Pub Date : 2026-01-01Epub Date: 2026-07-02DOI: 10.1016/j.proci.2026.106087
Allen Hsing, Jose G. Rivera Lizarralde, Mario Sánchez-Sanz, Abhishek Saha
Hydrogen-ammonia blends are increasingly considered as promising carbon-free fuels for future combustion systems. In this study, we investigate the dynamics of cellular instabilities in hydrogen-ammonia Bunsen flames. While previous studies have examined such instability phenomena in hydrocarbon and pure hydrogen flames, the influence of ammonia addition, particularly under flow-driven configuration, remains less explored. Using chemiluminescence and Mie-scattering imaging, we characterize the effect of ammonia concentration on the transition between stable and unstable flame regimes and on the morphology of the resulting polyhedral structures. Our results show that increasing ammonia content narrows the range of unstable operating conditions due to its lower chemical reactivity, whereas higher hydrogen fractions promote flashback, thereby limiting stable flame operation. The observed transitions and modifications in flame morphology are further interpreted using scaling analyses derived from the corresponding dispersion relations.
Novelty and significance statement: While hydrogen-ammonia blends are widely regarded as promising carbon-free fuels, important questions remain regarding their suitability for practical engine operation. Pure hydrogen flames, especially under fuel-lean conditions, are susceptible to cellular instabilities that induce intrinsic self-acceleration and thereby, elevated propagation speeds. This study demonstrates how the addition of ammonia to hydrogen modulates the transition between stable and unstable regimes.
While cellular instabilities are most often studied in freely expanding flames, we explore them in a burner-stabilized configuration. To the authors’ best knowledge, this is the first study to systematically investigate how varying the degree of ammonia blending in hydrogen affects the morphology, stability, and transition dynamics of polyhedral flames. In addition, the study also confirms that intense local stretch near the cusps of polyhedral ridges can trigger localized extinction. Beyond its fundamental significance, the experimental observations and accompanying scaling analysis offer a practical framework for identifying operating conditions that suppress such instabilities.
{"title":"Characteristics of polyhedral hydrogen-ammonia Bunsen flames","authors":"Allen Hsing, Jose G. Rivera Lizarralde, Mario Sánchez-Sanz, Abhishek Saha","doi":"10.1016/j.proci.2026.106087","DOIUrl":"10.1016/j.proci.2026.106087","url":null,"abstract":"<div><div>Hydrogen-ammonia blends are increasingly considered as promising carbon-free fuels for future combustion systems. In this study, we investigate the dynamics of cellular instabilities in hydrogen-ammonia Bunsen flames. While previous studies have examined such instability phenomena in hydrocarbon and pure hydrogen flames, the influence of ammonia addition, particularly under flow-driven configuration, remains less explored. Using chemiluminescence and Mie-scattering imaging, we characterize the effect of ammonia concentration on the transition between stable and unstable flame regimes and on the morphology of the resulting polyhedral structures. Our results show that increasing ammonia content narrows the range of unstable operating conditions due to its lower chemical reactivity, whereas higher hydrogen fractions promote flashback, thereby limiting stable flame operation. The observed transitions and modifications in flame morphology are further interpreted using scaling analyses derived from the corresponding dispersion relations.</div><div><em>Novelty and significance statement:</em> While hydrogen-ammonia blends are widely regarded as promising carbon-free fuels, important questions remain regarding their suitability for practical engine operation. Pure hydrogen flames, especially under fuel-lean conditions, are susceptible to cellular instabilities that induce intrinsic self-acceleration and thereby, elevated propagation speeds. This study demonstrates how the addition of ammonia to hydrogen modulates the transition between stable and unstable regimes.</div><div>While cellular instabilities are most often studied in freely expanding flames, we explore them in a burner-stabilized configuration. To the authors’ best knowledge, this is the first study to systematically investigate how varying the degree of ammonia blending in hydrogen affects the morphology, stability, and transition dynamics of polyhedral flames. In addition, the study also confirms that intense local stretch near the cusps of polyhedral ridges can trigger localized extinction. Beyond its fundamental significance, the experimental observations and accompanying scaling analysis offer a practical framework for identifying operating conditions that suppress such instabilities.</div></div>","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106087"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148582489","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-01-01Epub Date: 2026-07-20DOI: 10.1016/j.proci.2026.106249
Wendi Dong, Andrea Nobili, Hai Wang
Data-assimilated combustion reaction models must evolve as new fundamental combustion property data become available. This work introduces an adaptive learning framework, built on the Method of Uncertainty Minimization by Polynomial Chaos Expansions (MUM-PCE), that enables established data-assimilated models to efficiently incorporate additional data without requiring a complete re-optimization of the original database. The method is demonstrated by assimilating new hydrogen laminar flame speed measurements at elevated pressure and unburned gas temperature into the Foundational Fuel Chemistry Model version 2 (FFCM-2). Using the proposed adaptive optimization strategy, the updated model successfully reconciles the newly introduced data while remaining consistent with the rate parameter uncertainty space defined by the original FFCM-2 optimization. The approach also enables analysis of the effects of different inert flame diluents, revealing the degree of orthogonality and correlation among flame-speed measured with various diluents and among their corresponding third-body efficiencies. These results illustrate how adaptive learning can maintain model fidelity while progressively expanding the assimilated data base, leading to a more reliable reaction model for combustion simulations.
Novelty and significance statement: This work introduces an adaptive learning framework for updating data-assimilated combustion reaction models as new combustion property experiments become available. Unlike previous approaches that require re-optimizing against the entire experimental database, the proposed method efficiently assimilates new measurements while preserving the parameter uncertainty constraints established in the original optimization. Demonstrated using new hydrogen laminar flame speed data added to the FFCM-2 database, the framework maintains model consistency and reveals correlations among measurements with different diluents and their associated third-body efficiencies. The methodology provides a scalable strategy for continuously updating large kinetic models, enabling combustion reaction models to evolve systematically with emerging experimental data while retaining computational efficiency and predictive reliability.
{"title":"Adaptive learning of data assimilated combustion reaction model","authors":"Wendi Dong, Andrea Nobili, Hai Wang","doi":"10.1016/j.proci.2026.106249","DOIUrl":"10.1016/j.proci.2026.106249","url":null,"abstract":"<div><div>Data-assimilated combustion reaction models must evolve as new fundamental combustion property data become available. This work introduces an adaptive learning framework, built on the Method of Uncertainty Minimization by Polynomial Chaos Expansions (MUM-PCE), that enables established data-assimilated models to efficiently incorporate additional data without requiring a complete re-optimization of the original database. The method is demonstrated by assimilating new hydrogen laminar flame speed measurements at elevated pressure and unburned gas temperature into the Foundational Fuel Chemistry Model version 2 (FFCM-2). Using the proposed adaptive optimization strategy, the updated model successfully reconciles the newly introduced data while remaining consistent with the rate parameter uncertainty space defined by the original FFCM-2 optimization. The approach also enables analysis of the effects of different inert flame diluents, revealing the degree of orthogonality and correlation among flame-speed measured with various diluents and among their corresponding third-body efficiencies. These results illustrate how adaptive learning can maintain model fidelity while progressively expanding the assimilated data base, leading to a more reliable reaction model for combustion simulations.</div><div><em>Novelty and significance statement</em>: This work introduces an adaptive learning framework for updating data-assimilated combustion reaction models as new combustion property experiments become available. Unlike previous approaches that require re-optimizing against the entire experimental database, the proposed method efficiently assimilates new measurements while preserving the parameter uncertainty constraints established in the original optimization. Demonstrated using new hydrogen laminar flame speed data added to the FFCM-2 database, the framework maintains model consistency and reveals correlations among measurements with different diluents and their associated third-body efficiencies. The methodology provides a scalable strategy for continuously updating large kinetic models, enabling combustion reaction models to evolve systematically with emerging experimental data while retaining computational efficiency and predictive reliability.</div></div>","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106249"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148634405","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 : 2026-01-01Epub Date: 2026-07-29DOI: 10.1016/j.proci.2026.106293
Zachary T. Negrette, Mark Short, Scott I. Jackson
Flame dynamics in narrow channels are governed by thermal and momentum interactions at the walls, which become increasingly dominant as the surface-area-to-volume ratio increases. Understanding these near-quenching phenomena is critical for the development of small-scale combustion devices, such as microcombustors and flame arresters. This study investigates the influence of low-thermal-conductivity polyimide aerogel walls (Airloy H116) on freely propagating, constant-pressure ethylene–air flames, benchmarking the results against aluminum, PMMA, and soda-lime glass. Experiments were conducted in rectangular channels (25.4 mm depth, 305 mm length) with widths varying from 1.5 to 12.7 mm. High-speed imaging and synchronized pressure measurements revealed that while flame speeds were relatively insensitive to channel width for conventional materials above 4.7 mm, aerogel-confined flames exhibited a dramatic velocity enhancement in excess of 300%, just prior to the critical quenching width. This pronounced acceleration is attributed to the aerogel’s exceptionally low thermal effusivity, which elevates gas-wall interface temperatures and preheats the near-wall reactants. These findings demonstrate that strategic selection of wall materials can transform boundary layers from traditional loss mechanisms into combustion enhancers. Such extreme variation in flame speed has significant implications for the thermal management, efficiency, and safety parameters of microscale propulsion and power systems.
Novelty and significance statement: This study reports an unprecedented enhancement in flame propagation speeds near quenching limits, identifying a phenomenon unique to low-effusivity aerogel walls. We demonstrate that near-adiabatic boundary layers can fundamentally transform from a loss mechanism into a combustion enhancer. These findings provide a novel framework for “tuning” microscale devices, where millimeter-scale adjustments to channel geometry toggle between total flame extinction and peak thermal performance.
{"title":"Flame speed enhancement in narrow channels of low effusivity aerogel","authors":"Zachary T. Negrette, Mark Short, Scott I. Jackson","doi":"10.1016/j.proci.2026.106293","DOIUrl":"10.1016/j.proci.2026.106293","url":null,"abstract":"<div><div>Flame dynamics in narrow channels are governed by thermal and momentum interactions at the walls, which become increasingly dominant as the surface-area-to-volume ratio increases. Understanding these near-quenching phenomena is critical for the development of small-scale combustion devices, such as microcombustors and flame arresters. This study investigates the influence of low-thermal-conductivity polyimide aerogel walls (Airloy H116) on freely propagating, constant-pressure ethylene–air flames, benchmarking the results against aluminum, PMMA, and soda-lime glass. Experiments were conducted in rectangular channels (25.4 mm depth, 305 mm length) with widths varying from 1.5 to 12.7 mm. High-speed imaging and synchronized pressure measurements revealed that while flame speeds were relatively insensitive to channel width for conventional materials above 4.7 mm, aerogel-confined flames exhibited a dramatic velocity enhancement in excess of 300%, just prior to the critical quenching width. This pronounced acceleration is attributed to the aerogel’s exceptionally low thermal effusivity, which elevates gas-wall interface temperatures and preheats the near-wall reactants. These findings demonstrate that strategic selection of wall materials can transform boundary layers from traditional loss mechanisms into combustion enhancers. Such extreme variation in flame speed has significant implications for the thermal management, efficiency, and safety parameters of microscale propulsion and power systems.</div><div><em>Novelty and significance statement:</em> This study reports an unprecedented enhancement in flame propagation speeds near quenching limits, identifying a phenomenon unique to low-effusivity aerogel walls. We demonstrate that near-adiabatic boundary layers can fundamentally transform from a loss mechanism into a combustion enhancer. These findings provide a novel framework for “tuning” microscale devices, where millimeter-scale adjustments to channel geometry toggle between total flame extinction and peak thermal performance.</div></div>","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106293"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148656876","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-01-01Epub Date: 2026-09-12DOI: 10.1016/j.proci.2026.106377
Junfeng Bai, Lidong Zhang, Lei Zhou, Peng Zhang
The high-level ab initio chemical kinetics of N2H2/NNH with NO2 and OH and its application to modeling hypergolic ignition of the hydrazine-based fuels and NO2 system were investigated in this work. The CCSD(T)/CBS//M06-2X/6–311++G(d,p) single-reference method and the MRCI(7e,6o)/CBS//M06–2X/6–311++G(d,p) multi-reference method was employed for the calculations of the potential energy surfaces (PESs). The temperature- and pressure-dependent rate constants are determined by using the RRKM/Master Equation approach with the transition state theory (TST), and the variable reaction coordinate transition state theory (VRC-TST) for barrierless reactions, and the variational transition state theory (VTST) for the reactions with rather low barriers. The calculated rate constants and thermodynamics data were incorporated into the reaction mechanisms of bipropellant systems involving monomethylhydrazine(MMH) with nitrogen dioxide (NO2) and hydrazine (N2H4) with nitrogen dioxide (NO2) to evaluate their influence on modeling the hypergolic ignition of these systems. With the updated rate constants, the consumptions of MMH and NO2 were found to be highly promoted due to the fast consumption of N2H2. Important reaction pathways were identified by the rate of production and the sensitivity analyses. Moreover, the new N2H4/NO2 mechanism captures the gas-phase kinetic limits for the N2H4/NO2 system, although below 400 K, multiphase effects completely dominate.
Novelty and Significance Statement
This study presents the first high-level theoretical investigation of N2H2/NNH reactions with NO2 and OH radicals using both single-reference (CCSD(T)/CBS) and multi-reference (MRCI/CBS) methods coupled with variational transition state theory. The calculated accurate rate constants and thermochemistry enable critical updates to MMH/NO2 and N2H4/NO2 chemical mechanisms, substantially improving predictive capability for low-temperature hypergolic ignition. These findings resolve long-standing discrepancies in ignition simulations and provide the combustion community with essential kinetic parameters for hydrazine-based propulsion systems. The methodological framework underscores the importance of multi-reference calculations in complex radical reactions, establishing a foundation for optimizing bipropellant combustion efficiency and controlling ignition characteristics in aerospace applications.
{"title":"The role of ab-initio chemical kinetics of N2H2/NNH with NO2 and OH in gas-phase modeling hypergolic ignition of monomethylhydrazine/hydrazine and nitrogen dioxide","authors":"Junfeng Bai, Lidong Zhang, Lei Zhou, Peng Zhang","doi":"10.1016/j.proci.2026.106377","DOIUrl":"10.1016/j.proci.2026.106377","url":null,"abstract":"<div><div>The high-level ab initio chemical kinetics of N<sub>2</sub>H<sub>2</sub>/NNH with NO<sub>2</sub> and OH and its application to modeling hypergolic ignition of the hydrazine-based fuels and NO<sub>2</sub> system were investigated in this work. The CCSD(T)/CBS//M06-2X/6–311++G(d,p) single-reference method and the MRCI(7e,6o)/CBS//M06–2X/6–311++G(d,p) multi-reference method was employed for the calculations of the potential energy surfaces (PESs). The temperature- and pressure-dependent rate constants are determined by using the RRKM/Master Equation approach with the transition state theory (TST), and the variable reaction coordinate transition state theory (VRC-TST) for barrierless reactions, and the variational transition state theory (VTST) for the reactions with rather low barriers. The calculated rate constants and thermodynamics data were incorporated into the reaction mechanisms of bipropellant systems involving monomethylhydrazine(MMH) with nitrogen dioxide (NO<sub>2</sub>) and hydrazine (N<sub>2</sub>H<sub>4</sub>) with nitrogen dioxide (NO<sub>2</sub>) to evaluate their influence on modeling the hypergolic ignition of these systems. With the updated rate constants, the consumptions of MMH and NO<sub>2</sub> were found to be highly promoted due to the fast consumption of N<sub>2</sub>H<sub>2</sub>. Important reaction pathways were identified by the rate of production and the sensitivity analyses. Moreover, the new N<sub>2</sub>H<sub>4</sub>/NO<sub>2</sub> mechanism captures the gas-phase kinetic limits for the N<sub>2</sub>H<sub>4</sub>/NO<sub>2</sub> system, although below 400 K, multiphase effects completely dominate.</div><div><strong>Novelty and Significance Statement</strong></div><div>This study presents the first high-level theoretical investigation of N<sub>2</sub>H<sub>2</sub>/NNH reactions with NO<sub>2</sub> and OH radicals using both single-reference (CCSD(T)/CBS) and multi-reference (MRCI/CBS) methods coupled with variational transition state theory. The calculated accurate rate constants and thermochemistry enable critical updates to MMH/NO<sub>2</sub> and N<sub>2</sub>H<sub>4</sub>/NO<sub>2</sub> chemical mechanisms, substantially improving predictive capability for low-temperature hypergolic ignition. These findings resolve long-standing discrepancies in ignition simulations and provide the combustion community with essential kinetic parameters for hydrazine-based propulsion systems. The methodological framework underscores the importance of multi-reference calculations in complex radical reactions, establishing a foundation for optimizing bipropellant combustion efficiency and controlling ignition characteristics in aerospace applications.</div></div>","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106377"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148951579","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 : 2026-01-01Epub Date: 2026-09-16DOI: 10.1016/j.proci.2026.106450
Simone Vari, Sarah N. Elliott, Yuri Georgievskii, Stephen J. Klippenstein, Carlo Cavallotti
<div><div>Accurate determination of the rate coefficients and branching ratios for the various channels in a barrierless reaction process are critical for reliable mechanistic predictions, but they are often treated approximately due to several substantial challenges. The gold standard for computing rate constants of barrierless reactions is variable reaction coordinate transition state theory (VRC-TST), with accuracies approaching those of experimental measurements. However, the statistical assumptions that provide the basis for VRC-TST may not adequately describe certain dynamical effects, such as transition state recrossing fractions, especially for reactions with multiple bottlenecks, and roaming pathways. Although these processes can be adequately described with rigid-body trajectory simulations, doing so requires a sufficiently accurate potential energy surface (PES). Direct trajectory simulations come at a huge computational cost, requiring up to millions of single point energy evaluations and are thus unfeasible for the vast majority of chemical reactions. In this work we investigate whether it is possible to determine a PES suitable for trajectory simulations using a novel machine learning approach that exploits ab-initio calculations to feed a physics-inspired neural network (PHINN) architecture. The main result of this work is that the PHINN PES is sufficiently accurate not only to carry out VRC-TST simulations, but also to perform trajectory simulations, thus opening the path to the systematic use of trajectory simulations to study the kinetics of barrierless reactions. The developed framework is validated via a study of formaldehyde decomposition, providing roaming fractions in agreement with existing studies. After validation, the PHINN is employed to compute temperature-dependent recrossing coefficients for the recombination reactions between CH<sub>3</sub> + H and NH<sub>2</sub> + CH<sub>3</sub>, providing in the latter case a framework for validating the accuracy of two-state transition state theory. The developed computational approach is then used to study the long-range recombination roaming dynamics between H and phenoxy radical, providing site-specific branching fractions among the three possible addition sites.</div></div><div><h3>Novelty and significance statement</h3><div>In this work, we demonstrate for the first time that rate constants and roaming fluxes can be computed using trajectory simulations combined with neural network potential energy surfaces, enabling new approaches to reaction kinetics. We present a robust framework to (i) compute temperature-dependent recrossing coefficients, (ii) assess the accuracy of 2-state transition state theory, and (iii) determine roaming branching fractions, which are extremely difficult to evaluate accurately with existing methods. These advances are critical for improving the understanding of barrierless reactions, typically involving radical–radical recombination, which are pre
{"title":"Exploring long range dynamics using neural network potentials: From recrossing coefficients to roaming","authors":"Simone Vari, Sarah N. Elliott, Yuri Georgievskii, Stephen J. Klippenstein, Carlo Cavallotti","doi":"10.1016/j.proci.2026.106450","DOIUrl":"10.1016/j.proci.2026.106450","url":null,"abstract":"<div><div>Accurate determination of the rate coefficients and branching ratios for the various channels in a barrierless reaction process are critical for reliable mechanistic predictions, but they are often treated approximately due to several substantial challenges. The gold standard for computing rate constants of barrierless reactions is variable reaction coordinate transition state theory (VRC-TST), with accuracies approaching those of experimental measurements. However, the statistical assumptions that provide the basis for VRC-TST may not adequately describe certain dynamical effects, such as transition state recrossing fractions, especially for reactions with multiple bottlenecks, and roaming pathways. Although these processes can be adequately described with rigid-body trajectory simulations, doing so requires a sufficiently accurate potential energy surface (PES). Direct trajectory simulations come at a huge computational cost, requiring up to millions of single point energy evaluations and are thus unfeasible for the vast majority of chemical reactions. In this work we investigate whether it is possible to determine a PES suitable for trajectory simulations using a novel machine learning approach that exploits ab-initio calculations to feed a physics-inspired neural network (PHINN) architecture. The main result of this work is that the PHINN PES is sufficiently accurate not only to carry out VRC-TST simulations, but also to perform trajectory simulations, thus opening the path to the systematic use of trajectory simulations to study the kinetics of barrierless reactions. The developed framework is validated via a study of formaldehyde decomposition, providing roaming fractions in agreement with existing studies. After validation, the PHINN is employed to compute temperature-dependent recrossing coefficients for the recombination reactions between CH<sub>3</sub> + H and NH<sub>2</sub> + CH<sub>3</sub>, providing in the latter case a framework for validating the accuracy of two-state transition state theory. The developed computational approach is then used to study the long-range recombination roaming dynamics between H and phenoxy radical, providing site-specific branching fractions among the three possible addition sites.</div></div><div><h3>Novelty and significance statement</h3><div>In this work, we demonstrate for the first time that rate constants and roaming fluxes can be computed using trajectory simulations combined with neural network potential energy surfaces, enabling new approaches to reaction kinetics. We present a robust framework to (i) compute temperature-dependent recrossing coefficients, (ii) assess the accuracy of 2-state transition state theory, and (iii) determine roaming branching fractions, which are extremely difficult to evaluate accurately with existing methods. These advances are critical for improving the understanding of barrierless reactions, typically involving radical–radical recombination, which are pre","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106450"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148995089","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-01-01Epub Date: 2026-09-17DOI: 10.1016/j.proci.2026.106454
Malte Döntgen
<div><div>The present work investigates the reactions of chemically activated oxosilanol formed via the reduction of silicon dioxide (SiO<sub>2</sub>) with hydrogen (H<sub>2</sub>), focusing on its potential as a greener alternative to traditional carbon-based processes. The study highlights the theoretical framework for understanding the potential energy surface (PES) of oxosilanol and its kinetics, comparing it to the well-studied formic acid. It is found that the reaction pathway leading to silicon monoxide (SiO) and water (H<sub>2</sub>O) is significantly faster for oxosilanol than for formic acid, despite oxosilanol having a flipped order of reactivity. This is due to, first, a smaller barrier height of <span><math><mrow><msub><mrow><mtext>SiO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mo>+</mo><msub><mrow><mtext>H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow></math></span> compared to <span><math><mrow><msub><mrow><mtext>CO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mo>+</mo><msub><mrow><mtext>H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow></math></span> and, second, <span><math><mrow><msub><mrow><mtext>SiO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mo>+</mo><msub><mrow><mtext>H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow></math></span> being entropically favored and therefore populating higher energies when forming oxosilanol. The research further explores the impact of non-Boltzmann energy distributions on well-skipping, demonstrating how these distributions can enhance reactivity in low-pressure environments. Through rigorous computational methods, including Master Equation simulations, this study provides insights into optimizing hydrogen-based silica reduction processes and suggests potential temperature reductions of up to 240 K when utilizing non-Boltzmann effects. Compared to a thermally enhanced reactivity, i.e., by increasing the temperature, the selective depositing of energy through non-Boltzmann energy distributions gives up to five orders of magnitude stronger chemical activation of the intermediate, oxosilanol. These findings lay foundation for future investigations into sustainable silicon production techniques that could contribute to carbon-neutral technologies in chemical engineering and materials science.</div><div><em>Novelty and significance statement:</em> This work presents a novel investigation of reactions involving chemically activated oxosilanol, formed via the reduction of silicon dioxide (SiO<sub>2</sub>) with hydrogen (H<sub>2</sub>). By comparing its kinetics and potential energy surfaces to the carbon analog formic acid, significantly faster well-skipping to the corresponding monoxide and water is revealed. The impact of non-Boltzmann energy distributions is explored, demonstrating potential temperature reductions in hydrogen-based silica reduction. By highlighting the enhanced kinetics under non-thermal conditions, this study paves the way for alt
{"title":"Reactions of chemically activated oxosilanol formed via SiO2+H2","authors":"Malte Döntgen","doi":"10.1016/j.proci.2026.106454","DOIUrl":"10.1016/j.proci.2026.106454","url":null,"abstract":"<div><div>The present work investigates the reactions of chemically activated oxosilanol formed via the reduction of silicon dioxide (SiO<sub>2</sub>) with hydrogen (H<sub>2</sub>), focusing on its potential as a greener alternative to traditional carbon-based processes. The study highlights the theoretical framework for understanding the potential energy surface (PES) of oxosilanol and its kinetics, comparing it to the well-studied formic acid. It is found that the reaction pathway leading to silicon monoxide (SiO) and water (H<sub>2</sub>O) is significantly faster for oxosilanol than for formic acid, despite oxosilanol having a flipped order of reactivity. This is due to, first, a smaller barrier height of <span><math><mrow><msub><mrow><mtext>SiO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mo>+</mo><msub><mrow><mtext>H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow></math></span> compared to <span><math><mrow><msub><mrow><mtext>CO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mo>+</mo><msub><mrow><mtext>H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow></math></span> and, second, <span><math><mrow><msub><mrow><mtext>SiO</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><mo>+</mo><msub><mrow><mtext>H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow></math></span> being entropically favored and therefore populating higher energies when forming oxosilanol. The research further explores the impact of non-Boltzmann energy distributions on well-skipping, demonstrating how these distributions can enhance reactivity in low-pressure environments. Through rigorous computational methods, including Master Equation simulations, this study provides insights into optimizing hydrogen-based silica reduction processes and suggests potential temperature reductions of up to 240 K when utilizing non-Boltzmann effects. Compared to a thermally enhanced reactivity, i.e., by increasing the temperature, the selective depositing of energy through non-Boltzmann energy distributions gives up to five orders of magnitude stronger chemical activation of the intermediate, oxosilanol. These findings lay foundation for future investigations into sustainable silicon production techniques that could contribute to carbon-neutral technologies in chemical engineering and materials science.</div><div><em>Novelty and significance statement:</em> This work presents a novel investigation of reactions involving chemically activated oxosilanol, formed via the reduction of silicon dioxide (SiO<sub>2</sub>) with hydrogen (H<sub>2</sub>). By comparing its kinetics and potential energy surfaces to the carbon analog formic acid, significantly faster well-skipping to the corresponding monoxide and water is revealed. The impact of non-Boltzmann energy distributions is explored, demonstrating potential temperature reductions in hydrogen-based silica reduction. By highlighting the enhanced kinetics under non-thermal conditions, this study paves the way for alt","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106454"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"149003774","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}
This study aims to develop an early detection method for ignition based on the framework of bifurcation theory in dynamical systems. The proposed method exploits the characteristic feature of critical slowing down, namely the decrease in the recovery rate of temperature perturbations near the equilibrium between heat-release and heat-loss rates. A series of governing equations describing the temperature evolution near the ignition threshold was derived from the thermal balance, leading to a discrete-time representation of the Ornstein-Uhlenbeck (OU) process. However, the OU process assumes fluctuations around a fixed equilibrium point. In contrast, in the present system, the equilibrium temperature slowly evolves due to continuous heating. To account for this non-stationarity, we employed the measured temperature itself to compute the autocorrelation coefficient, standard deviation, and dynamic marker (DM). This formulation captures not only the local stochastic fluctuations but also the gradual drift of the equilibrium temperature. As ignition approached, the time evolution of the autocorrelation coefficient and standard deviation exhibited a transition from decreasing to increasing behavior in the ignition cases. This distinction became more pronounced when using the dynamic marker, defined as the product of the autocorrelation coefficient and the normalized standard deviation. The thresholds of DM, its temporal derivative, and the required number of consecutive detections were determined using receiver operating characteristic (ROC) analysis and probability density function (PDF) analysis. Using the optimized thresholds, ignition was detected with a lead time on the order of minutes before the actual event with an accuracy exceeding 90% for the present ignition problem. These results demonstrate that the proposed method provides sufficient lead time for early fire-response actions and offers a practical framework for ignition-risk mitigation in fire safety engineering.
Novelty and significance statement: This study presents a novel framework that treats ignition as a nonlinear bifurcation phenomenon. The key contribution of this work is the development of an early-ignition detection system based on early-warning signal (EWS) theory, implemented through statistical analysis of measured temperature data. Conventional EWS theory assumes fluctuations around a stationary equilibrium point and has therefore not been directly applicable to systems in which the equilibrium state slowly drifts. Extending the framework to accommodate such non-stationary behavior represents a significant theoretical advancement. The developed system provides a lead time on the order of minutes prior to ignition, offering sufficient time for initial fire-response actions. These results demonstrate the high practical significance and performance of the proposed ignition-detection method.
{"title":"A novel early detection method for ignition of flammable gases heated by a hot surface","authors":"Tomohiko Imamura, Keita Yamada, Jun-ichi Suematsu, Masato Nakazawa, Kazunori Kuwana","doi":"10.1016/j.proci.2026.106003","DOIUrl":"10.1016/j.proci.2026.106003","url":null,"abstract":"<div><div>This study aims to develop an early detection method for ignition based on the framework of bifurcation theory in dynamical systems. The proposed method exploits the characteristic feature of critical slowing down, namely the decrease in the recovery rate of temperature perturbations near the equilibrium between heat-release and heat-loss rates. A series of governing equations describing the temperature evolution near the ignition threshold was derived from the thermal balance, leading to a discrete-time representation of the Ornstein-Uhlenbeck (OU) process. However, the OU process assumes fluctuations around a fixed equilibrium point. In contrast, in the present system, the equilibrium temperature slowly evolves due to continuous heating. To account for this non-stationarity, we employed the measured temperature itself to compute the autocorrelation coefficient, standard deviation, and dynamic marker (DM). This formulation captures not only the local stochastic fluctuations but also the gradual drift of the equilibrium temperature. As ignition approached, the time evolution of the autocorrelation coefficient and standard deviation exhibited a transition from decreasing to increasing behavior in the ignition cases. This distinction became more pronounced when using the dynamic marker, defined as the product of the autocorrelation coefficient and the normalized standard deviation. The thresholds of DM, its temporal derivative, and the required number of consecutive detections were determined using receiver operating characteristic (ROC) analysis and probability density function (PDF) analysis. Using the optimized thresholds, ignition was detected with a lead time on the order of minutes before the actual event with an accuracy exceeding 90% for the present ignition problem. These results demonstrate that the proposed method provides sufficient lead time for early fire-response actions and offers a practical framework for ignition-risk mitigation in fire safety engineering.</div><div><strong>Novelty and significance statement:</strong> This study presents a novel framework that treats ignition as a nonlinear bifurcation phenomenon. The key contribution of this work is the development of an early-ignition detection system based on early-warning signal (EWS) theory, implemented through statistical analysis of measured temperature data. Conventional EWS theory assumes fluctuations around a stationary equilibrium point and has therefore not been directly applicable to systems in which the equilibrium state slowly drifts. Extending the framework to accommodate such non-stationary behavior represents a significant theoretical advancement. The developed system provides a lead time on the order of minutes prior to ignition, offering sufficient time for initial fire-response actions. These results demonstrate the high practical significance and performance of the proposed ignition-detection method.</div></div>","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106003"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148567012","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 : 2026-01-01Epub Date: 2026-06-26DOI: 10.1016/j.proci.2026.106098
Yuxuan Ma, Shiyu Ding, Zhengda Guo, Yan Gu, Longhua Hu
<div><div>Nitrogen (N₂), argon (Ar), and carbon dioxide (CO₂) are commonly used as diluent gases to reduce electrical fire risks in unattended environments and as clean agents in fire-suppression. This work investigated the limiting oxygen concentrations (LOCs) of low-density polyethylene (LDPE) electrical wires in N₂, Ar, and CO₂ diluted opposed flows. The results showed that CO₂ exhibited the strongest suppression effect, yielding the highest LOC, whereas Ar produced the lowest. Molten LDPE droplets formed and periodically detached from the wire surface, which may quench the flame on NiCr-core wires. The critical droplet volume, and the heat rate required to initiate droplet motion and prevent flame quenching, were quantified. A dual-criterion model was developed by coupling the stable solutions of the gas- and solid-phase energy balance equations. The gas criterion (LOC<sub>g</sub>) defines the minimum oxygen concentration required to sustain flame thermal equilibrium, while the solid criterion (LOC<sub>s</sub>) describes the balance among flame heat feedback, sensible heating, solid-surface re-radiation, and droplet cooling. The global limit is defined as LOC = max [LOC<sub>g</sub>, LOC<sub>s</sub>], and the omission of either criterion may cause errors. The predicted LOC curves and flame temperatures agreed well with experiments, validating both the overall model and its sub-models. Extinction-regime maps show that, under normal gravity, NiCr-core wires are mainly limited by droplet-induced quenching, while Cu-core wires are strongly affected by solid-surface re-radiation. Under microgravity, radiative losses become more important: NiCr-core wires shift from blowoff at high flow velocities to solid-surface re-radiation at low velocities, whereas Cu-core wires remain strongly influenced by solid-surface re-radiation over a broad velocity range. These findings provide a physics-based basis for improving flammability-limit prediction of solid materials, and for selecting and screening clean suppressants by linking each diluent gas to a distinct extinction pathway in normal- and microgravity.</div></div><div><h3>Novelty and significance statement</h3><div>This study establishes a dual-criterion model to predict the LOCs of electrical wires in diluent-gas environments. Unlike conventional scaling models based only on solid-phase energy balance, the model couples the stable solutions of gas- and solid-phase energy equations. It captures both flame thermal equilibrium and solid-surface heat balance, and introduces a thermal-inertia term to represent droplet-motion effects. Model parameters are trained and validated against experiments. The extinction-regime maps clearly show why classical models may miss the true minimum LOC (MLOC), where gas-phase radiation, flame stretch, and chemical kinetics decouple from the solid response. The model reproduces LOC trends across environments and guides fire-safety design for electrical and spacecraft systems.<
氮气（N₂）、氩气（Ar）和二氧化碳（CO₂）通常用作稀释气体，以减少无人值勤环境中的电气火灾风险，并在灭火中用作清洁剂。本文研究了低密度聚乙烯（LDPE）电线在N₂、Ar和CO₂稀释的反向流动中的极限氧浓度（LOCs）。结果表明，CO₂的抑制作用最强，LOC最高，Ar最低。熔融的LDPE液滴形成并周期性地从导线表面脱落，这可能会熄灭nicr芯线上的火焰。量化了临界液滴体积，以及启动液滴运动和防止火焰淬火所需的热速率。通过耦合气固两相能量平衡方程的稳定解，建立了双准则模型。气体判据（LOCg）定义了维持火焰热平衡所需的最低氧浓度，而固体判据（LOCs）描述了火焰热反馈、显热、固体表面再辐射和液滴冷却之间的平衡。全局限制定义为LOC = max [log, los]，遗漏任何一个标准都可能导致错误。预测的LOC曲线和火焰温度与实验结果吻合较好，验证了整体模型及其子模型的正确性。消光状态图显示，在正常重力下，nicr芯线主要受液滴诱导淬火的限制，而cu芯线则受固体表面再辐射的强烈影响。在微重力条件下，辐射损失变得更加重要：nicr芯导线从高流速下的吹散转向低速下的固体表面再辐射，而cu芯导线在较宽的流速范围内仍然受到固体表面再辐射的强烈影响。这些发现为改善固体材料的可燃性极限预测，以及通过将每个稀释气体连接到正常和微重力下不同的熄灭途径来选择和筛选清洁抑制剂提供了物理基础。新颖性和意义声明本研究建立了一种双准则模型来预测稀释气体环境下电线的loc。与传统的仅基于固相能量平衡的标度模型不同，该模型耦合了气相和固相能量方程的稳定解。它捕获了火焰热平衡和固体表面热平衡，并引入了热惯性项来表示液滴运动效应。对模型参数进行了训练，并通过实验进行了验证。消光状态图清楚地显示了为什么经典模型可能会错过真正的最小LOC (MLOC)，其中气相辐射，火焰拉伸和化学动力学与固体响应分离。该模型再现了各种环境的LOC趋势，并指导电气和航天器系统的防火安全设计。
{"title":"A dual-criterion model for predicting LDPE electrical wire flammability limits in diluent gas environments","authors":"Yuxuan Ma, Shiyu Ding, Zhengda Guo, Yan Gu, Longhua Hu","doi":"10.1016/j.proci.2026.106098","DOIUrl":"10.1016/j.proci.2026.106098","url":null,"abstract":"<div><div>Nitrogen (N₂), argon (Ar), and carbon dioxide (CO₂) are commonly used as diluent gases to reduce electrical fire risks in unattended environments and as clean agents in fire-suppression. This work investigated the limiting oxygen concentrations (LOCs) of low-density polyethylene (LDPE) electrical wires in N₂, Ar, and CO₂ diluted opposed flows. The results showed that CO₂ exhibited the strongest suppression effect, yielding the highest LOC, whereas Ar produced the lowest. Molten LDPE droplets formed and periodically detached from the wire surface, which may quench the flame on NiCr-core wires. The critical droplet volume, and the heat rate required to initiate droplet motion and prevent flame quenching, were quantified. A dual-criterion model was developed by coupling the stable solutions of the gas- and solid-phase energy balance equations. The gas criterion (LOC<sub>g</sub>) defines the minimum oxygen concentration required to sustain flame thermal equilibrium, while the solid criterion (LOC<sub>s</sub>) describes the balance among flame heat feedback, sensible heating, solid-surface re-radiation, and droplet cooling. The global limit is defined as LOC = max [LOC<sub>g</sub>, LOC<sub>s</sub>], and the omission of either criterion may cause errors. The predicted LOC curves and flame temperatures agreed well with experiments, validating both the overall model and its sub-models. Extinction-regime maps show that, under normal gravity, NiCr-core wires are mainly limited by droplet-induced quenching, while Cu-core wires are strongly affected by solid-surface re-radiation. Under microgravity, radiative losses become more important: NiCr-core wires shift from blowoff at high flow velocities to solid-surface re-radiation at low velocities, whereas Cu-core wires remain strongly influenced by solid-surface re-radiation over a broad velocity range. These findings provide a physics-based basis for improving flammability-limit prediction of solid materials, and for selecting and screening clean suppressants by linking each diluent gas to a distinct extinction pathway in normal- and microgravity.</div></div><div><h3>Novelty and significance statement</h3><div>This study establishes a dual-criterion model to predict the LOCs of electrical wires in diluent-gas environments. Unlike conventional scaling models based only on solid-phase energy balance, the model couples the stable solutions of gas- and solid-phase energy equations. It captures both flame thermal equilibrium and solid-surface heat balance, and introduces a thermal-inertia term to represent droplet-motion effects. Model parameters are trained and validated against experiments. The extinction-regime maps clearly show why classical models may miss the true minimum LOC (MLOC), where gas-phase radiation, flame stretch, and chemical kinetics decouple from the solid response. The model reproduces LOC trends across environments and guides fire-safety design for electrical and spacecraft systems.<","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106098"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148567184","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-01-01Epub Date: 2026-06-19DOI: 10.1016/j.proci.2026.106042
Zhaofan Zhu, Haiou Wang, Evatt R. Hawkes, Shibo Gu, Haiwang Li, Kun Luo, Jianren Fan
<div><div>Accurately predicting the boundary layer flashback limit is crucial for the safety of combustion systems. In the present study, two-dimensional numerical simulations of laminar boundary layer flashback are conducted for a series of <span><math><msub><mrow><mi>NH</mi></mrow><mrow><mn>3</mn></mrow></msub></math></span>/<span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span>/air mixtures with varying equivalence ratios and hydrogen blending ratios. The critical velocity gradient for flashback is efficiently determined using an inlet velocity gradient active-control approach, which significantly reduces the computational cost compared to previous methods. The flow and flame characteristics at the flashback limit are examined. It is observed that the streamwise velocity upstream of the flame decreases due to the combustion-induced local pressure rise. The streamlines bend upward ahead of the flame under the influence of the adverse pressure gradient. In the cases with large penetration distances, the streamlines noticeably bend downward within the penetration distance as they approach the flame, further amplifying the reduction in streamwise velocity. The flame speed at the penetration distance is influenced by differential diffusion, flame stretch, and wall heat loss. The discrepancy between the flame speed at the penetration distance and the flame speed predicted by the linear stretch model is found to be negatively correlated with the ratio of the penetration distance to the flame diffusion thickness, <em>i.e</em>, <span><math><mrow><msub><mrow><mi>δ</mi></mrow><mrow><mi>p</mi></mrow></msub><mo>/</mo><msub><mrow><mi>δ</mi></mrow><mrow><mi>D</mi></mrow></msub></mrow></math></span>. An improved critical velocity gradient model is therefore proposed, which accounts for the flow velocity reduction ahead of the flame, and the variation in flame speed at the flame leading point. Compared to the classical criterion, the proposed model significantly improves the accuracy of flashback limit prediction.</div><div><em>Novelty and significance statement:</em> Accurately predicting the boundary layer flashback is a critical challenge in the combustion applications of <span><math><msub><mrow><mi>NH</mi></mrow><mrow><mn>3</mn></mrow></msub></math></span>/<span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span> mixtures. This study investigates the boundary layer flashback limit for a series of laminar <span><math><msub><mrow><mi>NH</mi></mrow><mrow><mn>3</mn></mrow></msub></math></span>/<span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span>/air flames with varying equivalence ratios and <span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span> blending ratios. An inlet velocity gradient active-control approach is proposed to obtain the critical velocity gradient for flashback, significantly reducing the computational cost compared to pre
{"title":"An improved critical velocity gradient model for laminar boundary layer flashback of NH3/H2/air flames","authors":"Zhaofan Zhu, Haiou Wang, Evatt R. Hawkes, Shibo Gu, Haiwang Li, Kun Luo, Jianren Fan","doi":"10.1016/j.proci.2026.106042","DOIUrl":"10.1016/j.proci.2026.106042","url":null,"abstract":"<div><div>Accurately predicting the boundary layer flashback limit is crucial for the safety of combustion systems. In the present study, two-dimensional numerical simulations of laminar boundary layer flashback are conducted for a series of <span><math><msub><mrow><mi>NH</mi></mrow><mrow><mn>3</mn></mrow></msub></math></span>/<span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span>/air mixtures with varying equivalence ratios and hydrogen blending ratios. The critical velocity gradient for flashback is efficiently determined using an inlet velocity gradient active-control approach, which significantly reduces the computational cost compared to previous methods. The flow and flame characteristics at the flashback limit are examined. It is observed that the streamwise velocity upstream of the flame decreases due to the combustion-induced local pressure rise. The streamlines bend upward ahead of the flame under the influence of the adverse pressure gradient. In the cases with large penetration distances, the streamlines noticeably bend downward within the penetration distance as they approach the flame, further amplifying the reduction in streamwise velocity. The flame speed at the penetration distance is influenced by differential diffusion, flame stretch, and wall heat loss. The discrepancy between the flame speed at the penetration distance and the flame speed predicted by the linear stretch model is found to be negatively correlated with the ratio of the penetration distance to the flame diffusion thickness, <em>i.e</em>, <span><math><mrow><msub><mrow><mi>δ</mi></mrow><mrow><mi>p</mi></mrow></msub><mo>/</mo><msub><mrow><mi>δ</mi></mrow><mrow><mi>D</mi></mrow></msub></mrow></math></span>. An improved critical velocity gradient model is therefore proposed, which accounts for the flow velocity reduction ahead of the flame, and the variation in flame speed at the flame leading point. Compared to the classical criterion, the proposed model significantly improves the accuracy of flashback limit prediction.</div><div><em>Novelty and significance statement:</em> Accurately predicting the boundary layer flashback is a critical challenge in the combustion applications of <span><math><msub><mrow><mi>NH</mi></mrow><mrow><mn>3</mn></mrow></msub></math></span>/<span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span> mixtures. This study investigates the boundary layer flashback limit for a series of laminar <span><math><msub><mrow><mi>NH</mi></mrow><mrow><mn>3</mn></mrow></msub></math></span>/<span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span>/air flames with varying equivalence ratios and <span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub></math></span> blending ratios. An inlet velocity gradient active-control approach is proposed to obtain the critical velocity gradient for flashback, significantly reducing the computational cost compared to pre","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106042"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148567574","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 : 2026-01-01Epub Date: 2026-06-19DOI: 10.1016/j.proci.2026.106020
Yupan Bao, Liang Ji, Raphael Vogeley, Kaii Ri, Wenbin Xu, Zhiyuan Zhang, Yijie Xu, Bowen Mei, Huixin He, Christian Brackmann, Tao Li, Yiguang Ju
Surface nitridation induced by ammonia combustion can alter material properties, reducing lifetime and performance of a practical combustor. Although ammonia-induced surface nitriding has been studied at low and moderate temperatures, its behavior at temperatures above 1200 K in flames remains unexplored. This study investigates high-temperature nitriding and the impact of NH radical of nickel rods exposed to NH3/H2 flames above 1500 K. The presence of abundant radicals and complex chemical reactions in NH3/H2 flames, together with flame–wall interactions between reactive gases and metal surfaces, impose great challenges in understanding the underlying nitridation mechanism. In this work, gas-phase characterization was performed using in-situ planar laser-induced fluorescence (PLIF) for NH radical distribution and Rayleigh scattering for temperature, while surface morphology and chemical composition were analyzed by scanning electron microscopy (SEM) and high-resolution X-ray photoelectron spectroscopy (HRXPS). The results show that nickel nitridation can occur within minutes of flame exposure and is strongly dependent on NH concentration at a same flame temperature. In addition, no distinct nitride layer was observed after more than one hour of flame exposure, while pore structures beneath the nickel surface were detected using ion-beam cutting under high vacuum. These observations suggest that high-temperature nitridation under ammonia combustion conditions may proceed through mechanisms that differ from those typically reported at lower temperatures.
Novelty and significance statement
This study presents the first high-temperature experimental investigation of nickel nitridation under ammonia/hydrogen combustion conditions exceeding 1500 K, moving beyond the moderate-temperature (<1200 K) and non-flame studies that dominate existing literature. By combining operando planar laser-induced fluorescence (PLIF) for NH radicals with ex-situ high-resolution X-ray photoelectron spectroscopy (HRXPS) and scanning electron microscopy (SEM) analysis, the study directly links radical chemistry in the reacting flow to surface phase evolution and microstructural transformation. The findings, characterized by rapid nitride formation, radical-mediated nitrogen incorporation, and lattice instability, highlight that the flame-wall interactions at 1600 K are different from low and moderate temperatures. These insights are essential for predicting material degradation in ammonia-based energy systems and for guiding the design of high-temperature metal components with improved resistance to nitridation and/or oxidation.
{"title":"Surface nitridation of nickel by NH3/H2 rich flames at elevated temperature","authors":"Yupan Bao, Liang Ji, Raphael Vogeley, Kaii Ri, Wenbin Xu, Zhiyuan Zhang, Yijie Xu, Bowen Mei, Huixin He, Christian Brackmann, Tao Li, Yiguang Ju","doi":"10.1016/j.proci.2026.106020","DOIUrl":"10.1016/j.proci.2026.106020","url":null,"abstract":"<div><div>Surface nitridation induced by ammonia combustion can alter material properties, reducing lifetime and performance of a practical combustor. Although ammonia-induced surface nitriding has been studied at low and moderate temperatures, its behavior at temperatures above 1200 K in flames remains unexplored. This study investigates high-temperature nitriding and the impact of NH radical of nickel rods exposed to NH<sub>3</sub>/H<sub>2</sub> flames above 1500 K. The presence of abundant radicals and complex chemical reactions in NH<sub>3</sub>/H<sub>2</sub> flames, together with flame–wall interactions between reactive gases and metal surfaces, impose great challenges in understanding the underlying nitridation mechanism. In this work, gas-phase characterization was performed using in-situ planar laser-induced fluorescence (PLIF) for NH radical distribution and Rayleigh scattering for temperature, while surface morphology and chemical composition were analyzed by scanning electron microscopy (SEM) and high-resolution X-ray photoelectron spectroscopy (HRXPS). The results show that nickel nitridation can occur within minutes of flame exposure and is strongly dependent on NH concentration at a same flame temperature. In addition, no distinct nitride layer was observed after more than one hour of flame exposure, while pore structures beneath the nickel surface were detected using ion-beam cutting under high vacuum. These observations suggest that high-temperature nitridation under ammonia combustion conditions may proceed through mechanisms that differ from those typically reported at lower temperatures.</div></div><div><h3>Novelty and significance statement</h3><div>This study presents the first high-temperature experimental investigation of nickel nitridation under ammonia/hydrogen combustion conditions exceeding 1500 K, moving beyond the moderate-temperature (<1200 K) and non-flame studies that dominate existing literature. By combining operando planar laser-induced fluorescence (PLIF) for NH radicals with ex-situ high-resolution X-ray photoelectron spectroscopy (HRXPS) and scanning electron microscopy (SEM) analysis, the study directly links radical chemistry in the reacting flow to surface phase evolution and microstructural transformation. The findings, characterized by rapid nitride formation, radical-mediated nitrogen incorporation, and lattice instability, highlight that the flame-wall interactions at 1600 K are different from low and moderate temperatures. These insights are essential for predicting material degradation in ammonia-based energy systems and for guiding the design of high-temperature metal components with improved resistance to nitridation and/or oxidation.</div></div>","PeriodicalId":408,"journal":{"name":"Proceedings of the Combustion Institute","volume":"42 ","pages":"Article 106020"},"PeriodicalIF":4.6,"publicationDate":"2026-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148567521","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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