Pub Date : 2025-02-01DOI: 10.1016/j.combustflame.2024.113901
Jiasen Wei, Alessandro Bottaro, Jan O. Pralits
Clean combustion, particularly premixed hydrogen combustion aimed at reducing NOx emissions, is prone to thermoacoustic instabilities that can cause structural vibrations and equipment failures. This study focuses on a low-order model for a thermoacoustic prototype, a simple quasi-one-dimensional combustor comprising a plenum, premixing duct, and combustion chamber. Resonant modes of the combustor are identified by solving a nonlinear eigenvalue problem. Using an adjoint-based sensitivity analysis, the impact of uncertainties in base flow parameters on resonant frequencies and linear growth rates is assessed. The results obtained highlight the significant influence of variations in cold gas density within the plenum and premixing duct on the linear growth rates, potentially explaining discrepancies with literature data. Additionally, structural sensitivities in both the base and the perturbation flow are examined to evaluate the effects of a generic feedback mechanism on the eigenvalues. Structural sensitivities at the base-flow level are evaluated as a function of the flame position, identifying effective stabilizing mechanisms such as heat addition and mass flow rate reduction at duct intersections. The most stabilizing feedback mechanism is identified as mass fluctuations proportional to pressure perturbation at the end of the plenum, an effect achievable with Helmholtz resonators. Adjoint analyses permit uncertainty quantification of base-state parameters and gradient information for optimization strategies aimed at mitigating thermoacoustic instabilities through efficient and low-cost calculations.
Novelty and significance statement
The novelty of this research lies in its development of a comprehensive adjoint analysis framework for three types of sensitivity analyses within a thermoacoustic premixed combustor model. This paper uses base-state sensitivity to quantify the significant effect of base flow uncertainties, such as cold gas properties in the premixer, on the unstable resonant mode growth rates. In addition to structural perturbation sensitivity analysis, it uniquely applies structural sensitivity to base flow modifications, uncovering effective steady control mechanisms like mass suction and heating. The findings identify efficient approaches to mitigate thermoacoustic instabilities in premixed combustion systems and broaden the scope of potential control strategies.
{"title":"Adjoint-based mean-flow uncertainty and feedback-forcing analyses of a thermoacoustic model system","authors":"Jiasen Wei, Alessandro Bottaro, Jan O. Pralits","doi":"10.1016/j.combustflame.2024.113901","DOIUrl":"10.1016/j.combustflame.2024.113901","url":null,"abstract":"<div><div>Clean combustion, particularly premixed hydrogen combustion aimed at reducing NOx emissions, is prone to thermoacoustic instabilities that can cause structural vibrations and equipment failures. This study focuses on a low-order model for a thermoacoustic prototype, a simple quasi-one-dimensional combustor comprising a plenum, premixing duct, and combustion chamber. Resonant modes of the combustor are identified by solving a nonlinear eigenvalue problem. Using an adjoint-based sensitivity analysis, the impact of uncertainties in base flow parameters on resonant frequencies and linear growth rates is assessed. The results obtained highlight the significant influence of variations in cold gas density within the plenum and premixing duct on the linear growth rates, potentially explaining discrepancies with literature data. Additionally, structural sensitivities in both the base and the perturbation flow are examined to evaluate the effects of a generic feedback mechanism on the eigenvalues. Structural sensitivities at the base-flow level are evaluated as a function of the flame position, identifying effective stabilizing mechanisms such as heat addition and mass flow rate reduction at duct intersections. The most stabilizing feedback mechanism is identified as mass fluctuations proportional to pressure perturbation at the end of the plenum, an effect achievable with Helmholtz resonators. Adjoint analyses permit uncertainty quantification of base-state parameters and gradient information for optimization strategies aimed at mitigating thermoacoustic instabilities through efficient and low-cost calculations.</div><div><strong>Novelty and significance statement</strong></div><div>The novelty of this research lies in its development of a comprehensive adjoint analysis framework for three types of sensitivity analyses within a thermoacoustic premixed combustor model. This paper uses base-state sensitivity to quantify the significant effect of base flow uncertainties, such as cold gas properties in the premixer, on the unstable resonant mode growth rates. In addition to structural perturbation sensitivity analysis, it uniquely applies structural sensitivity to base flow modifications, uncovering effective steady control mechanisms like mass suction and heating. The findings identify efficient approaches to mitigate thermoacoustic instabilities in premixed combustion systems and broaden the scope of potential control strategies.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"272 ","pages":"Article 113901"},"PeriodicalIF":5.8,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143102611","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 : 2025-02-01DOI: 10.1016/j.combustflame.2024.113905
Siqi Cai, Wenquan Yang, Jianlong Wan
The lean premixed combustion LPC can achieve clean combustion of natural gas and reduce emissions of harmful gases. However, this combustion mode is usually difficult to sustain. To improve the anchoring performance of lean premixed flame LPF, the high-temperature bluff-body HTB with 900 K is employed in this work. Unexpectedly, a stable residual flame of the methane-air ultra-lean premixed mixture is found experimentally and computationally near the blow-off limit. A deeper insight into its anchoring mechanism is necessary to further promote the LPF stability. At first, the residual flame structure is revealed quantitatively, and it is found that the diffusion dominates the reactant flux which arrives at the residual flame rather than the convection. Then, the anchoring mechanism of the ultra-lean residual flame is revealed in terms of the effects of the preferential transport, stretch, and conjugate heat exchange. The recirculation zone right behind the HTB provides a good anchoring location for the residual flame base. The small value of the stretch rate contributes to the residence of the residual flame tip. The enhanced preferential transport effect by the HTB contributes to maintaining the residual flame by generating a relatively fuel-richer region compared with the incoming fresh mixture around it. In addition, the enhanced pre-heated fresh reactants by the HTB provide good ignition and combustion conditions around the residual flame, which contributes to its residence. To the best of our knowledge, such a detailed visualization of the main factors responsible for anchoring the residual flame stabilized by the HTB has not been reported yet. This study provides a new scheme to improve LPC performance. This study expands our understanding of the LPF dynamics stabilized by the bluff-body.
{"title":"Insight into the ultra-lean residual flame stabilized on a high-temperature bluff-body","authors":"Siqi Cai, Wenquan Yang, Jianlong Wan","doi":"10.1016/j.combustflame.2024.113905","DOIUrl":"10.1016/j.combustflame.2024.113905","url":null,"abstract":"<div><div>The lean premixed combustion LPC can achieve clean combustion of natural gas and reduce emissions of harmful gases. However, this combustion mode is usually difficult to sustain. To improve the anchoring performance of lean premixed flame LPF, the high-temperature bluff-body HTB with 900 K is employed in this work. Unexpectedly, a stable residual flame of the methane-air ultra-lean premixed mixture is found experimentally and computationally near the blow-off limit. A deeper insight into its anchoring mechanism is necessary to further promote the LPF stability. At first, the residual flame structure is revealed quantitatively, and it is found that the diffusion dominates the reactant flux which arrives at the residual flame rather than the convection. Then, the anchoring mechanism of the ultra-lean residual flame is revealed in terms of the effects of the preferential transport, stretch, and conjugate heat exchange. The recirculation zone right behind the HTB provides a good anchoring location for the residual flame base. The small value of the stretch rate contributes to the residence of the residual flame tip. The enhanced preferential transport effect by the HTB contributes to maintaining the residual flame by generating a relatively fuel-richer region compared with the incoming fresh mixture around it. In addition, the enhanced pre-heated fresh reactants by the HTB provide good ignition and combustion conditions around the residual flame, which contributes to its residence. To the best of our knowledge, such a detailed visualization of the main factors responsible for anchoring the residual flame stabilized by the HTB has not been reported yet. This study provides a new scheme to improve LPC performance. This study expands our understanding of the LPF dynamics stabilized by the bluff-body.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"272 ","pages":"Article 113905"},"PeriodicalIF":5.8,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143102614","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 : 2025-02-01DOI: 10.1016/j.combustflame.2024.113902
Qianlong Wang , Siming Xiong , Zilin Deng , Guillaume legros , Haifeng Liu , Zibang Zhang
This paper initially utilizes a Fourier single-pixel imaging (FSI) optical method to measure the soot volume fraction () field in flames, which is based on the theorems of Fourier transform and Beer-Lambert law. Unlike the conventional two-dimensional sensor used for imaging, a spatially unresolvable detector, such as a photomultiplier tube (PMT), is utilized to reconstruct flame images. The current prototype optical measurement system is detailed and further validated by a proof-of-concept experiment on a benchmark laminar diffusion flame. It is found that the sample rate and the loop number significantly affect the quality of flame image reconstruction, and it is recommended to use thresholds of 25 % and 20 for these two parameters. In addition, the maximum standard deviation of 0.025, as calculated through error propagation in five repeated experiments, demonstrates the robustness of the FSI technique. Nevertheless, the present optical layout could be further optimized in terms of improving the quality of reconstructed images, shortening the sampling duration, and replacing the near-infrared light source to achieve more precise distributions and reduce uncertainties. Moreover, the potential for single-shot resolution improvements deserves further investigation of temporal flame measurements in the near future.
{"title":"A novel single-pixel imaging method for two-dimensional soot volume fraction measurements in axisymmetric flames","authors":"Qianlong Wang , Siming Xiong , Zilin Deng , Guillaume legros , Haifeng Liu , Zibang Zhang","doi":"10.1016/j.combustflame.2024.113902","DOIUrl":"10.1016/j.combustflame.2024.113902","url":null,"abstract":"<div><div>This paper initially utilizes a Fourier single-pixel imaging (FSI) optical method to measure the soot volume fraction (<span><math><msub><mi>f</mi><mi>v</mi></msub></math></span>) field in flames, which is based on the theorems of Fourier transform and Beer-Lambert law. Unlike the conventional two-dimensional sensor used for imaging, a spatially unresolvable detector, such as a photomultiplier tube (PMT), is utilized to reconstruct flame images. The current prototype optical measurement system is detailed and further validated by a proof-of-concept experiment on a benchmark laminar diffusion flame. It is found that the sample rate and the loop number significantly affect the quality of flame image reconstruction, and it is recommended to use thresholds of 25 % and 20 for these two parameters. In addition, the maximum standard deviation <span><math><msub><mi>f</mi><mi>v</mi></msub></math></span> of 0.025, as calculated through error propagation in five repeated experiments, demonstrates the robustness of the FSI technique. Nevertheless, the present optical layout could be further optimized in terms of improving the quality of reconstructed images, shortening the sampling duration, and replacing the near-infrared light source to achieve more precise <span><math><msub><mi>f</mi><mi>v</mi></msub></math></span> distributions and reduce uncertainties. Moreover, the potential for single-shot resolution improvements deserves further investigation of temporal flame measurements in the near future.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"272 ","pages":"Article 113902"},"PeriodicalIF":5.8,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143102615","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}
Measuring the ignition temperature of micron-sized iron particles can verify the ignition mechanism and aid in designing efficient iron powder combustion devices. This study captured the diameter, morphology and ignition status of iron particles with diameter range from 17 to 45 μm entering a stable high-temperature environment by high-speed cameras. The ignition frequency of iron particles at different ambient temperatures and oxygen concentration were recorded. Defining the ignition temperature as the ambient temperature at which the ignition frequency of iron particles exceeds 0.9, it was found that the ignition temperature of iron particles heated from room temperature and closer to a spherical shape is approximately 1140 K, while the non-spherical iron particles is around 1120 K. The ignition temperature is independent of particle diameter and ambient oxygen concentration. The theoretical method for estimating ignition temperature (X.C. Mi, A. Fujinawa, J.M. Bergthorson, 2022) aligns well with the experimental results. Theoretical analysis indicates that the oxidation mechanism at low temperatures (below 800 K) does not affect the ignition temperature, preheating does not effectively reduce the ignition temperature, and iron particles with high specific surface areas, such as sponge iron powder, exhibit significantly lower ignition temperatures.
Novelty and significance statement
This study, for the first time integrates experimental investigation with theoretical models to systematically examine the ignition temperature of individual micron-sized iron particles under diverse conditions. The experimental approach allows precise in-situ characterization of particle diameter, particle morphology in different ambient oxygen concentration, providing insights into their respective effects on ignition temperature. Through comprehensive theoretical discussion and experimental validation, the ignition mechanism of iron particles is verified, offering crucial parameters for the design and optimization of efficient iron particle combustion systems.
{"title":"A detailed experimental and numerical study on the ignition temperature of single micron-sized spherical iron particles","authors":"Liulin Cen , Zekang Lyu , Yong Qian , Wenjun Zhong , Xingcai Lu","doi":"10.1016/j.combustflame.2024.113909","DOIUrl":"10.1016/j.combustflame.2024.113909","url":null,"abstract":"<div><div>Measuring the ignition temperature of micron-sized iron particles can verify the ignition mechanism and aid in designing efficient iron powder combustion devices. This study captured the diameter, morphology and ignition status of iron particles with diameter range from 17 to 45 μm entering a stable high-temperature environment by high-speed cameras. The ignition frequency of iron particles at different ambient temperatures and oxygen concentration were recorded. Defining the ignition temperature as the ambient temperature at which the ignition frequency of iron particles exceeds 0.9, it was found that the ignition temperature of iron particles heated from room temperature and closer to a spherical shape is approximately 1140 K, while the non-spherical iron particles is around 1120 K. The ignition temperature is independent of particle diameter and ambient oxygen concentration. The theoretical method for estimating ignition temperature (X.C. Mi, A. Fujinawa, J.M. Bergthorson, 2022) aligns well with the experimental results. Theoretical analysis indicates that the oxidation mechanism at low temperatures (below 800 K) does not affect the ignition temperature, preheating does not effectively reduce the ignition temperature, and iron particles with high specific surface areas, such as sponge iron powder, exhibit significantly lower ignition temperatures.</div></div><div><h3>Novelty and significance statement</h3><div>This study, for the first time integrates experimental investigation with theoretical models to systematically examine the ignition temperature of individual micron-sized iron particles under diverse conditions. The experimental approach allows precise in-situ characterization of particle diameter, particle morphology in different ambient oxygen concentration, providing insights into their respective effects on ignition temperature. Through comprehensive theoretical discussion and experimental validation, the ignition mechanism of iron particles is verified, offering crucial parameters for the design and optimization of efficient iron particle combustion systems.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"272 ","pages":"Article 113909"},"PeriodicalIF":5.8,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143102616","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 : 2025-02-01DOI: 10.1016/j.combustflame.2024.113887
Mingrui Wang , Ruoyue Tang , Xinrui Ren , Hongqing Wu , Yuxin Dong , Ting Zhang , Song Cheng
Significant efforts have been committed to understanding the fundamental combustion chemistry of ammonia at high-pressure and low-temperature conditions with or without blending with other fuels, as these are promising to improve ammonia combustion performance and reduce NOx emission. A commonly used fundamental reactor is the jet-stirred reactor (JSR). However, modeling of high-pressure JSR experiments has been conducted assuming complete ideal gas behaviors, which might lead to misinterpreted or completely wrong results. Therefore, this study proposes, for the first time, a novel framework coupling high-order Virial equation of state, ab initio multi-body intermolecular potential, and real-fluid governing equations. The framework is further applied to investigate NH3 oxidation under supercritical conditions in jet-stirred reactors, where the real-fluid effects on NH3 oxidation characteristics are quantified and compared, via simulated species profiles and relative changes in simulated mole fractions at various temperatures, pressures, diluents, dilution ratios, equivalence ratios, and with or without blending with H2 and CH4. Strong promoting effects on NH3 oxidation from real-fluid effects are revealed, with significant shifts in simulated species profiles observed for both fuel, intermediates, and product species. Sensitivity analyses are also conducted based on the new framework, with diverse influences of real-fluid effects on the contributions of the most sensitive pathways highlighted. It is found that, without considering real-fluid behaviors, the error introduced in simulated species mole fractions can reach ±85 % at the conditions investigated in this study. Propagation of such levels of error to chemical kinetic mechanisms can disqualify them for any meaningful modeling work. These errors can now be excluded using the framework developed in this study.
{"title":"Investigation of real-fluid effects on NH3 oxidation and blending characteristics at supercritical conditions via high-order Virial equation of state coupled with ab initio intermolecular potentials","authors":"Mingrui Wang , Ruoyue Tang , Xinrui Ren , Hongqing Wu , Yuxin Dong , Ting Zhang , Song Cheng","doi":"10.1016/j.combustflame.2024.113887","DOIUrl":"10.1016/j.combustflame.2024.113887","url":null,"abstract":"<div><div>Significant efforts have been committed to understanding the fundamental combustion chemistry of ammonia at high-pressure and low-temperature conditions with or without blending with other fuels, as these are promising to improve ammonia combustion performance and reduce NOx emission. A commonly used fundamental reactor is the jet-stirred reactor (JSR). However, modeling of high-pressure JSR experiments has been conducted assuming complete ideal gas behaviors, which might lead to misinterpreted or completely wrong results. Therefore, this study proposes, for the first time, a novel framework coupling high-order Virial equation of state, <em>ab initio</em> multi-body intermolecular potential, and real-fluid governing equations. The framework is further applied to investigate NH<sub>3</sub> oxidation under supercritical conditions in jet-stirred reactors, where the real-fluid effects on NH<sub>3</sub> oxidation characteristics are quantified and compared, via simulated species profiles and relative changes in simulated mole fractions at various temperatures, pressures, diluents, dilution ratios, equivalence ratios, and with or without blending with H<sub>2</sub> and CH<sub>4</sub>. Strong promoting effects on NH<sub>3</sub> oxidation from real-fluid effects are revealed, with significant shifts in simulated species profiles observed for both fuel, intermediates, and product species. Sensitivity analyses are also conducted based on the new framework, with diverse influences of real-fluid effects on the contributions of the most sensitive pathways highlighted. It is found that, without considering real-fluid behaviors, the error introduced in simulated species mole fractions can reach ±85 % at the conditions investigated in this study. Propagation of such levels of error to chemical kinetic mechanisms can disqualify them for any meaningful modeling work. These errors can now be excluded using the framework developed in this study.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"272 ","pages":"Article 113887"},"PeriodicalIF":5.8,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143103019","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 : 2025-02-01DOI: 10.1016/j.combustflame.2024.113894
Shangpeng Li, Huangwei Zhang
<div><div>Liquid sprays consisting of interacting droplets with diverse compositions are prevalent in engineering applications and everyday life. Although extensive research has been conducted on multi-droplet interactions, most studies concentrate on scenarios involving identical compositions. In this study, we theoretically investigate the quasi-steady evaporation of compositionally distinct droplet pairs using the mass-flux potential model and a bispherical-coordinate approach. Two droplet temperature models under contrasting conditions – no-mixing and rapid-mixing – are examined. Based on these models, the realistic equilibrium temperatures of droplet pairs, accounting for evaporative cooling effects, are clarified. Asymptotic analysis, conducted for relatively large inter-droplet spacings, yields theoretical solutions for the distributions of potential function, temperature, and various components in the gas phase. The equilibrium droplet temperatures, local and cumulative evaporation rates, and interaction coefficients between droplets are determined. Findings reveal that in dual-droplet systems with varying compositions, the surface evaporation potential, temperature, and vapor mass fraction become non-uniform. This non-uniformity, in contrast to the uniform distribution observed in existing studies on identical compositions, intensifies with greater differences in droplet volatilities or reduced inter-droplet spacing. Furthermore, the equilibrium droplet temperatures and evaporation rates decrease due to the shielding effect from adjacent droplets, with smaller or less volatile droplets experiencing more significant impacts. The theoretical results align well with numerical simulations across a wide range of parameters, including liquid compositions, droplet sizes, and inter-droplet spacings. This study enhances the understanding of realistic evaporation dynamics in multi-droplet systems, especially those with diverse compositions. Additionally, while the current analysis focuses on pure vaporization, the flame-sheet assumption provides a theoretical basis for potentially extending this work to include combustion scenarios.</div><div>Novelty and Significance Statement: This study contributes novel theoretical insights into the quasi-steady evaporation processes of compositionally distinct droplet pairs, a subject that has received less attention compared to the extensive research on identical compositions. It unveils detailed asymptotic solutions that expose the non-uniform distributions of temperatures, vapor concentrations, and evaporation rates on droplet surfaces, presenting a significant contrast to the uniformity observed in prior studies. These findings deepen our understanding of evaporation dynamics in multi-droplet systems with varying compositions, which are prevalent in practical settings. The insights obtained from this study could influence the design and operational strategies of technologies in fields such as spray evapora
{"title":"Theoretical analysis of quasi-steady evaporation in compositionally distinct droplet pairs","authors":"Shangpeng Li, Huangwei Zhang","doi":"10.1016/j.combustflame.2024.113894","DOIUrl":"10.1016/j.combustflame.2024.113894","url":null,"abstract":"<div><div>Liquid sprays consisting of interacting droplets with diverse compositions are prevalent in engineering applications and everyday life. Although extensive research has been conducted on multi-droplet interactions, most studies concentrate on scenarios involving identical compositions. In this study, we theoretically investigate the quasi-steady evaporation of compositionally distinct droplet pairs using the mass-flux potential model and a bispherical-coordinate approach. Two droplet temperature models under contrasting conditions – no-mixing and rapid-mixing – are examined. Based on these models, the realistic equilibrium temperatures of droplet pairs, accounting for evaporative cooling effects, are clarified. Asymptotic analysis, conducted for relatively large inter-droplet spacings, yields theoretical solutions for the distributions of potential function, temperature, and various components in the gas phase. The equilibrium droplet temperatures, local and cumulative evaporation rates, and interaction coefficients between droplets are determined. Findings reveal that in dual-droplet systems with varying compositions, the surface evaporation potential, temperature, and vapor mass fraction become non-uniform. This non-uniformity, in contrast to the uniform distribution observed in existing studies on identical compositions, intensifies with greater differences in droplet volatilities or reduced inter-droplet spacing. Furthermore, the equilibrium droplet temperatures and evaporation rates decrease due to the shielding effect from adjacent droplets, with smaller or less volatile droplets experiencing more significant impacts. The theoretical results align well with numerical simulations across a wide range of parameters, including liquid compositions, droplet sizes, and inter-droplet spacings. This study enhances the understanding of realistic evaporation dynamics in multi-droplet systems, especially those with diverse compositions. Additionally, while the current analysis focuses on pure vaporization, the flame-sheet assumption provides a theoretical basis for potentially extending this work to include combustion scenarios.</div><div>Novelty and Significance Statement: This study contributes novel theoretical insights into the quasi-steady evaporation processes of compositionally distinct droplet pairs, a subject that has received less attention compared to the extensive research on identical compositions. It unveils detailed asymptotic solutions that expose the non-uniform distributions of temperatures, vapor concentrations, and evaporation rates on droplet surfaces, presenting a significant contrast to the uniformity observed in prior studies. These findings deepen our understanding of evaporation dynamics in multi-droplet systems with varying compositions, which are prevalent in practical settings. The insights obtained from this study could influence the design and operational strategies of technologies in fields such as spray evapora","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"272 ","pages":"Article 113894"},"PeriodicalIF":5.8,"publicationDate":"2025-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143103023","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}
In this work, Mie-scattering imaging, chemiluminescence and PIV are applied in various optically accessible Porous Media Burners (PMBs). The present PMBs are generated using Triply Periodic Minimal Surfaces (TPMS) and produced via Additive Manufacturing (AM). These topologies feature optical pathways that are both orthogonal and coincident, so that one can be used for illumination and the other for imaging. This enables the application of laser diagnostics in fully 3D structures whilst avoiding altering the geometry of the porous medium. These techniques are applied in homogeneous porous burners where the position of the flame is determined by the operating conditions. First, Mie-scattering imaging is combined with chemiluminescence to analyze the influence of the flame position on the preheating distance of the reactants. For that, the flow is seeded with micrometric oil droplets that evaporate at approximately 500 K. The light scattered by these particles delineates an evaporation front in the Mie-scattering images and this can be compared to the actual location of the reaction region, which is deduced from chemiluminescence images. Then, Mie-scattering imaging and PIV are used to obtain the flame shape and the velocity field in the unburned gas region for inlet-anchored flames. The influence of the hydrogen content, the pore-size and the burner topology is analyzed. The topology is found to have a major impact on the interstitial flow and flame stabilization. A new topological parameter, namely the linear porosity, is proposed to quantify the influence of local hydrodynamic effects on flame stabilization.
Novelty and Significance Statement
The novelty of this work is the accomplishment of Mie-scattering imaging and PIV measurements in a Porous Media Burner (PMB). The application of these laser techniques in an homogeneous PMB requires two orthogonal and coincident optical accesses. In this work, this is achieved via TPMS-based topologies and Additive Manufacturing (AM) techniques. It is significant because it opens the door for the application of laser diagnostics in 3D porous structures without altering the topology. Mie-scattering imaging on small oil droplets seeded in the flow offers a new way to study heat recirculation in PMBs via characterization of the solid-diffusion distance. PIV measurements reveal the importance of linear porosity and its influence on the interstitial flow and on the hydrodynamic stabilization of flames within the pores.
{"title":"Mie-scattering imaging and μPIV in porous media burners with TPMS-based topologies","authors":"Enrique Flores-Montoya , Sébastien Cazin , Thierry Schuller , Laurent Selle","doi":"10.1016/j.combustflame.2025.113990","DOIUrl":"10.1016/j.combustflame.2025.113990","url":null,"abstract":"<div><div>In this work, Mie-scattering imaging, <span><math><msup><mrow><mi>CH</mi></mrow><mrow><mi>⋆</mi></mrow></msup></math></span> chemiluminescence and <span><math><mi>μ</mi></math></span>PIV are applied in various optically accessible Porous Media Burners (PMBs). The present PMBs are generated using Triply Periodic Minimal Surfaces (TPMS) and produced via Additive Manufacturing (AM). These topologies feature optical pathways that are both orthogonal and coincident, so that one can be used for illumination and the other for imaging. This enables the application of laser diagnostics in fully 3D structures whilst avoiding altering the geometry of the porous medium. These techniques are applied in homogeneous porous burners where the position of the flame is determined by the operating conditions. First, Mie-scattering imaging is combined with <span><math><msup><mrow><mi>CH</mi></mrow><mrow><mi>⋆</mi></mrow></msup></math></span> chemiluminescence to analyze the influence of the flame position on the preheating distance of the reactants. For that, the flow is seeded with micrometric oil droplets that evaporate at approximately 500 K. The light scattered by these particles delineates an evaporation front in the Mie-scattering images and this can be compared to the actual location of the reaction region, which is deduced from chemiluminescence images. Then, Mie-scattering imaging and <span><math><mi>μ</mi></math></span>PIV are used to obtain the flame shape and the velocity field in the unburned gas region for inlet-anchored flames. The influence of the hydrogen content, the pore-size and the burner topology is analyzed. The topology is found to have a major impact on the interstitial flow and flame stabilization. A new topological parameter, namely the linear porosity, is proposed to quantify the influence of local hydrodynamic effects on flame stabilization.</div><div><strong>Novelty and Significance Statement</strong></div><div>The novelty of this work is the accomplishment of Mie-scattering imaging and <span><math><mi>μ</mi></math></span>PIV measurements in a Porous Media Burner (PMB). The application of these laser techniques in an homogeneous PMB requires two orthogonal and coincident optical accesses. In this work, this is achieved via TPMS-based topologies and Additive Manufacturing (AM) techniques. It is significant because it opens the door for the application of laser diagnostics in 3D porous structures without altering the topology. Mie-scattering imaging on small oil droplets seeded in the flow offers a new way to study heat recirculation in PMBs via characterization of the solid-diffusion distance. <span><math><mi>μ</mi></math></span>PIV measurements reveal the importance of linear porosity and its influence on the interstitial flow and on the hydrodynamic stabilization of flames within the pores.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"274 ","pages":"Article 113990"},"PeriodicalIF":5.8,"publicationDate":"2025-01-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143193351","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 : 2025-01-30DOI: 10.1016/j.combustflame.2025.114005
Srujan Gubbi , Renee Cole , Cristian D. Avila Jimenez , Ben Emerson , David Noble , Robert Steele , Wenting Sun , Tim Lieuwen
Ammonia (NH3) is being evaluated as a carbon-free energy carrier. However, combustion of NH3 leads to potentially significant amounts of NOx emissions as well as flame stabilization challenges. For both reasons, there is interest in partially cracking NH3 and combusting some blend of NH3/H2/N2. Our prior work has evaluated the minimum theoretical NOx emissions from pure NH3 combustion, which is a useful benchmark for evaluating fundamental limits, as well as to evaluate the performance of a given combustion system relative to these theoretical limits. This work is aimed to evaluate the fundamental minimum NOx emissions of partially and fully cracked NH3. Significant NOx benefits are possible with 100% cracked NH3 – i.e., H2/N2 combustion – and the optimal combustion architecture is a lean premixed strategy. However, this lean premixed strategy obviously does not work for partially cracked NH3 combustion. NOx emissions for intermediate cracking fractions exhibit both a highly nonlinear and, in certain pressure regions, a non-monotonic dependence upon cracking fraction – in other words, NOx emissions do not necessarily, linearly decrease with increased cracking. In general, partial cracking does provide NOx benefits in a manner that is highly pressure dependent; for example, minimum theoretical NO emissions decrease by around 90% and 40% between pure NH3 and 90% cracked NH3 at 1 and 20 bar for a system with 20 ms residence time, but a 2% increase in NO is observed for the same system at 4 bar. It is only at cracking levels exceeding about 99% that major NO benefits occur, with minimum NO reaching sub-30 ppm (15% O2 dry) values for all pressures. Moreover, these results show that rich-lean staged systems lead to optimal NOx emissions over cracking fractions from about 0 – 99.9%; it is only above 99.9% cracking ratio that traditional lean premixed combustion strategies show comparable results. These results indicate that only if nearly complete cracking is possible, that NH3 utilization will require retrofitting low NOx combustors from lean premixed systems to rich-lean staged systems. The sensitivity of these results to the choice of kinetic models is also addressed in this work.
{"title":"Investigation of minimum NOx emissions for cracked ammonia combustion","authors":"Srujan Gubbi , Renee Cole , Cristian D. Avila Jimenez , Ben Emerson , David Noble , Robert Steele , Wenting Sun , Tim Lieuwen","doi":"10.1016/j.combustflame.2025.114005","DOIUrl":"10.1016/j.combustflame.2025.114005","url":null,"abstract":"<div><div>Ammonia (NH<sub>3</sub>) is being evaluated as a carbon-free energy carrier. However, combustion of NH<sub>3</sub> leads to potentially significant amounts of NO<sub>x</sub> emissions as well as flame stabilization challenges. For both reasons, there is interest in partially cracking NH<sub>3</sub> and combusting some blend of NH<sub>3</sub>/H<sub>2</sub>/N<sub>2</sub>. Our prior work has evaluated the minimum theoretical NO<sub>x</sub> emissions from pure NH<sub>3</sub> combustion, which is a useful benchmark for evaluating fundamental limits, as well as to evaluate the performance of a given combustion system relative to these theoretical limits. This work is aimed to evaluate the fundamental minimum NO<sub>x</sub> emissions of partially and fully cracked NH<sub>3</sub>. Significant NO<sub>x</sub> benefits are possible with 100% cracked NH<sub>3</sub> – i.e., H<sub>2</sub>/N<sub>2</sub> combustion – and the optimal combustion architecture is a lean premixed strategy. However, this lean premixed strategy obviously does not work for partially cracked NH<sub>3</sub> combustion. NO<sub>x</sub> emissions for intermediate cracking fractions exhibit both a highly nonlinear and, in certain pressure regions, a non-monotonic dependence upon cracking fraction – in other words, NO<sub>x</sub> emissions do not necessarily, linearly decrease with increased cracking. In general, partial cracking does provide NO<sub>x</sub> benefits in a manner that is highly pressure dependent; for example, minimum theoretical NO emissions decrease by around 90% and 40% between pure NH<sub>3</sub> and 90% cracked NH<sub>3</sub> at 1 and 20 bar for a system with 20 ms residence time, but a 2% increase in NO is observed for the same system at 4 bar. It is only at cracking levels exceeding about 99% that major NO benefits occur, with minimum NO reaching sub-30 ppm (15% O<sub>2</sub> dry) values for all pressures. Moreover, these results show that rich-lean staged systems lead to optimal NO<sub>x</sub> emissions over cracking fractions from about 0 – 99.9%; it is only above 99.9% cracking ratio that traditional lean premixed combustion strategies show comparable results. These results indicate that only if nearly complete cracking is possible, that NH<sub>3</sub> utilization will require retrofitting low NO<sub>x</sub> combustors from lean premixed systems to rich-lean staged systems. The sensitivity of these results to the choice of kinetic models is also addressed in this work.</div></div>","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"274 ","pages":"Article 114005"},"PeriodicalIF":5.8,"publicationDate":"2025-01-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143193333","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 : 2025-01-30DOI: 10.1016/j.combustflame.2025.113982
Louis Benteux , Xinyu Hu , Wenkai Liang , Chung K. Law , Damir M. Valiev
The present work numerically investigates the role of wall heat loss on the propagation of stoichiometric dimethyl ether (DME)/air premixed cool flames in channels, with emphasis on the extinction limits, transition from cool to hot flames and the possibility of observing both cool and hot flame propagation in a channel of a given width and wall thermal condition. The premixed DME/air flame is simulated in a two-dimensional (2D) semi-open narrow channel employing a detailed kinetic model. The study focuses on the impact of the channel width, heat loss intensity and the initial hot kernel temperature on the formation of a steady cool or hot flame, the relevant quenching channel half-width and the transition between cool and hot flames. The results indicate that, as the wall heat loss intensity is increased, the quenching channel width for the cool flames increases as well. Transition to hot flame is observed for larger channel widths. For such a transition, the propagation of a cool flame front starts first, then a hot flame front appears behind it, and subsequently catches up with it. The critical channel width for the cool to hot flame transition (two-stage ignition) increases with heat loss intensity. As for the effect of the initial kernel temperature on the flame regime, stable cool flames were obtained for a certain range of channel widths for lower initial kernel temperatures. A non-monotonic dependence of the critical channel width for the two-stage ignition on the initial kernel temperature is shown to be related to NTC effects. Initial kernel temperatures larger than 1100 K resulted in the direct formation of a hot flame. In a preliminary study for non-stocihiometric mixtures, it was demonstrated that double cool and hot flame configuration can be observed for very lean or rich mixtures.
Novelty and Significance Statement
Propagation of premixed DME/air flames ignited at a closed end of a micro-channel in the presence of wall heat loss was studied numerically. For the first time, it was observed that both stable cool flame and stable hot flame can be established independently in a channel of a given width and wall thermal conditions. Regime diagrams for the emergence of cool and hot flames were obtained numerically for different channel widths, thermal resistance coefficients, and initial kernel temperatures. Despite active research into cool flames in DME-air mixtures in the past years, there are very few studies on the regimes of cool and hot DME flame propagation in channels, both numerical and experimental. This is the first systematic parametric study of DME-air premixed flames in micro-channels of different widths.
本研究以数值方法研究了壁面热损失对化学计量二甲醚（DME）/空气预混合冷火焰在通道中传播的影响，重点是熄灭极限、冷火焰向热火焰的过渡以及在给定宽度和壁面热条件的通道中同时观察冷火焰和热火焰传播的可能性。采用详细的动力学模型模拟了二甲醚/空气预混合火焰在二维（2D）半开放式狭窄通道中的情况。研究重点是通道宽度、热损失强度和初始热核温度对稳定冷焰或热焰的形成、相关淬火通道半宽以及冷焰和热焰之间过渡的影响。结果表明，随着壁面热损失强度的增加，冷焰的淬火通道宽度也随之增加。通道宽度越大，火焰越热。在这种过渡过程中，首先是冷火焰前沿开始传播，然后在其后面出现热火焰前沿，最后赶上冷火焰前沿。冷焰向热焰过渡（两级点火）的临界通道宽度随着热损失强度的增加而增加。至于初始内核温度对火焰状态的影响，在较低的初始内核温度下，在一定的通道宽度范围内可以获得稳定的冷火焰。两阶段点火的临界通道宽度与初始内核温度的非单调依赖关系与 NTC 效应有关。初始内核温度大于 1100 K 会直接形成高温火焰。对非吸热混合物的初步研究表明，在极贫或极富的混合物中可以观察到双冷焰和热焰配置。研究首次发现，在给定宽度和壁面热条件的通道中，可以独立形成稳定的冷火焰和稳定的热火焰。在不同的通道宽度、热阻系数和初始内核温度下，通过数值计算得到了冷焰和热焰出现的时序图。尽管在过去几年中对二甲醚-空气混合物中的冷火焰进行了积极研究，但对二甲醚火焰在通道中的冷和热传播机制进行的数值和实验研究却很少。这是首次对不同宽度微通道中的二甲醚-空气预混合火焰进行系统的参数研究。
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Pub Date : 2025-01-30DOI: 10.1016/j.combustflame.2025.113988
H. Pers , T. Schuller
<div><div>The ability of an aperture to prevent flame propagation, <em>i.e.</em>, to quench it, is essential to the design of flashback-resistant premixed burners, making the determination of hydrogen-air flame quenching distances crucial. However, current experimental methods for estimating quenching distances are not readily applicable to multi-perforated burners and do not account for the aperture geometry. This study presents a novel method for determining quenching widths, here applied to lean hydrogen-air flames stabilized over narrow oblong slits with aspect ratios varying from 1, <em>i.e.</em> circular holes, to values exceeding 20, representing elongated slits over 10 mm in length. Quenching widths <span><math><msub><mrow><mi>W</mi></mrow><mrow><mi>Q</mi></mrow></msub></math></span> are determined for hydrogen-air mixtures with equivalence ratios <span><math><mrow><mn>0</mn><mo>.</mo><mn>45</mn><mo>≤</mo><mi>ϕ</mi><mo>≤</mo><mn>0</mn><mo>.</mo><mn>75</mn></mrow></math></span> at <span><math><mrow><msub><mrow><mi>T</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>=</mo><mn>300</mn></mrow></math></span> K. Results show that transitioning from circular holes to elongated slits reduces the quenching width <span><math><msub><mrow><mi>W</mi></mrow><mrow><mi>Q</mi></mrow></msub></math></span> by a factor of two, consistently across the range of equivalence ratios, with a smaller reduction only observed for the leanest flames at <span><math><mrow><mi>ϕ</mi><mo>=</mo><mn>0</mn><mo>.</mo><mn>45</mn></mrow></math></span>. A physics-based model is developed to generalize these findings and predict quenching widths across different aperture geometries. Predictions are compared with previous measurements. A quenching Peclet number <span><math><mrow><msub><mrow><mtext>Pe</mtext></mrow><mrow><msub><mrow><mi>D</mi></mrow><mrow><mi>h</mi></mrow></msub></mrow></msub><mo>=</mo><msub><mrow><mi>D</mi></mrow><mrow><mi>h</mi></mrow></msub><mo>/</mo><msub><mrow><mi>δ</mi></mrow><mrow><mi>f</mi></mrow></msub></mrow></math></span>, defined as the ratio of the hydraulic diameter <span><math><msub><mrow><mi>D</mi></mrow><mrow><mi>h</mi></mrow></msub></math></span> of the aperture (with a width <span><math><msub><mrow><mi>W</mi></mrow><mrow><mi>Q</mi></mrow></msub></math></span>) to the flame thickness <span><math><mrow><msub><mrow><mi>δ</mi></mrow><mrow><mi>f</mi></mrow></msub><mo>=</mo><mi>α</mi><mo>/</mo><msub><mrow><mi>S</mi></mrow><mrow><mi>L</mi></mrow></msub></mrow></math></span>, is shown to effectively collapse the experimental data for all apertures and underscores the influence of slit geometry on flame quenching by a cold wall. These insights could inform the design of safer premixed hydrogen-air burners by optimizing aperture geometry.</div><div><strong>Novelty and significance statement</strong></div><div>This work advances the understanding of hydrogen-air flame quenching by multi perforated plates, offering essential insights for designing safer, flashbac
{"title":"Impact of hole geometry on quenching and flashback of laminar premixed hydrogen-air flames","authors":"H. Pers , T. Schuller","doi":"10.1016/j.combustflame.2025.113988","DOIUrl":"10.1016/j.combustflame.2025.113988","url":null,"abstract":"<div><div>The ability of an aperture to prevent flame propagation, <em>i.e.</em>, to quench it, is essential to the design of flashback-resistant premixed burners, making the determination of hydrogen-air flame quenching distances crucial. However, current experimental methods for estimating quenching distances are not readily applicable to multi-perforated burners and do not account for the aperture geometry. This study presents a novel method for determining quenching widths, here applied to lean hydrogen-air flames stabilized over narrow oblong slits with aspect ratios varying from 1, <em>i.e.</em> circular holes, to values exceeding 20, representing elongated slits over 10 mm in length. Quenching widths <span><math><msub><mrow><mi>W</mi></mrow><mrow><mi>Q</mi></mrow></msub></math></span> are determined for hydrogen-air mixtures with equivalence ratios <span><math><mrow><mn>0</mn><mo>.</mo><mn>45</mn><mo>≤</mo><mi>ϕ</mi><mo>≤</mo><mn>0</mn><mo>.</mo><mn>75</mn></mrow></math></span> at <span><math><mrow><msub><mrow><mi>T</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>=</mo><mn>300</mn></mrow></math></span> K. Results show that transitioning from circular holes to elongated slits reduces the quenching width <span><math><msub><mrow><mi>W</mi></mrow><mrow><mi>Q</mi></mrow></msub></math></span> by a factor of two, consistently across the range of equivalence ratios, with a smaller reduction only observed for the leanest flames at <span><math><mrow><mi>ϕ</mi><mo>=</mo><mn>0</mn><mo>.</mo><mn>45</mn></mrow></math></span>. A physics-based model is developed to generalize these findings and predict quenching widths across different aperture geometries. Predictions are compared with previous measurements. A quenching Peclet number <span><math><mrow><msub><mrow><mtext>Pe</mtext></mrow><mrow><msub><mrow><mi>D</mi></mrow><mrow><mi>h</mi></mrow></msub></mrow></msub><mo>=</mo><msub><mrow><mi>D</mi></mrow><mrow><mi>h</mi></mrow></msub><mo>/</mo><msub><mrow><mi>δ</mi></mrow><mrow><mi>f</mi></mrow></msub></mrow></math></span>, defined as the ratio of the hydraulic diameter <span><math><msub><mrow><mi>D</mi></mrow><mrow><mi>h</mi></mrow></msub></math></span> of the aperture (with a width <span><math><msub><mrow><mi>W</mi></mrow><mrow><mi>Q</mi></mrow></msub></math></span>) to the flame thickness <span><math><mrow><msub><mrow><mi>δ</mi></mrow><mrow><mi>f</mi></mrow></msub><mo>=</mo><mi>α</mi><mo>/</mo><msub><mrow><mi>S</mi></mrow><mrow><mi>L</mi></mrow></msub></mrow></math></span>, is shown to effectively collapse the experimental data for all apertures and underscores the influence of slit geometry on flame quenching by a cold wall. These insights could inform the design of safer premixed hydrogen-air burners by optimizing aperture geometry.</div><div><strong>Novelty and significance statement</strong></div><div>This work advances the understanding of hydrogen-air flame quenching by multi perforated plates, offering essential insights for designing safer, flashbac","PeriodicalId":280,"journal":{"name":"Combustion and Flame","volume":"274 ","pages":"Article 113988"},"PeriodicalIF":5.8,"publicationDate":"2025-01-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143193331","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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