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The interactions between cool flames and flames with repetitive extinction and ignition (FREI) of stoichiometric n -heptane/air mixture were studied using a micro flow reactor with a controlled temperature profile from 373 to 1300 K. Two different flame dynamics with and without cool flames were observed in reactors with inner diameters d inner of 1 and 2 mm. Cool flames and FREI are spatially separated at d inner = 1 mm, whereas interactions between cool flames and FREI are observed at d inner = 2 mm. At d inner = 1 mm, the brightness intensity from cool flames depends on the inlet velocity (u inlet). Approximately above u inlet = 10 cm/s, the brightness intensity from cool flames decreases with increasing inlet velocity, despite a large amount of mixture input. This is because before low temperature ignition occurs under higher inlet velocity conditions, the mixture archives temperature where negative temperature coefficient is dominant. Reaction front propagation speed of FREI decreases monotonically due to heat loss because the extinction points of FREI are located in higher temperatures than the cool flame region. At d inner = 2 mm, the acceleration of the reaction front in the cool flame region is confirmed experimentally, as predicted in our previous two-dimensional numerical simulations. Additionally, the instantaneous reaction front speed after autoignition is analyzed at d inner = 1 mm. The instantaneous reaction front speed decreases as the time from extinction to ignition t ex _ ig becomes longer because a moderate mixing zone of reactants and products is formed. [ABSTRACT FROM AUTHOR]

To investigate (fuel-)lean/rich limits and essential stoichiometries, i.e., the borders of lean/rich combustion, one-dimensional steady computations with detailed chemistry for flame balls, counterflow flames, and stretch-free planar flames were conducted using a CH 4 /O 2 /Xe mixture that has been used in microgravity experiments. As continuous converged solutions were obtained under lean/rich conditions, it was suggested that the existence of flame ball not only under lean but also under rich condition. Flame radii and temperatures of flame balls decreased and increased toward the lean/rich limits from their maximum and minimum values, respectively. The lean limits were wider in the order of the flame ball, counterflow flame, and stretch-free planar flame. Therefore, the lean flammability limit corresponded to the lean limit of the flame ball in the mixture. Conversely, the rich limits were wider in the order of the counterflow flame, stretch-free planar flame, and flame ball. Thus, the rich flammability limit corresponded to the rich limit of the counterflow flame in the mixture. Essential stoichiometry, which represents the actual stoichiometry depending on the dominant transport in near-flame front, was not uniquely determined as conventional stoichiometry (ϕ = 1); it was located between the equivalence ratio of ϕ = 1 and ϕ c , where ϕ c denotes the critical equivalence ratio is evaluated using the fuel and oxidizer Lewis number of a target mixture. The results indicated that the essential stoichiometry of the stretch-free planar flame corresponded to ϕ = 1, that of the flame ball corresponded to ϕ = ϕ c , and that of the stretched flame was located between ϕ = 1 and ϕ c depending on the stretch rate. [ABSTRACT FROM AUTHOR]

The R410A refrigerant (a blend of CH 2 F 2 and C 2 HF 5 with a mass ratio of 50:50) has been extensively used because of its small impact on the environment. We investigated the effects of the CH 2 F 2 -to-C 2 HF 5 blending ratio in a stoichiometric CH 2 F 2 /C 2 HF 5 /air mixture on the ignition and combustion properties using a microflow-reactor with a controlled temperature profile. Experiments with weak flames showed two separated reaction zones of CH 2 F 2 and blend of CH 2 F 2 and C 2 HF 5 (50:50), while a single reaction zone was observed for C 2 HF 5. This was computationally reproduced using the Linteris mechanism. According to computations for different CH 2 F 2 -to-C 2 HF 5 ratios with steps of 10 wt%, two heat release rate (HRR) peaks were observed under all conditions. The first HRR peak at the lower temperature and second HRR peak at the higher temperature corresponded to the consumption of refrigerant and CO oxidation, respectively. The second HRR peak was larger than the first peak for CH 2 F 2. The second HRR peak shifted to a higher temperature and decreased with the increase in C 2 HF 5 mass fraction up to 50 wt%, because the OH production by the H 2 –O 2 reaction decreased owing to the increased HF production in the low-temperature zone. At C 2 HF 5 mass fractions larger than 50 wt%, the first HRR peak became dominant because CO oxidation did not occur. The first HRR peak shifted to a lower temperature with the increase in C 2 HF 5 mass fraction in the range of 60 to 100 wt% because the reactions contributing to the production of OH from H 2 O increased in the low-temperature zone, instead of the H 2 –O 2 reaction. A large change in HRR was observed at an F/H ratio in the refrigerant blend of 1.8, owing to the change in OH production reactions. The tendency of the reactivities of the refrigerants is useful to prevent unwanted ignition. [ABSTRACT FROM AUTHOR]

For the understanding of ball-like flame behavior in counterflow field, transient three-dimensional computations with thermal-diffusion model were conducted for a low-Lewis number mixture near lean limit. Three types of flame behaviors were confirmed: stable spherical ball-like flame (spherical BLF) in A ≤ 0.010, stable non-spherical ball-like flame (non-spherical BLF) in 0.01 < A < 0.089 and splitting ball-like flame (splitting BLF) in A ≥ 0.089, where A is ordinary stretch rate normalized with laminar burning velocity S L and thermal diffusivity α. Analysis of flame structure for non-spherical BLF located its center at the stagnation point showed that the maximum temperature on the stagnation plane was higher than that on the counterflow axis because of the small difference between the flame curvatures on the stagnation plane and that on the counterflow axis. With the increase of stretch rate, the maximum temperature of the non-spherical BLF on the stagnation plane increased and the position of maximum temperature got away from the stagnation point. The maximum temperature on the counterflow axis decreased and the position of maximum temperature got closer to the stagnation point. Existence of unburned fuel was also confirmed near the stagnation point at A = 0.085. Thus, net fuel velocity was newly introduced to evaluate the effect of the unburned fuel diffusion. The profile of the net fuel velocity revealed two peaks in the case of A < 0.050 and four peaks in the case of A > 0.050. In the case A > 0.050, the inner two peaks were found to be due to the diffusion of unburned fuel to the outward direction. The analyses on the peak positions showed that the flame splitting occurs when the positions of the inner two peaks of the net fuel velocity are located outside of the reference flame ball radius. [ABSTRACT FROM AUTHOR]

Two-dimensional axisymmetric numerical simulation reproduced flames with repetitive extinction and ignition (FREI) in a micro flow reactor with a controlled temperature profile with a stoichiometric n -heptane/air mixture, which have been observed in the experiment. The ignition of hot flame occurred from consumption reactions of CO that was remained in the previous cycle of FREI. Between extinction and ignition locations of hot flames, several other heat release rate peaks related to cool and blue flames were observed for the first time. After the extinction of the hot flame, cool flame by the low-temperature oxidation of n -heptane appeared first and was stabilized in a low wall temperature region. In the downstream of the stable cool flame, a blue flame by the consumption reactions of cool flame products of CH 2 O and H 2 O 2 appeared. After that, the hot flame ignition occurred from the remaining CO in the downstream of the blue flame. Then after the next hot flame ignition, the blue flame was swept away by the propagating hot flame. Soon before the hot flame merged with the stable cool flame, the hot flame propagation was intensified by the cool flame. After the hot flame merged with the stable cool flame, the hot flame reacted with the incoming fresh mixture of n -C 7 H 16 and O 2. [ABSTRACT FROM AUTHOR]

Nitromethane has many applications, such as in racing, as a gasoline fuel additive, and as a monopropellant. Despite a large number of studies and the small size of the molecule, the combustion chemistry of nitromethane is still not well understood. To improve models, the pyrolysis of nitromethane (CH 3 NO 2) was investigated experimentally in shock tubes and in a micro flow reactor with a controlled temperature profile (MFR), under dilute conditions. Several spectroscopic diagnostics were used in the shock tubes to follow the concentration time histories of CO, H 2 O (both using IR laser absorption), and CH 3 NO 2 (UV light absorption). A quadrupole mass spectrometer was used to measure CH 3 NO 2 , NO 2 , CH 4 , C 2 H 4 , and C 2 H 2 at various temperatures with the MFR. These unique experimental results were compared to modern, detailed kinetics models from the literature, and no mechanism was able to reproduce these data over the wide range of conditions investigated. Predictions for the CO and H 2 O levels were generally inaccurate, and the CH 4 , C 2 H 4 , and C 2 H 2 predictions were poor in most cases for the MFR data. Importantly, all models largely differ in their predictions. A numerical analysis was performed to identify ways to improve the next generation of nitromethane models. Results indicate that nitromethane decomposition needs to be improved below 1050 K, and that hydrocarbon-NOx interactions still need to be further investigated. [ABSTRACT FROM AUTHOR]