Suchergebnisse

Compared to partially premixed combustion (or combustion of non-homogeneous reactants in general), fully premixed and diffusion flames represent only two asymptotic limits of combustion modes. However, the deep knowledge accumulated over the years on these two elementary and archetypal flame prototypes is such that they remain the cornerstone and reference building blocks of most combustion modelling proposals. Therefore, from a general point of view, being able to distinguish between premixed and non-premixed modes of combustion thanks to a flame index appears as a quite appealing but challenging task that still concentrates many research efforts. Indeed, the availability of such an index is not only appealing to proceed with the analysis of either experimental or computational data issued from DNS (or highly resolved LES) databases. It is also an essential ingredient to elaborate advanced flamelet-based multiregime combustion models on the basis of single regime tabulated flamelet databases. In the present study, a new definition of the premixedness index ζPF is proposed for partially premixed combustion. It is based on a weighted form of the cross-scalar dissipation rate of the mixture fraction Yξ and progress variable Yc, i.e. quantities that have been previously identified as relevant parameters to describe partially premixed combustion regimes. The relevance of the corresponding index is assessed through a detailed computational procedure that includes three successive validation subsets: counterflow flames (including premixed, rich partially-premixed, and diffusion flames), (ii) stabilised triple flames for three distinct values of the inlet mixture fraction gradient, and finally (iii) unsteady flame kernel developments in non-homogeneous mixtures of fresh reactants, which are characterised by various initial levels of the segregation rate between the fuel and oxidiser. The proposed premixedness index ζPF and its counterpart ζDF=1−ζPF are used as the weighting coefficients between tabulated premixed flamelets (TPF) and tabulated diffusion flamelets (TDF) data, which have been parameterised as functions of Yξ and Yc. It is noteworthy that, in contrast to some previous proposals of the literature, the present flame index does not require the consideration of any other quantities in addition to those already used to parameterise the flamelets databases, i.e. Yξ and Yc. The validation procedure makes use of steady and unsteady processes with a priori and a posteriori analyses. In both cases, the comparisons between the results obtained with the proposed flame partitioning and detailed chemistry (DC) computations lead to a satisfactory level of agreement and, from a general viewpoint, the level of agreement is better than the one obtained with either premixed or diffusion flamelet-based models. ; Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 422037413 – TRR 287. Daniel Mira acknowledges the funding received through the Spanish Ministry of Economy and Competitiveness in the frame of the CHEST project (TRA2017-89139-C2-2-R) and the ESTiMatE project from the Clean Sky 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation programme under grant agreement No 821418. Conflict of Interest: Daniel Mira acknowledges the Juan de la Cierva personal grant IJCI-2015-26686. ; Peer Reviewed ; Postprint (author's final draft)

The present paper focuses on the large eddy simulation (LES) of the n-heptane and n-dodecane flames of the Cambridge swirl flames data repository (Sidey et al., 2017). The applied tabulated chemistry spray combustion model reproduces well the key phenomena controlling flame dynamics. The distinct volatility of the fuels is identified as the source of main difference between the n-heptane and n-dodecane flames. ; We acknowledge the funding from the CoEC project through the European Union's Horizon 2020 research and innovation programme under grant agreement No 952181. The authors thankfully acknowledge the computer resources at MareNostrum and the technical support provided by Barcelona Supercomputing Center (FI-2019-1-0045, IM-2019-3-0019, IM-2020- 1-0011, IM-2020-2-0025.) ; Peer Reviewed ; Postprint (published version)

Large-eddy simulation of the reacting flow field in a combustion-based mitigation system to reduce the emissions of methane contained in ventilation air methane is presented. The application is based on the preheating and combustion of ventilation air methane. Effects of preheating and methane concentration are examined in five computational cases. The results indicate that the oxidation of the ventilation air methane can take place in a co-annular jet configuration provided that the preheating temperature is as high as 500 K for mixtures containing a low methane concentration of 0.5%. It is found that the oxidation process that eventually leads to reaction and combustion is controlled by the methane concentration and the level of preheating. ; The authors would like to thank the European Union's Research Programme of the Research Fund for Coal and Steel (RFCS) research programme under Grant agreement Number RFCR-CT-2010-00004 and EPSRC Grant EP/G062714/2 for funding the activities under- taken for this study. ; Peer Reviewed ; Postprint (author's final draft)

Large-eddy simulation of the reacting flow field in a combustion-based mitigation system to reduce the emissions of methane contained in ventilation air methane is presented. The application is based on the preheating and combustion of ventilation air methane. Effects of preheating and methane concentration are examined in five computational cases. The results indicate that the oxidation of the ventilation air methane can take place in a co-annular jet configuration provided that the preheating temperature is as high as 500 K for mixtures containing a low methane concentration of 0.5%. It is found that the oxidation process that eventually leads to reaction and combustion is controlled by the methane concentration and the level of preheating. ; The authors would like to thank the European Union's Research Programme of the Research Fund for Coal and Steel (RFCS) research programme under Grant agreement Number RFCR-CT-2010-00004 and EPSRC Grant EP/G062714/2 for funding the activities under- taken for this study. ; Peer Reviewed ; Postprint (author's final draft)

This study investigates the impact of chemical kinetic uncertainties on biogas combustion using a Uncertainty Quantification (UQ)-based methodology. The results indicate that the variation of physicochemical properties introduced by composition variability introduces smaller uncertainties in the resulting flame properties than the Arrhenius parameters involved in the kinetics used to describe the oxidation process. We demonstrate that the use of reduced mechanisms for methane-air oxidation could be a starting point to develop optimized schemes for biogas combustion. In that regard, we adopted an embedded discrepancy approach to understanding the limits of the use of a reduced mechanism for methane/air in this renewable fuel. This strategy provides a way to reduce systematically the cost of reaction kinetics in simulations, while quantifying the accuracy of predictions of important target quantities. Finally, we develop a surrogate model for biogas flame propagation using machine learning techniques to make feasible a broader UQ analysis. ; The research leading to these results had received funding from the European Union's Horizon2020 Programme (2014-2020) and from Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project (www.hpc4e.eu), grant agreement number 689772 ; Peer Reviewed ; Postprint (author's final draft)

This work addresses the prediction of the reacting flow field in a swirl stabilized gas turbine model combustor using large-eddy simulation. The modeling of the combustion chemistry is based on laminar premixed flamelets and the effect of turbulence-chemistry interaction is considered by a presumed shape probability density function. The prediction capabilities of the presented combustion model for perfectly premixed and partially premixed conditions are demonstrated. The effect of partial premixing for the prediction of the reacting flow field is assessed by comparison of a perfectly premixed and partially premixed simulation. Even though significant mixture fraction fluctuations are observed, only small impact of the non-perfect premixing is found on the flow field and flame dynamics. Subsequently, the effect of heat loss to the walls is assessed assuming perfectly premixing. The adiabatic baseline case is compared to heat loss simulations with adiabatic and non-adiabatic chemistry tabulation. The results highlight the importance of considering the effect of heat loss on the chemical kinetics for an accurate prediction of the flow features. Both heat loss simulations significantly improve the temperature prediction, but the non-adiabatic chemistry tabulation is required to accurately capture the chemical composition in the reacting layers. ; The research leading to these results has received funding through the People Program (Marie Curie Actions) of the European Union's Seventh Framework Program (FP7, 2007-2013) for the project COPA-GT (grant agreement No. FP7-290042) as well as the European Union's Horizon 2020 Program (2014-2020) and the Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project (grant agreement No. 689772). Furthermore, computer resources and technical assistance has been provided by the Red Española de Supercomputación (RES) (grant numbers FI-2015-1-0002, FI-2015-3-0013, FI-2016-1-0001). ; Peer Reviewed ; Postprint (published version)

This work addresses the prediction of the reacting flow field in a swirl stabilized gas turbine model combustor using large-eddy simulation. The modeling of the combustion chemistry is based on laminar premixed flamelets and the effect of turbulence-chemistry interaction is considered by a presumed shape probability density function. The prediction capabilities of the presented combustion model for perfectly premixed and partially premixed conditions are demonstrated. The effect of partial premixing for the prediction of the reacting flow field is assessed by comparison of a perfectly premixed and partially premixed simulation. Even though significant mixture fraction fluctuations are observed, only small impact of the non-perfect premixing is found on the flow field and flame dynamics. Subsequently, the effect of heat loss to the walls is assessed assuming perfectly premixing. The adiabatic baseline case is compared to heat loss simulations with adiabatic and non-adiabatic chemistry tabulation. The results highlight the importance of considering the effect of heat loss on the chemical kinetics for an accurate prediction of the flow features. Both heat loss simulations significantly improve the temperature prediction, but the non-adiabatic chemistry tabulation is required to accurately capture the chemical composition in the reacting layers. ; The research leading to these results has received funding through the People Program (Marie Curie Actions) of the European Union's Seventh Framework Program (FP7, 2007-2013) for the project COPA-GT (grant agreement No. FP7-290042) as well as the European Union's Horizon 2020 Program (2014-2020) and the Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project (grant agreement No. 689772). Furthermore, computer resources and technical assistance has been provided by the Red Española de Supercomputación (RES) (grant numbers FI-2015-1-0002, FI-2015-3-0013, FI-2016-1-0001). ; Peer Reviewed ; Postprint (published version)

The present work addresses the coupling of a flamelet database, to a low-Mach approximation of the Navier–Stokes equations using scalar controlling variables. The model is characterized by the chemistry tabulation based on laminar premixed flamelets in combination with an optimal choice of the reaction progress variable, which is determined based on the computational singular perturbation (CSP) method. The formulation of the model focuses on turbulent premixed flames taking into account the effect of heat losses, but it is easily extended to partially premixed and non-premixed regimes. The model is designed for applications in both, Reynolds-averaged Navier–Stokes (RANS) as well as large-eddy simulations (LES) and results for the two methods are compared. A priori analysis of the database is presented to demonstrate the influence of the reaction progress definition and the chemistry tabulation is validated against a one-dimensional premixed laminar flame. The validation of the turbulent case is performed using a tur- bulent premixed confined jet flame subject to strong heat losses, in which the model shows a good overall performance. ; The research leading to these results has received funding through the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7, 2007– 2013) under the grant agreement No. FP7-290042 for the project COPA-GT. The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the Red Española de Supercomputación (RES). Finally, the authors would like to thank O. Lammel for kindly providing the data for comparison. ; Peer Reviewed ; Postprint (author's final draft)

The present work addresses the coupling of a flamelet database, to a low-Mach approximation of the Navier–Stokes equations using scalar controlling variables. The model is characterized by the chemistry tabulation based on laminar premixed flamelets in combination with an optimal choice of the reaction progress variable, which is determined based on the computational singular perturbation (CSP) method. The formulation of the model focuses on turbulent premixed flames taking into account the effect of heat losses, but it is easily extended to partially premixed and non-premixed regimes. The model is designed for applications in both, Reynolds-averaged Navier–Stokes (RANS) as well as large-eddy simulations (LES) and results for the two methods are compared. A priori analysis of the database is presented to demonstrate the influence of the reaction progress definition and the chemistry tabulation is validated against a one-dimensional premixed laminar flame. The validation of the turbulent case is performed using a tur- bulent premixed confined jet flame subject to strong heat losses, in which the model shows a good overall performance. ; The research leading to these results has received funding through the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7, 2007– 2013) under the grant agreement No. FP7-290042 for the project COPA-GT. The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the Red Española de Supercomputación (RES). Finally, the authors would like to thank O. Lammel for kindly providing the data for comparison. ; Peer Reviewed ; Postprint (author's final draft)

This study is focused on the numerical investigation of reduced chemical schemes for high pressure spray flames of oxymethylene ethers (OMEx). A reduced mechanism using Path Flux Analysis (PFA) technique is tested using an a priori and a posteriori evaluation under the ECN Spray A nominal conditions. The global flame metrics for spray A along with the flame shape are well reproduced by the reduced mechanism, which encourages its use for practical applications. ; The research leading to these results has received funding through the ENERXICO project from the European Union's Horizon 2020 Programme, grant agreement n 828947, and from the Mexican Department of Energy, CONACYT-SENER Hidrocarburos grant agreement n B-S-69926 and the Spanish Ministry of Economy and Competitiveness in the frame of the CHEST project (TRA2017-89139-C2-2- R). The authors thankfully acknowledge the computer resources at MareNostrum and the technical support provided by Barcelona Supercomputing Center (IM- 2021-1-0014, IM-2020-3-0029, IM-2020-2-0023,IM- 2020-1-0013). ; Peer Reviewed ; Postprint (published version)

The presented work addresses the investigation of the heat loss of a confined turbulent jet flame in a lab-scale combustor using a conjugate-heat transfer approach and large-eddy simulation. The analysis includes the assessment of the principal mechanisms of heat transfer in this combustion chamber: radiation, convection and conduction of heat over walls. A staggered approach is used to couple the reactive flow field to the heat conduction through the solid and both domains are solved using two implementations of the same code. Numerical results are compared against experimental data and an assessment of thermal boundary conditions to improve the prediction of the reactive flow field is given. ; The research leading to these results has received funding through the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7, 2007–2013) under the Grant agreement No. FP7-290042 for the project COPA-GT as well as the European Union's Horizon 2020 Programme (2014–2020) and from Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project, Grant agreement No. 689772. The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the Red Española de Supercomputación (RES). Finally, the authors would like to thank O. Lammel for the useful discussions and kindly providing the data for the comparison. ; Peer Reviewed ; Postprint (published version)

The presented work addresses the investigation of the heat loss of a confined turbulent jet flame in a lab-scale combustor using a conjugate-heat transfer approach and large-eddy simulation. The analysis includes the assessment of the principal mechanisms of heat transfer in this combustion chamber: radiation, convection and conduction of heat over walls. A staggered approach is used to couple the reactive flow field to the heat conduction through the solid and both domains are solved using two implementations of the same code. Numerical results are compared against experimental data and an assessment of thermal boundary conditions to improve the prediction of the reactive flow field is given. ; The research leading to these results has received funding through the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7, 2007–2013) under the Grant agreement No. FP7-290042 for the project COPA-GT as well as the European Union's Horizon 2020 Programme (2014–2020) and from Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project, Grant agreement No. 689772. The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the Red Española de Supercomputación (RES). Finally, the authors would like to thank O. Lammel for the useful discussions and kindly providing the data for the comparison. ; Peer Reviewed ; Postprint (published version)

The presented work addresses the investigation of the heat loss of a confined turbulent jet flame in a lab-scale combustor using a conjugate-heat transfer approach and large-eddy simulation. The analysis includes the assessment of the principal mechanisms of heat transfer in this combustion chamber: radiation, convection and conduction of heat over walls. A staggered approach is used to couple the reactive flow field to the heat conduction through the solid and both domains are solved using two implementations of the same code. Numerical results are compared against experimental data and an assessment of thermal boundary conditions to improve the prediction of the reactive flow field is given. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CCBY license (http://creativecommons.org/licenses/by/4.0/). ; The research leading to these results has received funding through the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7, 2007–2013) under the Grant agreement No. FP7-290042 for the project COPA-GT as well as the European Union's Horizon 2020 Programme (2014–2020) and from Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project, Grant agreement No. 689772. The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the Red Española de Super-computación (RES). Finally, the authors would like to thank O. Lammel for the useful discussions and kindly providing the data for the comparison. ; Peer Reviewed

Larger supercomputers allow the simulation of more complex phenomena with increased accuracy. Eventually this requires finer and thus also larger geometric discretizations. In this context, and extrapolating to the Exascale paradigm, meshing operations such as generation, deformation, adaptation/regeneration or partition/load balance, become a critical issue within the simulation workflow. In this paper we focus on mesh partitioning. In particular, we present a fast and scalable geometric partitioner based on Space Filling Curves (SFC), as an alternative to the standard graph partitioning approach. We have avoided any computing or memory bottleneck in the algorithm, while we have imposed that the solution achieved is independent (discounting rounding off errors) of the number of parallel processes used to compute it. The performance of the SFC-based partitioner presented has been demonstrated using up to 4096 CPU-cores in the Blue Waters supercomputer. ; The research leading to these results has received funding from the European Union Horizon 2020 Programme (2014-2020) and the Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project (grant agreement No. 689772). This work is also part of the PRAC "Simulations of Aircraft Engines" supported by the National Science Foundation. It has also been financially supported by the PRACE preparatory access projects funded in part by the EU Horizon 2020 research and innovation programme (2014-2020) under grant agreement 653838. J.C. Cajas acknowledges the nancial sup- port of the `Consejo Nacional de Ciencia y Tecnolog a (CONACyT, M exico)' grant number 231588 290790. Ricard Borrell and Daniel Mira acknowledge the Juan de la Cierva postdoctoral grants with codes IJCI-2014-21034 and IJCI-2015-26686, respectively. ; Postprint (author's final draft)

Larger supercomputers allow the simulation of more complex phenomena with increased accuracy. Eventually this requires finer and thus also larger geometric discretizations. In this context, and extrapolating to the Exascale paradigm, meshing operations such as generation, deformation, adaptation/regeneration or partition/load balance, become a critical issue within the simulation workflow. In this paper we focus on mesh partitioning. In particular, we present a fast and scalable geometric partitioner based on Space Filling Curves (SFC), as an alternative to the standard graph partitioning approach. We have avoided any computing or memory bottleneck in the algorithm, while we have imposed that the solution achieved is independent (discounting rounding off errors) of the number of parallel processes used to compute it. The performance of the SFC-based partitioner presented has been demonstrated using up to 4096 CPU-cores in the Blue Waters supercomputer. ; The research leading to these results has received funding from the European Union Horizon 2020 Programme (2014-2020) and the Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project (grant agreement No. 689772). This work is also part of the PRAC "Simulations of Aircraft Engines" supported by the National Science Foundation. It has also been financially supported by the PRACE preparatory access projects funded in part by the EU Horizon 2020 research and innovation programme (2014-2020) under grant agreement 653838. J.C. Cajas acknowledges the nancial sup- port of the `Consejo Nacional de Ciencia y Tecnolog a (CONACyT, M exico)' grant number 231588 290790. Ricard Borrell and Daniel Mira acknowledge the Juan de la Cierva postdoctoral grants with codes IJCI-2014-21034 and IJCI-2015-26686, respectively. ; Postprint (author's final draft)