Pub Date : 2026-04-01Epub Date: 2026-01-19DOI: 10.1016/j.combustflame.2026.114789
Zhiyong Wu , Weitian Wang , Edouard Berrocal , Marcus Aldén , Zhongshan Li
This study presents the first direct measurement of aluminum monohydride (AlH) distribution and dynamics during aluminum combustion. Single micron-sized aluminum droplets were burned in a controlled H₂O/N₂/O₂ environment to ensure repeatable conditions. A dual-wavelength laser absorption imaging system is used to quantify the AlH concentration with high temporal and spatial resolution. The results show that AlH concentration peaks near the droplet surface and decreases from about 1.2% to a negligible level within the condensation layer. As combustion proceeds, AlH extends outward from the droplet surface, and its distribution area stabilizes approximately 12 ms after ignition. This work demonstrates a robust technique for AlH quantification and provides novel data which is critical to understand the aluminum combustion mechanism.
{"title":"Transient AlH distribution around a burning micron-sized Al droplet quantified by laser absorption imaging","authors":"Zhiyong Wu , Weitian Wang , Edouard Berrocal , Marcus Aldén , Zhongshan Li","doi":"10.1016/j.combustflame.2026.114789","DOIUrl":"10.1016/j.combustflame.2026.114789","url":null,"abstract":"<div><div>This study presents the first direct measurement of aluminum monohydride (AlH) distribution and dynamics during aluminum combustion. Single micron-sized aluminum droplets were burned in a controlled H₂O/N₂/O₂ environment to ensure repeatable conditions. A dual-wavelength laser absorption imaging system is used to quantify the AlH concentration with high temporal and spatial resolution. The results show that AlH concentration peaks near the droplet surface and decreases from about 1.2% to a negligible level within the condensation layer. As combustion proceeds, AlH extends outward from the droplet surface, and its distribution area stabilizes approximately 12 ms after ignition. This work demonstrates a robust technique for AlH quantification and provides novel data which is critical to understand the aluminum combustion mechanism.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114789"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146036470","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-04-01Epub Date: 2026-01-21DOI: 10.1016/j.combustflame.2026.114796
Yujia Huo , Xin He , Xin Huang , Hongqing Zhu , Xiaomeng Zhou
Aiming to address the limitations of traditional mine inhibitors, including low inhibition efficiency and insufficient responsiveness, an antioxidant synergistic composite inhibitor (P&C) was developed. First, water was selected as the solvent, magnesium chloride (MgCl₂) as the physical inhibitor, dibutyl hydroxytoluene (BHT) as the primary antioxidant, triphenyl phosphite (TPPI) as the auxiliary antioxidant, and polyethylene glycol (PEG-400) as the functional additive. Subsequently, the composite formulation was optimized using response surface methodology, and the inhibition performance of P&C was evaluated through synchronous thermal analysis, crossing-point temperature experiments, and low-temperature oxidation tests. Finally, the synergistic inhibition mechanism of P&C was investigated via quantum chemical calculations, supported by moisture absorption and retention experiments, BET analysis, in situ infrared spectroscopy, and in situ EPR experiments. The results indicate that the optimal inhibition effect was achieved when the concentrations of MgCl₂, BHT, TPPI, and PEG-400 are 10.26%, 3.15%, 2.09%, and 0.58%, respectively. P&C can significantly increase the crossing point temperature (CPT), characteristic temperature points, and apparent activation energy while reducing heat release, and the inhibition rate is notably higher than that of the conventional inhibitor CaCl₂. Mechanism analysis reveals that MgCl₂ suppresses oxygen diffusion through moisture absorption, cooling, and pore blockage; BHT and TPPI inhibit the chain reaction of coal spontaneous combustion by scavenging free radicals and decomposing peroxides; and PEG-400 enhances the dispersion and permeability of the components in P&C. The P&C system forms a synergistic physical–chemical inhibition effect: physical inhibition provides reaction time for chemical inhibition, while chemical inhibition maintains the stability of the physical inhibition layer. These findings offer new insights into the development of high-efficiency composite inhibitors and hold significant application potential for mine fire prevention.
{"title":"Study on the effectiveness and mechanism of antioxidant synergistic compounds in inhibiting coal spontaneous combustion","authors":"Yujia Huo , Xin He , Xin Huang , Hongqing Zhu , Xiaomeng Zhou","doi":"10.1016/j.combustflame.2026.114796","DOIUrl":"10.1016/j.combustflame.2026.114796","url":null,"abstract":"<div><div>Aiming to address the limitations of traditional mine inhibitors, including low inhibition efficiency and insufficient responsiveness, an antioxidant synergistic composite inhibitor (P&C) was developed. First, water was selected as the solvent, magnesium chloride (MgCl₂) as the physical inhibitor, dibutyl hydroxytoluene (BHT) as the primary antioxidant, triphenyl phosphite (TPPI) as the auxiliary antioxidant, and polyethylene glycol (PEG-400) as the functional additive. Subsequently, the composite formulation was optimized using response surface methodology, and the inhibition performance of P&C was evaluated through synchronous thermal analysis, crossing-point temperature experiments, and low-temperature oxidation tests. Finally, the synergistic inhibition mechanism of P&C was investigated via quantum chemical calculations, supported by moisture absorption and retention experiments, BET analysis, in situ infrared spectroscopy, and in situ EPR experiments. The results indicate that the optimal inhibition effect was achieved when the concentrations of MgCl₂, BHT, TPPI, and PEG-400 are 10.26%, 3.15%, 2.09%, and 0.58%, respectively. P&C can significantly increase the crossing point temperature (CPT), characteristic temperature points, and apparent activation energy while reducing heat release, and the inhibition rate is notably higher than that of the conventional inhibitor CaCl₂. Mechanism analysis reveals that MgCl₂ suppresses oxygen diffusion through moisture absorption, cooling, and pore blockage; BHT and TPPI inhibit the chain reaction of coal spontaneous combustion by scavenging free radicals and decomposing peroxides; and PEG-400 enhances the dispersion and permeability of the components in P&C. The P&C system forms a synergistic physical–chemical inhibition effect: physical inhibition provides reaction time for chemical inhibition, while chemical inhibition maintains the stability of the physical inhibition layer. These findings offer new insights into the development of high-efficiency composite inhibitors and hold significant application potential for mine fire prevention.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114796"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146036405","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-04-01Epub Date: 2026-02-09DOI: 10.1016/j.combustflame.2026.114864
Shuguo Shi , Justin Knubel , Tao Li , Robin Schultheis , Robert S. Barlow , Dirk Geyer , Andreas Dreizler
<div><div>The effects of turbulent mixing and mixture inhomogeneity on the flame structures of turbulent hydrogen–air multi-mode flames stabilized on a modified Darmstadt multi-regime burner are investigated in this study. Near-simultaneous one-dimensional Raman/Rayleigh and two-dimensional Rayleigh scattering measurements are used to quantify the internal flame structures and flame topologies, whereas simultaneous two-dimensional laser-induced fluorescence of hydroxyl radicals and particle image velocimetry are performed to characterize the macroscopic flame structures and flow fields. Quantitative multi-scalar data, including temperature and major species mole fractions, combined with two-dimensional flame topologies, enable characterization of the local thermochemical states. The examined hydrogen–air multi-mode flames consist of a lifted central jet reaction zone, a primary recirculation zone, a secondary recirculation zone, and an outer reaction zone. Quantitative multi-scalar results reveal an intense jet flame reaction zone characterized by local temperature and water mole fraction peaks. These burning behaviors, which differ from previously investigated methane–air multi-mode flames, are attributed to the higher reactivity and the wider flammability range of hydrogen. Global thermochemical state data indicate significant different reaction trajectories in flames with either different turbulent mixing levels or mixture inhomogeneities. Instantaneous thermochemical states conditioned on the central jet flame front demonstrate the variety of reaction trajectories spanning a wide range of equivalence ratios in the flame with the highest initial jet equivalence ratio. Combined with the heat release rate results derived from one-dimensional simulations, the example single-shot data suggest premixed and stratified combustion modes near the jet flame stabilization position. Local thermochemical state results indicate that an increasing air flow velocity from a surrounding slot increases the jet flame lift-off height and modifies the local equivalence ratio distribution, while a higher jet equivalence ratio promotes a broader diversity of reaction trajectories.</div><div><strong>Novelty and significance statement</strong></div><div>Comprehensive experimental investigations of hydrogen–air flames featuring multi-mode characteristics, which are relevant to practical rich-quench-lean operating conditions in gas turbines, have been scarcely reported in the literature. In this work, the effects of turbulent mixing and mixture inhomogeneity on the global flame structures and the local thermochemical states of multi-mode hydrogen–air flames are experimentally investigated using laser-based optical diagnostics. Quantitative multi-scalar results resolve the internal flame structures and provide insight into the various local reaction trajectories in the central reaction zone. To the best of the authors’ knowledge, this work presents the first such set of q
{"title":"Analysis of local thermochemical states in turbulent H2-air multi-mode flames by Raman/Rayleigh spectroscopy","authors":"Shuguo Shi , Justin Knubel , Tao Li , Robin Schultheis , Robert S. Barlow , Dirk Geyer , Andreas Dreizler","doi":"10.1016/j.combustflame.2026.114864","DOIUrl":"10.1016/j.combustflame.2026.114864","url":null,"abstract":"<div><div>The effects of turbulent mixing and mixture inhomogeneity on the flame structures of turbulent hydrogen–air multi-mode flames stabilized on a modified Darmstadt multi-regime burner are investigated in this study. Near-simultaneous one-dimensional Raman/Rayleigh and two-dimensional Rayleigh scattering measurements are used to quantify the internal flame structures and flame topologies, whereas simultaneous two-dimensional laser-induced fluorescence of hydroxyl radicals and particle image velocimetry are performed to characterize the macroscopic flame structures and flow fields. Quantitative multi-scalar data, including temperature and major species mole fractions, combined with two-dimensional flame topologies, enable characterization of the local thermochemical states. The examined hydrogen–air multi-mode flames consist of a lifted central jet reaction zone, a primary recirculation zone, a secondary recirculation zone, and an outer reaction zone. Quantitative multi-scalar results reveal an intense jet flame reaction zone characterized by local temperature and water mole fraction peaks. These burning behaviors, which differ from previously investigated methane–air multi-mode flames, are attributed to the higher reactivity and the wider flammability range of hydrogen. Global thermochemical state data indicate significant different reaction trajectories in flames with either different turbulent mixing levels or mixture inhomogeneities. Instantaneous thermochemical states conditioned on the central jet flame front demonstrate the variety of reaction trajectories spanning a wide range of equivalence ratios in the flame with the highest initial jet equivalence ratio. Combined with the heat release rate results derived from one-dimensional simulations, the example single-shot data suggest premixed and stratified combustion modes near the jet flame stabilization position. Local thermochemical state results indicate that an increasing air flow velocity from a surrounding slot increases the jet flame lift-off height and modifies the local equivalence ratio distribution, while a higher jet equivalence ratio promotes a broader diversity of reaction trajectories.</div><div><strong>Novelty and significance statement</strong></div><div>Comprehensive experimental investigations of hydrogen–air flames featuring multi-mode characteristics, which are relevant to practical rich-quench-lean operating conditions in gas turbines, have been scarcely reported in the literature. In this work, the effects of turbulent mixing and mixture inhomogeneity on the global flame structures and the local thermochemical states of multi-mode hydrogen–air flames are experimentally investigated using laser-based optical diagnostics. Quantitative multi-scalar results resolve the internal flame structures and provide insight into the various local reaction trajectories in the central reaction zone. To the best of the authors’ knowledge, this work presents the first such set of q","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114864"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146184818","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-04-01Epub Date: 2026-02-14DOI: 10.1016/j.combustflame.2026.114871
Yuan Fang , Wenjing Qu , Zelong Xie , Liyan Feng
Adding H2 and n-heptane to ammonia improves flame speed and autoignition reactivity, respectively. Using n-heptane as a pilot fuel to ignite NH3/H2 mixtures has emerged as a promising strategy to reduce carbon emissions in engine applications. In this work, ignition delay times (IDT) of NH3/H2/n-heptane ternary mixtures with n-heptane molar fractions from 0 to 0.12 and NH3/H2 ratios from 90/10 to 0/100 were measured in a rapid compression machine (RCM) at compressed pressures of 15 and 30 bar, compressed temperatures of 650 to 1050 K, and equivalence ratios (ϕ) of 0.5, 1.0, and 2.0. Results show that the introduction of n-heptane significantly enhances reactivity and dominates the ignition behavior, thereby diminishing the influence of H2 on IDTs compared to binary NH3/H2 mixtures. Furthermore, a novel convolutional neural network (CNN)-augmented hybrid model is proposed to predict IDTs by introducing compression-related features in RCM experiments. These features, combined with mixture composition, thermodynamic conditions, and reaction-rate multipliers, serve as inputs for an integrated artificial neural network (ANN). The model accurately captures complex input–output relationships and yields robust predictions. By coupling this surrogate model with the advanced Success-History based Adaptive Differential Evolution with Linear Population Size Reduction (L-SHADE) optimization algorithm and incorporating a variety of experimental data, a robust mechanism optimization framework is developed. The final optimized reduced mechanism, validated against extensive in-house and literature data, demonstrates strong predictive capability and compactness, making it suitable for engine simulations applications.
{"title":"Experimental study of NH3/H2/n-heptane combustion and reduced mechanism optimization via a CNN-augmented neural network and the L-SHADE algorithm","authors":"Yuan Fang , Wenjing Qu , Zelong Xie , Liyan Feng","doi":"10.1016/j.combustflame.2026.114871","DOIUrl":"10.1016/j.combustflame.2026.114871","url":null,"abstract":"<div><div>Adding H<sub>2</sub> and n-heptane to ammonia improves flame speed and autoignition reactivity, respectively. Using n-heptane as a pilot fuel to ignite NH<sub>3</sub>/H<sub>2</sub> mixtures has emerged as a promising strategy to reduce carbon emissions in engine applications. In this work, ignition delay times (IDT) of NH<sub>3</sub>/H<sub>2</sub>/n-heptane ternary mixtures with n-heptane molar fractions from 0 to 0.12 and NH<sub>3</sub>/H<sub>2</sub> ratios from 90/10 to 0/100 were measured in a rapid compression machine (RCM) at compressed pressures of 15 and 30 bar, compressed temperatures of 650 to 1050 K, and equivalence ratios (<em>ϕ</em>) of 0.5, 1.0, and 2.0. Results show that the introduction of n-heptane significantly enhances reactivity and dominates the ignition behavior, thereby diminishing the influence of H<sub>2</sub> on IDTs compared to binary NH<sub>3</sub>/H<sub>2</sub> mixtures. Furthermore, a novel convolutional neural network (CNN)-augmented hybrid model is proposed to predict IDTs by introducing compression-related features in RCM experiments. These features, combined with mixture composition, thermodynamic conditions, and reaction-rate multipliers, serve as inputs for an integrated artificial neural network (ANN). The model accurately captures complex input–output relationships and yields robust predictions. By coupling this surrogate model with the advanced Success-History based Adaptive Differential Evolution with Linear Population Size Reduction (L-SHADE) optimization algorithm and incorporating a variety of experimental data, a robust mechanism optimization framework is developed. The final optimized reduced mechanism, validated against extensive in-house and literature data, demonstrates strong predictive capability and compactness, making it suitable for engine simulations applications.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114871"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146184842","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-04-01Epub Date: 2026-01-28DOI: 10.1016/j.combustflame.2026.114817
Hongqing Wu , Guojie Liang , Tianzhou Jiang , Fan Li , Yang Li , Rongpei Jiang , Ruoyue Tang , Song Cheng
The interaction between unsaturated hydrocarbons and N2O has attracted considerable attention in recent years due to their important role as potential propellants for advanced propulsion systems (e.g. Nitrous oxide fuel blend (NOFBX)) and key combustion intermediates in exhaust gas recirculation systems. Although experimental studies and kinetic models have been developed to investigate its fuel chemistry, discrepancies remain between modeled and measured ignition delay times at low temperatures. In this work, we characterize previously unreported direct interaction pathways between N2O and unsaturated hydrocarbons (C2H4, C3H6, C2H2, C3H4-A, and C3H4-P) through quantum chemistry calculations, comprehensive kinetic modeling, and experimental validation. These reactions proceed via O-atom addition from N2O to unsaturated hydrocarbons, forming five-membered ring intermediates that decompose into N2 and hydrocarbon-specific products. Distinct differences are identified between alkenes and dienes and alkynes, arising from the disparity in N–C bond lengths within the intermediates (∼1.480 Å for alkenes and 1.429 Å for dienes vs. ∼1.381 Å for alkynes), which governs their decomposition pathways. The corresponding rate coefficients are determined and implemented into multiple kinetic models, with autoignition simulations showing a pronounced promoting effect on model reactivity and improved agreement with experiments, especially at low temperatures. Comprehensive uncertainty analyses of the potential energy surfaces, rate coefficients, and ignition delay times are conducted to ensure the robustness and reliability of the findings. Flux analysis further reveals that the new pathways suppress conventional inhibiting channels while enabling aldehyde- and ketone-forming pathways that enhance overall reactivity, with JSR simulations further confirming the feasibility of validating these pathways through experiments. This work provides a more complete description of N2O–hydrocarbon interactions and reveals other important N2O–hydrocarbon interaction chemistries that need to be further studied via both theoretical and experimental investigations.
{"title":"Unravelling the unique kinetic interactions between N2O and unsaturated hydrocarbons","authors":"Hongqing Wu , Guojie Liang , Tianzhou Jiang , Fan Li , Yang Li , Rongpei Jiang , Ruoyue Tang , Song Cheng","doi":"10.1016/j.combustflame.2026.114817","DOIUrl":"10.1016/j.combustflame.2026.114817","url":null,"abstract":"<div><div>The interaction between unsaturated hydrocarbons and N<sub>2</sub>O has attracted considerable attention in recent years due to their important role as potential propellants for advanced propulsion systems (e.g. Nitrous oxide fuel blend (NOFBX)) and key combustion intermediates in exhaust gas recirculation systems. Although experimental studies and kinetic models have been developed to investigate its fuel chemistry, discrepancies remain between modeled and measured ignition delay times at low temperatures. In this work, we characterize previously unreported direct interaction pathways between N<sub>2</sub>O and unsaturated hydrocarbons (C<sub>2</sub>H<sub>4</sub>, C<sub>3</sub>H<sub>6</sub>, C<sub>2</sub>H<sub>2</sub>, C<sub>3</sub>H<sub>4</sub>-A, and C<sub>3</sub>H<sub>4</sub>-P) through quantum chemistry calculations, comprehensive kinetic modeling, and experimental validation. These reactions proceed via O-atom addition from N<sub>2</sub>O to unsaturated hydrocarbons, forming five-membered ring intermediates that decompose into N<sub>2</sub> and hydrocarbon-specific products. Distinct differences are identified between alkenes and dienes and alkynes, arising from the disparity in N–C bond lengths within the intermediates (∼1.480 Å for alkenes and 1.429 Å for dienes vs. ∼1.381 Å for alkynes), which governs their decomposition pathways. The corresponding rate coefficients are determined and implemented into multiple kinetic models, with autoignition simulations showing a pronounced promoting effect on model reactivity and improved agreement with experiments, especially at low temperatures. Comprehensive uncertainty analyses of the potential energy surfaces, rate coefficients, and ignition delay times are conducted to ensure the robustness and reliability of the findings. Flux analysis further reveals that the new pathways suppress conventional inhibiting channels while enabling aldehyde- and ketone-forming pathways that enhance overall reactivity, with JSR simulations further confirming the feasibility of validating these pathways through experiments. This work provides a more complete description of N<sub>2</sub>O–hydrocarbon interactions and reveals other important N<sub>2</sub>O–hydrocarbon interaction chemistries that need to be further studied via both theoretical and experimental investigations.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114817"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146075181","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-04-01Epub Date: 2026-01-29DOI: 10.1016/j.combustflame.2026.114806
Cristian C. Mejía-Botero , Florent Virot , Luis Fernando Figueira da Silva , Josué Melguizo-Gavilanes
<div><div>We investigated the effect of fundamental combustion properties (FCP) on the 3D morphology and dynamics of flames and shocks during acceleration and transition to detonation in unobstructed channels. To achieve this, an extensive experimental campaign was conducted using a simultaneous schlieren visualization setup. The effect of selected FCP was assessed by evaluating nine different mixtures of hydrogen, methane, and hydrogen/methane blends, using oxygen with and without dilution by nitrogen, helium, or argon. The experimental results revealed two characteristic flame evolution behaviors during flame acceleration (FA), depending on the mixtures: (i) a symmetric flame inversion (tulip flame) during the early stages of FA, followed by a short, symmetric flame in the later stages, with the formation of a precursor compression wave located relatively far from the flame, and (ii) an asymmetric, wrinkled flame during the early stages, which develops into a longer flame with the tip inclined toward a corner of the channel, accompanied by the formation of multiple precursor compression waves ahead of the flame in the later stages of FA. For a more robust statistical analysis, a morphology database was compiled from literature sources reporting similar flame morphologies to those observed in our experiments. This database was analyzed using the Feature Elimination Technique in conjunction with the Logistic Regression Model, which enabled the identification of FCP boundaries between the observed flame morphologies. The analysis showed that the pairs of properties most influencing flame morphology are the expansion ratio and the ratio of the laminar flame speed to the sound speed in the combustion products, i.e., <span><math><mrow><mo>(</mo><mi>σ</mi><mo>,</mo><mi>σ</mi><msub><mrow><mi>s</mi></mrow><mrow><mi>L</mi></mrow></msub><mo>/</mo><msub><mrow><mi>c</mi></mrow><mrow><mi>b</mi></mrow></msub><mo>)</mo></mrow></math></span>, as well as the latter ratio with the heat capacity ratio, i.e., <span><math><mrow><mo>(</mo><mi>σ</mi><msub><mrow><mi>s</mi></mrow><mrow><mi>L</mi></mrow></msub><mo>/</mo><msub><mrow><mi>c</mi></mrow><mrow><mi>b</mi></mrow></msub><mo>,</mo><mi>γ</mi><mo>)</mo></mrow></math></span>. Additionally, this methodology helped to identify experimental conditions where little or no data is available in the literature, such as for mixtures with Lewis numbers smaller than unity, which are expected to be affected by thermodiffusive instabilities. These boundaries can, therefore, serve as guidelines for selecting experimental conditions that develop specific flame and shock morphologies and dynamics.</div><div><strong>Novelty and significance statement</strong></div><div>This study establishes, for the first time, a direct link between fundamental combustion properties (FCP) and the observed flame and shock morphologies during flame acceleration in unobstructed channels from ignition to detonation onset. The results offer predictive in
{"title":"Flame morphology boundaries and fundamental combustion properties in unobstructed channels","authors":"Cristian C. Mejía-Botero , Florent Virot , Luis Fernando Figueira da Silva , Josué Melguizo-Gavilanes","doi":"10.1016/j.combustflame.2026.114806","DOIUrl":"10.1016/j.combustflame.2026.114806","url":null,"abstract":"<div><div>We investigated the effect of fundamental combustion properties (FCP) on the 3D morphology and dynamics of flames and shocks during acceleration and transition to detonation in unobstructed channels. To achieve this, an extensive experimental campaign was conducted using a simultaneous schlieren visualization setup. The effect of selected FCP was assessed by evaluating nine different mixtures of hydrogen, methane, and hydrogen/methane blends, using oxygen with and without dilution by nitrogen, helium, or argon. The experimental results revealed two characteristic flame evolution behaviors during flame acceleration (FA), depending on the mixtures: (i) a symmetric flame inversion (tulip flame) during the early stages of FA, followed by a short, symmetric flame in the later stages, with the formation of a precursor compression wave located relatively far from the flame, and (ii) an asymmetric, wrinkled flame during the early stages, which develops into a longer flame with the tip inclined toward a corner of the channel, accompanied by the formation of multiple precursor compression waves ahead of the flame in the later stages of FA. For a more robust statistical analysis, a morphology database was compiled from literature sources reporting similar flame morphologies to those observed in our experiments. This database was analyzed using the Feature Elimination Technique in conjunction with the Logistic Regression Model, which enabled the identification of FCP boundaries between the observed flame morphologies. The analysis showed that the pairs of properties most influencing flame morphology are the expansion ratio and the ratio of the laminar flame speed to the sound speed in the combustion products, i.e., <span><math><mrow><mo>(</mo><mi>σ</mi><mo>,</mo><mi>σ</mi><msub><mrow><mi>s</mi></mrow><mrow><mi>L</mi></mrow></msub><mo>/</mo><msub><mrow><mi>c</mi></mrow><mrow><mi>b</mi></mrow></msub><mo>)</mo></mrow></math></span>, as well as the latter ratio with the heat capacity ratio, i.e., <span><math><mrow><mo>(</mo><mi>σ</mi><msub><mrow><mi>s</mi></mrow><mrow><mi>L</mi></mrow></msub><mo>/</mo><msub><mrow><mi>c</mi></mrow><mrow><mi>b</mi></mrow></msub><mo>,</mo><mi>γ</mi><mo>)</mo></mrow></math></span>. Additionally, this methodology helped to identify experimental conditions where little or no data is available in the literature, such as for mixtures with Lewis numbers smaller than unity, which are expected to be affected by thermodiffusive instabilities. These boundaries can, therefore, serve as guidelines for selecting experimental conditions that develop specific flame and shock morphologies and dynamics.</div><div><strong>Novelty and significance statement</strong></div><div>This study establishes, for the first time, a direct link between fundamental combustion properties (FCP) and the observed flame and shock morphologies during flame acceleration in unobstructed channels from ignition to detonation onset. The results offer predictive in","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114806"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146075185","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-04-01Epub Date: 2026-01-31DOI: 10.1016/j.combustflame.2026.114843
Simin Ren, Zhongqi Wang, Qi Zhang
As an advanced form of conventional explosive energy, fuel mists react with ambient oxygen and can deliver high energy density. In fuel–air explosive (FAE) devices, the central detonation generates high pressure and high temperature: the former drives rapid fuel dispersion, whereas the latter can ignite the evolving fuel–air mixture during dispersion, leading to premature ignition and reduced effective cloud energy utilization. Premature ignition during dispersion involves strongly coupled unsteady processes, including flow, turbulence, heat and mass transfer, droplet-field evolution, and chemical reactions. In this study, numerical simulations together with experimental validation are employed to identify the critical conditions for premature ignition in a typical explosion-driven dispersion configuration and to elucidate the underlying physico-chemical mechanisms. The results show that ignition activity preferentially appears near the upper and lower ends of the device in the early stage, and then migrates toward the ±45° directions relative to the X-axis (defined as 0°) in the middle stage, consistent with the evolving temperature and mixing fields. For a 2 kg propylene-oxide FAE device, no premature-ignition occurs at a central charge ratio of 1.0%, whereas ratios of 2.0% or higher lead to sustained premature ignition. A central charge ratio of 1.5% is identified as the critical condition, with additional cases at 1.25% and 1.75% used to bracket this boundary. This critical boundary can be interpreted by an ignition-in-motion rate-competition criterion, Da = RA/Rcritical≈1; within the present single-step framework, the associated effective critical reaction-rate level is about 0.5 kgmol/m3s. The present results provide a baseline for the studied configuration under controlled ambient conditions. For a 2.0% central charge ratio, premature ignition initiates at the upper edge of the cloud, where the local fuel concentration is about 300 g/m3 and the explosion-driven temperature at the ignition site is about 1146 K.
Novelty and significance statement
Significance: Premature-ignition is a critical bottleneck limiting the energy efficiency of fuel-air explosive (FAE) systems. A fundamental understanding of this process is essential for optimizing FAE design to overcome incomplete energy release and maximize performance, providing a basis for developing more advanced energetic systems.
Novelty: Moving beyond previous work limited to static parameters, this study reveals the fundamental cause of premature-ignition. It is the first to elucidate the dynamic, multi-field coupling between an evolving high-temperature field and a transient fuel cloud, establishing a previously unreported transient ignition mechanism.
{"title":"Ignition mechanism and laws of explosion-driven thermal field and fuel dispersion flow","authors":"Simin Ren, Zhongqi Wang, Qi Zhang","doi":"10.1016/j.combustflame.2026.114843","DOIUrl":"10.1016/j.combustflame.2026.114843","url":null,"abstract":"<div><div>As an advanced form of conventional explosive energy, fuel mists react with ambient oxygen and can deliver high energy density. In fuel–air explosive (FAE) devices, the central detonation generates high pressure and high temperature: the former drives rapid fuel dispersion, whereas the latter can ignite the evolving fuel–air mixture during dispersion, leading to premature ignition and reduced effective cloud energy utilization. Premature ignition during dispersion involves strongly coupled unsteady processes, including flow, turbulence, heat and mass transfer, droplet-field evolution, and chemical reactions. In this study, numerical simulations together with experimental validation are employed to identify the critical conditions for premature ignition in a typical explosion-driven dispersion configuration and to elucidate the underlying physico-chemical mechanisms. The results show that ignition activity preferentially appears near the upper and lower ends of the device in the early stage, and then migrates toward the ±45° directions relative to the X-axis (defined as 0°) in the middle stage, consistent with the evolving temperature and mixing fields. For a 2 kg propylene-oxide FAE device, no premature-ignition occurs at a central charge ratio of 1.0%, whereas ratios of 2.0% or higher lead to sustained premature ignition. A central charge ratio of 1.5% is identified as the critical condition, with additional cases at 1.25% and 1.75% used to bracket this boundary. This critical boundary can be interpreted by an ignition-in-motion rate-competition criterion, Da = R<sub>A</sub>/R<sub>critical</sub>≈1; within the present single-step framework, the associated effective critical reaction-rate level is about 0.5 kgmol/m<sup>3</sup>s. The present results provide a baseline for the studied configuration under controlled ambient conditions. For a 2.0% central charge ratio, premature ignition initiates at the upper edge of the cloud, where the local fuel concentration is about 300 g/m<sup>3</sup> and the explosion-driven temperature at the ignition site is about 1146 K.</div><div>Novelty and significance statement</div><div>Significance: Premature-ignition is a critical bottleneck limiting the energy efficiency of fuel-air explosive (FAE) systems. A fundamental understanding of this process is essential for optimizing FAE design to overcome incomplete energy release and maximize performance, providing a basis for developing more advanced energetic systems.</div><div>Novelty: Moving beyond previous work limited to static parameters, this study reveals the fundamental cause of premature-ignition. It is the first to elucidate the dynamic, multi-field coupling between an evolving high-temperature field and a transient fuel cloud, establishing a previously unreported transient ignition mechanism.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114843"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146075137","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Hydrogen is a promising energy carrier for power and propulsion systems. While its combustion in air or oxygen has been well studied, hydrogen flames with nitrous oxide as the oxidizer remain less explored. This study presents a systematic numerical investigation of the linear instability characteristics of premixed H2/N2O/N2 flames using detailed numerical simulations. A multi-wavelength perturbation method is employed to extract the dispersion relations, providing insights into the linear stability characteristics of the flame front. The effects of equivalence ratio and N2O concentration are quantified. The equivalence ratio primarily affects the Lewis number, and thus thermo-diffusive instability, with lean mixtures exhibiting greater instability. The N2O concentration has opposing effects on lean and rich flames: higher N2O content reduces instability for lean mixtures but exerts little effect on rich mixtures. In addition, the flame stability is also strongly influenced by the thermodynamic state: higher unburned temperatures stabilize the flame by reducing the thermal expansion ratio, whereas elevated pressures destabilize it by influencing both the thermal expansion ratio and Zeldovich number. At the end, correlations between the numerical dispersion relations and asymptotic theory are quantified, and empirical fits are derived to capture the dependence of growth rates and cutoff wavenumbers on mixture composition, providing practical tools for reduced-order stability modeling. Collectively, these findings advance the fundamental understanding of hydrogen–nitrous oxide combustion.
Novelty and Significance Statement
Flames involving H2 and N2O exhibit a high susceptibility to instabilities due to the high diffusivity of H2 and the exothermic decomposition of N2O. This study presents the first systematic analysis of the linear instability characteristics of premixed planar H2/N2O/N2 flames using high-fidelity detailed numerical simulations. Quantitative dispersion relations are obtained over a wide range of temperatures, pressures, and mixture compositions, which are absent in the existing literature. Another key novelty of this work lies in the systematic evaluation of fundamental non-dimensional parameters, including the thermal expansion ratio, Zeldovich number, and effective Lewis number, and in the examination of correlations between theoretical and numerical dispersion relations. Together, these analyses elucidate how thermodynamic and compositional variations govern the instability growth rate and cutoff wavenumber in H2/N2O/N2 flames.
{"title":"Linear stability analysis of laminar premixed planar H2/N2O/N2 flames","authors":"Shumeng Xie , Christine Mounaïm-Rousselle , Huangwei Zhang","doi":"10.1016/j.combustflame.2026.114846","DOIUrl":"10.1016/j.combustflame.2026.114846","url":null,"abstract":"<div><div>Hydrogen is a promising energy carrier for power and propulsion systems. While its combustion in air or oxygen has been well studied, hydrogen flames with nitrous oxide as the oxidizer remain less explored. This study presents a systematic numerical investigation of the linear instability characteristics of premixed H<sub>2</sub>/N<sub>2</sub>O/N<sub>2</sub> flames using detailed numerical simulations. A multi-wavelength perturbation method is employed to extract the dispersion relations, providing insights into the linear stability characteristics of the flame front. The effects of equivalence ratio and N<sub>2</sub>O concentration are quantified. The equivalence ratio primarily affects the Lewis number, and thus thermo-diffusive instability, with lean mixtures exhibiting greater instability. The N<sub>2</sub>O concentration has opposing effects on lean and rich flames: higher N<sub>2</sub>O content reduces instability for lean mixtures but exerts little effect on rich mixtures. In addition, the flame stability is also strongly influenced by the thermodynamic state: higher unburned temperatures stabilize the flame by reducing the thermal expansion ratio, whereas elevated pressures destabilize it by influencing both the thermal expansion ratio and Zeldovich number. At the end, correlations between the numerical dispersion relations and asymptotic theory are quantified, and empirical fits are derived to capture the dependence of growth rates and cutoff wavenumbers on mixture composition, providing practical tools for reduced-order stability modeling. Collectively, these findings advance the fundamental understanding of hydrogen–nitrous oxide combustion.</div><div><strong>Novelty and Significance Statement</strong></div><div>Flames involving H<sub>2</sub> and N<sub>2</sub>O exhibit a high susceptibility to instabilities due to the high diffusivity of H<sub>2</sub> and the exothermic decomposition of N<sub>2</sub>O. This study presents the first systematic analysis of the linear instability characteristics of premixed planar H<sub>2</sub>/N<sub>2</sub>O/N<sub>2</sub> flames using high-fidelity detailed numerical simulations. Quantitative dispersion relations are obtained over a wide range of temperatures, pressures, and mixture compositions, which are absent in the existing literature. Another key novelty of this work lies in the systematic evaluation of fundamental non-dimensional parameters, including the thermal expansion ratio, Zeldovich number, and effective Lewis number, and in the examination of correlations between theoretical and numerical dispersion relations. Together, these analyses elucidate how thermodynamic and compositional variations govern the instability growth rate and cutoff wavenumber in H<sub>2</sub>/N<sub>2</sub>O/N<sub>2</sub> flames.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114846"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146075138","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-04-01Epub Date: 2026-02-02DOI: 10.1016/j.combustflame.2026.114818
Renaud Gablier , Joey Kim Soriano , Jean-Baptiste Perrin-Terrin , Yuji Ikeda , Christophe O. Laux
<div><div>Plasma-assisted combustion (PAC) is widely studied to increase the operability limits of combustion systems. Nonequilibrium plasmas are advantageous in PAC as they offer strong chemical effects with low Joule heating. Several sources of nonequilibrium plasma are used in PAC, notably Nanosecond Repetitively Pulsed (NRP) and microwave (MW) discharges. MW discharges have the advantage of requiring only one electrode, which facilitates integration in industrial applications. MW discharges have also already shown promising results for flame ignition, but studies are lacking for lean flame stabilization. On the other hand, NRP discharges have been much more studied for lean flame stabilization, ignition, and lean blow-off (LBO) limit extension. However, quantitative comparisons of these two sources on the same burner are hard to perform as their integrations in combustors are challenging and differ a lot. In this work, we adapted a premixed CH<sub>4</sub>-air bluff-body stabilized burner, historically operated with NRP discharges, to surface MW discharges. We compare quantitatively the performances of NRP and MW discharges for lean flame stabilization and ignition. We found that MW can extend the LBO limit by ∼5-10% for flame powers ranging from 2 to 16 kW with 60 W of plasma power. For the same plasma power, NRP discharges extend the LBO by ∼17-45%. This difference is attributed to the larger plasma volume of the NRP discharges, as it fills a wider portion of the recirculation zone, whereas MW discharges remain close to the bluff body surface. The minimum ignition energy is measured for both discharges: MW discharges require 500 mJ and NRP discharges 13 mJ. NRP discharges require less energy for breakdown as they present a stronger electric field than MW discharges. Finally, optical emission spectroscopy of the MW plasma validates that the MW plasma is in nonequilibrium with moderate gas heating (<700 K) and low ionization degree (<0.3%).<br></div><div><strong>Novelty and significance statement</strong><br></div><div>This work quantitatively compares the effect of microwave (MW) and nanosecond repetitively pulsed (NRP) discharges on lean blow-off limit extension and flame ignition. To the author’s knowledge, this is the first quantitative comparison of plasma-assisted combustion (PAC) with MW and NRP discharges on the same burner. Experimental rigs are often built around a particular type of discharge, each one having its own integration challenges (electrode geometry and position, integration in the burner, etc.). It is thus not common to have facilities capable of switching from one source to another. This work is significant because it illustrates the advantages and drawbacks of the two technologies and quantifies their effects on the same facility. This type of comparison can guide the industry toward the best plasma sources for large-scale applications. This work also shows that the MW plasma in a flame is in a state of thermochem
{"title":"Comparison between nanosecond repetitively pulsed and surface microwave discharges for flame stabilization and ignition","authors":"Renaud Gablier , Joey Kim Soriano , Jean-Baptiste Perrin-Terrin , Yuji Ikeda , Christophe O. Laux","doi":"10.1016/j.combustflame.2026.114818","DOIUrl":"10.1016/j.combustflame.2026.114818","url":null,"abstract":"<div><div>Plasma-assisted combustion (PAC) is widely studied to increase the operability limits of combustion systems. Nonequilibrium plasmas are advantageous in PAC as they offer strong chemical effects with low Joule heating. Several sources of nonequilibrium plasma are used in PAC, notably Nanosecond Repetitively Pulsed (NRP) and microwave (MW) discharges. MW discharges have the advantage of requiring only one electrode, which facilitates integration in industrial applications. MW discharges have also already shown promising results for flame ignition, but studies are lacking for lean flame stabilization. On the other hand, NRP discharges have been much more studied for lean flame stabilization, ignition, and lean blow-off (LBO) limit extension. However, quantitative comparisons of these two sources on the same burner are hard to perform as their integrations in combustors are challenging and differ a lot. In this work, we adapted a premixed CH<sub>4</sub>-air bluff-body stabilized burner, historically operated with NRP discharges, to surface MW discharges. We compare quantitatively the performances of NRP and MW discharges for lean flame stabilization and ignition. We found that MW can extend the LBO limit by ∼5-10% for flame powers ranging from 2 to 16 kW with 60 W of plasma power. For the same plasma power, NRP discharges extend the LBO by ∼17-45%. This difference is attributed to the larger plasma volume of the NRP discharges, as it fills a wider portion of the recirculation zone, whereas MW discharges remain close to the bluff body surface. The minimum ignition energy is measured for both discharges: MW discharges require 500 mJ and NRP discharges 13 mJ. NRP discharges require less energy for breakdown as they present a stronger electric field than MW discharges. Finally, optical emission spectroscopy of the MW plasma validates that the MW plasma is in nonequilibrium with moderate gas heating (<700 K) and low ionization degree (<0.3%).<br></div><div><strong>Novelty and significance statement</strong><br></div><div>This work quantitatively compares the effect of microwave (MW) and nanosecond repetitively pulsed (NRP) discharges on lean blow-off limit extension and flame ignition. To the author’s knowledge, this is the first quantitative comparison of plasma-assisted combustion (PAC) with MW and NRP discharges on the same burner. Experimental rigs are often built around a particular type of discharge, each one having its own integration challenges (electrode geometry and position, integration in the burner, etc.). It is thus not common to have facilities capable of switching from one source to another. This work is significant because it illustrates the advantages and drawbacks of the two technologies and quantifies their effects on the same facility. This type of comparison can guide the industry toward the best plasma sources for large-scale applications. This work also shows that the MW plasma in a flame is in a state of thermochem","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114818"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146184669","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-04-01Epub Date: 2026-01-16DOI: 10.1016/j.combustflame.2026.114788
Yinan Yang, Tsukasa Hori, Shinya Sawada, Fumiteru Akamatsu
Ammonia co-combustion with hydrocarbon fuels has emerged as a promising pathway toward carbon neutrality. However, as a nitrogen-containing fuel, ammonia combustion at high temperatures inevitably leads to the formation of both thermal and a substantial amount of fuel NOx. To distinguish the nitrogen sources within ammonia co-combustion furnaces, a three-dimensional numerical analysis employing the nitrogen element-tracking method was conducted. Combustion characteristics and nitrogen oxide emission behaviors of a 10-kW ammonia co-combustion furnace were investigated under various total air ratios (λtotal = 1.0 – 1.5) and ammonia co-firing ratios (ENH3 = 0 % – 100 %). Results indicate that under air-staged combustion, increasing the total air ratio from 1.0 to 1.2 improves combustion performance with a slight increase in fuel NO (N*O), whereas a further increase to 1.5 leads to an approximately threefold rise in N*O at the furnace outlet. Thermal NO predominantly forms downstream in the furnace and shows pronounced sensitivity above temperatures of approximately 1800 K. Regarding the ammonia co-firing ratio, increasing the ammonia content enhances N*O reduction reactions but reduces the overall reaction intensity and furnace temperature, resulting in a parabolic variation of N*O emissions, with a peak occurring at an ammonia co-firing ratio of approximately 50 %. In contrast, thermal NO emissions continually decline and become negligible at ammonia co-firing ratios above 40 %. Reaction pathway analysis reveals that N*O formation is dominated by the decomposition of HN*O intermediates driven by H and OH radicals, while direct oxidation of N* and the conversion of N*H to N*O also contribute under high ammonia co-firing ratios or elevated total air ratios. The numerical results provide valuable theoretical insights for optimizing NOx emission control strategies in ammonia co-combustion systems.
{"title":"Simulation-based study of nitrogen sources and reaction pathways for NO formation in a 10-kW ammonia co-combustion furnace","authors":"Yinan Yang, Tsukasa Hori, Shinya Sawada, Fumiteru Akamatsu","doi":"10.1016/j.combustflame.2026.114788","DOIUrl":"10.1016/j.combustflame.2026.114788","url":null,"abstract":"<div><div>Ammonia co-combustion with hydrocarbon fuels has emerged as a promising pathway toward carbon neutrality. However, as a nitrogen-containing fuel, ammonia combustion at high temperatures inevitably leads to the formation of both thermal and a substantial amount of fuel NOx. To distinguish the nitrogen sources within ammonia co-combustion furnaces, a three-dimensional numerical analysis employing the nitrogen element-tracking method was conducted. Combustion characteristics and nitrogen oxide emission behaviors of a 10-kW ammonia co-combustion furnace were investigated under various total air ratios (<em>λ</em><sub><em>total</em></sub> = 1.0 – 1.5) and ammonia co-firing ratios (<em>E</em><sub><em>NH3</em></sub> = 0 % – 100 %). Results indicate that under air-staged combustion, increasing the total air ratio from 1.0 to 1.2 improves combustion performance with a slight increase in fuel NO (N*O), whereas a further increase to 1.5 leads to an approximately threefold rise in N*O at the furnace outlet. Thermal NO predominantly forms downstream in the furnace and shows pronounced sensitivity above temperatures of approximately 1800 K. Regarding the ammonia co-firing ratio, increasing the ammonia content enhances N*O reduction reactions but reduces the overall reaction intensity and furnace temperature, resulting in a parabolic variation of N*O emissions, with a peak occurring at an ammonia co-firing ratio of approximately 50 %. In contrast, thermal NO emissions continually decline and become negligible at ammonia co-firing ratios above 40 %. Reaction pathway analysis reveals that N*O formation is dominated by the decomposition of HN*O intermediates driven by H and OH radicals, while direct oxidation of N* and the conversion of N*H to N*O also contribute under high ammonia co-firing ratios or elevated total air ratios. The numerical results provide valuable theoretical insights for optimizing NOx emission control strategies in ammonia co-combustion systems.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"286 ","pages":"Article 114788"},"PeriodicalIF":6.2,"publicationDate":"2026-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145969329","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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