form shown below; this fit reconciles experimental data on the title reaction at low, intermediate, and high temperatures (200-1670 K).The reaction of OH with CH 2 O was also studied using quantum chemical methods at the CCSD(T) level of theory using the 6-311++G(d,p) basis set. The transition state for the H-atom metathesis reaction was located, and reaction rate coefficients were calculated. Reasonable agreement with the experimental measurements was obtained.