Pub Date : 2022-06-28DOI: 10.1080/13647830.2022.2085181
Donald Scott Stewart, Kibaek Lee, A. Hernández
Through the use of carefully designed numerical experiments on an explosive system, that use predictive models for subcomponents and multi-material simulation, we demonstrate enhanced reactivity by energy trapping in regions of the reactive flow that were previously shocked. Particles and inclusions are placed in designed patterns in an explosive matrix. New capabilities in additive manufacture make it possible to consider novel designs, that we refer to as ‘reactive metamaterials’. For a fixed amount of energy delivered by a shock impactor, an explosive that normally would not detonate, will detonate when particles are included. Enhanced reactivity correlates precisely with a change in the partition of energy from kinetic to internal, via reflective processes and flow stagnation in high pressure systems. We analyse cases associated with high shock impedance tantalum particles, and void inclusions, individually and placed in a test array. High impedance reflectors trap energy in regions of pre-shocked material. Whereas void shock collapse causes depressurisation of the material along with rapid material flow, and high pressure spot formation related to the jet impact blast. We analyse how these limiting cases of high impedance particle arrays and void arrays partition specific kinetic and internal energy, during the shock impact transient on the system of matrix explosive and embedded particle/voids. Both generate specific flow fields, pressure and temperature cycles in the matrix material over interval times, determined by the particle/void size and placement. Design variations of the configurations presented here can be tested by both experiment and simulation, and can be searched for optimal designs, aided by modern machine learning search methods.
{"title":"Enhanced reactivity by energy trapping in shocked materials: reactive metamaterials for controllable output","authors":"Donald Scott Stewart, Kibaek Lee, A. Hernández","doi":"10.1080/13647830.2022.2085181","DOIUrl":"https://doi.org/10.1080/13647830.2022.2085181","url":null,"abstract":"Through the use of carefully designed numerical experiments on an explosive system, that use predictive models for subcomponents and multi-material simulation, we demonstrate enhanced reactivity by energy trapping in regions of the reactive flow that were previously shocked. Particles and inclusions are placed in designed patterns in an explosive matrix. New capabilities in additive manufacture make it possible to consider novel designs, that we refer to as ‘reactive metamaterials’. For a fixed amount of energy delivered by a shock impactor, an explosive that normally would not detonate, will detonate when particles are included. Enhanced reactivity correlates precisely with a change in the partition of energy from kinetic to internal, via reflective processes and flow stagnation in high pressure systems. We analyse cases associated with high shock impedance tantalum particles, and void inclusions, individually and placed in a test array. High impedance reflectors trap energy in regions of pre-shocked material. Whereas void shock collapse causes depressurisation of the material along with rapid material flow, and high pressure spot formation related to the jet impact blast. We analyse how these limiting cases of high impedance particle arrays and void arrays partition specific kinetic and internal energy, during the shock impact transient on the system of matrix explosive and embedded particle/voids. Both generate specific flow fields, pressure and temperature cycles in the matrix material over interval times, determined by the particle/void size and placement. Design variations of the configurations presented here can be tested by both experiment and simulation, and can be searched for optimal designs, aided by modern machine learning search methods.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"916 - 942"},"PeriodicalIF":1.3,"publicationDate":"2022-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48641275","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2022-06-26DOI: 10.1080/13647830.2022.2090443
Pabitra Badhuk, R. Ravikrishna
Phosphorus-based chemical compounds such as trimethylphosphate (TMP) and dimethylmethylphosphonate (DMMP) are widely used as fire suppressants. The detailed chemical kinetic mechanism by Jayaweera et al. [1] is frequently used to describe the flame inhibition process. The elementary reaction steps can be categorised into inhibitor molecule decomposition steps and radical recombination steps. The present work shows that the inhibitor decomposition process can be adequately represented by a single irreversible step for TMP. Subsequently, graph-based mechanism reduction techniques and sensitivity analysis are employed to extract the key catalytic inhibition reactions. The resultant skeletal kinetic mechanism consists of 4 species and 7 reactions. The present work also proposes a global mechanism containing 3 species and 3 reactions. In the global model, flame inhibition is described by a 2-step model. These models are validated in premixed and diffusion flame environments. Excellent agreement with the experimental measurements and detailed model predictions are obtained. Development of the skeletal/global models reduces the computational time by around 82% compared to the detailed model.
以磷为基础的化合物，如三甲基磷酸盐(TMP)和二甲基膦酸盐(DMMP)被广泛用作灭火剂。Jayaweera et al.[1]详细的化学动力学机理常被用来描述抑焰过程。基本反应步骤可分为抑制剂分子分解步骤和自由基重组步骤。本研究表明，缓蚀剂的分解过程可以用一个不可逆的步骤来充分表征。随后，采用基于图的机理还原技术和灵敏度分析提取关键的催化抑制反应。生成的骨架动力学机制由4种物质和7种反应组成。本工作还提出了一个包含3种物质和3种反应的全局机制。在全局模型中，火焰抑制用两步模型来描述。这些模型在预混火焰和扩散火焰环境下得到了验证。与实验测量结果和详细的模型预测结果非常吻合。与详细模型相比，骨架/全局模型的开发减少了约82%的计算时间。
{"title":"Development and validation of skeletal/global mechanisms describing TMP-based flame inhibition","authors":"Pabitra Badhuk, R. Ravikrishna","doi":"10.1080/13647830.2022.2090443","DOIUrl":"https://doi.org/10.1080/13647830.2022.2090443","url":null,"abstract":"Phosphorus-based chemical compounds such as trimethylphosphate (TMP) and dimethylmethylphosphonate (DMMP) are widely used as fire suppressants. The detailed chemical kinetic mechanism by Jayaweera et al. [1] is frequently used to describe the flame inhibition process. The elementary reaction steps can be categorised into inhibitor molecule decomposition steps and radical recombination steps. The present work shows that the inhibitor decomposition process can be adequately represented by a single irreversible step for TMP. Subsequently, graph-based mechanism reduction techniques and sensitivity analysis are employed to extract the key catalytic inhibition reactions. The resultant skeletal kinetic mechanism consists of 4 species and 7 reactions. The present work also proposes a global mechanism containing 3 species and 3 reactions. In the global model, flame inhibition is described by a 2-step model. These models are validated in premixed and diffusion flame environments. Excellent agreement with the experimental measurements and detailed model predictions are obtained. Development of the skeletal/global models reduces the computational time by around 82% compared to the detailed model.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"968 - 987"},"PeriodicalIF":1.3,"publicationDate":"2022-06-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48570377","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2022-06-16DOI: 10.1080/13647830.2022.2086068
Pragneshkumar Rajubhai Rana, K. Narayanaswamy, Sivaram Ambikasaran
Ignition delay time (IDT) is an important global combustion property that affects the thermal efficiency of the engine and emissions (particularly NO ). IDT is generally measured by performing experiments using Shock-tube and Rapid Compression Machine (RCM). The numerical calculation of IDT is a computationally expensive and time-consuming process. Arrhenius type empirical correlations offer an inexpensive alternative to obtain IDT. However, such correlations have limitations as these typically involve ad-hoc parameters and are valid only for a specific fuel and particular range of temperature/pressure conditions. This study aims to formulate a data-driven scientific way to obtain IDT for new fuels without performing detailed numerical calculations or relying on ad-hoc empirical correlations. We propose a rigorous, well-validated data-driven study to obtain IDT for new fuels using a regression-based clustering algorithm. In our model, we bring in the fuel structure through the overall activation energy ( ) by expressing it in terms of the different bonds present in the molecule. Gaussian Mixture Model (GMM) is used for the formation of clusters, and regression is applied over each cluster to generate models. The proposed algorithm is used on the ignition delay dataset of straight-chain alkanes (C H for n = 4 to 16). The high level of accuracy achieved demonstrates the applicability of the proposed algorithm over IDT data. The algorithm and framework discussed in this article are implemented in python and made available at https://doi.org/10.5281/zenodo.5774617.
{"title":"A data-driven framework to predict ignition delays of straight-chain alkanes","authors":"Pragneshkumar Rajubhai Rana, K. Narayanaswamy, Sivaram Ambikasaran","doi":"10.1080/13647830.2022.2086068","DOIUrl":"https://doi.org/10.1080/13647830.2022.2086068","url":null,"abstract":"Ignition delay time (IDT) is an important global combustion property that affects the thermal efficiency of the engine and emissions (particularly NO ). IDT is generally measured by performing experiments using Shock-tube and Rapid Compression Machine (RCM). The numerical calculation of IDT is a computationally expensive and time-consuming process. Arrhenius type empirical correlations offer an inexpensive alternative to obtain IDT. However, such correlations have limitations as these typically involve ad-hoc parameters and are valid only for a specific fuel and particular range of temperature/pressure conditions. This study aims to formulate a data-driven scientific way to obtain IDT for new fuels without performing detailed numerical calculations or relying on ad-hoc empirical correlations. We propose a rigorous, well-validated data-driven study to obtain IDT for new fuels using a regression-based clustering algorithm. In our model, we bring in the fuel structure through the overall activation energy ( ) by expressing it in terms of the different bonds present in the molecule. Gaussian Mixture Model (GMM) is used for the formation of clusters, and regression is applied over each cluster to generate models. The proposed algorithm is used on the ignition delay dataset of straight-chain alkanes (C H for n = 4 to 16). The high level of accuracy achieved demonstrates the applicability of the proposed algorithm over IDT data. The algorithm and framework discussed in this article are implemented in python and made available at https://doi.org/10.5281/zenodo.5774617.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"943 - 967"},"PeriodicalIF":1.3,"publicationDate":"2022-06-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47743440","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2022-06-16DOI: 10.1080/13647830.2022.2083525
S. Tomasch, N. Swaminathan, Christoph Spijker, I. S. Ertesvåg
This study presents an algebraic combustion closure for Large eddy simulation (LES) exhibiting attributes of simplicity and simultaneous accuracy under realistic combustion conditions. The model makes use of the interlink between the reaction and dissipation rates in premixed turbulent combustion but relaxes the thin flame assumption by considering finite-rate chemistry effects in the small-scale turbulence structure. The core idea of the approach is to approximate the reaction progress in the unresolved spectrum of wave lengths and to use it within a filtered reaction rate expression. The model is implemented in OpenFOAM 4.0 and is tested on a turbulent, premixed flame behind a bluff-body, applying an LES approach for turbulence modelling. The cross comparison of velocity, temperature and composition data with experiments and a well-investigated combustion model in literature reveals competitive performance of the new model. Especially in the near-field of the bluff body flame, corresponding to thin and moderately thickened flame regions, its ability to capture the flame structure is highly promising. The chosen, partly explicit approach to recover the temperature from the transported sensible enthalpy, involving a strong coupling between filtered reaction and heat release rate, also shows advantages over obtaining the temperature from presumed probability density functions.
{"title":"Development of a turbulence dissipation based reaction rate model for progress variable in turbulent premixed flames","authors":"S. Tomasch, N. Swaminathan, Christoph Spijker, I. S. Ertesvåg","doi":"10.1080/13647830.2022.2083525","DOIUrl":"https://doi.org/10.1080/13647830.2022.2083525","url":null,"abstract":"This study presents an algebraic combustion closure for Large eddy simulation (LES) exhibiting attributes of simplicity and simultaneous accuracy under realistic combustion conditions. The model makes use of the interlink between the reaction and dissipation rates in premixed turbulent combustion but relaxes the thin flame assumption by considering finite-rate chemistry effects in the small-scale turbulence structure. The core idea of the approach is to approximate the reaction progress in the unresolved spectrum of wave lengths and to use it within a filtered reaction rate expression. The model is implemented in OpenFOAM 4.0 and is tested on a turbulent, premixed flame behind a bluff-body, applying an LES approach for turbulence modelling. The cross comparison of velocity, temperature and composition data with experiments and a well-investigated combustion model in literature reveals competitive performance of the new model. Especially in the near-field of the bluff body flame, corresponding to thin and moderately thickened flame regions, its ability to capture the flame structure is highly promising. The chosen, partly explicit approach to recover the temperature from the transported sensible enthalpy, involving a strong coupling between filtered reaction and heat release rate, also shows advantages over obtaining the temperature from presumed probability density functions.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"896 - 915"},"PeriodicalIF":1.3,"publicationDate":"2022-06-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48252076","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2022-05-30DOI: 10.1080/13647830.2022.2080122
Zhuang Ma, Chen Wang, Gaofeng Wang, Tao Cui, Yao Zheng
Critical slowing down phenomena occur during the transition process of various dynamical states, such as bifurcations. The eigenvalues of dynamical systems can be regarded as an indicator of critical slowing down of impending bifurcation. Adaptive, locally linear models can extract local eigenvalues of the nonlinear dynamical system by segmenting a full-time series into multi-windows, and eigenvalue spectrum analysis is based on the eigenvalues. The state transition between different combustion states occurs through bifurcation processes. In this paper, we investigate critical slowing down in the bifurcation process of backward-facing step combustor. We performed a series of experiments by fixing the air mass flow and varying the fuel mass flow from the lean blowout condition to a thermoacoustic instability condition with a quasi-steady change, and the combustion state varying versus the change of the operation conditions exhibit a quasi-Hopf bifurcation process. The measured pressure fluctuations were treated by the local linear model to analyse the eigenvalue spectrum. The real parts of the eigenvalues approximate zero gradually when the equivalence ratio increases, and this tendency corresponds to the critical slowing down. Furthermore, the commonly used early warning signals were also used to support the analysis results of the eigenvalue spectrum.
{"title":"Experimental investigation on critical slowing down of premixed combustion in a backward-facing step combustor","authors":"Zhuang Ma, Chen Wang, Gaofeng Wang, Tao Cui, Yao Zheng","doi":"10.1080/13647830.2022.2080122","DOIUrl":"https://doi.org/10.1080/13647830.2022.2080122","url":null,"abstract":"Critical slowing down phenomena occur during the transition process of various dynamical states, such as bifurcations. The eigenvalues of dynamical systems can be regarded as an indicator of critical slowing down of impending bifurcation. Adaptive, locally linear models can extract local eigenvalues of the nonlinear dynamical system by segmenting a full-time series into multi-windows, and eigenvalue spectrum analysis is based on the eigenvalues. The state transition between different combustion states occurs through bifurcation processes. In this paper, we investigate critical slowing down in the bifurcation process of backward-facing step combustor. We performed a series of experiments by fixing the air mass flow and varying the fuel mass flow from the lean blowout condition to a thermoacoustic instability condition with a quasi-steady change, and the combustion state varying versus the change of the operation conditions exhibit a quasi-Hopf bifurcation process. The measured pressure fluctuations were treated by the local linear model to analyse the eigenvalue spectrum. The real parts of the eigenvalues approximate zero gradually when the equivalence ratio increases, and this tendency corresponds to the critical slowing down. Furthermore, the commonly used early warning signals were also used to support the analysis results of the eigenvalue spectrum.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"879 - 895"},"PeriodicalIF":1.3,"publicationDate":"2022-05-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47650839","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
To build a set of complete kinetic parameters of oxygenated fuels kinetic model on Pt catalyst, methanol was used as an example to carry out the catalytic oxidation kinetics experiment of oxygenated fuels on Pt/ZSM-5 catalyst. The Power law model and Langmuir–Hinshelwood (L–H) model were established to characterise the catalytic oxidation reaction of methanol. Then the oxidation kinetics of methanol, ethanol, dimethyl ether (DME) and n-butanol on Pt/ZSM-5 was studied under the same experimental conditions. It was found that the reaction orders of fuel molecules (methanol is −0.14) were much less than that of oxygen (1.23) in Power law model. The adsorption constants of fuel molecules were higher than that of oxygen in L–H model. The adsorption characteristics of alcohols on Pt were similar, but the reaction orders of alcohols were not consistent. The adsorption constants and adsorption heat of dimethyl ether were much larger than that of alcohols. The intrinsic reaction rates of four oxygenated fuels on Pt/ZSM-5 were compared at the same input power: r methanol r DME r ethanol r n-butanol. In general, methanol is a suitable oxygenated fuel in the design and development of catalytic micro-combustor.
{"title":"Kinetics of catalytic oxidation of oxygenated fuels on Pt/ZSM-5 catalyst","authors":"Yanyi Yao, Wei-juan Yang, Xing Zhang, Xiaoyu Zhu, Jun Cheng, Junhu Zhou","doi":"10.1080/13647830.2022.2063194","DOIUrl":"https://doi.org/10.1080/13647830.2022.2063194","url":null,"abstract":"To build a set of complete kinetic parameters of oxygenated fuels kinetic model on Pt catalyst, methanol was used as an example to carry out the catalytic oxidation kinetics experiment of oxygenated fuels on Pt/ZSM-5 catalyst. The Power law model and Langmuir–Hinshelwood (L–H) model were established to characterise the catalytic oxidation reaction of methanol. Then the oxidation kinetics of methanol, ethanol, dimethyl ether (DME) and n-butanol on Pt/ZSM-5 was studied under the same experimental conditions. It was found that the reaction orders of fuel molecules (methanol is −0.14) were much less than that of oxygen (1.23) in Power law model. The adsorption constants of fuel molecules were higher than that of oxygen in L–H model. The adsorption characteristics of alcohols on Pt were similar, but the reaction orders of alcohols were not consistent. The adsorption constants and adsorption heat of dimethyl ether were much larger than that of alcohols. The intrinsic reaction rates of four oxygenated fuels on Pt/ZSM-5 were compared at the same input power: r methanol r DME r ethanol r n-butanol. In general, methanol is a suitable oxygenated fuel in the design and development of catalytic micro-combustor.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"728 - 745"},"PeriodicalIF":1.3,"publicationDate":"2022-05-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48000950","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2022-05-20DOI: 10.1080/13647830.2022.2072237
Seyed Mehdi Ashrafizadeh, C. Devaud
The modelling of soot formation is investigated for two turbulent flames, at atmospheric and 3 atm pressure conditions. For the first time, a semi-empirical soot formulation that accounts for soot inception, coagulation, surface growth, and oxidation processes is coupled with the turbulent combustion model, Conditional Source-term Estimation (CSE) using Reynolds-Averaged Navier–Stokes equations. Detailed chemistry is included and an optically thin radiation model is considered. Non-adiabatic chemistry tabulations are created. Good agreement with the experiments is found for turbulent mixing and temperature fields in both flames, with some discrepancies believed to be due to the turbulence modelling approach. At 1 atm, the soot volume fractions are in reasonable agreement with the experiments, but typically smaller than the measurements with the centerline peak locating closer to the fuel exit. At 3 atm, good agreement between the numerical predictions and experimental data is achieved for the soot volume fraction within the experimental error. The centerline peak location is observed slightly farther downstream. Possible sources of discrepancies are examined and comparison with previously published numerical results is also undertaken. Differential diffusion and modified soot chemistry constants may bring further improvement. Without any particular tuning of soot chemistry, soot modelling within CSE is shown to be a promising approach.
{"title":"Investigation of conditional source-term estimation coupled with a semi-empirical model for soot predictions in two turbulent flames","authors":"Seyed Mehdi Ashrafizadeh, C. Devaud","doi":"10.1080/13647830.2022.2072237","DOIUrl":"https://doi.org/10.1080/13647830.2022.2072237","url":null,"abstract":"The modelling of soot formation is investigated for two turbulent flames, at atmospheric and 3 atm pressure conditions. For the first time, a semi-empirical soot formulation that accounts for soot inception, coagulation, surface growth, and oxidation processes is coupled with the turbulent combustion model, Conditional Source-term Estimation (CSE) using Reynolds-Averaged Navier–Stokes equations. Detailed chemistry is included and an optically thin radiation model is considered. Non-adiabatic chemistry tabulations are created. Good agreement with the experiments is found for turbulent mixing and temperature fields in both flames, with some discrepancies believed to be due to the turbulence modelling approach. At 1 atm, the soot volume fractions are in reasonable agreement with the experiments, but typically smaller than the measurements with the centerline peak locating closer to the fuel exit. At 3 atm, good agreement between the numerical predictions and experimental data is achieved for the soot volume fraction within the experimental error. The centerline peak location is observed slightly farther downstream. Possible sources of discrepancies are examined and comparison with previously published numerical results is also undertaken. Differential diffusion and modified soot chemistry constants may bring further improvement. Without any particular tuning of soot chemistry, soot modelling within CSE is shown to be a promising approach.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"856 - 878"},"PeriodicalIF":1.3,"publicationDate":"2022-05-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43759005","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2022-05-19DOI: 10.1080/13647830.2022.2070549
F. Chitgarha, F. Ommi, M. Farshchi
The reaction progress variable is a crucial concept in the advanced flamelet combustion models. As a controlling variable, a well-defined progress variable must consider the essential features of the combustion process. It is usually a heuristically defined linear combination of some major chemical species mass fractions. However, such a simple definition could lead to inaccurate results for the fuel-rich reactive mixtures or complicated fuels, due to the vast number of chemical species in the combustion process. In this paper, a new method for generating a reaction progress variable is proposed through solving a constrained optimisation problem. The proposed method uses a genetic algorithm with new constraints. The major new constraint is the minimisation of the inverse of a progress variable-based Damköhler number in addition to the minimisation of the gradients of a collection of chemical species concentrations, as used in the previous methods. Hence, this scheme increases the Damköhler number defined based on the progress variable. The applicability and performance of the current optimised progress variable are evaluated for ethanol–air partially premixed flames in an axisymmetric two-dimensional counterflow burner and a two-dimensional plugged flow triple-flame burner. The effects of the number of chemical species included in the progress variable and the flow field strain rate on a partially premixed ethanol–air flame prediction are investigated. Results indicate that including the progress variable Damköhler number in the determination of the progress variable has a considerable effect on the accuracy of Flamelet Generated Manifold (FGM) model prediction of fuel-rich and lean reactive mixtures, especially at higher strain rates. Also, it is shown that the inclusion of the critical chemical species for ignition and fuel decomposition processes, such as CH3O2, CH3CHO, sC2H4OH, HO2, H and H2O2, in the definition of progress variable has a significant effect on the accuracy of the ethanol–air flame predictions.
{"title":"Assessment of optimal reaction progress variable characteristics for partially premixed flames","authors":"F. Chitgarha, F. Ommi, M. Farshchi","doi":"10.1080/13647830.2022.2070549","DOIUrl":"https://doi.org/10.1080/13647830.2022.2070549","url":null,"abstract":"The reaction progress variable is a crucial concept in the advanced flamelet combustion models. As a controlling variable, a well-defined progress variable must consider the essential features of the combustion process. It is usually a heuristically defined linear combination of some major chemical species mass fractions. However, such a simple definition could lead to inaccurate results for the fuel-rich reactive mixtures or complicated fuels, due to the vast number of chemical species in the combustion process. In this paper, a new method for generating a reaction progress variable is proposed through solving a constrained optimisation problem. The proposed method uses a genetic algorithm with new constraints. The major new constraint is the minimisation of the inverse of a progress variable-based Damköhler number in addition to the minimisation of the gradients of a collection of chemical species concentrations, as used in the previous methods. Hence, this scheme increases the Damköhler number defined based on the progress variable. The applicability and performance of the current optimised progress variable are evaluated for ethanol–air partially premixed flames in an axisymmetric two-dimensional counterflow burner and a two-dimensional plugged flow triple-flame burner. The effects of the number of chemical species included in the progress variable and the flow field strain rate on a partially premixed ethanol–air flame prediction are investigated. Results indicate that including the progress variable Damköhler number in the determination of the progress variable has a considerable effect on the accuracy of Flamelet Generated Manifold (FGM) model prediction of fuel-rich and lean reactive mixtures, especially at higher strain rates. Also, it is shown that the inclusion of the critical chemical species for ignition and fuel decomposition processes, such as CH3O2, CH3CHO, sC2H4OH, HO2, H and H2O2, in the definition of progress variable has a significant effect on the accuracy of the ethanol–air flame predictions.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"797 - 830"},"PeriodicalIF":1.3,"publicationDate":"2022-05-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45543332","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2022-05-10DOI: 10.1080/13647830.2022.2069601
T. Shen, Huahua Xiao
This paper studies premixed flame dynamics in half-open tubes by solving the two-dimensional, fully compressible, reactive Navier-Stokes equations on a dynamically adapting mesh using a high-order algorithm. A simplified chemical-diffusive model was used to describe the chemical reaction and diffusive transports in a stoichiometric hydrogen-air mixture. The influence of the length scale was examined by considering four tube heights at a fixed aspect ratio α = 7. The numerical simulations show that the flame evolves into a tulip flame (TF) in all the tubes shortly after being ignited at the open end. Variation in tube size leads to differences in the evolution of TF and generation of expansion waves. In a sufficiently large tube (d > 0.5 cm), the TF further develops into a series of more unstable distorted tulip flames (DTFs). By contrast, in a small tube (d < 0.5 cm), the TF shape remains until the end of the combustion. In addition, both the flame and pressure oscillate significantly almost in the entire process of flame propagation in the large tubes, while the oscillating behaviour in flame or pressure is negligible in the small tube after TF forms. It was found that the TF formation mechanism is length-scale dependent even for the same type of geometry and condition. A detailed examination of the interactions between flame, boundary layer, and pressure waves showed two mechanisms of TF formation: (1) boundary layer effect for the larger tubes (d ≥ 0.5 cm), and (2) Rayleigh–Taylor instability driven by compression waves for the smallest tube (d = 0.25 cm). The DTF formation in the half-open tubes is closely related to the expansion waves generated by the collapse of the TF cusp. The expansion waves cause a reverse flow in the boundary layer ahead of the flame front and consequently initiate the DTF.
{"title":"Numerical study of the stability of premixed flames propagating in half-open tubes","authors":"T. Shen, Huahua Xiao","doi":"10.1080/13647830.2022.2069601","DOIUrl":"https://doi.org/10.1080/13647830.2022.2069601","url":null,"abstract":"This paper studies premixed flame dynamics in half-open tubes by solving the two-dimensional, fully compressible, reactive Navier-Stokes equations on a dynamically adapting mesh using a high-order algorithm. A simplified chemical-diffusive model was used to describe the chemical reaction and diffusive transports in a stoichiometric hydrogen-air mixture. The influence of the length scale was examined by considering four tube heights at a fixed aspect ratio α = 7. The numerical simulations show that the flame evolves into a tulip flame (TF) in all the tubes shortly after being ignited at the open end. Variation in tube size leads to differences in the evolution of TF and generation of expansion waves. In a sufficiently large tube (d > 0.5 cm), the TF further develops into a series of more unstable distorted tulip flames (DTFs). By contrast, in a small tube (d < 0.5 cm), the TF shape remains until the end of the combustion. In addition, both the flame and pressure oscillate significantly almost in the entire process of flame propagation in the large tubes, while the oscillating behaviour in flame or pressure is negligible in the small tube after TF forms. It was found that the TF formation mechanism is length-scale dependent even for the same type of geometry and condition. A detailed examination of the interactions between flame, boundary layer, and pressure waves showed two mechanisms of TF formation: (1) boundary layer effect for the larger tubes (d ≥ 0.5 cm), and (2) Rayleigh–Taylor instability driven by compression waves for the smallest tube (d = 0.25 cm). The DTF formation in the half-open tubes is closely related to the expansion waves generated by the collapse of the TF cusp. The expansion waves cause a reverse flow in the boundary layer ahead of the flame front and consequently initiate the DTF.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"774 - 795"},"PeriodicalIF":1.3,"publicationDate":"2022-05-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43777883","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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 : 2022-05-10DOI: 10.1080/13647830.2022.2071170
W. Jayasuriya, T. C. Mulky, Kyle E. Niemeyer
Smouldering combustion plays a key role in wildfires in forests, grasslands, and peatlands due to its common occurrence in porous fuels like peat and duff. As a consequence, understanding smouldering behaviour in these fuels is crucial. Such fuels are generally composed of cellulose, hemicellulose, and lignin. Here we present an updated computational model for simulating smouldering combustion in cellulose and hemicellulose mixtures. We used this model to examine changes in smouldering propagation speed and peak temperatures with varying fuel composition and density. For a given fuel composition, increases in density decrease the propagation speed and increase mean peak temperature; for a given density, increases in hemicellulose content increase both propagation speed and peak temperature. We also examined the role of natural fuel expansion with the addition of water. Without expansion, addition of moisture content reduces the propagation speed primarily due to increasing (wet) fuel density. However, with fuel expansion similar to that observed in peat, the propagation speed increases due to the overall drop in fuel density. Finally, we studied the influence of fuel composition on critical moisture content of ignition and extinction: mixtures dominated by hemicellulose have 10% higher critical moisture content due to the increase in peak temperature.
{"title":"Smouldering combustion in cellulose and hemicellulose mixtures: Examining the roles of density, fuel composition, oxygen concentration, and moisture content","authors":"W. Jayasuriya, T. C. Mulky, Kyle E. Niemeyer","doi":"10.1080/13647830.2022.2071170","DOIUrl":"https://doi.org/10.1080/13647830.2022.2071170","url":null,"abstract":"Smouldering combustion plays a key role in wildfires in forests, grasslands, and peatlands due to its common occurrence in porous fuels like peat and duff. As a consequence, understanding smouldering behaviour in these fuels is crucial. Such fuels are generally composed of cellulose, hemicellulose, and lignin. Here we present an updated computational model for simulating smouldering combustion in cellulose and hemicellulose mixtures. We used this model to examine changes in smouldering propagation speed and peak temperatures with varying fuel composition and density. For a given fuel composition, increases in density decrease the propagation speed and increase mean peak temperature; for a given density, increases in hemicellulose content increase both propagation speed and peak temperature. We also examined the role of natural fuel expansion with the addition of water. Without expansion, addition of moisture content reduces the propagation speed primarily due to increasing (wet) fuel density. However, with fuel expansion similar to that observed in peat, the propagation speed increases due to the overall drop in fuel density. Finally, we studied the influence of fuel composition on critical moisture content of ignition and extinction: mixtures dominated by hemicellulose have 10% higher critical moisture content due to the increase in peak temperature.","PeriodicalId":50665,"journal":{"name":"Combustion Theory and Modelling","volume":"26 1","pages":"831 - 855"},"PeriodicalIF":1.3,"publicationDate":"2022-05-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42177999","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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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