Combustion and Flame最新文献

Preferential diffusion plays an important role especially in hydrogen flames. Flame stretch significantly affects the flame structure and induces preferential diffusion. A problematic phenomenon occurring in real combustion devices is flashback, which is influenced by non-adiabatic effects, such as wall heat loss. In this paper, an extended flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion, flame stretch, and non-adiabatic effects is proposed. In this method, the diffusion terms in the transport equations of scalars, viz. the progress variable, mixture fraction, and enthalpy, are formulated employing non-unity Lewis numbers that are variable in space and different for each chemical species. The applicability of the extended FGM method to hydrogen flames is investigated using two- and three-dimensional numerical simulations of hydrogen-air flame flashback in channel flows. The results of the extended FGM method are compared with those of detailed calculations and other FGM methods. The two-dimensional numerical simulations show that considering both preferential diffusion and flame stretch improves the prediction accuracy of the mixture fraction distribution and flashback speed. The three-dimensional numerical simulations show that the prediction accuracy of the flashback speed, backflow region, and distributions of physical quantities near the flame front is improved by employing the extended FGM method, compared with the FGM method that considers only the heat loss effect. In particular, the extended FGM method successfully reproduced the relationship between the reaction rate and curvature. These results demonstrate the effectiveness of the extended FGM method.

Novelty and Significance Statement The novelty of this research is the development of a flamelet-generated mani-fold (FGM) method that explicitly considers preferential diffusion, flame stretch, and non-adiabatic effects. To the best of the authors’ knowledge, no studies have performed numerical simulations of pure hydrogen flames using such an FGM method. The developed FGM method was applied to numerical simula- tions of hydrogen-air premixed flame flashback at an equivalence ratio of 0.5 and reproduced the flashback speed of lean hydrogen-air premixed flame. The applicability of the FGM method to the numerical simulation of hydrogen-air flashback is reported first. This research is significant because the FGM method is one of the most widely used combustion models for premixed combustion, and the development of an accurate FGM method will contribute to the engineering field. The accurate prediction of the flame flashback attempted in this study is particularly important for the development of hydrogen-fueled combustion devices.

This study explores the relationship between knocking and particulate matter emissions in both single-cylinder and multi-cylinder direct injection spark ignition (DISI) engines. The concentration of particulate matter emissions rapidly increases after the onset of knock with advancing spark timing. Single-cylinder engine experiments were conducted with variations in intake temperature and coolant temperature. The results showed that knock plays a role in generating particulate matter emissions. Moreover, knock intensity had a greater effect on the increase in particulate emissions compared to the variation of in-cylinder temperature. Experiments conducted on a multi-cylinder turbocharged engine validated the existence of this phenomenon in production engines. The phenomenon is shown to exist across diverse engine conditions and a range of gasoline fuel compositions (anti-knock index rating and oxygenate/aromatic/alkylate content). The knock-soot correlation was seen more prominently at lower engine speeds where the magnitudes of knock intensity were higher. The fuel study showed that while the AKI rating of the fuel directly correlated to the propensity to knock, it also indirectly correlated to the particulate emissions concentration. When running on high AKI-rated fuels, the engine emitted lower concentrations of particulate emissions irrespective of whether the fuel was heavily doped with ethanol or aromatics. At similar AKI ratings, aromatic blended gasoline operation led to higher particulate emissions than ethanol blended gasoline operation. The study concludes by suggesting empirical theories for the existence of the knock-soot correlation.

In this study, the influence of a direct-current electric field under high pressures on the burning rate and combustion efficiency of a propellant were investigated, and the results were verified by solid rocket motor (SRM) experiments. The effects of the electric field on combustion of propellant were studied at various ambient pressures (1–7 MPa) and different applied voltages (−5 to 5 kV). It was found that an applied electric field had a catalytic effect on the propellant combustion at different ambient pressures. When applying an electric field at 1 MPa, as the applied voltage increased from 0 to 5 kV, the burning rate and combustion efficiency increased from 3.390 to 3.884 mm/s, combustion efficiency increased from 95.13 % to 97.45 %, respectively. However, a high ambient pressure weakened the burning rate catalytic influence of the applied electric field. Hot firing of the SRM verified that the electric field could increase the thrust and the specific impulse of the motor by 6.1N(25.3 %) and 7.5s(3.3 %), respectively. Additionally, the forward electric field had a greater catalytic effect on the propellant combustion than the reverse electric field. The high-strength applied electric field was found to catalyze particle charge acceleration, the rupture of aluminum droplets, and ion excitation. By utilizing this phenomenon, the combustion, and energetic properties of solid rocket motors (SRMs) can be finely adjusted using an electric field. The results of this research contribute to the advancement of a more effective approach for thrust control in SRMs.

Novelty and significance

This study investigates the effects of an electric field on aluminum droplets and combustion flames in propellants. An experimental setup was designed to regulate the combustion of solid propellants using an electric field, and this setup was tested in a solid rocket motor. A specialized electric field experiment apparatus and an electric field-regulated motor were designed to validate the influence of the electric field on propellant combustion. The results show that the electric field increased the propellant's combustion rate by up to 8.8 % and improved combustion efficiency by up to 1.81 %. An electric field of 1.5 kV increased the motor's thrust by 6.1 N and the specific impulse by 7.5 s. This research elucidates the impact of electric field strength and direction on propellant combustion characteristics and analyzes the underlying mechanisms from the perspective of combustion flames and aluminum particle behavior. The study provides a new approach for energy management in solid rocket motors.

The paper presents an experimental study investigating the impact of under-oxygenated conditions on the burning characteristics and flame extinction of horizontal transparent poly(methyl-methacrylate) (PMMA) slabs subjected to external heating. Small-sized solids are exposed to various constant levels of radiant heat flux in a nitrogen-diluted environment at ambient pressure. The study aims to understand the multivariable dependence of PMMA combustion on oxygen and heating levels. The analysis goes beyond the study of global and local variables to provide an understanding of the mechanisms involved, revealing a notable effect on burning characteristics and flame extinction mechanisms by blow-off. The results provide a detailed assessment of the surface energy balance and flame bulk properties. The results demonstrate good repeatability up to a certain oxygen limit, beyond which the flame combustion regime exhibits stochastic behaviour. The study of heat transfer mechanisms has revealed that flame radiation and external heating play a predominant role in the heating of horizontal solid fuels, In contrast, within the flame, convection is the main contributor to solid heating, rather than flame radiation. Solid characteristics show linear trends proportional to oxygen depletion, while gas phase characteristics show monotonic trends strongly influenced by the changing combustion regime. The surface temperature of the solid is highly sensitive to the test conditions that affect the energy balance. Dimensionless parameters are developed and data are correlated with existing literature. The analysis demonstrates that the linear or monotonic trends are maintained regardless of external heating. Finally, models are proposed and validated to predict both the extinction limit and the burning rate independently of irradiance and oxygen levels.

This communication aims to explore the cyanide kinetics during spark-plug discharge with a focus on updating its kinetic cycle. A plasmochemical model augmenting oxygen-CO$2$ interaction was utilized for this purpose, within an axially symmetric domain to solve the fluid and kinetic equations. The simulations reveal an amplification of CN production in the discharge region, showing that the cyanide density reached a peak exceeding the safety limit for humans.

Novelty and significance statement

Numerical simulations of spark plug discharge are essential for predicting their outcomes. Crispim et al. (2021) did not consider crucial processes in dry air-CO${}_2$ and other factors affecting CN consumption and CO production, requiring a review of CN kinetics. To address this, we used a new chemical cycle with previously unconsidered processes to investigate cyanide concentration. Our simulations assess the percentage impact of each physical–chemical process on species concentration during electrical discharge, identifying the most influential processes on cyanide consumption and carbon monoxide production. We present the main processes and percentages of CN consumption and CO production in Ar-CO${}_2$, which were not previously addressed. Moreover, the new CN(A${}^2\Pi$) concentration values are significantly higher than the previous theoretical results.

The inter-scale transfer of kinetic energy is analyzed in fuel-lean turbulent premixed swirl flames with varying equivalence ratio, velocity, and swirl number. Measurements are made using tomographic particle image velocimetry (TPIV) and formaldehyde planar laser induced fluorescence (PLIF) to measure three-dimensional velocity fields and deduce planar flame properties, respectively. It first is demonstrated that velocity measurements that spatially filter the true velocity fields at the TPIV interrogation box size can be used to deduce inter-scale energy transfer at scales sufficiently above the TPIV resolution. Mean small-to-large scale transfer of kinetic energy – i.e. mean backscatter or an inverse cascade – occurs across all conditions and length-scales studied, in contrast to the classical production-driven forward cascade described by Kolmogorov’s equilibrium concepts. The magnitude of backscatter increases with increasing turbulence kinetic energy and equivalence ratio (in these lean flames), but decreased with increasing swirl number. Normalization by a dimensionless thermal expansion parameter that relates the pressure difference and kinetic energy difference over the flame (O’Brien et al., (2017)) collapses the data across equivalence ratios at a given flow condition. Further normalization by a characteristic parameter describing the swirl-induced radial pressure gradient largely collapses the data across swirl numbers and flow rates. The existence of the inverse cascade at the studied conditions and the relationship with the swirl-induced pressure gradient indicate that large-scale, geometry-driven pressure fields play an important role in turbulent kinetic energy dynamics for practical systems.

Novelty and Significance Statement

This paper presents a novel extension of previous analyzes of the inverse turbulent kinetic energy cascade in turbulent premixed flames to high Karlovitz numbers and large spatial scales. It provides the first demonstration – either experimentally or computationally – that energy backscatter reaches to scales beyond the turbulent flame brush thickness and occurs for $\mathrm{Ka}>100$. By studying multiple swirl numbers, it provides novel insight regarding the influence of geometry-scale pressure fields on kinetic energy dynamics in flames. It also demonstrates new analytical techniques for experimentally quantifying backscatter.

The manuscript is significant because it provides some of the most compelling evidence to-date that standard models for turbulence and turbulence-flame interactions – which are based on Kolmogorov’s equilibrium hypothesis – are missing key physical phenomena. It, therefore, motivates development and use of more accurate LES closure models.

2,2,4,6,6-pentamethylheptane (PMH) stands out as a highly branched alkane with promising applications in sustainable aviation fuels, making it a notable candidate for ongoing efforts to replace conventional jet fuels. This study contributes significantly to our understanding of PMH combustion by providing a comprehensive dataset of new high-temperature experimental data and an optimized detailed high-temperature kinetic mechanism. The experimental results exhibit a strong agreement with the predictions of the refined model. The shock tube experiments covered a range of conditions, including equivalence ratios of 0.5 and 1.0, pressures from 1 to 20 atm, and temperatures from 850 to 1450 K. One noteworthy observation is the significant two-stage heat release phenomenon in the high-temperature region, particularly in highly branched alkanes like PMH. Although the heat release in the first stage is weak and its impact on the actual engine may be limited, it holds scientific significance in optimizing the chemical reaction kinetics model. The detailed kinetic analysis, incorporating sensitivity analysis and reaction path analysis based on the optimized model, offers a profound insight into the combustion characteristics of PMH. This analytical approach deepens our understanding of the observed first-stage heat release phenomenon from a kinetic perspective, shedding light on the intricate combustion behavior of PMH under high-temperature conditions. The findings contribute not only to the fundamental knowledge of PMH combustion but also hold practical relevance for the development and optimization of alternative fuels.