Heinz Pitsch

Large-eddy simulation of a turbulent piloted methane/air diffusion flame (Sandia flame D)

The Lagrangian Flamelet Model is formulated as a combustion model for large-eddy simulations of turbulent jet diffusion flames. The model is applied in a large-eddy simulation of a piloted partially premixed methane/air diffusion flame (Sandia flame D). The results of the simulation are compared to experimental data of the mean and RMS of the axial velocity and the mixture fraction and the unconditional and conditional averages of temperature and various species mass fractions, including CO and NO. All quantities are in good agreement with the experiments. The results indicate in accordance with experimental findings that regions of high strain appear in layer like structures, which are directed inwards and tend to align with the reaction zone, where the turbulence is fully developed. The analysis of the conditional temperature and mass fractions reveals a strong influence of the partial premixing of the fuel.

Development of an Experimental Database and Kinetic Models for Surrogate Diesel Fuels

Computational fluid dynamic (CFD) simulations that include realistic combustion/emissions chemistry hold the promise of significantly shortening the development time for advanced high-efficiency, low-emission engines. However, significant challenges must be overcome to realize this potential. This paper discusses these challenges in the context of diesel combustion and outlines a technical program based on the use of surrogate fuels that sufficiently emulate the chemical complexity inherent in conventional diesel fuel. The essential components of such a program are discussed and include: (a) surrogate component selection; (b) the acquisition or estimation of requisite elementary chemical kinetic, thermochemical, and physical property data; (c) the development of accurate predictive chemical kinetic models, together with the measurement of the necessary fundamental laboratory data to validate these mechanisms; and (d) mechanism reduction tools to render the coupled chemistry/flow calculations feasible. In parallel to these efforts, the need exists to develop similarly robust models for fuel injection and spray processes involving multicomponent mixtures of wide distillation character, as well as methodologies to include all of these high fidelity submodels in computationally efficient CFD tools. Near- and longerterm research plans are proposed based on an application target of premixed diesel combustion. In the near term, the recommended surrogate components include n-decane, iso-octane, methylcyclohexane, and toluene. For the longer term, n-hexadecane, heptamethylnonane, n-decylbenzene, and 1-methylnaphthalene are proposed.

High order conservative finite difference scheme for variable density low Mach number turbulent flows

The high order conservative finite difference scheme of Morinishi et al. Y. Morinishi, O.V. Vasilyev, T. Ogi, Fully conservative finite difference scheme in cylindrical coordinates for incompressible flow simulations, J. Comput. Phys. 197 (2004) 686] is extended to simulate variable density flows in complex geometries with cylindrical or cartesian non-uniform meshes. The formulation discretely conserves mass, momentum, and kinetic energy in a periodic domain. In the presence of walls, boundary conditions that ensure primary conservation have been derived, while secondary conservation is shown to remain satisfactory. In the case of cylindrical coordinates, it is desirable to increase the order of accuracy of the convective term in the radial direction, where most gradients are often found. A straightforward centerline treatment is employed, leading to good accuracy as well as satisfactory robustness. A similar strategy is introduced to increase the order of accuracy of the viscous terms. The overall numerical scheme obtained is highly suitable for the simulation of reactive turbulent flows in realistic geometries, for it combines arbitrarily high order of accuracy, discrete conservation of mass, momentum, and energy with consistent boundary conditions. This numerical methodology is used to simulate a series of canonical turbulent flows ranging from isotropic turbulence to a variable density round jet. Both direct numerical simulation (DNS) and large eddy simulation (LES) results are presented. It is observed that higher order spatial accuracy can improve significantly the quality of the results. The error to cost ratio is analyzed in details for a few cases. The results suggest that high order schemes can be more computationally efficient than low order schemes.

A Consistent Flamelet Formulation for Non-Premixed Combustion Considering Differential Diffusion Effects

A flamelet formulation for non-premixed combustion that allows an exact description of differential diffusion has been developed. The main difference to previous formulations is the definition of a mixture fraction variable, which is not related directly to any combination of the reactive scalars, but defined from the solution of a conservation equation with an arbitrary diffusion coefficient and appropriate boundary conditions. Using this definition flamelet equations with the mixture fraction as the independent coordinate are derived without any assumptions about the Lewis numbers for chemical species. The formulation is shown to be exact if the scalar dissipation rate is prescribed as a function of the mixture fraction. Different approximations of the scalar dissipation rate that had been derived from analytic solutions for special cases are investigated by varying the diffusion coefficient of the mixture fraction transport equation. As examples, counterflow flames of hydrogen and n-heptane, which have much larger and much smaller diffusivities than oxygen and nitrogen, are considered. It is shown that the use of equal thermal and mixture fraction diffusivities yields a sufficiently well-described flame structure and is therefore recommended for the definition of the mixture fraction diffusion coefficient. Finally, the possibility of using constant species Lewis numbers has been examined. It has been found that once an appropriate set of Lewis numbers is determined, good results are achieved over wide ranges of the parameters, such as scalar dissipation rate, pressure, and oxidizer temperature.

LARGE-EDDY SIMULATION OF PREMIXED TURBULENT COMBUSTION USING A LEVEL-SET APPROACH

In the present study, we have formulated the G equation concept for large-eddy simulation (LES) of premixed turbulent combustion. The developed model for the subgrid burning velocity is shown to cor- rectly reflect Damkohlers limits for large- and small-scale turbulence. From the discussion of the regime diagram for turbulent premixed combustion, it is shown that given a particular configuration of flow pa- rameters, changes in the LES filter width result in changes along constant Karlovitz number lines. The Karlovitz number is chosen as the horizontal axis to construct a new regime diagram for LES of premixed turbulent combustion, where changes in the filter width are represented by vertical lines. In addition, some new regimes appear in the new diagram, which are related to the numerical treatment. An important conclusion is that changes in the filter width cannot result in changes of the combustion regime among the corrugated flamelets, thin reaction zones, and broken reaction zones regimes. This is a consistency requirement for the model, since the choice of the filter width cannot change the fundamental combustion mode. With decreasing filter width, changes from corrugated to wrinkled flamelets, or in general to a laminar regime, are possible. In applying the model in a numerical simulation of a turbulent Bunsen burner experiment, it is shown that the results predict the mean flame front location, and thereby the turbulent burning velocity, and the influence of the heat release on the flow field in good agreement with experimental data.

Extinction and Autoignition of n-Heptane in Counterflow Configuration

A study is performed to elucidate the mechanisms of extinction and autoignition of n-heptane in strained laminar flows under nonpremixed conditions. A previously developed detailed mechanism made UP of 2540 reversible elementary reactions among 557 species is the starting point for the study. The detailed mechanism was previously used to calculate ignition delay times in homogeneous reactors, and concentration histories of a number of species in plug-flow and jet-stirred reactors. An intermediate mechanism made up of 1282 reversible elementary reactions among 282 species and a short mechanism made up of 770 reversible elementary reactions among 160 species are assembled from this detailed mechanism. Ignition delay times in an isochoric homogeneous reactor calculated using the intermediate and the short mechanism are found to agree well with those calculated using the detailed mechanism. The intermediate and the short mechanism are used to calculate extinction and autoignition of n-heptane in strained laminar flows. Steady laminar flow of two counter flowing Streams toward a stagnation plane is considered. One stream made up of prevaporized n-heptane and nitrogen is injected from the fuel boundary and the other stream made up of air and nitrogen is injected from the oxidizer boundary. Critical conditions of extinction and autoignition given by the strain rate, temperature and concentrations of the reactants at the boundaries, are calculated. The results are found to agree well with experiments. Sensitivity analysis is carried out to evaluate the influence of various elementary reactions on autoignition. At all values of the strain rate investigated here, high temperature chemical processes are found to control autoignition. In general, the influence of low temperature chemistry is found to increase with decreasing strain. A key finding of the present study is that strain has more influence on low temperature chemistry than the temperature of the reactants.

Unsteady flamelet modeling of turbulent hydrogen-air diffusion flames

The unsteady flamelet model is applied in numerical simulations of a steady, turbulent, nitrogen-diluted hydrogen-air diffusion flame. An unsteady flamelet is solved interactively with a CFD solver for the turbulent flow and the mixture fraction field. Transient effects occurring in steady jet diffusion flames are discussed in terms of the relevant timescales. It is shown that radiation can be neglected and that the flame structure is hardly influenced by transient effects for the present case. However, for predictions of slow processes, like the formation of NO, unsteady effects have to be considered. The results predicted by the model are in reasonable agreement to experimental data for temperature, major species mass fractions, OH, and NO mole fractions. On the contrary, the use of steady flamelet libraries yields good results for flame structure and even OH concentrations, but NO is overpredicted by an order of magnitude. However, reasonably well-predicted NO concentrations can also be obtained by solving an unsteady flamelet as a postprocessing mode.

An accurate conservative level set/ghost fluid method for simulating turbulent atomization

This paper presents a novel methodology for simulating incompressible two-phase flows by combining an improved version of the conservative level set technique introduced in [E. Olsson, G. Kreiss, A conservative level set method for two phase flow, J. Comput. Phys. 210 (2005) 225-246] with a ghost fluid approach. By employing a hyperbolic tangent level set function that is transported and re-initialized using fully conservative numerical schemes, mass conservation issues that are known to affect level set methods are greatly reduced. In order to improve the accuracy of the conservative level set method, high order numerical schemes are used. The overall robustness of the numerical approach is increased by computing the interface normals from a signed distance function reconstructed from the hyperbolic tangent level set by a fast marching method. The convergence of the curvature calculation is ensured by using a least squares reconstruction. The ghost fluid technique provides a way of handling the interfacial forces and large density jumps associated with two-phase flows with good accuracy, while avoiding artificial spreading of the interface. Since the proposed approach relies on partial differential equations, its implementation is straightforward in all coordinate systems, and it benefits from high parallel efficiency. The robustness and efficiency of the approach is further improved by using implicit schemes for the interface transport and re-initialization equations, as well as for the momentum solver. The performance of the method is assessed through both classical level set transport tests and simple two-phase flow examples including topology changes. It is then applied to simulate turbulent atomization of a liquid Diesel jet at Re=3000. The conservation errors associated with the accurate conservative level set technique are shown to remain small even for this complex case.

Development of an Experimental Database and Chemical Kinetic Models for Surrogate Gasoline Fuels

The development of surrogate mixtures that represent gasoline combustion behavior is reviewed. Combustion chemistry behavioral targets that a surrogate should accurately reproduce, particularly for emulating homogeneous charge compression ignition (HCCI) operation, are carefully identified. Both short and long term research needs to support development of more robust surrogate fuel compositions are described. Candidate component species are identified and the status of present chemical kinetic models for these components and their interactions are discussed. Recommendations are made for the initial components to be included in gasoline surrogates for near term development. Components that can be added to refine predictions and to include additional behavioral targets are identified as well. Thermodynamic, thermochemical and transport properties that require further investigation are discussed.

Large eddy simulation inflow conditions for coupling with reynolds-averaged flow solvers

Hybrid approaches using a combination of Reynolds-averaged Navier-Stokes (RANS) approaches and large eddy simulations (LES) have become increasingly popular. One way to construct a hybrid approach is to apply separate flow solvers to components of a complex system and to exchange information at the interfaces of the domains. For the LES flow solver, boundary conditions then have to be defined on the basis of the Reynolds-averaged flow statistics delivered by a RANS flow solver. This is a challenge, which also arises, for instance, when defining LES inflow conditions from experimental data. The problem for the coupled RANS-LES computations is further complicated by the fact that the mean flow statistics at the interface may vary in time and are not known a priori but only from the RANS solution. The present study defines a method to provide LES inflow conditions through auxiliary, a priori LES computations, where an LES inflow database is generated. The database is modified to account for the unsteadiness of the interface flow statistics