{"title":"Numerical studies on intrinsically unstable H2/CO2 flames: Effect of CO2 dilution, equivalence ratio, temperature and pressure","authors":"Mayank Pandey, Krishnakant Agrawal, Anjan Ray","doi":"10.1016/j.combustflame.2025.114307","DOIUrl":null,"url":null,"abstract":"<div><div>Direct combustion of H<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span>/CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span> gas mixtures obtained from the gasification and steam reforming processes can help avoid CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span> separation costs and safety concerns associated with the fast combustion of pure hydrogen as a fuel. This work studies the effect of varying CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span> concentrations in hydrogen-air flames through detailed numerical simulations (DNS) as well as canonical flame calculations. The species transport budget calculated using one-dimensional freely propagating flames shows dominance of diffusion against convection with increase in CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span> percentage. Similarly, an increase in reaction rate and negative Markstein length is observed for flames under strain, suggesting enhanced thermodiffusive response. Quantification of thermodiffusive instability in linear and non-linear flame propagation regimes has been performed for CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span>-rich hydrogen-air mixtures using two-dimensional freely propagating flames in a classical inflow, outflow and periodic domain for a range of equivalence ratios (0.8–1.1), unburned temperatures (300 K–700 K) and pressures (1 atm–8 atm). The growth rate of perturbation amplitude increases with decreasing the equivalence ratio and temperature, and increases with CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span> dilution and pressure, enhancing intrinsic instability. The flame propagates with a finger-like structure with increasing propensity of subadiabatic and superadiabatic regions for mixtures corresponding to higher growth rates. These findings will further the understanding of burning characteristics of H<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span>/CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span>/air mixtures when used in practical combustion devices.</div><div><strong>Novelty and significance statement</strong></div><div>Direct combustion of H<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span>/CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span> fuel mixtures can provide different low-cost pathways for grey hydrogen utilization. Such mixtures are known to exhibit intrinsic flame instabilities, especially of a thermodiffusive nature. This work presents a one-dimensional assessment of the convective, diffusive, reactive balance of H<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span>/CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span> fuel-air flames and their response to flow-stretch, which confirms the sensitivity of their thermodiffusive nature towards instabilities. The main novelty lies in the demonstration of thermodiffusive instabilities in H<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span>/CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span> fuel-air mixtures using two dimensional direct numerical simulations (DNS) and assessing sensitivities of instability growth rates and overall consumption speed to mixture equivalence ratio, unburnt temperature and pressure. Further, the relative impact of flame wrinkling and local differential diffusion on consumption speed enhancement for such mixtures has been quantified for the first time. Such insights make this study significant for understanding the combustion of H<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span>/CO<span><math><msub><mrow></mrow><mrow><mn>2</mn></mrow></msub></math></span>/air mixture when used in practical applications.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"279 ","pages":"Article 114307"},"PeriodicalIF":5.8000,"publicationDate":"2025-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Combustion and Flame","FirstCategoryId":"5","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0010218025003451","RegionNum":2,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"ENERGY & FUELS","Score":null,"Total":0}
引用次数: 0
Abstract
Direct combustion of H/CO gas mixtures obtained from the gasification and steam reforming processes can help avoid CO separation costs and safety concerns associated with the fast combustion of pure hydrogen as a fuel. This work studies the effect of varying CO concentrations in hydrogen-air flames through detailed numerical simulations (DNS) as well as canonical flame calculations. The species transport budget calculated using one-dimensional freely propagating flames shows dominance of diffusion against convection with increase in CO percentage. Similarly, an increase in reaction rate and negative Markstein length is observed for flames under strain, suggesting enhanced thermodiffusive response. Quantification of thermodiffusive instability in linear and non-linear flame propagation regimes has been performed for CO-rich hydrogen-air mixtures using two-dimensional freely propagating flames in a classical inflow, outflow and periodic domain for a range of equivalence ratios (0.8–1.1), unburned temperatures (300 K–700 K) and pressures (1 atm–8 atm). The growth rate of perturbation amplitude increases with decreasing the equivalence ratio and temperature, and increases with CO dilution and pressure, enhancing intrinsic instability. The flame propagates with a finger-like structure with increasing propensity of subadiabatic and superadiabatic regions for mixtures corresponding to higher growth rates. These findings will further the understanding of burning characteristics of H/CO/air mixtures when used in practical combustion devices.
Novelty and significance statement
Direct combustion of H/CO fuel mixtures can provide different low-cost pathways for grey hydrogen utilization. Such mixtures are known to exhibit intrinsic flame instabilities, especially of a thermodiffusive nature. This work presents a one-dimensional assessment of the convective, diffusive, reactive balance of H/CO fuel-air flames and their response to flow-stretch, which confirms the sensitivity of their thermodiffusive nature towards instabilities. The main novelty lies in the demonstration of thermodiffusive instabilities in H/CO fuel-air mixtures using two dimensional direct numerical simulations (DNS) and assessing sensitivities of instability growth rates and overall consumption speed to mixture equivalence ratio, unburnt temperature and pressure. Further, the relative impact of flame wrinkling and local differential diffusion on consumption speed enhancement for such mixtures has been quantified for the first time. Such insights make this study significant for understanding the combustion of H/CO/air mixture when used in practical applications.
期刊介绍:
The mission of the journal is to publish high quality work from experimental, theoretical, and computational investigations on the fundamentals of combustion phenomena and closely allied matters. While submissions in all pertinent areas are welcomed, past and recent focus of the journal has been on:
Development and validation of reaction kinetics, reduction of reaction mechanisms and modeling of combustion systems, including:
Conventional, alternative and surrogate fuels;
Pollutants;
Particulate and aerosol formation and abatement;
Heterogeneous processes.
Experimental, theoretical, and computational studies of laminar and turbulent combustion phenomena, including:
Premixed and non-premixed flames;
Ignition and extinction phenomena;
Flame propagation;
Flame structure;
Instabilities and swirl;
Flame spread;
Multi-phase reactants.
Advances in diagnostic and computational methods in combustion, including:
Measurement and simulation of scalar and vector properties;
Novel techniques;
State-of-the art applications.
Fundamental investigations of combustion technologies and systems, including:
Internal combustion engines;
Gas turbines;
Small- and large-scale stationary combustion and power generation;
Catalytic combustion;
Combustion synthesis;
Combustion under extreme conditions;
New concepts.