The quantification of the prediction accuracy in large eddy simulations (LES) is very challenging due to various interacting errors associated with this approach. When dealing with errors in LES using implicit filtering, numerical and modeling errors have drawn the interest of many researchers. Little attention has been paid to other sources of discrepancies between LES results and reference data, namely sampling errors, influence of the initial conditions, improper boundary conditions or uncertainties issuing from reference data. A framework of metrics that includes all these issues is addressed in the present paper to study subgrid-scale (SGS) models for LES and to quantify their prediction accuracy and computational costs. The method is applied to a simple wall-bounded turbulent flow at moderate Reynolds number. It turns out from the results obtained with six commonly used SGS models that wall-adapting models (WALE and SIGMA) and localized dynamic models reproduce the physics of the flow field more faithfully, reveal a superior prediction accuracy and have a similar computational cost than models using van Driest wall damping. Especially at the viscous wall region (\(r^+<50\)), wall-adapting and localized dynamic models are more accurate, reflecting the proper near wall behavior of such models. Relying on the analysis of sources of various errors, uncertainties in LES are estimated and systematically assessed, and their influence on simulation results is quantified. Finally, engineering estimations of the required averaging time to obtain basic estimates of statistical quantities with a predetermined degree of accuracy are suggested.

Taking into account detailed chemical kinetics and therefore allowing for a detailed representation of the flame's microstructure at reduced computational cost make flamelet-based tabulation approaches such as the flamelet-generated manifold (FGM) a commonly used method for turbulent combustion simulations. However, there has been little focus on analyzing such models for fuel blends, including hydrogen. One reason for that is the challenging inclusion of differential diffusion effects into FGM, which may become crucial for highly diffusive fuels such as hydrogen. This paper presents an extension of the FGM approach that takes into account differential diffusion to assess the importance of differential diffusion for methane hydrogen blends. To this end, an extended model containing five controlling variables can be derived. However, the high correlation of certain controlling variables and the number of control variables could be reduced to three controlling variables in this study. These models are coupled to the artificially thickened flame (ATF) approach to facilitate large-eddy simulations (LESs). To ensure the consistency of the coupling between FGM and ATF when differential diffusion is considered, the model is thoroughly verified and validated using freely propagating and stratified laminar one-dimensional flames. Finally, simulations of the turbulent premixed stratified burner operated with a hydrogen methane blend are performed. The validation of the modeling framework is performed by comparing the simulation results to extensive experimental data, allowing an in-depth analysis of the macro- and microstructure of the flame.

This paper analyzes the liquid atomization mechanisms of an N-heptane spray jet emerging from a swirl simplex atomizer using numerical and experimental techniques. In particular, a direct seamless coupled Volume of Fluid and Lagrangian Particle Tracking approach together with adaptive mesh refinement within the Large Eddy Simulation framework offers a suitable way to accurately simulate the complex behavior of spray atomization, spray evolution, and droplet dispersion as a whole while using manageable computational cost. The achieved simulation results are first presented in terms of qualitative properties, characteristics of liquid sheet, air core generation, flow recirculation zones, and vortex patterns. For validation purposes, the numerical results are then compared with detailed experimental data obtained by a two-component Phase Doppler Anemometry technique. The assessment includes especially droplet statistics which strongly determine subsequent possible spray combustion process and related product and species emissions. The overall reported agreement demonstrates the capability of the adopted methodology in predicting and comprehensively investigating the complex phenomena associated with a pressure swirl fuel atomizer. In particular, it is found out that the normalized number-based probability density function of droplet size fits well with a lognormal distribution.

The entropy generation analysis of adiabatic combustion systems was performed to quantify the exergy losses which are mainly the exergy destroyed during combustion inside the chamber and in the exhaust gases. The purpose of the present work was therefore: (a) to extend the exergy destruction analysis by including the exhaust gas exergy while applying the hybrid filtered Eulerian stochastic field (ESF) method coupled with the FGM chemistry tabulation strategy; (b) to introduce a novel method for evaluating the exergy content of exhaust gases; and (c) to highlight a link between exhaust gas exergy and combustion emissions. In this work, the adiabatic Sandia flames E and F were chosen as application combustion systems. First, the numerical results of the flow and scalar fields were validated by comparison with the experimental data. The under-utilization of eight stochastic fields (SFs), the flow field results and the associated scalar fields for the flame E show excellent agreement contrary to flame F. Then, the different exergy losses were calculated and analyzed. The heat transfer and chemical reaction are the main factors responsible for the exergy destruction during combustion. The chemical exergy of the exhaust gases shows a strong relation between the exergy losses and combustion emission as well as the gas exhaust temperature.

Different strategies to account for the heat and mass transfer between liquid droplets and their carrier phase within the Artificially Thickened Flame (ATF) approach are analyzed and compared. Herein, two approaches are introduced to take into account the droplet movement relative to the thickened flame front orientation. While the first approach achieves this behavior through scalar modifications in the droplet temperature and mass evolution equations, the second one introduces a trajectory modification within the thickened flame front. Both approaches, referred to as projection and refraction correction, are first compared to state of the art methods in a simplified two-dimensional configuration, and then in a complex turbulent spray flame. The investigated spray flame corresponds to the operating condition EtF6 of the Sydney Spray Burner. The analysis showed that: (1) A consideration of a simplified configuration is insufficient to fully uncover the performance of the different approaches. (2) While the proposed approaches performed best in the two-dimensional configuration, only the projection method outperforms the remaining ones in the turbulent spray flame. (3) The formulation to consider the flame thickening has a strong effect on global flame properties, combustion regime distribution as well as carrier and liquid phase statistics.

Evaporative cooling effects and turbulence flame interaction are analyzed in the large eddy simulation (LES) context for an ethanol turbulent spray flame. Investigations are conducted with the artificially thickened flame (ATF) approach coupled with an extension of the mixture adaptive thickening procedure to account for variations of enthalpy. Droplets are tracked in a Euler–Lagrangian framework, in which an evaporation model accounting for the inter-phase non-equilibrium is applied. The chemistry is tabulated following the flamelet generated manifold (FGM) method. Enthalpy variations are incorporated in the resulting FGM database in a universal fashion, which is not limited to the heat losses caused by evaporative cooling effects. The relevance of the evaporative cooling is evaluated with a typically applied setting for a flame surface wrinkling model. Using one of the resulting cases from the evaporative cooling analysis as a reference, the importance of the flame wrinkling modeling is studied. Besides its novelty, the completeness of the proposed modeling strategy allows a significant contribution to the understanding of the most relevant phenomena for the turbulent spray combustion modeling.