The unsteady two-dimensional conditional moment closure (CMC) model with first-order closure of t... more The unsteady two-dimensional conditional moment closure (CMC) model with first-order closure of the chemistry and supplied with standard models for the conditional convection and turbulent diffusion terms has been interfaced with a commercial engine CFD code and analyzed ...
Abstract Compact and computationally efficient reaction models capable of accurately predicting i... more Abstract Compact and computationally efficient reaction models capable of accurately predicting ignition delay and heat release rates are a prerequisite for the development of strategies to control and optimize HCCI engines. In particular for full boiling range fuels ...
ABSTRACT Three-dimensional Computational Fluid Dynamics (CFD) has become an integral part in anal... more ABSTRACT Three-dimensional Computational Fluid Dynamics (CFD) has become an integral part in analyzing engine in-cylinder processes since it provides detailed information on flow and combustion inside internal combustion engines, hence allowing for improvements during the development of modern engine concepts. The predictive capability of simulation tools depends largely on the accuracy, fidelity and robustness of the various model used, in particular concerning turbulence and combustion, and, in some cases, two-phase flow. Almost all combustion models currently used in engine design require some level of parameter tuning to obtain a reasonable match between measured and computed in-cylinder pressure evolution. However, if the model parameters are closely tied to the physics of the problem then one might be able to eliminate this model tuning, specifically for the combustion sub-modeling part. The model employed here has seen successful application to laboratory type flames, i.e. freely propagating flames, spherically expanding methane– and hydrogen-air flames as well as spark-ignited, turbulent swirling stoichiometric propane-air flames at engine relevant conditions in a constant volume combustion chamber. In this study, the model is applied for the first time to intermittent combustion in an internal combustion engine in the context of a RANS (k-ε) turbulence treatment. Validation is performed by means of experimental data from a gasoline port fuel injected, single-cylinder, four-stroke spark-ignited engine operated at 3,500 RPM close to full load (9 bar BMEP) at stoichiometric conditions. A simple ‘energy deposition’ approach was adopted to model spark ignition for which the sensitivity of the predictions is discussed. The combustion modeling framework used in this study is based on the flamelet concept, where complex chemistry can be separated from fluid dynamics. In this revised flamelet model, the dissipation rate of reaction progress variable variance plays a central role and its influence on the overall combustion process is felt through a presumed probability density function (PDF) for the reaction progress variable. Look-up tables constructed using a skeletal (29 species and 49 reactions) and detailed (1389 species and 5935 reactions) chemical mechanisms for iso-octane–air combustion for the chosen engine operating condition are employed. This first application to IC engines confirms that the adopted modeling approach gives a reasonably good agreement in terms of in-cylinder pressure evolution compared to the experimental data without having to change any combustion sub-model related parameters, provided the chemical mechanism correctly predicts the laminar flame characteristics for the given mixture conditions.
This paper presents experimental data on the autoignition of hydrogen jets in a turbulent co-flow... more This paper presents experimental data on the autoignition of hydrogen jets in a turbulent co-flow of hot air and associated modelling using the Conditional Moment Closure. The aim is to understand better how turbulence affects autoignition of non-premixed flows and to validate modelling approaches. It is demonstrated that the turbulent mixing causes randomness in the ignition location, that it can delay the onset of autoignition and that the autoignition time has a higher sensitivity to temperature than that expected in homogeneous mixtures. The CMC method successfully reproduces the measured autoignition times.
ABSTRACT The dynamics and evolution of the turbulent flow inside an experimentally investigated e... more ABSTRACT The dynamics and evolution of the turbulent flow inside an experimentally investigated engine-like geometry consisting of a flat-top cylinder head with a fixed, axis-centered valve and low-speed piston were studied numerically by means of Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES), with a particular focus on Cycle-to-Cycle Variability (CCV). DNS was performed by the spectral element code nek5000 on a 58M points grid, whereas LES was carried out by the finite volume software OpenFOAM on a 4.6M hexahedral mesh. Results obtained by DNS and LES are compared with respect to the velocity means and fluctuations, as well with other derived quantities, achieving good agreement between simulations and experiments. The cyclic variability and complex unsteady flow features like the laminar-to-turbulent transition and the evolution of the tumble vortices were studied by time-resolved analysis and Proper Orthogonal Decomposition (POD). Simulations show that during the first half of the intake stroke the flow field is dominated by the dynamics of the incoming jet and the vortex rings it creates. With decreasing piston speed, the large central ring becomes the dominant flow feature until the top dead center. The flow field at the end of the previous cycle is found to have a strong effect on the jet breakup process and the dynamics of the vortex ring below the valve of the subsequent cycle as well as on the observed significant cyclic variations.
The unsteady two-dimensional conditional moment closure (CMC) model with first-order closure of t... more The unsteady two-dimensional conditional moment closure (CMC) model with first-order closure of the chemistry and supplied with standard models for the conditional convection and turbulent diffusion terms has been interfaced with a commercial engine CFD code and analyzed ...
Abstract Compact and computationally efficient reaction models capable of accurately predicting i... more Abstract Compact and computationally efficient reaction models capable of accurately predicting ignition delay and heat release rates are a prerequisite for the development of strategies to control and optimize HCCI engines. In particular for full boiling range fuels ...
ABSTRACT Three-dimensional Computational Fluid Dynamics (CFD) has become an integral part in anal... more ABSTRACT Three-dimensional Computational Fluid Dynamics (CFD) has become an integral part in analyzing engine in-cylinder processes since it provides detailed information on flow and combustion inside internal combustion engines, hence allowing for improvements during the development of modern engine concepts. The predictive capability of simulation tools depends largely on the accuracy, fidelity and robustness of the various model used, in particular concerning turbulence and combustion, and, in some cases, two-phase flow. Almost all combustion models currently used in engine design require some level of parameter tuning to obtain a reasonable match between measured and computed in-cylinder pressure evolution. However, if the model parameters are closely tied to the physics of the problem then one might be able to eliminate this model tuning, specifically for the combustion sub-modeling part. The model employed here has seen successful application to laboratory type flames, i.e. freely propagating flames, spherically expanding methane– and hydrogen-air flames as well as spark-ignited, turbulent swirling stoichiometric propane-air flames at engine relevant conditions in a constant volume combustion chamber. In this study, the model is applied for the first time to intermittent combustion in an internal combustion engine in the context of a RANS (k-ε) turbulence treatment. Validation is performed by means of experimental data from a gasoline port fuel injected, single-cylinder, four-stroke spark-ignited engine operated at 3,500 RPM close to full load (9 bar BMEP) at stoichiometric conditions. A simple ‘energy deposition’ approach was adopted to model spark ignition for which the sensitivity of the predictions is discussed. The combustion modeling framework used in this study is based on the flamelet concept, where complex chemistry can be separated from fluid dynamics. In this revised flamelet model, the dissipation rate of reaction progress variable variance plays a central role and its influence on the overall combustion process is felt through a presumed probability density function (PDF) for the reaction progress variable. Look-up tables constructed using a skeletal (29 species and 49 reactions) and detailed (1389 species and 5935 reactions) chemical mechanisms for iso-octane–air combustion for the chosen engine operating condition are employed. This first application to IC engines confirms that the adopted modeling approach gives a reasonably good agreement in terms of in-cylinder pressure evolution compared to the experimental data without having to change any combustion sub-model related parameters, provided the chemical mechanism correctly predicts the laminar flame characteristics for the given mixture conditions.
This paper presents experimental data on the autoignition of hydrogen jets in a turbulent co-flow... more This paper presents experimental data on the autoignition of hydrogen jets in a turbulent co-flow of hot air and associated modelling using the Conditional Moment Closure. The aim is to understand better how turbulence affects autoignition of non-premixed flows and to validate modelling approaches. It is demonstrated that the turbulent mixing causes randomness in the ignition location, that it can delay the onset of autoignition and that the autoignition time has a higher sensitivity to temperature than that expected in homogeneous mixtures. The CMC method successfully reproduces the measured autoignition times.
ABSTRACT The dynamics and evolution of the turbulent flow inside an experimentally investigated e... more ABSTRACT The dynamics and evolution of the turbulent flow inside an experimentally investigated engine-like geometry consisting of a flat-top cylinder head with a fixed, axis-centered valve and low-speed piston were studied numerically by means of Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES), with a particular focus on Cycle-to-Cycle Variability (CCV). DNS was performed by the spectral element code nek5000 on a 58M points grid, whereas LES was carried out by the finite volume software OpenFOAM on a 4.6M hexahedral mesh. Results obtained by DNS and LES are compared with respect to the velocity means and fluctuations, as well with other derived quantities, achieving good agreement between simulations and experiments. The cyclic variability and complex unsteady flow features like the laminar-to-turbulent transition and the evolution of the tumble vortices were studied by time-resolved analysis and Proper Orthogonal Decomposition (POD). Simulations show that during the first half of the intake stroke the flow field is dominated by the dynamics of the incoming jet and the vortex rings it creates. With decreasing piston speed, the large central ring becomes the dominant flow feature until the top dead center. The flow field at the end of the previous cycle is found to have a strong effect on the jet breakup process and the dynamics of the vortex ring below the valve of the subsequent cycle as well as on the observed significant cyclic variations.
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