Proceedings of the Combustion Institute最新文献

Understanding the ignition of iron particle combustion in hot environments is critical for harnessing the use of metal fuels in clean energy production. In this research work, a new single particle burner is implemented to disperse and burn iron particles using hot air coflow. The burner disperses iron particles through Coulomb forces between two air-spaced capacitor electrodes. The oxidizer coflow is heated up as it passes through a newly designed induction gas heater. Electromagnetic induction is used to heat up multiple metallic and porous heat discs by Joule heating. When the coflow gas passes through the pores of the discs, it carries the heat generated within the porous discs and delivers it to the travelling iron particles which provides ultimate heat transfer effectiveness. Coflow temperature homogeneity is examined using Schlieren imaging and confirmed by thermocouple measurements at the outlet of the coflow tube. Particle ignition and combustion occur in open space, providing excellent conditions for optical diagnostics. Two synchronized high-speed cameras were used to simultaneously determine the particle size using green laser shadowgraphy and detect iron particles ignition. Iron particles ignition experiments were conducted for a sieved patch (45–53 μm) at different hot coflow temperatures starting at 700℃ up to 900℃. The burner showed extensive capabilities to maintains a stable and homogeneous hot coflow for a wide range of temperatures up to 900℃. Ignition experiments showed 5.67 % of ejected particles are burnt at 700 ℃, while full ignition (99.33 %) can be achieved at 900 ℃.

Liftoff height behaviors are experimentally investigated in nonpremixed coflow jets of dimethyl ether (DME) diluted with nitrogen, by varying initial fuel mole fraction (X_F,0), jet velocity (U₀ ≤ 1.0 m/s), and initial temperature (up to T₀ = 500 K). As jet velocity increases, three distinct liftoff height (H_L) behaviors are observed depending on X_F,0; monotonically-increasing H_L for small X_F,0, monotonically-decreasing H_L for large X_F,0, and decreasing and then increasing H_L (U-shaped behavior) for intermediate X_F,0. Such characteristics of exhibiting all three types of behaviors for a specified fuel and monotonically decreasing behavior have not been previously observed. For relatively-large jet velocities at small X_F,0, where the jet momentum is sufficiently larger than buoyancy, the liftoff height increases with jet velocity. The monotonically-decreasing H_L behavior for large X_F,0 is attributed to strong buoyancy. As the jet velocity decreases, a blowout occurs for all X_F,0 tested. In the increasing liftoff height regime with jet velocity, the normalized liftoff height is characterized in terms of U₀ scaled by laminar burning velocity (S_L⁰), while in the decreasing regime, in terms of the Richardson number. The blowout velocities are characterized in terms of the density difference between the fuel and burnt gas, which have distinct ranges among the regimes, emphasizing the role of the density difference on the three liftoff height behaviors.

Recent experiments in a micro flow reactor with a controlled temperature profile (MFR) have realized the simultaneous observations of cool flames and flames with repetitive extinction and ignition (FREI) for an n-heptane/air mixture under atmospheric pressure. The primary objective of this study is to numerically reproduce the experimental observations and to investigate the effects of the cool flame on the FREI dynamics through one-dimensional transient reactive flow simulations with various reactor diameters and inlet velocities. The simulation results show that the reaction front speed of FREI decreases monotonically in the case of non-interaction with the cool flame at an inner diameter of 1 mm. Conversely, the reaction front speed temporarily increases due to thermal–chemical effects from stabilized cool flame for 2 mm diameter. Previous experimental and numerical studies in MFR have revealed strong pressure dependence of the three-stage oxidation reactions, comprising cool flame (CF), blue flame (BF) and hot flame (HF) in steady weak flame regime at low inflow velocity conditions. The second objective is to extend pressure-dependent flame response diagram up to FREI regime observed at higher inflow velocity conditions. Simulations demonstrate that there are pressure-dependent four distinct flame regimes: HF-dominant weak flame, HF-driven FREI, BF-dominant weak flame, and BF-driven FREI emerge with increasing pressure. In addition, a notable difference in ignition modes between HF-driven and BF-driven FREIs is observed. The HF-driven FREI exhibits a single reaction front propagating upstream, whereas the BF-driven FREI exhibits bifurcation in ignition, leading to the reaction fronts that propagate both upstream and downstream. Overall, the findings in this study provide a comprehensive understanding of the interactions between flame dynamics and low to intermediate temperature ignition chemistry in the thermally stratified flow fields.

Recent studies have demonstrated that several oxygenated aromatics, including oxygenated polycyclic aromatic hydrocarbons (OPAHs) possessing different functional groups, are formed during anisole combustion. However, a quantitative analysis for these species is still very limited. This limitation inhibits the development and validation of formation kinetic mechanisms for these toxic air pollutants. This study addresses this gap by investigating quantitatively, a fuel-rich anisole-doped laminar premixed flame stabilized on a Holthuis burner at atmospheric pressure with an equivalence ratio of 1.90. Gas samples were extracted from the flame using a quartz nozzle and analyzed by gas chromatography (GC) preceded by a special online pre-concentration trap system, which decreases the detection limit by a factor of over 1000 compared to a conventional GC. Major species (reactants, CO, etc.), 32 small intermediates (C₁-C₅ like formaldehyde, acetaldehyde, acetylene, cyclopentadiene, etc.), 12 non-oxygenated aromatics (benzene, naphthalene, phenanthrene, etc.), and especially 24 oxygenated aromatics (phenol, 2,2-biphenol, dibenzofuran, 9H-xanthene…) including several OPAHs at ppb concentration levels were quantified. Interestingly, flame structure analysis shows that oxygenated aromatics peak closer to the burner surface as compared to non-oxygenated aromatics. The number of rings for non-oxygenated aromatics was observed to increase with the height above the burner, indicating that one-ring aromatics form before two- or three-ring aromatics. However, this is not always the case for oxygenated aromatics. Three-ring OPAHs are almost as abundant as the three-ring PAHs in terms of quantity, some three-ring OPAHs are even twice as abundant as their analogous PAHs (e.g., benzofuran vs fluorene, 9H-xanthene vs anthracene, etc.), which emphasizes their importance and certainly implies that these species need to be considered in kinetic studies. However, unlike PAHs, only less than half of the quantified OPAHs are currently present in literature models for anisole combustion.

Flickering of laminar diffusion flames is known to be caused by the buoyancy-induced toroidal vortices attached to the outer flame surface, making a classical model for understanding flame-vortex interactions. This work experimentally investigates the impacts of ambient coflow on the flickering behaviors of a buoyant jet diffusion flame. Utilizing a simultaneous high-speed flame imaging/OH^⁎ chemiluminescence imaging/particle image velocimetry measurement system, the evolutions and interactions of coherent flame and flow structures are resolved. The results demonstrate that the periodic deformation of the flame surface (i.e., flame flickering) arises from the periodic formation, growth, and shedding of the buoyancy-induced outer vortex rings (OVRs), manifesting as a hydrodynamic instability developing along the outer shear layer (OSL). Upon applying a weak-to-moderate coflow, the flame bulge's size gradually reduces, and the flickering frequency slightly increases with increasing coflow velocity. The notable frequency jump phenomenon is confirmed as the coflow velocity rises to a threshold value such that the flickering frequency undergoes a sudden increase of 3–4 Hz; this is also accompanied by a significant reduction in the flame bulge's size and a delay in flame pinch-off. Moreover, a second frequency jump of 2–3 Hz occurs at a higher coflow velocity. Based on quantitative analysis of the OVR core's evolution, we find that the frequency jumps result from a sudden downstream shift of the initial OVR core, essentially indicating the jump of the instability onset point of the OSL. From a hydrodynamic instability perspective, this phenomenon can be explained as a stabilization effect of the coflow on the OSL, resulting in the re-establishment of the instability condition in a downstream location.

The Flame Surface Density (FSD) model is an affordable combustion model that has been widely used in simulating turbulent premixed flames. In Large Eddy Simulations (LES) with FSD, the combined effect of reaction and diffusion is governed by the Filtered Flame Front Displacement (FFFD) term. While the existing modelling approaches for this term are computationally cost-effective, their predictions still show inconsistencies in certain cases. This study aims to address these inconsistencies by generating Machine Learning (ML) models for the FFFD and FSD terms using the DNS data of a turbulent premixed jet flame. With this approach, the relevance of certain input parameters as well as certain modelling assumptions used for the FFFD term are assessed. Overall, it is found that the resolved curvature term is the most important input parameter to consider and that the resolved progress variable should also be considered in the models. It is shown that the ML models perform significantly better than legacy, algebraic formulations using a priori testing. To further assess the performance of ML, one of the ML models is employed in a a posteriori LES and compared against simulations with the algebraic model. The ML simulation is stable and yields encouraging improvements on key physical parameters regarding the flame length and the FFFD distribution.

Novelty and Significance Statement: This research is of importance because it answers fundamental and practical questions related to the use of combustion modelling approaches, specifically the Flame Surface Density (FSD) and the Filtered Flame Front Displacement (FFFD) models, by means of Machine Learning (ML) algorithms. From a fundamental aspect, we show that two features which are typically not considered as inputs in combustion models, i.e., the progress variable and the resolved curvature, are key to consider for improved predictions of the model, more so than features which are typically used in FSD modelling, i.e., $u_{\Delta }^{\prime },\Delta ,$ and $|\nabla \overline{c}|$. From a practical standpoint, we demonstrate a framework to use the developed ML combustion model a posteriori in a LES, without any stability issues. Overall, these findings are key to guide further traditional and ML improvement efforts on combustion models.

This work proposes a new method to estimate the Markstein number by investigating the thermoacoustic parametric instability of laminar premixed flames propagating downwards in an open-closed tube. Methane-air flames were propagated in a combustion tube to capture the flame evolution in transition to parametric instability. By synchronized high-speed imaging filming both lateral and longitudinal views of flame propagation, cellular flame wavenumbers are measured from the clear transition of flat flames to cellular flames visualized in the present approach. The Markstein number is indirectly determined from wavenumber measurements at the onset of parametric instability by employing a laminar flame model under acoustic excitation treating the Markstein number as a free parameter. Acoustic velocity fluctuation amplitudes obtained from pressure fluctuation measurements were determined to further evaluate the validity of wavenumber-derived Markstein numbers. Markstein numbers obtained in the present work are compared with Markstein numbers relative to fresh gases in the literature from indirect and computational methods.

The oxidation of methylamine (CH₃NH₂) and methane mixtures has been studied by experiments in a flow reactor at atmospheric pressure and temperatures of 350–1450 K. In addition to temperature, stoichiometry (ranging from fuel-rich to fuel-lean conditions) and the presence of NO have been evaluated. Several diagnostic techniques have been used to experimentally quantify many different species: gas chromatography, Fourier Transform Infra-red spectroscopy (FTIR) and an infra-red NO analyzer. Results show a negligible influence of stoichiometry both on the conversion of MEA and CH₄ in the absence of NO, while the presence of NO acts to inhibit the conversion of CH₄ with no appreciable influence on MEA conversion. This indicates the complex interaction occurring in the MEA/CH₄/NO mixtures, for which the mechanism is not able to properly predict the conversion of CH₄ in the presence of NO, while the rest of compounds are well reproduced both in the absence and presence of NO. This fact, together with the probable formation of species containing C and N, due to the presence of additional unidentified species and the deep analysis of the mass balances carried out, supports the idea of the formation of C-N species, not clearly identified so far. The literature mechanism used in simulations has provided good results in reproducing most of the species and conditions considered. The largest discrepancy has been observed for CH₄ conversion in the presence of NO, supporting the existence of missing interactions in the mechanism.

Intrinsic combustion instabilities that onset in NH${}_3$–H${}_2$ and NH${}_3$–CH${}_4$ non-premixed flames are experimentally characterized. A unique research burner capable of creating a good approximation of the classical one-dimensional chambered non-premixed configuration is used, enabling direct comparison with theoretical stability models based on this simple configuration. Starting from a stable flame near the Burke-Schumann limit, the Damköhler number is gradually reduced by decreasing the fuel concentration, going through the marginal stability state where instabilities onset. Both CO${}_2$ and $N_2$ dilution are considered, and global stability limits are provided for a wide range of fuel blends. The instabilities are diffusive-thermal in nature, where it is shown that unstable NH${}_3$–CH${}_4$ flames exhibit pulsations in their reaction rates due to their large Lewis numbers. The characteristic pulsation frequency is highly dependent on the NH${}_3$ fraction in the fuel. A peculiar phenomenon is reported when H${}_2$ with a small Lewis number is added to the less mobile NH${}_3$ species. Superimposed cellular-pulsating instabilities form in NH${}_3$–H${}_2$ flames, which are thoroughly characterized as a function of the NH${}_3$ fraction.

Isoprene holds significant relevance in the realm of atmospheric and combustion chemistry due to its widespread occurrence in both natural and anthropogenic sources. Despite the pivotal role of isoprene in global emissions and combustion scenarios, a detailed understanding of its speciation during thermal decomposition is lacking. Leveraging recent advancements, our focus is on time-resolved speciation behind reflected shock waves, providing precise quantification of the mole fraction time histories of major products. Using a low-pressure shock tube, we investigate isoprene pyrolysis over temperatures of 1280–1780 K and pressures ranging 2.8 to 3.2 bar. Employing multi-wavelength analysis technique, five laser beams are co-aligned through the shock tube to measure the evolution of five major hydrocarbons, namely isoprene, methane, ethylene, acetylene, and 1,3-butadiene, offering a comprehensive overview of isoprene pyrolysis. Comparison of the measured data with the predictions of literature kinetic models shows the inadequacy of existing models. Our proposed model, featuring an updated isoprene pyrolysis subset, enhances predictability and highlights the intricate chemistry of isoprene pyrolysis. Rate of production and sensitivity analyses are used to illustrate key pathways responsible for the formation of observed species. This work will help advance our understanding of isoprene's role in combustion chemistry and pollutant formation, facilitating the optimization of future energy systems.