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The biodiesel surrogate, methyl propanoate (MP), is more reactive than methane. Mixtures of the two can be used to control combustion initiation in various combustion systems. Reported here is a shock tube study of the influence of chemical interactions resulting from mixing the two fuels on observable combustion properties, such as global chemical time scales and species time histories. Experiments are carried out at pressures of about 4, 7.4, and 10 atm covering a temperature window of 1000 to 1500 K. Using direct laser absorption, CO time histories during MP pyrolysis are obtained. The CO absorbance is further used to determine pyrolysis times by means of which the effect of temperature on MP pyrolysis is probed. Reactivity differences are first examined with the fuel concentration maintained at 3% and then with the oxygen concentration fixed at 10%. The evidence of chemical interactions during ignition is observed through a reduction of methane ignition delay times caused by MP addition. The influence...

Methane (CH4) and bio alcohols have different ignition properties. These have been extensively studied and the resulting experimental data have been used to validate chemical kinetic models. Methane is the main component of natural gas, which is of interest because of its relative availability and lower emissions compared to other hydrocarbon fuels. Given growing interest in fuel-flexible systems, there can be situations in which the combustion properties of natural gas need to be modified by adding biofuels, such as bio alcohols. This can occur in dual fuel internal combustion engines or gas turbines with dual fuel capabilities. The combustion behavior of such blends can be understood by studying the auto ignition properties in fundamental combustion experiments. Studies of the ignition of such blends are very limited in the literature. In this work, the auto ignition of methane and bio alcohol fuel blends is investigated using a shock tube facility. The chosen bio alcohols are ethanol (C2H5OH) and n-propanol (NC3H7OH). Experiments are carried out at 3 atm and 10 atm for stoichiometric and lean mixtures of fuel, oxygen, and argon. The ignition delay times of the pure fuels are first established at conditions of constant oxygen concentration and comparable pressures. The ignition delay times of blends with 50% methane are then measured. The pyrolysis kinetics of the blends is further explored by measuring CO formation during pyrolysis of the alcohol and methane-alcohol blends. The resulting experimental data are compared with the predictions of selected chemical kinetic models to establish the ability of these models to predict the disproportionate enhancement of methane ignition by the added alcohol.

This study presents four separate reduced chemical kinetic models of methanol/ethanol, propanol isomers, n- and iso-butanol, and n- and s-butanol isomers, derived from a comprehensive chemical kinetic model of C1-C5 alcohols using the Alternate Species Elimination approach. It is motivated by complexity of the detailed model (comprising 600 species and 4100 elementary reactions) and the need for simpler kinetic models for analysis of combustion of smaller alcohols. The reduced models are obtained on the basis of ignition delay time simulations with imposed thresholds on the resulting normalized changes in ignition delay times. The following reduced models are obtained: methanol/ethanol: 38 species and 197 reactions; propanol isomers: 68 species and 419 reactions; n- and iso-butanol: 140 species and 745 reactions; and n- and s-butanol: 134 species and 739 reactions. Predictions of ignition delay times by the reduced models are found to be in good with the detailed models. The reduced models are further tested against other relevant combustion properties. These properties include burning velocities of laminar premixed flames, global pyrolysis time scales, and heat release timing in Homogeneous Charge Compression Ignition engines. This verification shows that reduced models can replace the comprehensive model in combustion analysis without loss of predictive performance. The reduced models can also serve as starting models for developing combined chemical kinetic models of gasoline/diesel and alcohol blends.

This study presents an investigation of the ignition and pyrolysis kinetics of the two propanol isomers. Experiments are carried out behind reflected shock waves at average pressures of 3.5, 5.0, and 11 atm over a temperature range of 1150–1550 K. Oxidation experiments are carried out using mixtures with equivalence ratios of 0.5, 1.0, and 2.0. Laser absorption measurement of CO is carried out near 4.6 µm on the basis of which a characteristic pyrolysis time scale is defined. By means of this pyrolysis time scale, the effects of reactor temperature, pressure, and fuel concentration on pyrolysis are established. Contrast is further made between the global kinetics of pyrolysis and purely oxidative kinetics during ignition. The measured ignition and pyrolysis time scales are also compared with predictions of three chemical kinetic models from the literature. The expected lower reactivity of iso -propanol relative to n -propanol during ignition and pyrolysis is observed. The study also reveals a distinctly higher global temperature sensitivity of the pyrolysis process (global activation energy > 60 kcal/mol) compared to the typical global activation energy of ignition (global activation energy in the range of 20–40 kcal/mol). This difference leads to a cross-over effect at a characteristic reactor temperature between ignition that is controlled by pyrolysis and one that is controlled by predominantly oxidative kinetics. While this is observed for the chosen molecular systems, the cross-over effect is a general feature of high-temperature fuel reaction kinetics. At lower temperatures the chemical time scales of predominantly pyrolytic reactions are much longer than the time scales of oxidative radical processes which dominate the ignition process whereas at higher temperatures the time scales of pyrolytic processes become shorter and dominate the global ignition kinetics. Selected CO concentration profiles obtained during pyrolysis are also compared with predictions of current chemical kinetic models, revealing varying degrees of agreement and discrepancies among the models. The study advances characterization of propanol combustion and demonstrates the use of global chemical time scales to characterize fuel pyrolysis kinetics.

A combined experimental and chemical kinetic modeling study of the high-temperature ignition and pyrolysis of 1,3-dimethylcyclohexane (13DMCH) is presented. Ignition delay times are measured behind reflected shock waves over a temperature range of 1049–1544 K and pressures of 3.0–12 atm. Pyrolysis is investigated at average pressures of 4.0 atm at temperatures of 1238, 1302, and 1406 K. By means of mid-infrared direct laser absorption at 3.39 μm, fuel concentration time histories are measured under ignition and pyrolytic conditions. A detailed chemical kinetic model for 13DMCH combustion is developed. Ignition measurements show that the ignition delay times of 13DMCH are longer than those of its isomer, ethylcyclohexane. The proposed chemical kinetic model predicts reasonably well the effects of equivalence ratio and pressure, with overall good agreement between predicted and measured ignition delay times, except at low dilution levels and high pressures. Simulated fuel concentration profiles agree reason...

The ignition behavior of methyl furan (2-MF) and methyl tetrahydrofuran (2-MTHF) is investigated using the shock tube technique. Experiments are carried out using homogeneous gaseous mixtures of fuel, oxygen, and argon with equivalence ratios, ϕ, of 0.5, 1.0, and 2.0 at average pressures of 3 and 12 atm over a temperature range of 1060–1300 K. In addition to ignition delay time measurements, fuel concentration time histories during ignition and pyrolysis of 2-MTHF are obtained by means of laser absorption spectroscopy using a He–Ne laser at a fixed wavelength of 3.39 µm. With respect to ignition delay times, it is observed that under similar conditions of equivalence ratio and argon/oxygen ratio (D), 2-MTHF has longer ignition delay times than 2-MF at 3 atm. In addition, 2-MTHF has longer ignition delay times than 2-MF at higher temperatures for the case of 12 atm and under the same conditions of ϕ and D. The higher reactivity of 2-MF, as indicated by shorter ignition delay times, is attributed to differences in chemical structure, whereby weaker C–H bond sites are more readily susceptible to radical attack than in 2-MTHF. It is observed that ignition delay times of 2-MTHF decrease with increasing equivalence ratio at 12 atm for fixed argon/oxygen ratio. Ignition delay times are compared with model predictions using recent chemical kinetic models of both fuels, showing that both models generally predict shorter ignition delay times than measured. The relatively higher absorption cross section of 2-MTHF at 3.39 µm allows for its concentration time histories to be determined and compared to model predictions. In line with the observed discrepancy in ignition predictions, predicted 2-MTHF concentration profiles are such that the fuel is shown to be more rapidly consumed than observed in the experiments. The study advances understanding of the combustion chemistry of these cyclic ethers that are potential alternative fuels.

Limiting solar power is the inability to cost effectively store energy. The most cost effective means to store solar energy is thermally in the ground, which can then be used for direct conversion to electricity. However, doing so is limited by a historically poor thermal efficiency of such engines. A novel Stirling engine is posed which more closely mimics a Carnot heat engine. It does this through the use of a new passive thermal ‘switch’ which permits heat flow into the expansion chamber of the Stirling engine only when the temperature of the chamber is above a desired value. Ideally heat would be added only at the end of the compression stroke and the beginning of the expansion stroke. Central to this thermal switch is the use of a vanadium dioxide (VO2) low mass heat exchanger internal to the expansion chamber. This low mass heat exchanger allows the film material to track and react to the temperature changes within the expansion chamber, permitting it to transfer heat only when needed. An adiabatic model of this enhanced solar Stirling engine is developed. Results indicate that the thermal efficiency can be nearly doubled, delivering a second law efficiency of over 0.6. Further, a year round overall efficiency accounting for losses in the Stirling engine and solar thermal collectors of 7% appears to be feasible when this engine is integrated with ground solar storage, providing the necessary power to meet loads in a low energy residence. Such results demonstrate promise for future application of this technology.Copyright © 2013 by ASME