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Abstract The current work presents new experimental autoignition and speciation data on the twocis‐hexene isomers:cis‐2‐hexene andcis‐3‐hexene. The new data provide insights on the effects of carbon‐carbon double bond location and stereoisomeric structures on ignition delay times and reaction pathways for linear hexene isomers. Experiments were performed using the University of Michigan rapid compression facility to determine ignition delay times from pressure‐time histories. Stoichiometric (ϕ = 1.0) mixtures at dilution levels of inert gas to O₂ = 7.5:1 (mole basis) were investigated at an average pressure of 11 atm and temperatures from 809 to 1052 K. Speciation experiments were conducted atT = 900 K for the twocis‐hexene isomers, where fast‐gas sampling and gas chromatography were used to identify and quantify the twocis‐hexene isomers and stable intermediate species. The ignition delay time data showed negligible sensitivity to the location of the carbon‐carbon double bond and the stereoisomeric structure (cis‐trans), and the species data showed no correlation with the stereoisomeric structure, but there was a strong correlation of some of the measured species with the location of the double bond in the hexene isomer. In particular, 2‐hexene showed strong selectivity to propene, acetaldehyde, and 1,3‐butadiene, and 3‐hexene showed selectivity to propanal. Model predictions of ignition delay times were in excellent agreement with the experimental data. There was generally good agreement for the model predictions of the species data for 2‐hexene; however, the mechanism overpredicted some of the small aldehyde (C₂‐C₄) species for 3‐hexene. Reaction pathway analysis indicates the hexenes are almost exclusively consumed by H‐atom abstraction reactions at the conditions studied (P = 11 atm,T > 900 K), and not by C₃‐C₄scission as observed in high‐temperature (>1300 K) hexene ignition studies. Improved estimates for 3‐hexene + OH reactions may improve model predictions for the species measured in this work. 

Autoignition delay time data are one important means to develop, quantify, and validate fundamental understanding of combustion chemistry at low temperatures (T<1200 K). However, low-temperature chemistry often has higher uncertainties and scatter in the experimental data compared with high-temperature ignition data (T>1200 K). In this study, autoignition properties of propane and oxygen mixtures were investigated using the University of Michigan rapid compression facility in order to understand the effects of ignition regimes on low-temperature ignition data. For the first time for propane, autoignition delay times were determined from pressure histories, and autoignition characteristics were simultaneously recorded using high-speed imaging of the test section through a transparent end-wall. Propane mixtures with fuel-to-O2 equivalence ratios of ϕ = 0.25 and ϕ = 0.5 and O2-to-inert gas molar ratios of 1:3.76 were studied over the pressure range of 8.9 to 11.3 atm and the temperature range of 930 – 1070 K. The results showed homogeneous or strong autoignition occurred for all ϕ = 0.25 experiments, and inhomogeneous or mixed autoignition occurred for all ϕ = 0.5 experiments. While a limited temperature range is covered in the study, importantly the data span predicted transitions in autoignition behavior, allowing validation of autoignition regime hypotheses. Specifically, the results agree well with strong-autoignition limits proposed based on the Sankaran Criterion. The autoignition delay time data at the strong-ignition conditions are in excellent agreement with predictions using a well-validated detailed reaction mechanism from the literature and a zero-dimensional modeling assumption. However, the experimental data at the mixed autoignition conditions were systematically faster than the model predictions, particularly at lower temperatures (T< ~970 K). The results are an important addition to the growing body of data in the literature that show mixed autoignition phenomena are important sources of the higher scatter observed in the low-temperature autoignition data for propane and other fuels. The results are discussed in terms of different methods to capture the effects of pre-autoignition heat release associated with mixed autoignition conditions and thereby address some of the discrepancies between kinetic modeling and experimental measurements.