Combustion and Flame最新文献

Atmospheric oxidation experiments of iso-propylamine (IPA) were conducted in a jet-stirred reactor over the temperature range from 550 to 870 K at fuel-equivalence ratios of 0.5 and 2.0. A combination of synchrotron vacuum ultraviolet (SVUV) photoionization and time-of-flight mass spectrometry (TOFMS) was utilized to identify and quantify oxidation products and intermediates. Compared to the previous n-propylamine (NPA) low-temperature oxidation [Proc. Combust. Inst., 39 (2023) 295–303.], some intermediates and products were newly observed among the 34 detected species, including nitrosyl hydride, methylamine, acetonitrile, nitrous oxide, ethyl isocyanide, propanenitrile, 2-propanimine, n-methylformamide and 2-methylallylamine. A kinetic model consisting of 815 species and 4402 reactions was developed based on the iso-propanol model [Prog. Energy Combust. Sci. 67 (2018) 31–68.] and an ethylamine model [Prog. Energy Combust. Sci. 44 (2014) 40–102.]. The onset temperature of IPA and O₂ consumption is 750 K under both lean and rich conditions. Rate-of-production (ROP) and sensitivity analyses were performed at 800 K to illustrate the reacting paths from parent fuel to major intermediates and products, and identify the most sensitive reactions. H₂O₂(+M) = 2OH(+M) is the most sensitive reaction with a promoting effect on IPA consumption while 2HO₂ = H₂O₂+O₂ is the most inhibiting one. Important N-containing pollutants like NH₃, NOx, HCN, CH₃NH₂ and so on were analyzed with respect to their reaction routes. (CH₃)₂CNH is abundant due to the H-abstraction reactions between tC₃H₆NH₂ radical with O₂. This work was made for gaining a more comprehensive insight into the oxidation of IPA and make a foundation for further exploring amine chemistry.

Developing a novel high-efficiency coal combustion technology with ultra-low NOx emission is urgently needed to sustain the good ecological environment. Here, the targeted regulation of local microenvironment around fuel-N, such as functional groups, radicals, molecular configurations, and reaction atmosphere, is realized by the selective oxidation. The molecular configurations, including pore networks and microcrystalline structure in coal, are well characterized through synchrotron radiation-induced SAXS (small angle X-ray scattering) and WAXS (wide angle X-ray scattering) simultaneously. Furthermore, by combining density functional theory (DFT) and experiments, the effects of the local microenvironment on the nitrogen transformation and NO evolution during the thermal conversion are focused on. The results indicate that for the PPA oxidation, the H radicals attack the adjacent carbon to pyrrole/pyridine nitrogen, promoting the conversion of fuel-N to HCN. On the other hand, for the H₂O₂ oxidation, disrupting the π bond electron cloud by the CO and C = O on the ortho carbon of pyrrole/pyridine dominates the NH₃ generation. Additionally, the increased L_a (average graphene layer extent), a₃ (average interlayer spacing), σ₃ (standard deviation of interlayer spacing) and σ₁ (standard deviation of the first-neighbor distribution) induce massive smaller pores, promoting the generation of abundant reaction defects inside the particles. Importantly, the intensified adsorption on abundant active sites lead to the decreased HCN and increased NH₃ evolution, which is adverse for the interaction between homogeneous and heterogeneous NO reduction. Interestingly, the PAA selective oxidation can reduce NO emission by 31.72 % - 34.30 % during the air combustion, which is far better than the H₂O₂ oxidation. Overall, the attack of free radicals on nitrogen-containing heterocycles promotes the conversion of fuel-N to HCN, the adsorption of which on char surfaces can further enhance the heterogeneous reduction in a lean oxygen atmosphere. The work here provides a novel route for developing high-efficiency and low-NOx combustion technologies.

Boron, renowned for its high-energy potential but challenged by combustion difficulties, emerges as an ideal fuel for solid-fuel scramjet engines. This study improved the ignition and combustion model for boron particles in wet air by refining the Penn State University extension models based on the dynamic experimental phenomena. Under atmospheric pressure, the transition in combustion mode for boron particles occurs within the diameter range of 3.5–4.9 µm, with increased ambient temperature or H₂O concentration promoting the shift towards diffusion-controlled mode. Larger particles exhibit a sequential combustion mode, transitioning from kinetics-controlled to diffusion-controlled, and back to kinetics-controlled, while smaller particles consistently remain kinetics-controlled. The ignition delay proportion increases with the particle diameter but generally stays below 10 %. Increasing the temperature significantly shortens the ignition time, while increasing the pressure significantly shortens the combustion time. Taking the combustion of 1 µm boron particles at atmospheric pressure as an example, as the temperature increases from 1700 K to 3500 K, the ignition time decreases to 0.08 %, and as the pressure increases from 0.5 atm to 15 atm, the combustion time decreases to 1.6 %. Increasing the O₂ concentration significantly shortens the combustion time, with a lesser effect on the ignition time. The addition of H₂O can reduce both ignition and combustion times, especially for boron particles with an approximate diameter of 5 µm in low temperature environments. However, once XH₂O exceeds 15 %, the combustion time stabilizes in both combustion modes. Lower ambient temperatures and smaller particles enhance the impact of solidification on the combustion of boron particles.

With the increasing interest in ammonia combustion technologies, using cracked ammonia gas is becoming an essential strategy. This study examined the stabilization characteristics of non-premixed lifted flames using mixtures of methane and cracked ammonia gas. A surrogate fuel of cracked ammonia consisting of 75 % H₂ and 25 % N₂ was used. Significant variations in the lift-off characteristics were observed, including the transition from laminar to turbulent regime. Flame structures at the base were investigated by capturing simultaneous Schlieren and OH-PLIF images, and the flame stabilization mechanisms were investigated based on the interaction between the fuel jet flow and the flame structures. Stable flames were formed below the mixing core in the laminar flame regime. Their behavior can be explained in terms of flame quenching. Flame stabilization below the turbulent core regime was also investigated in detail. In addition, the experimental results with cracked ammonia gas were significantly different than the previous relationship between lift-off height and fuel jet velocity in the fully turbulent regime. Thus, an improved relationship is proposed to estimate reasonable lift-off heights for all mixtures of methane, propane, hydrogen, and cracked ammonia gas. Finally, the blowout limits were explained by the decrease in flame propagation velocity due to the reduced turbulent intensity along the jet stream.

Ammonia as an eco-friendly and carbon-free energy has been greatly concerned at home and abroad. It has been utilized in the ammonia and coal co-firing process, demonstrating robust market viability. Nonetheless, due to the element N within ammonia causing the production of NO_x, the catalytic effect and reaction mechanics of AAEMs in coal ash on the NH₃ oxidation remain elusive. In this study, the catalytic effects of AAEMs (K, Ca, Na and Mg) on NH₃ oxidation were investigated through experiments, and the reaction mechanism was elucidated from a microscopic perspective by density functional theory (DFT). The experimental results show that the addition of AAEMs does enhance the oxidation potential of NH₃ during coal and ammonia co-firing process, and promote the transformation of NH₃ to NO, N₂O and N₂. However, the promotion effect of different AAEMs is different, consequently influencing the selectivity of NH₃ oxidation pathway. At 600–800 °C, alkaline earth metals (Ca and Mg) facilitate the oxidation of NH₃ while alkali metals (K and Na) have the opposite effect. Mg has the highest promoting effect on the conversion of NH₃ to NO at 600–800 °C, but the catalytic effect of K and Na is more dominant from 800 °C to 900 °C. The transformation from NH₃ to N₂O remains the same, but the catalytic impact of Ca is mostly pronounced. AAEMs can reduce thermal decomposition activation energy of N₂O, and the decomposition of N₂O can be promoted at 800 °C. The DFT results indicate that AAEMs can enhance the adsorption capacity of NH₃ on the active site of the coal ash surface, and reduce the activation energy of NH₃ oxidation, thereby accelerating the transformation of NH₃ to other nitrogen-containing species. This study provides new insight into the evidence for the enhanced catalytic effect of AAEMs on NH₃ oxidation during ammonia and coal co-firing process.

To better understand the mechanism of carbon deposition, the ex-situ catalytic pyrolysis of polyethylene (PE) was explored over a FeNi/Al₂O₃ catalyst, and the evolution of volatiles as well as the yield and structure of carbon products at different temperatures were analyzed in depth by an in-situ atmospheric pressure photoionization high-resolution mass spectrometry (APPI HRMS) combined with a fixed-bed reactor. The results show that raising the pyrolysis temperature (from 500 to 800 °C) reduces carbon product yield and H₂ production, and the proportion of carbon nanotubes (CNTs) in the carbon products decreases. The source of carbon products gradually shifts from liquid-phase carbon sources to gaseous-phase carbon sources. Below 700 °C, both the outer diameter and carbon interlayer spacing of CNTs decrease, while above 700 °C, they both increase along with increased distortion of carbon layers. It is also observed that the carbon layers are connected to the catalyst lattice stripes, and the interlayer spacing of carbon layers is greater than the catalyst lattice spacing. In situ mass spectrometry reveals a significant increase in the carbon number and double bond equivalent (DBE) values of volatiles with increasing pyrolysis temperature, but the rate of increase slows down after 700 °C. Volatiles from lower pyrolysis temperatures undergo severe polymerization, upon entering the catalytic zone at higher temperatures, minimizing temperature-induced distinctions. Nevertheless, it remains evident that volatiles from lower pyrolysis temperatures exhibit less pronounced polymerization when exposed to higher catalytic temperatures.

Ammonia (NH₃) is a promising alternative clean fuel due to its carbon-free and high hydrogen content, along with the well-established infrastructure for storage and distribution. To overcome the issue of the low reactivity of NH₃, a feasible strategy is co-burning ammonia with highly reactive fuels. Diethyl ether (DEE) is considered a promising alternative and biomass-oxygenated clean diesel substitute. Therefore, the NH₃/DEE blend is also a promising carbon neutral alternative fuel. In this work, we measured the ignition delay times (IDTs) of NH₃/DEE mixtures with DEE fractions (X_DEE) of 0.05, 0.10, 0.30, and 1.00 at equivalence ratios of 0.5, 1.0, and 2.0, pressures of 1.75 and 10 bar, and temperature ranges from 1102 K to 1673 K in a shock tube. The DEE-NH₃ model was proposed in this work, which included the DEE sub-model, NH₃ sub-model, and some new cross-reactions between N-containing species and C-containing species. The DEE sub-model from Shrestha et al. (Fuel Communications, 2022) was modified in this work, NH₃ sub-model was from our previous work (Reaction Chemistry & Engineering, 2023). The DEE-NH₃ model was extensively validated by the IDTs, laminar flame speeds (LFSs), and species profiles (SPs) of NH₃/DEE mixtures as well as pure DEE and NH₃. The comparison of the prediction performance between the DEE-NH₃ model and the Shrestha model was conducted for the ignition, flame propagation, and NH₃ consumption. The effects of the cross-reactions on the NH₃/DEE ignition and combustion were studied in detail.

Ignition delay times (IDTs) were measured in a shock tube facility for NH₃/n-heptane mixtures with NH₃ concentrations in the blending fuel ranging from 0.3 to 0.95 by molar fraction. The measurements were conducted under low pressure of 2 atm and intermediate temperatures of 1350–1500 K at equivalence ratios of 0.5, 1, and 2. With the increase of n-heptane content or equivalence ratio, there is a decrease in the IDTs of NH₃/n-heptane mixtures at intermediate temperatures. A detailed mechanism was updated in this study based on the mechanism of Dong et al. Subsequently, the proposed mechanism was compared to existing blending mechanisms of ammonia and n-heptane in terms of laminar burning velocities (LBVs), IDTs, and species profiles reported in literature. The present model improved the predictions in reproducing the performed experimental measurements compared to previous mechanisms. Finally, rate of production (ROP), sensitivity analysis, and instantaneous and cumulative reaction path analysis were performed to interpret the experiment observations and deepen the understanding of auto-ignition kinetics of ammonia and n-heptane. The results indicate that intermediate species, such as C₂H₄ and C₃H₆, characterized by long lifespans and high concentrations during n-heptane decomposition, play a crucial role at elevated temperatures, while the significance of n-heptane dehydrogenation by NH₂ diminishes with increasing temperature.

The spatio-temporal evolution of laser induced spark ignition of non-premixed gaseous methane and oxygen is investigated. The reactants are injected into an optically accessible combustion chamber from an oxidizer centered shear-coaxial injector. High-speed schlieren imaging and measurements of laser energy deposited are used to characterize ignition behavior at various spark locations throughout the chamber. A spatial map of ignition probability is generated from tests at multiple axial and radial locations from the point of propellant injection. The rate of pressure rise from successful tests reveals two modes of ignition dependent on the location of the laser-induced breakdown, direct and indirect, based on laser-induced breakdown occurring within reactant jet or in the recirculation region, respectively. Physical processes occurring over multiple timescales throughout the direct ignition method are first discussed. Ignition outcomes associated with the indirect method are determined by the hydrodynamic ejection protruding from the laser spark, the behavior of which is explored in detail using flow statistics extracted from images taken at a single spark location. The spatial–temporal progression of this jet is found to be dependent on deposited laser energy, leading to a relationship between the amount of energy added to the flow and ignition outcome. General bounds for spark location, deposited laser energy, and ejection behavior are identified to predict ignition outcomes. Cases that are an exception from these bounds are investigated in detail to understand the cause of unique behavior. These cases lie within regions of the variable space that are susceptible to stochastic elements of the flow.

Novelty and Significance Statement: This work investigates laser induced spark ignition of O₂ and CH₄ issuing from a shear-coaxial injector in a model rocket combustor. Probability of ignition as a function of deposited energy and spatial location is studied with high-speed schlieren imaging and pressure measurements. Two distinct ignition modes that have previously not been identified, direct and indirect, are dependent on the laser deposition location with respect to the reactant jet. For indirect cases, where laser deposition is at a distance from the reactants, the hydrodynamic jet ejection generated from the breakdown of the plasma plays a critical role on ignition probability. The behavior of the ejection jet is characterized to provide a phenomenological description of the ignition process. The results of this research show the important interaction between chemical, fluid dynamic, and thermodynamic processes and their impact on laser ignition.

Premixed flames in narrow heated circular channels subjected to a Poiseuille flow are investigated within the constant density model for various Lewis numbers using irreversible one-step Arrhenius kinetics. A global stability analysis of steady-state axisymmetric solutions is carried out, together with time-dependent direct numerical simulations. The analysis reveals the criteria for the appearance of oscillatory and three-dimensional cellular flame structures. The problem is also studied separately within the framework of the narrow-channel approximation.

Among the results obtained, the following can be singled out as the main ones. First, the multiplicity of stable dynamic modes, oscillatory and steady-state, taking place for the same set of parameters for flames with $Le<1$ is demonstrated. The actual occurrence of one mode or another depends on the initial conditions. Second, the appearance of chaotic regimes is shown for flames with $Le>1$. The chaotic dynamics occurs in a narrow range of values of the flow rate, with Feigenbaum period-doubling cascades taking place both before and after this interval. The results of this study could be useful in the development and use of small-scale combustion devices.

Novelty and significance statement: A systematic study of premixed flames in narrow heated circular channels in the presence of Poiseuille flow is carried out for various Lewis numbers using irreversible one-step Arrhenius kinetics. One of the novelties presented in the paper is the linear global stability analysis of steady state axisymmetric solutions of this problem, which has not been reported before. Also, for the first time, the existence of multiple stable dynamic modes, oscillatory and time-independent, which occurs at the same parameter values, is demonstrated for flames with Lewis number smaller than one. For flames with a Lewis number greater than one, cases with chaotic dynamics are found that manifest themselves in a narrow range of the flow rate. Finally, it is demonstrated that the Feigenbaum cycle-doubling cascade can appear before and after this interval of chaotic dynamics. Such analysis has not been reported before for this problem of flames in partially heated channels.