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Numerical study on flame propagation in n-heptane/air mixture with the use of a gradient LES combustion model

100%

Jaworski P. , Teodorczyk A.

tom Vol. 91, nr 3

114-121

This paper presents the results of numerical simulations with a combustion model using Large Eddy Simulation (LES). The objective is to check whether the proposed combustion model is capable of representing the laminar reacting flow. The numerical results are compared with flame front propagation data gained from experiments. The combustion model is based on the gradient method, which determines flame propagation. The gradient is calculated from the mass fraction of fuel or products. Laminar burning velocity is described by empirical correlation. Flame generated turbulence is used in this study to represent the nonlinear flame propagation effects in the laminar reacting flow. From the results it is concluded that flame generated turbulence can be used for laminar reacting flows and is important for representation of the combustion process in numerical simulations. The gradient combustion model for turbulence reacting flow is capable of proper representation of the flame front in laminar reacting flows. The gradient combustion model for LES did not increase the time needed for calculation, making it an attractive method in full engine cycle simulations.

Numerical simulations of n-heptane spray in high pressure and temperature environments

80%

Smuga W. , Kapusta Ł. J. , Teodorczyk A.

tom Vol. 97, nr 1

1--6

In this study n-heptane spray in supercritical environments was simulated using commercial CFD (Computational Fluid Dynamic) software AVL Fire. The numerical results were analyzed in terms of global spray parameter, and spray penetration. The results obtained were compared with experimental data available at Sandia National Laboratories. N-heptane spray simulations were performed in the same conditions as in the Sandia experiments. The goal of the study was to assess whether the Lagrangian approach performs well in engine relevant conditions in terms of spray global parameters. Not included in this assessment was the influence of supercritical mixing on liquid-gas interphase. The major element was the potential for practical application of the commercial CFD code in terms of properly representing global spray parameters and thus mixture formation in supercritical conditions, which is one of the core aspects in whole engine process simulation. The key part of the study was mesh optimization. Therefore, the influence of mesh density on both the accuracy of calculations and the calculation time was determined, taking into consideration detailed experimental data as initial conditions for the subsequent calculations. This served as a basis to select the optimal mesh with regard to both accuracy of the results obtained and time duration of the calculations. As a determinant of accuracy, the difference within a range of evaporated fuel stream was used. Using selected mesh the set of numerical calculations were performed and the results were compared with experimental ones taken from the literature. Several spray parameters were compared: spray tip penetration, temperature of the gaseous phase and mixture fraction in the gaseous phase. The numerical results were very consistent in respect of spray tip penetration. The other parameters were influenced by specific features of the Lagrangian approach. Nevertheless the results obtained showed that the Lagrangian approach may be used for engine relevant conditions.

Numerical investigation on diesel combustion and emissions with a standard combustion model and detailed chemistry

80%

Pyszczek R. , Schmalhorst C. , Teodorczyk A.

tom R. 54, nr 3

19--33

The aim of this study was to determine possibilities of the soot and NOx emissions reduction from an existing heavy-duty compression-ignition (CI) engine based only on in-cylinder techniques. To that end numerical simulations of such processes as a multiphase fuel flow through injector nozzles, a liquid fuel jet breakup and evaporation, combustion and emissions formation were performed in AVL Fire 3D CFD software. The combustion process was calculated with the ECFM-3Z model and with the detailed n-heptane oxidation scheme that consisted of 76 species and 349 reactions. Both approaches of combustion modeling were validated against experimental data from the existing engine working under 75% and 100% loads. As for the reduction of the NOx emission an introduction of exhaust gas recirculation (EGR) was investigated. As for the soot concentration reduction such measures as an increased rail pressure, application of a post-injection and an increased injector nozzles conicity were investigated. Finally the ECFM-3Z model with emissions models, as well as the n-heptane mechanism predicted that it is possible to reach specified emissions limits with application of EGR, post-injection and increased nozzles conicity.