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Abstract Anthropogenic Greenhouse Gases (GHG) emissions have increased since the pre-industrial era, driven by the economic and population growth. In the last decades, significant efforts have been made to develop cleaner and more efficient technologies able to control GHG emissions. In the field of combustion burners, this commitment strongly relies on the development of new and detailed numerical models, which allow a deep investigation of complex burner performance and a significant reduction of full-scale experimental costs. The knowledge previously acquired on the TEA-C coal burner CFD study is here expanded in the framework of the Be4GreenS project, targeting to the development of a new generation of burners. The new implemented model accounts for the detailed chemical and kinetic characterization of the pulverized coal together with the exact inner volume geometry of the experimental combustion chamber and the actual extension of the heat exchanging and refractory surfaces. In addition, the secondary and tertiary air registers are here numerically investigated. The outcome from the CFD analysis is validated against the experimental data. The NOx formation analysis is also provided. [ABSTRACT FROM AUTHOR]

Abstract In recent years, the research community devoted many resources to define accurate methodologies to model the real physics behind turbulent combustion. Such effort aims at reducing the need for case-by-case calibration in internal combustion engine simulations. In the present work two of the most widespread combustion models in the engine modelling community are compared, namely ECFM-3Z and G-equation. The interaction of turbulent flows with combustion chemistry is investigated and understood. In particular, the heat release rate characterizing combustion, and therefore the identification of a flame front, is analysed based on flame surface density concept rather than algebraic correlations for turbulent burn rate. In the first part, spark-ignition (S.I.) combustion is simulated in an optically accessible GDI single-cylinder research engine in firing conditions. The turbulent combustion regime is mapped on the Borghi-Peters diagram for all the conditions experienced by the engine flame, and the consistency of the two combustion models is critically analysed. In the second part, a simple test case is defined to test the two combustion models in an ideally turbulence-controlled environment: this allows to fully understand the main differences between the two combustion models under well-monitored conditions. and results are compared against experimental databases of turbulent burn rate for wide ranges of Damkohler (Da) and Karlovitz (Ka) numbers. The joint experimental and numerical study presented in this paper evaluates different approaches within the unified flamelet/non-flamelet framework for modelling turbulent combustion in SI engines. It also indicates guidelines for reduced calibration effort in widespread combustion models. [ABSTRACT FROM AUTHOR]

The present numerical study aims to assess the performance of an Eulerian Stochastic Field (ESF) model in simulating spray flames produced by three fuel injectors with different nozzle diameters of 100 µm, 180 µm and 363 µm. A comparison to the measurements shows that although the simulated ignition delay times are consistently overestimated, the relative differences remain below 28%. Furthermore, the change of the averaged pressure rise with respect to the variation of nozzle diameter is captured by the model. The simulated flame lift-off lengths also agree with the measurements, with a maximum relative difference of 13%. The spray flame produced by a larger nozzle diameter has a fuel-richer premixed core region despite the longer lift-of length, indicating that the higher fueling rate used with the larger nozzle diameter is a more dominating factor than the lift-off length is in influencing the air entrainment into the upstream of the spray flames. In addition, the simulated normalised flame lengths are found to decrease when the nozzle diameters increase. These predictions are in good qualitative agreement with the experimental observation. This work proves that the ESF model can serve as an important tool for the simulation of spray flames in marine diesel engines, where fuel injectors with different nozzle diameters are applied for pilot and main injections. [ABSTRACT FROM AUTHOR]

The environmental and economic objectives for the reduction of fuel consumption and reduction in the polluting emissions are always of crucial news. Thus, the elucidation of new techniques of combustion using the diffusion flame (more secure) in a confined environment and the comprehension of the involved physical phenomena remains of capital challenge. The dilution of the combustion air by CO 2 has confirmed its technological interest; the objective of this work is to quantify numerically the polluting emissions in a turbulent diffusion flame. The air is diluted by CO 2 and attention has been paid to its thermal and chemical effects on the internal structure of the flame. It is found that CO 2 addition reduces flame temperature by both thermal and chemical effects. Most of the emissions are also reduced by CO 2 increasing in the air. [ABSTRACT FROM AUTHOR]

Mild combustion in a lab-scale furnace has been experimentally and numerically studied. The furnace was operated with Dutch natural gas (DNG) at 10 kW and at an equivalence ratio of 0.8. OH* chemiluminescence images were taken to characterize the reaction zone. The chemiluminescence intensity is relatively low compared to conventional flames and relatively uniformly distributed in the reaction zone due to the dilution effects of recirculated burnt gases. Visible flames were not observed. To characterize the dilution effects of burnt gases on reactions, flamelets generated with diluted fuel and diluted air, instead of flamelets based on pure fuel and air, were applied in an extended Flamelet Generated Manifold (FGM) approach. Burnt gases at stoichiometric mixture fraction rather than those at global equivalence ratio were considered as diluent, which is more appropriate for furnaces operating at lean condition. The numerical simulations were performed using the open source CFD package -- OpenFOAM. [ABSTRACT FROM AUTHOR]

Ultra-Lean operating conditions have a great potential toward the increase of engine thermal efficiency and control of pollutant emissions in Internal Combustion Engines; however, cyclic variability and stability are still critical issues to be solved. The use of natural gas may partially solve these issues due to a more efficient mixing process, widening the flammability limits. An analysis of natural gas ultra-lean combustion process is presented in this work using local charge stratification to stabilize the process. In particular, the predictive capabilities of the numerical model are here validated up to ultra-lean (λ=2.0) mixtures. Moreover, an analysis of the ignition timing has been performed, to find the optimal trade-off between flame quenching and turbulent kinetic energy decay as produced by the jet in the vicinity of the spark location. The optimal ignition timing has been calculated in the range of 5ms. [ABSTRACT FROM AUTHOR]

The present work describes the main findings derived from CFD simulations of the swirl stabilized spray flame experimentally investigated by Sheen [1] . Scale Adaptive Simulations (SAS) have been performed using Flamelet Generated Manifold (FGM) for combustion modelling and a Eulerian-Lagrangian approach for liquid phase description. Results highlight the capabilities of SAS in predicting the main characteristics of the analysed turbulent spray flame, leading to appreciable enhancements with respect to RANS results in terms of both velocity and temperature distributions. [ABSTRACT FROM AUTHOR]

Differential molecular diffusion effects are expected to be of particular importance in the combustion of hydrogen and H 2 -enriched fuels. The mixing between fuel and oxidizer can be significantly influenced, implying changed chemical reaction rates and overall heat release. Here, the LEM3D model based on the Linear Eddy Model (LEM) is employed to simulate differential diffusion effects in a turbulent round jet of H 2 and Freon 22 issuing into air. Input to LEM3D, generated from ANSYS Fluent, consists of velocity profiles and profiles for the turbulent diffusivity, the Kolmogorov scale and the integral length scale. In this paper, the LEM3D-Fluent coupling is demonstrated by a coarse steady-state RANS simulation in Fluent with a one-to-one correspondence between the RANS grid cells and the LEM3D control volumes. The LEM3D simulation results are compared with previously obtained measurements of differential molecular diffusion for the given flow configuration. [ABSTRACT FROM AUTHOR]

The Partially Stratified Charge Spark Ignition (PSC-SI) combustion strategy is envisaged as a way of reducing fuel consumption and therefore polluting emissions; the improved fuel economy is mainly due to lean, stratified combustion, and to the reduction of pumping losses at partial load conditions. The aim of this work is to explore the potential capabilities of the PSC-SI combustion strategy over a wide flammability air-to-fuel ratio range with a CFD-based computational approach. A validated LES solver has been used to represent the main occurring phenomena into an experimentally implemented Constant Volume Combustion Chamber (CVCC). For different air fuel ratios, both homogeneous and non-homogeneous combustion processes have been simulated in order to compare and emphasize the benefits of the PSC-SI and the impact of the choice of operating conditions. [ABSTRACT FROM AUTHOR]

A numerical study on the turbulent combustion in porous media is conducted based on 2D staggered cylinders using the eddy dissipation concept model (EDC) with emphasis on characteristics of the turbulent premixed flame. The results show that as the velocity increases, the sheet of laminar flame is stretched and distorted gradually. A burned island forms at the recirculating zone due to the entrainment. Based on the parameter study, it is find that turbulent kinetic energy gradually decay after the reaction taking place. The pseudo thermal flame thickness is less than the length scale of a representative elementary volume. The Damköhler number at quasi-steady state is in the order of 10, indicating that the flame is in the flamelet regime. [ABSTRACT FROM AUTHOR]

Two-dimensional direct numerical simulation (2D-DNS) is used to investigate the effect of turbulence intensity and composition stratification on the ignition of H 2 /air mixture in a constant volume enclosure relevant to homogeneous charge compression ignition (HCCI) engines. A comprehensive H 2 chemical kinetic mechanism is adopted, involving 9 species and 21 elementary reactions. Different turbulence levels and composition fluctuations are simulated to investigate their influence on the combustion behavior. The results show that the ignition delay time tends to be prolonged and the heat release rate increased under higher turbulence intensity. Turbulence can affect the reaction zone, e.g., through wrinkling of reaction front. Higher composition stratification can smoothen the heat release rate. It is found that in HCCI engine flame propagation can co-exist with volumetric auto-ignition. [ABSTRACT FROM AUTHOR]

The LEM3D stochastic model for numerical simulation of turbulent mixing is used to simulate differential molecular diffusion effects in an isothermal jet of hydrogen and Freon 22 issued into air. The computations are compared with a published experimental study of the flow configuration. Salient features of the measured results are reproduced qualitatively, but the absence of spatial variation of the smallest eddy motion in LEM3D omits the streamwise variation of this length scale in the experimental configuration, resulting in a systematic deviation from the experimental trend. A first-principles basis for incorporating this missing physics into LEM3D is described, indicating the path forward for physically based quantitative prediction of differential diffusion effects, and turbulent combustion phenomenology more generally, using LEM3D. [ABSTRACT FROM AUTHOR]

Abstract: Our contribution consists of the demonstration of the advantages of LES-WALE coupled with PDF approach including progress variable (c) in reacting flow, by using the Fluent-CFD. The confirmation is based on the comparison of three parameters: mean longitudinal velocity, intensity of longitudinal velocity and lengths of recirculations zones. In line with what was observed by the experimental reference study, the simulation succeeds to detect the flame zone of the recirculating region and shows the differences between the different cases of flows in variable equivalence ratio (Φ=0.60 and 0.75) or in variable mass flow rate (Q= 65 and 195g/s). All cases of study considered the identical conditions from the tow supply channels of the burner. The main issue is the modeling of the closure of the turbulent combustion. In addition, the numerical simulation predicts as well as the asymmetry and the symmetry flow, respectively, for inert and reacting flows. [Copyright &y& Elsevier]