The response of laminar methane/air counterflow premixed flames under sinusoidal equivalence ratio oscillation was investigated numerically. The timescales of the oscillation were chosen to be sufficiently longer than the flame timescale so that the flame responds quasi-steadily. The response of periodically stratified flame (SF) with a detailed reaction mechanism exhibited the “back-support” effect, in that the consumption speed Sc response deviated increasingly from Sc of steady homogeneous flames (HFs) at higher oscillation frequencies. It was shown that even when the imposed oscillation timescale is much longer than the flame timescale, the flame response can still be delayed under a sufficiently large equivalence ratio gradient. Subsequently, the above results were compared with those obtained with a global four-step mechanism that omits back-diffusion radicals into the reaction zone. As a result, SFs with the global mechanism displayed a much smaller back-support effect in both lean and rich mixtures. Further analysis with modified diffusion coefficients revealed the dominant roles of H2 and radical species diffusion in inducing the back-support effect. Contrary to the previous findings, variations in burned gas temperature were found to play a negligible role in modifying Sc. Additionally, the hysteresis of the back-support effect under periodical stratification was found to be more prominent on the richer side because of the presence of a larger H2 pool.

Front propagation speeds from fully resolved unsteady one dimensional simulations with dimethyl-ether (DME)/methane (CH4)/air mixtures under engine relevant conditions are presented using complex kinetics and transport. Different time-scales of monochromatic inhomogeneities in DME concentration with varying DME/CH4 blending ratios are simulated to unravel the fundamental aspects of auto-ignition and flame propagation under the influence of reactivity stratification. To understand the influence of different stratification time-scales on the flame-ignition interaction, two sets of conditions are simulated such that low temperature chemistry is present in only one of them. For a given amplitude of stratification, it is found that the instantaneous propagation speed is significantly affected by the level of CH4 concentration in the binary fuel blend. Specifically, for cases with low temperature chemistry, at relatively smaller time-scales, the overall fluctuation in the instantaneous propagation speed is found to subside as the level of CH4 concentration in the mixture is increased. However, for both sets of conditions, at comparatively larger time-scales, a rapid change in the instantaneous propagation speed is observed with an increase in the level of CH4 concentration in the mixture. The intrinsic effects of stratification time-scales on the low temperature chemistry and the high temperature chemistry are further examined to assess the flame-ignition interaction. A displacement speed analysis is also carried out to elucidate the underlying combustion modes that are responsible for such a variation in flame response.

2,2,3-Trimethylbutane (i.e., triptane) is a potential gasoline octane booster with a research octane number (RON) of 112. Recent studies showed that it can be catalytically produced with high selectivity from methanol (CH3OH) and dimethyl ether (DME), which presents a promising route for utilizing biomass derivatives as transportation fuels. Understanding the ignition properties of triptane at engine relevant conditions is crucial for its further evaluation. In this work, a detailed kinetic model for triptane combustion is developed and validated. The rate rules for the low-temperature oxidation reactions are evaluated based on quantum chemistry calculations from literature, and thermochemical properties of all the species are assessed based on new thermodynamic group values with careful treatment of gauche interactions. In addition, alternative isomerization pathways for peroxy-alkylhydroperoxide species (ȮOQOOH) are incorporated in the model. The model is validated against new ignition delay data from facilities at King Abdullah University of Science and Technology (KAUST): rapid compression machine (RCM) experiments at pressures of 20 and 40 bar, equivalence ratios of 0.5 and 1 and across a temperature range of 620 to 1015 K, and shock tube experiments at 2 and 5 bar, 0.5 and 1 equivalence ratio and over 1000–1400 K. Moreover, the model prediction of various species is compared against species profiles from jet stirred reactor experiments at three equivalence ratios (0.5, 1 and 2) at atmospheric pressure. Finally, triptane is compared with its less branched isomers, n-heptane and 2-methylhexane, to evaluate the effect of branching on fuel reactivity and importance of alternative isomerization pathway.

The effect of DC electric field on the characteristics of flame spread over polyethylene-insulated twin electrical wires was studied by varying wire gap (S) and voltage (VDC). Under an applied electric field, the flame spread rate (FSR), flame width, leaning direction of the interacting twin flames varied substantially with varying the voltage and wire gap. The flame spread rate was initially larger for the wire with negative voltage (spreading flame with negative charge; SF−) than the wire with positive voltage (SF+), but the two eventually became the same in the developed region when a quasi-steady state was reached. The FSR behavior could be classified into two regimes; twin flame spread (regime I) and single flame spread (regime II) after the extinction of SF+. Under regime I, three sub-regimes were identified depending on the wire gap and voltage. For the twin flame spread, the flame spread rate initially decreased with increasing voltage as the flame leaned toward the burnt wire. As the two flames interacted, the flame spread rate increased because of the ionic wind effect, and eventually decreased because of the loss of molten PE mass and the electrospray phenomenon. In regime II after the extinction of SF+, the single flame spread showed a transient behavior since the influences of electric field from burnt and unburned wire sections of SF+ wire varied with flame spread. When the voltage was increased even further, SF– was extinguished by streamer generation and, at excessive voltages, an electrical short occurred. The flame spread rates for twin flame spread were best correlated with the electric field intensity in the form of |VDC|0.91/S0.72.

The blowout behavior of non-premixed turbulent coflow jet flames under microgravity environment was studied experimentally by utilizing a 3.6 s drop tower. Variations of flames leading to liftoff as well as blowout were examined by varying the coflow velocity and compared with those obtained under the normal gravity condition. A modeling work was conducted to incorporate the effects of the gravity (buoyancy) and coflow velocity on blowout behavior. Major findings include: (1) the flame length in microgravity was longer than that in normal gravity and decreased with increasing coflow velocity. The flame in microgravity showed more intense yellow luminosity with larger sooting zone; (2) the flame liftoff height increased with increasing coflow velocity in both gravity levels. The flame base was closer to the burner in microgravity as compared with that in normal gravity; (3) the blowout velocity in microgravity was appreciably larger than that obtained in normal gravity; and (4) a physical model based on Damköhler number was developed by using similarity solutions to characterize the differences in the blowout limits considering both the coflow and gravity (buoyancy) effects. The proposed model can successfully predict the experimental data. This work provided new data and basic scaling analysis for blowout limit of non-premixed turbulent jet flames considering both the coflow and gravity (buoyancy) effects.

Ignition delay times (IDTs) of fuels provide very important macro-information about the fuel reactivity and autoignition behavior. IDTs constitute a key metric for fuel/engine co-optimization studies. Chemical kinetic modeling pursuits rely on experimental IDTs as their primary validation target. There have been extensive works in literature on measuring, calculating, modeling and correlating IDTs of a wide range of hydrocarbons, oxygenates, mixtures of pure components and real fuels. Recently, some studies employed a simplified ignition model at high temperatures, comprising of a fast fuel decomposition step and a rate-determining small molecule oxidation step. This description suggests that high-temperature IDT is mainly controlled by the ignition of fuel fragments and is rather weakly dependent on the initial fuel composition. In this work, we study the validity of the hypothesis that IDT of multi-component fuels is weakly dependent on fuel composition under specific thermodynamic conditions. If so, high-temperature IDTs of practical fuels may be described by a universal Arrhenius type correlation. By combining experimental observations and chemical kinetic simulations, we determine the ranges of key parameters (temperature, pressure, equivalence ratio, composition) under which a universal IDT assumption is valid. We conclude that, for fairly random composition and within a P-T-ϕ constraint, IDTs of gasolines and jet fuels may be predicted with a high degree of certainty by the following modified Arrhenius expressions (P = 10–80 bar, P0 = 1 bar, ϕ = 0.5–2, fuel/air mixtures, units are ms, bar, K, mol, kcal): τgasoline=6.76*10−7( [Formula presented] )−1.01φ1.13− [Formula presented] exp( [Formula presented] ), forT> [Formula presented] τjetfuel=4.46*10−7( [Formula presented] )−1.21φ2.04− [Formula presented] *exp( [Formula presented] ), forT> [Formula presented]

In compression ignition engines, split-injection strategy has shown great benefits in reducing pollutant emissions and improving combustion efficiency. Spray–flame interaction involving in split injections is significantly complex, which affects the ignition process and even pollutant emissions. Therefore, the objective of this study is to investigate how the flame–spray interaction affects the subsequent ignition process and combustion features in split injections under diesel engine-like conditions. In this work, large eddy simulation coupled with a 54-species mechanism for split injections of n-dodecane is performed to study the effect of injection duration and dwell times (DTs) on spray–flame interactions and the ignition mechanism. The numerical model gives a reasonable agreement with the experiments in terms of the vapor penetration length, ignition delay times, mixture fraction distributions and the flame structures. The present study revealed that combustion for split injections is a multi-stage process and the ignition processes for the first and second injections are controlled by different mechanisms, namely autoignition for the first injection, and the accelerating ignition for the second injection due to the intermediate species and heating effect formed in the first injection. Moreover, the increase in dwell time between individual injections reduces the subsequently promoting ignition effect for the second injection and thus weakens the interacting process between the two injections. Consumption of the fuel in the first injection leads to a temperature increase and production of different species, which in turn accelerates the ignition of the second injection. Finally, the competition between the local flow timescale and chemical timescale is investigated based on the chemical explosive mode analysis (CEMA) methods. A balance between reaction and mixing processes dominates the combustion of the quasi-steady spray in the second injection with a short DT. However, the flame is controlled by autoignition when a longer DT is used.

Dominant physical processes that characterize the combustion of a lean methane/air mixture, diluted with exhaust gas recirculation (EGR), under turbulent MILD premixed conditions are identified using the combined approach of Computational Singular Perturbation (CSP) and Tangential Stretching Rate (TSR). TSR is a measure to combine the time scale and amplitude of all active modes and serves as a rational metric for the true dynamical characteristics of the system, especially in turbulent reacting flows in which reaction and turbulent transport processes compete. Applied to the MILD conditions where the flame structures exhibit nearly distributed combustion modes, the TSR metric was found to be an excellent diagnostic tool to depict the regions of important activities. In particular, the analysis of turbulent DNS data revealed that the system's dynamics is mostly dissipative in nature, as the chemically explosive modes are largely suppressed by the dissipative action of transport. On the other hand, the convective transport associated with turbulent eddies play a key role in bringing the explosive nature into the system. In the turbulent MILD conditions under study, the flame structure appears nearly in the distributed combustion regime, such that the conventional statistics conditioned over the progress variable becomes inappropriate, but TSR serves as an automated and systematic way to depict the topology of such complex flames. In addition, further analysis of the CSP modes revealed a strong competition between explosive and dissipative modes, the former favored by hydrogen-related reactions and the convection of CH4, and the latter by carbon-related processes. This competition results in a much smaller region of explosive dynamics in contrast to the widespread existence of explosive modes.

An improved polycyclic aromatic hydrocarbon (PAH) model is developed to predict the decomposition of indene and the formation of large PAHs under pyrolytic conditions. This model is developed based on experimental study of pyrolytic kinetics of indene in a flow reactor at low and atmospheric pressures (30 and 760 Torr) by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A general map of PAH growth is presented according to the observations in this study and those in literature. Indene dissociates via indanyl forming mono-cyclic aromatics and small intermediates, while its dominant decomposition product is indenyl. As a resonantly stabilized radical, indenyl serves as a platform molecule in PAH growth process which links small unsaturated hydrocarbons and mono-aromatic species to multi-cyclic ones. Reactions of indenyl radical are proposed to form commonly studied and recently observed PAHs. Rate constants of these reactions are evaluated by analyzing literature data of rate constant measurements, quantum chemical calculations and analogy to cyclopentadienyl radical. The main PAH formation pathways are the bi-molecular addition reactions of indenyl radical with indene and a series of intermediates, forming C10C18 and larger PAHs. Meanwhile, radical chain reactions provide huge passage for PAH growth form one resonantly stabilized radical (RSR) to larger ones. Particular contribution has been found from the reactions of RSRs that have five-member ring in their molecular structures, such as fluorenyl, benz-indenyl, cyclopenta-phenanthrenyl and benzo-fluorenyl.

Laser-Induced Grating Spectroscopy (LIGS) is applied to premixed CH4/air laminar flat flames under operating pressures of 1 to 6 bar. For the first time, temperature and water concentration have been acquired simultaneously in a reacting flow environment using LIGS. A 1064 nm pulsed laser is used as pump to generate a temporary stationary intensity grating in the probe volume. Water molecules in the flame products absorb the laser energy and generate a thermal grating if sufficiently high energies are delivered by the laser pulses, here more than 100 mJ per pulse. Such energies allow the electric field to polarize the dielectric medium, resulting in a detectable electrostrictive grating as well. This creates LIGS signals containing both the electrostrictive and the thermal contributions. The local speed of sound is derived from the oscillation frequency of LIGS signals, which can be accurately measured from the single shot power spectrum. Data show that the ratio between the electrostrictive and the thermal peak intensities is an indicator of the local water concentration. The measured values of speed of sound, temperature, and water concentration in the flames examined compare favorably with flame simulations with Chemkin, showing an estimated accuracy of 0.5 to 2.5% and a precision of 1.4–2%. These results confirm the potential for 1064 nm LIGS-based thermometry for high-precision temperature measurements of combustion processes.