Flame Development Characteristics and Control Mechanism of Equivalent Ratio Stratified Compression Combustion

There has been much recent attempt at the operation of engines on gaseous fuels, which has been motivated largely by the need to produce engines that can comply with the ever more demanding regulations for cleaner exhaust emissions. In this context, natural gas in a potential alternative to gasoline due to lower overall pollutant emissions and more excellent fuel economy. On the other hands, the lean burn approach has such a number of attractive features as high thermal efficiency, low likelihood for knock, reduced emissions especially of NOx. However there are number of difficulties to be tackled associated with lean burn operation, such as slower flame propagation, less complete combustion, increased cyclic variations and locally flame quenching. To overcome these problems, in past studies, hydrogen addition to mixtures has been proposed as one approach and combustion characteristics of hybrid fuels have been investigated such as for laminar flames. In our previous studies, hydrogen addition was found to be effective in improving lean burning not only for laminar flames but also for turbulent flames on fuel-lean sides. Although qualitative explanation was made to some extent, combustion mechanisms and flame structure have not been clarified yet. In our 1st report, the effects of hydrogen addition on stretched laminar methane flames were studied in terms of the effect on laminar burning velocity, and it was found that the effects of hydrogen addition depend on equivalence ratio. In this study, following 1st report, the effect of hydrogen addition on stretched laminar methane flames was investigated in terms of the flame structures, relevant chemical species and element reactions, for the explanation of the dependence of the effects of hydrogen addition on equivalence ratio.

ABSTRACTA comprehensive and hierarchical optimization of a joint hydrogen and syngas combustion mechanism has been carried out. The Kéromnès et al. (Combust Flame, 2013, 160, 995–1011) mechanism for syngas combustion was updated with our recently optimized hydrogen combustion mechanism (Varga et al., Proc Combust Inst, 2015, 35, 589–596) and optimized using a comprehensive set of direct and indirect experimental data relevant to hydrogen and syngas combustion. The collection of experimental data consisted of ignition measurements in shock tubes and rapid compression machines, burning velocity measurements, and species profiles measured using shock tubes, flow reactors, and jet‐stirred reactors. The experimental conditions covered wide ranges of temperatures (800–2500 K), pressures (0.5–50 bar), equivalence ratios (ϕ = 0.3–5.0), and C/H ratios (0–3). In total, 48 Arrhenius parameters and 5 third‐body collision efficiency parameters of 18 elementary reactions were optimized using these experimental data. A large number of directly measured rate coefficient values belonging to 15 of the reaction steps were also utilized. The optimization has resulted in a H2/CO combustion mechanism, which is applicable to a wide range of conditions. Moreover, new recommended rate parameters with their covariance matrix and temperature‐dependent uncertainty ranges of the optimized rate coefficients are provided. The optimized mechanism was compared to 19 recent hydrogen and syngas combustion mechanisms and is shown to provide the best reproduction of the experimental data.

Most nanothermite compositions utilise Al as a fuel, due to its low cost, high reactivity and availability. Nevertheless, aluminothermites exhibit high ignition temperature and low active metal content. In this paper, the combustion behaviour of Ti/CuO and Ti/CuO/NC systems is discussed. The compositions were prepared with a wet-mixing/sonication process followed by an electrospray technique and were examined in terms of their mechanical and radiation sensitivity, energetic parameters and morphology. The results exhibited a strong correlation between equivalence ratio and energetic parameters. The performed tests showed the crucial impact the addiction of the chosen energetic binder on the morphology and performance of the compositions. The results of our experiments indicate the occurrence of a different combustion mechanism than the one observed for Al-based nanothermites. In our case, the combustion mechanism involves a limitation by the diffusion of the oxidising agent and its decomposition products into the reactive fuel core.

Ammonia is an alternative fuel that has potential in much industrial equipment such as internal combustion engines, gas turbines, and boilers. In the present study, the influence of flue gas on the injection process of ammonia fuel was analyzed for the first time, and the influences of ammonia/methane fuel ratio and equivalence ratio on the diffusion process were considered. A Schlieren system was used to characterize the characteristics of ammonia jet, including tip penetration and jet angle. Meanwhile, the NO emissions during this progress were calculated based on the GRI 3.0 combustion mechanism. The results showed that the penetration distance was affected by both the diffusion effect and the chemical effect. The time evolution of penetration has three stages. The second stage of penetration in the flue gas environment develops faster with the decrease in the equivalence ratio. The increase in jet angle is accelerated by the flue gas environment and a decrease in equivalence ratio. Furthermore, the theoretical results showed that the emission process of NO is mainly affected by the fuel ratio and equivalence ratio. When the equivalence ratio increased from 0.8 to 1.2, NO emissions decreased by 85.7%. The distribution of NO along the axis is also affected by the fuel ratio. When the volume ratio of NH3 to CH4 decreases from 1 to 0.5, the peak value of NO concentration decreases by 29.4%.

The mechanism of mathane-air combustion on the surface of a porous ceramic plate was studied by experimental testing and analysis of a simplified theoretical model based on one-dimensional flow of methane-air mixture and the overall chemical reaction rate.The effects of such parameters as thickness of porous ceramic plates, equivalence ratio of mixed-gas and heat load on the combustion characteristics were examined.A thicker plate achieves higher surface temperature as premixed gas is preheated on the porous ceramic plate. The combustion zone is closest to the porous ceramic plate with equivalence ratio Φ = 1.2. The surface temperature has peak value at a certain heat load. It is observed that combustion begins just off the porous ceramic plate, and the flame is kept less than 1 mm from the surface. The position is influenced by the combustion conditions. These phenomena can be explained by a theoretical model and such aspects of the combustion mechanism as temperature profile of premixed gas and porous plate and chemical reaction on the plate are made clear.

A compact chemical kinetic mechanism for autoignition and combustion of methylcyclohexane (MCH) was developed and validated for a wide range of conditions, especially for low temperatures and high pressures that are most relevant to real engines. The mechanism was constructed in steps. An improved C5-C7 submechanism (26 species and 90 reactions) was first developed to describe fuel-cracking to form smaller fragments under high temperatures. Five modules of the C5-C7 submechanism were considered separately, and the rate constants were carefully estimated. A semiglobal low-temperature submechanism was developed to improve prediction of the negative temperature coefficient behaviors, which contained 4 species and 11 reactions. Isomers of intermediate radicals and fast reactions were lumped to obtain the minimal low-temperature submechanism. Combined with a simplified C0-C4 kernel (40 species and 276 reactions), the final mechanism consists of 70 species and 377 reactions. Validations of the newly developed mechanism were performed using amounts of experimental data, including ignition delays in shock tubes and rapid compression machines, under a wide range of temperatures (650–2000 K), pressures (1–50 atm), and equivalence ratios (0.5–2.0), and OH concentration histories in high pressure shock tubes. Furthermore, experimental data of species concentrations and flame speeds in laminar premixed flames were also used for validation. The present mechanism showed good accuracy in predicting ignition and combustion properties over a range of parameters. Simulations using other detailed MCH mechanisms were also carried out for comparison.

Numerical simulation of ammonia/methane/air combustion using reduced chemical kinetics models

The addition of ammonia and hydrogen into natural gas fuel is an effective method to reduce carbon emissions. This study aims to investigate the effect of adding ammonia and hydrogen on the mechanism of natural gas combustion and emission characteristics. Based on a self-developed mixed gas deflagrate experimental platform, the deflagrate characteristics, emission characteristics, and chemical reaction kinetics mechanism of mixed gas fuels under different composition ratios (natural gas 0-100%, hydrogen 10-85%, and ammonia 0-100%) were studied. The results indicate that the propagation of the deflagration shock wave can be categorized into an initial stage (L < 3 m) and a development stage (L > 3 m) based on the observed trend of shock wave intensity variation with distance. The intensity of the deflagration shock wave for the mixed gases increases monotonically as the hydrogen content ratio rises. In contrast, the impact of the ammonia content ratio on the shock wave intensity exhibits a distinct pattern that varies with changes in the equivalence ratio and hydrogen content ratio. In terms of carbon emissions per unit of heat value produced by the fuel, adding hydrogen to natural gas proves to be more effective at reducing carbon emissions than adding ammonia. When the ammonia content ratio is 50% and the hydrogen content ratio is 40%, the combustion performance of the mixed gas fuel is similar to that of natural gas, but its carbon emissions are lower than 30% of natural gas, making it a new type of mixed fuel with potential application value; the interaction between reflected pressure waves and flames is the main reason for the fluctuation of deflagrate shock wave pressure; ammonia lowers the temperature of the reaction system by reducing the concentration of OH radicals.

Direct numerical simulations (DNSs) on autoignition and flame propagation of inhomogeneous methane–air mixtures in a closed vessel are conducted with considering detailed kinetic mechanism and temperature dependence of transport and thermal properties. The mixtures with spatial inhomogeneity of temperature or equivalence ratio are investigated. Periodic condition for non-heatloss cases or isothermal wall condition for heatloss cases is imposed on the boundaries. From the DNS results without heatloss, effects of spatial inhomogeneity of temperature and equivalence ratio on mean heat release rate are clarified. Increase of spatial variations of temperature or equivalence ratio suppresses drastic rise of mean heat release rate and reduces its maximum value. Autoignition process is affected by temperature more strongly than equivalence ratio. In the cases with heatloss, ignition delay increases and the maximum mean heat release rate decreases. After autoignition process, propagating flame is formed along walls. Heat transfer characteristics in a closed vessel are also discussed with combustion mechanisms.

Reaction mechanisms of combustion in low pressure acetylene-oxygen flames

SummaryThis paper reports on the theoretical and experimental studies of combustion in a bidirectional swirling flow to develop its criterion base. Based on considering known aero‐thermochemical similarity criteria, turbulent combustion numbers were calculated for the bidirectional vortex combustor for the first time. Theoretical assessment and experimental studies of the probability of the known combustion mechanisms' implementation in a limited bidirectional swirling flow, containing the secondary flows and separation phenomena, were performed. This gave us a possibility to provide the new correlation of combustion mechanisms with emission dependencies. A joint review of these data allowed us to establish that the greatest environmental efficiency is observed in the range of the air‐fuel equivalence ratio of 1.5 &lt; λ &lt; 1.9. At the same time, it was defined that stable combustion of liquid fuel in the bidirectional swirling flow is available up to the value λ = 22. Moreover, criterion equations for lean and rich limits of stable combustion were obtained.

A comprehensive experimental and numerical study on turbulent jet ignition mechanisms of lean hydrogen mixture using a super-rich pre-chamber combustion

Combustion of ammonium perchlorate (20–90%) mixtures with ferrocene is studied. It is demonstrated that, depending on the ratio of the components in the examined compositions, in addition to the usual gas-phase combustion model, another possible combustion mechanism exists. A somewhat unusual condensed-phase (c-phase) model may be realized, in which the heat-generating reaction occurs in a foam/aerosol layer at the temperature of evaporation of the less volatile component, which the surface temperature is defined by evaporation of the more volatile component. The efficiency of ferrocene depends on the propellant combustion mechanism: in systems that obey the gas-phase combustion mechanism, the influence of ferrocene addition is higher than the influence of addition of a hydrocarbon fuel; in systems with the c-phase mechanism of combustion, ferrocene addition produces a significant effect. Depending on the ratio of the components, ferrocene first acts in these compositions simply as a highly reactive fuel; it is only in fuel-enriched compositions with a high equivalence ratio that the burning rate increases owing to catalysis of the combustion process by the ferric oxide on the soot skeleton.

Investigation of NCN and prompt-NO formation in low-pressure C1–C4 alkane flames

This work primarily focuses on a three-dimensional model of flame propagation and stable combustion in a scramjet chamber. The one-equation LES turbulence model is adopted to close the sub-grid-scale turbulent viscosity terms. The finite-rate combustion model, along with the Jachimowski detailed hydrogen reaction mechanism with eight components and nineteen steps, is used to analyze the flame propagation characteristics of hydrogen combustion in the scramjet combustion chamber. Initially, based on the combustion chamber model, the effect of different injection locations and equivalence ratios on flame kernel formation and the flame propagation process is analyzed. The relationship between different fuel injection conditions and the oxygen consumption rate of the combustion chamber, as well as the total pressure recovery coefficient changes, is investigated. The research focuses on changes in equivalence ratios and injection hole distributions, with injection holes arranged upstream, downstream, and inside of the cavity. The result indicated that when the injection holes were arranged downstream of the cavity, there was a phenomenon of flame backflow into the cavity, which was related to the size of the injection pressure. For this work, the pressure causing flame backflow was approximately 2 MPa. When the injection hole was arranged inside the cavity, the relative distance difference between the injection hole and the upper wall of the cavity led to the formation of two reaction zones in the combustion chamber. When the injection hole was arranged upstream of the cavity, different injection equivalence ratios affected the final stable position and structure of the flame. Therefore, the injection position, injection pressure, and injection equivalence ratio all had a certain impact on the flame kernel formation and flame propagation process.