Main Branching Reaction Research Articles

Numerical study and cellular instability analysis of E30-air mixtures at elevated temperatures and pressures

Ethanol is used as a gasoline blending component in numerous countries throughout the world. In the present work, the Chemkin Pro package was used to study the chemical kinetic behavior of E30 (a blended fuel containing 30% ethanol and 70% gasoline on volume basis). The effects of chemical kinetics and adiabatic flame temperature on the laminar flame propagation speed at different initial temperatures were studied and the relative importance of the thermal and chemical effects of ethanol at different initial temperatures was investigated. With increasing initial temperature, the rate of main branching reactions (R1:O2 + H ⇔ O + OH and R29: CO + OH ⇔ CO2 + H) increases much more than that of the main termination reactions (R13:H20 + M ⇔ H + OH + M, R15:O2 + H(+M) ⇔ HO2(+M)). The laminar flame propagation speed shows a linear relationship with the sum of the peak concentrations of H and OH. The chemical effect of ethanol was far greater than its thermal effect which was characterized by adiabatic flame temperature. In addition, the cellular instabilities of the E30-Air flame were studied. As the initial pressure and equivalence ratio change, the dominant instability shifts between hydrodynamic and diffusional-thermal. The theoretical critical Peclet number (Pecr) decreases with increased equivalence ratio and is insensitive to the initial temperature and pressure; these trends are consistent with the experimental findings. Furthermore, β(Leeff-1)Pe(σ-1)Q2 is the dominant parameter affecting Pecr with varying equivalence ratio.

Hydrogen, Methane, Ethylene and Propylene Blending on the Ignition Delay Time of n-Heptane/Toluene Mixtures under Homogeneous and Nonpremixed Counterflowing Conditions

ABSTRACT Effects of hydrogen (H2), methane (CH4), ethylene (C2H4) and propylene (C3H6) blending on the ignition delay time of n-heptane/toluene mixtures were numerically studied under both homogeneous and nonpremixed counterflowing conditions. In both configurations, results reveal that the addition of H2 and C2H4 is beneficial to promote ignition, while the addition of CH4 and C3H6 inhibits ignition at atmospheric pressure. The blending effects of H2, CH4 and C2H4 at elevated pressures are similar to that at atmospheric pressure. However, the inhibiting effect of C3H6 becomes weaker under higher pressures. At the same mole blending ratio, in a homogeneous system, C2H4 blending has stronger promotion effect than H2 blending, but in the nonpremixed counterflowing configuration, the results are opposite. The effect of light gas blending is barely caused by the changes of the thermophysical properties; instead, it is dominated by the changes of chemical kinetics, including the changes of the rates of the main endothermal branching reactions, main heat production reactions, and the radical formation reactions. Furthermore, for the homogeneous mixtures, H2 addition directly enhances the main branching reaction H + O2 <=> O + OH, while C2H4 addition directly enhances the main exothermic reactions C2H4 + OH <=> C2H3 + H2O. CH4 addition consumes OH and produces CH3 to inhibit ignition. C3H6 addition consumes H and OH to inhibit ignition. The ignition of a stretched nonpremixed flame begins on the high-temperature oxidizer side. Due to the stronger mass diffusivity of H2, the blended H2 permeates to the oxidizer side with a higher temperature than the blended C2H4 does. A small amount of H2 blending results in much higher H, OH, CH3 radicals, higher heat product rate, and thereby shorter ignition delay time than the C2H4 blending. The effect is enhanced with increasing strain rate. In addition, the results reveal that the ignition delay time is only sensitive to the molecular transport of the externally-added light gas components, rather than that of the gas molecules generated by the fuel decomposition.

Transcriptional Regulation of Energy Metabolism in Cancer Cells.

Cancer development, growth, and metastasis are highly regulated by several transcription regulators (TRs), namely transcription factors, oncogenes, tumor-suppressor genes, and protein kinases. Although TR roles in these events have been well characterized, their functions in regulating other important cancer cell processes, such as metabolism, have not been systematically examined. In this review, we describe, analyze, and strive to reconstruct the regulatory networks of several TRs acting in the energy metabolism pathways, glycolysis (and its main branching reactions), and oxidative phosphorylation of nonmetastatic and metastatic cancer cells. Moreover, we propose which possible gene targets might allow these TRs to facilitate the modulation of each energy metabolism pathway, depending on the tumor microenvironment.

Extinction studies of near-limit lean premixed syngas/air flames

Extinction studies of weakly-stretched near-limit lean premixed syngas/air flames were conducted in a twin-flame counterflow configuration. Experiments showed that buoyancy-induced natural convection at normal gravity strongly disturbed these flames. In order to validate the simulation, accurate extinction data was obtained at micro-gravity. Experimental data obtained from the 3.6 s micro-gravity drop tower showed that the extinction equivalence ratio increased with the increasing global stretch rate and decreased with the increasing H2 mole fraction in the fuel. Numerical simulation was conducted with CHEMKIN software using GRI 3.0 and USC-Mech II mechanisms. The predicted extinction limit trend was in agreement with the micro-gravity experimental data. Sensitivity analyses showed that the competition between the main branching reaction H + O2 ⇔ O + OH and the main termination reaction H + O2 + M ⇔ HO2 + M in the H2/O2 chemistry determined the extinction limits of the flames. The dominant species for syngas/air flame extinction was the H radical. The key exothermal reaction changed from OH + CO ⇔ H + CO2 to OH + H2 ⇔ H + H2O with the increasing H2 mole fraction in the fuel. Also, the mass diffusion played a more important role than chemical kinetics in the flame extinction. When the H2 mass diffusion was suppressed, the reaction zone was pushed to the stagnation plane and the flame became weaker; while H mass diffusion is suppressed, the reaction zone slightly shifted towards the upstream and the flame was slightly strengthened.

Effects of H2S addition on hydrogen ignition behind reflected shock waves: Experiments and modeling

Hydrogen sulfide is a common impurity that can greatly change the combustion properties of fuels, even when present in small concentrations. However, the combustion chemistry of H2S is still poorly understood, and this lack of understanding subsequently leads to difficulties in the design of emission-control and energy-production processes. During this study, ignition delay times were measured behind reflected shock waves for mixtures of 1% H2/1% O2 diluted in Ar and doped with various concentration of H2S (100, 400, and 1600ppm) over large pressure (around 1.6, 13, and 33atm) and temperature (1045–1860K) ranges. Results typically showed a significant increase in the ignition delay time due to the addition of H2S, in some cases by a factor of 4 or more over the baseline mixtures with no H2S. The magnitude of the increase is highly dependent on the temperature and pressure. A detailed chemical kinetics model was developed using recent, up-to-date detailed-kinetics mechanisms from the literature and by changing a few reaction rates within their reported error factor. This updated model predicts well the experimental data obtained during this study and from the shock-tube literature. However, flow reactor data from the literature were poorly predicted when H2S was a reactant. This study highlights the need for a better estimation of several reaction rates to better predict H2S oxidation chemistry and its effect on fuel combustion. Using the kinetics model for sensitivity analyses, it was determined that the decrease in reactivity in the presence of H2S is because H2S initially reacts before the H2 fuel does, mainly through the reaction H2S+H⇄SH+H2, thus taking H atoms away from the main branching reaction H+O2⇄OH+O and inhibiting the ignition process.

Flame propagation of mixtures of air with binary liquid fuel mixtures

The laminar flame speeds of mixtures of air with 80% n-dodecane+20% methylcyclohexane and 80% n-dodecane+20% toluene, on a per volume basis, were determined in the counterflow configuration over a wide range of equivalence ratio, at atmospheric pressure and 403K unburned mixture temperature. The choice of the fuel blends was dictated by their anticipated compositions in jet fuels surrogates. Phenomenological analysis shows that the laminar flame speeds of binary fuels mixtures can be estimated using the laminar flame speeds and adiabatic flame temperatures of the neat components. The propagation rates of various binary fuels blends were computed using detailed descriptions of chemical kinetics and molecular transport and were found to be in good agreement with the estimations. Although the fuel initial consumption pathways and the resulting intermediates and radicals may be different for each neat component, the propagation of flames of binary fuels is mostly sensitive to the flame temperature through its influence on the main branching reaction H+O2→OH+O. Thus, kinetic couplings resulting from the presence of two different fuels, appear to have minor effect on flame propagation.

Assessment of kinetic modeling for lean H2/CH4/O2/diluent flames at high pressures

Experimental measurements of burning rates, analysis of key reactions and kinetic pathways, and modeling studies were performed for H2/CH4/O2/diluent flames spanning a wide range of fuel-lean conditions: equivalence ratios from 0.30 to 1.0, flame temperatures from 1400 to 1800K, pressures from 1 to 25atm, CH4 fuel fractions from 0 to 0.1. The experimental data show negative pressure dependence of burning rate at high-pressure, low-flame-temperature conditions for all equivalence ratios and with CH4 addition. Substantial differences are observed between literature model predictions and the experimental data as well as among model predictions themselves – up to a factor of four at high pressures. Similar to our previous work that demonstrated that none of the recent kinetic models reproduced the measured pressure dependence of the mass burning rate for all diluent concentrations and medium to high equivalence ratios, here it is demonstrated that none reproduce the measured pressure dependence for very low equivalence ratios. The effect of pressure on the kinetics of lean flames is largely driven by competition of both H+O2(+M)=HO2(+M) and HO2+O/OH/HO2 with the main branching reactions, in contrast to rich mixtures that are largely driven by competition of both H+O2(+M)=HO2(+M) and HO2+H with the main branching reactions. Methane addition is shown to influence the pressure dependence mainly through reactions of CH3 with H and HO2. Given the nature of the modeling problem for high-pressure flames, it appears that a rigorous solution to improving predictive capabilities will require both empirical adjustments of multiple rate constant parameters as well as improved characterization of the functional temperature and pressure dependence of certain highly sensitive reactions. Furthermore, many of the reactions responsible for uncertainties in the pressure dependence of H2/O2 flames at high pressures are shown to contribute significantly to uncertainties in the pressure dependence of flames of hydrocarbon fuels.

Transient response of premixed methane flames

The response of premixed methane–air flames to transient strain and local variations in equivalence ratio is studied during isolated interactions between a line–vortex pair and a V-flame. The temporal evolution of OH and CH is measured with planar laser-induced fluorescence for N 2-diluted flames with equivalence ratios ranging from 0.8 to 1.2. One-dimensional laminar flame calculations are used to simulate the flame response to unsteady strain and variations in reactant composition. When the reactant composition of the vortex pair and the V-flame are identical, the measurements and predictions show that the peak mole fractions of OH and CH decay monotonically in lean, stoichiometric, and rich flames. We also investigate the effects of a vortex pair with a leaner composition than the V-flame. In a stoichiometric flame, the leaner vortex enhances the decay of both OH and CH. In a rich flame, we observe an abrupt increase in OH-LIF signal and a disappearance of CH-LIF signal that are consistent with a previous experimental investigation. Our results indicate that the previously observed OH burst and CH breakage were caused by a difference in the equivalence ratios of the vortex pair and the main reactant flow. A numerical study shows that N 2 dilution enhances the response of premixed flames to unsteady strain and variations in stoichiometry. Reaction-path and sensitivity analyses indicate that the peak OH and CH mole fractions exhibit significant sensitivity to the main branching reaction, H + O 2 ↔ OH + O. The sensitivity of OH and CH to this and other reactions is enhanced by N 2 dilution. As a result, N 2-diluted flames provide a good test case for studying the reliability of chemical kinetic and transport models.

The polyaddition, chain, and polycondensation machanisms of formation of networks based on bismaleimides

The kinetics and mechanisms of polymerization of equifunctional 4,4'-(N,N'-bismaleimide)-diphenylmethane/2,2'-diallyl-bisphenol A (BMDM/DABPA) and model (phenylmaleimide/2-allylphenol) (PMI/AP) systems have been studied in the temperature range 140 – 400 °C using IR-, 1H and 13C NMR spectroscopy, GC/MS, DSC, and isothermal calorimetry. It was established that the cure mechanism consisted of a unique combination of step-wise and chain polymerization of maleimide and propenyl groups generated by the first reaction. The last reaction was the main branching reaction. The second source of branching was the dehydration reaction of phenol groups (polycondensation reaction) that proceeds with the participation of the 1:1 “ene” adduct as one of the component. Homopolymerization of maleimide groups proceeded autocatalytically, initiated by free radicals generated by thermal decomposition of the maleimide-propenyl group's donor–acceptor pairs. The steric hindrance in 2,2'-diallyl-bisphenol A prevented the reversible Diels–Alder reaction, but the reaction proceeded in model systems. Some thermodynamic and kinetics parameters of the reactions have been determined.

Unsteady response of C 3H 3/Air laminar premixed flames submitted to mixture composition oscillations

Detailed numerical simulations were conducted on the effects of unsteadiness of the mixture composition on the structure and dynamics of one-dimensional, strained laminar premixed propane/air flames established in a stagnation flow configuration. The study is relevant to the flamelet behavior in direct-injection gasoline engines, where large- and small-scale charge stratifications develop and are encountered by the flame. Sinusoidal perturbations of the fuel concentration were imposed as boundary condition. Results show that the flame response is quasi-steady at low frequencies, whereas above a cutoff frequency f cutoff this response is substantially attenuated by diffusion, and a phase lag between cause and effect is established, in accord with previous studies. This phase lag corresponds to a finite time that is required for the fuel oscillation to be transported through the successive diffusive and reactive layers. The extent of both diffusional attenuation and phase lag of any property was found to depend on the thickness of the layer considered. For example, properties determined close to the burned gases, like the maximum temperature T max , exhibit greater attenuation and phase lag compared to those determined close to the unburned gases, like the integrated fuel consumption rate W f . It was also found that instantaneous flame properties, such as W f and T max , can be derived from the time history of the flame and through the use of steady flamelets libraries. Unsteadiness was also shown to extend the extinction limits as a result of the amplitude attenuation caused by the diffusion. Extinction was found to be controlled by the response of the main branching reaction. A criterion was developed capable of predicting extinction, based on instantaneous parameters defined in the layer of relevance to this reaction, and by using steady extinction libraries.