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Flamelet modeling allows the application of comprehensive chemical mechanisms, which. include all relevant chemical combustion processes that occur in a DI Diesel engine during autoignition, the burnout in the partially premixed phase, the transition to diffusive burning and formation of pollutants like NO, and soot. The highly nonlinear dependencies of the chemistry need not to be simplified, and the complete structure of the flame is preserved. Using the Representative Interactive Flamelet (RIF) model the one-dimensional unsteady set of partial differential equations is solved online with the 3-D CFD code. The flamelet solution is coupled to the flow and mixture field by the current boundary conditions (enthalpy, pressure, scalar dissipation rate). In return, the flamelet code yields the species concentrations, which are then used by the 3-D CFD code to compute the temperature field.

In Diesel engines combustion proceeds essentially under partially premixed and non-premixed conditions. In this study the flamelet model for non-premixed combustion is derived and its implementation into 3-D codes is discussed. The model is capable of describing auto-ignition, the following burnout of the partially premixed phase, and the transition to diffusive burning. Flamelet modeling has the advantage of separating the numerical effort associated with the resolution of fast chemical time scales from the fluid dynamics' scales occuring in the 3-D computation of the engine combustion cycle. Three additional scalar field equations have to be solved in the 3-D engine code, while the entire chemistry consisting of up to 1000 or more chemical reactions is simultaneously treated in a separate 1-D code describing the flamelet structure. A new aspect proposed here is to use so-called RIFs (Representative Interactive Flamelets), which are solved on-line with the 3D-code.

Early injection strategies in the case of part-load conditions are offering the possibility to enhance mixing and evaporation. Due to the early injection, ignition and evaporation are separated in time and space for that less rich pockets from where soot is formed are occurring. For reducing NOx, cooled EGR is a method to dilute the intake charge. The combustion is shifted to lower temperatures and less NOx is formed. More, the cooling of the intake charge and the higher heat capacity enhance the evaporation time for that ignition starts at later times and combustion is retarded. For the simulation of such engine cases using high rates of EGR with an early fuel injection, a CFD (Computational Fluid Dynamics) code is coupled interactively with the flamelet model that will be applied here as combustion model. That approach, known as RIF (Representative Interactive Flamelet) model, requires a re-evaluation of the chemical reaction mechanism.

The autoignition delay time and location of a n-heptane fueled high pressure and high temperature spray combustion chamber under Diesel engine conditions has been investigated numerically. The conservation equations for the fluid dynamics of sprays have been solved using the KIVA-II code with its standard spray models. A detailed chemical mechanism of 81 elementary reactions and 37 chemical species has been applied to describe the ignition and combustion of n-heptane. The coupling between complex chemistry and turbulence is treated by employing the Representative Interactive Flamelet (RIF) concept. Unsteady flamelets are computed using a separate flamelet code that interacts with the CFD solver at each time step. The scalar dissipation rate, which is an important parameter for the flamelet, has been studied numerically under different conditions.

Numerical simulations for an n-heptane fueled high pressure and high temperature chamber under Diesel engine conditions have been performed to study soot formation and oxidation processes. A kinetically based soot model has been applied, which accounts for the pyrolysis and oxidation of fuel and formation of polycyclic aromatic hydrocarbons (PAHs) by the use of a detailed kinetic mechanism. PAH growth and oxidation is modeled by a fast polymerization process, coagulation of PAHs leads to particle inception. The soot particles are allowed to coagulate with other particles and PAHs. The interaction of soot particles with the gas phase is modeled by heterogeneous surface reactions leading to particle growth due to acetylene addition and particle oxidation by hydroxyl radicals and molecular oxygen. The conservation equations for the fluid dynamics are solved with the KIVA II code and the coupling with chemistry is treated by employing the flamelet concept.

Combustion and pollutant formation in a DI-Diesel engine are numerically investigated using the Eulerian Particle Flamelet Model (EPFM). A baseline case for part load operating conditions is considered as well as an EGR variation. The surrogate fuel consisting of n-decane (70% liquid volume fraction) and α-methylnaphthalene (30% liquid volume fraction) is used in the simulation. Results are compared to experimental data that has been obtained using real diesel fuel. The effect of multiple flamelets on the simulation of the auto-ignition process and the pollutant formation is discussed and a converging behavior of the model with respect to the number of flamelets is found. The effect of homogenization of the three-dimensional mixture field is investigated and it has been included in the formulation of the scalar dissipation rate.

Correct prediction of mixture formation in fuel sprays is a prerequisite in the framework of 3D CFD engine simulations using a reliable combustion model. To understand the process of fuel evaporation and mixture formation in dense atomized sprays a simultaneous Mie/Shadow imaging technique and a 1D-linear Raman scattering technique are applied to investigate spray formation of high pressure direct injection. Mixture composition is one of the most important parameters in fuel sprays and is difficult to measure because liquid and vapor phases appear simultaneously. Ethanol is used as a model fuel since the phase-dependent spectral shift of the OH stretching vibration allows the Raman signal separation of liquid and vapor phase. The investigations are carried out in a high temperature high pressure injection chamber, where pressures and temperatures can be set up to 5MPa and 800 K.

Auto-ignition in Diesel engines, occurring essentially under non-premixed and partially premixed conditions, is considerably different to homogeneous ignition. In order to study the relevant chemistry--mixing interactions, it is assumed that the ignition of Diesel fuel can be described by using the single component model fuel n-heptane. Starting from a detailed chemical reaction scheme with about 1000 elementary reactions among 168 chemical components, a skeletal mechanism consisting of 98 reactions and 40 components is derived, which is still capable of describing the auto-ignition process under Diesel engine conditions and concentrations of NO, relevant intermediate components. Introducing steady state assumptions for intermediate species which are consumed rapidly leads to a reduced 14-step mechanism. The mechanism is validated with auto-ignition delay times from shock tube experiments by Adomeit for different temperatures, pressures, and equivalence ratios.

Engine noise pollution is as harmful as other forms of pollution to human health. Apart from the health effects, noise also has an adverse effect on the engine structure, thus requiring a sturdier construction to maintain long engine life. In a conventional direct injection diesel engine the fuel ignites spontaneously shortly after the beginning of injection. The Combustion process causes fluctuations in heat release and therefore, fluctuations in combustion chamber pressure. Combustion generated noise can be lowered by lowering the fluctuations in heat release or pressure. Which can be achieved by separating the fuel evaporation and fuel-air mixing from start of ignition in space and in time. The noise is mainly affected by the early part of the combustion process due to higher rates of heat release. Combustion noise generation in the early stage of combustion is not yet entirely understood.

The laminar burning velocity is one key parameter for the numerical simulation of gasoline engine combustion processes. In order to understand the effect of the laminar burning velocity of different fuel components on modern engine development it is of great interest to conduct experiments under high initial pressure and temperature. Initial conditions in this publication are a pressure of p = 10bar and a temperature of T = 373K. Special focus has been laid on the common C1 and C2 alcohols, methanol and ethanol, which are frequently used for blending components in standard gasoline. The experimental setup consists of a spherical closed pressurized combustion vessel with optical access. Schlieren measurements coupled with a high speed camera are used for image acquisition to track the expanding flame front. Finally, a post processing tool is used to extrapolate the measurements to zero stretch. Experiments were done at different fuel-air ratios between Φ = 0.8 and up to Φ = 1.2.

Diesel engine modeling by means of CFD (computational fluid dynamics) has become a more and more important tool in the development process for new engine design. An adequate and reliable Diesel engine model relies on many features. Beside the combustion and spray modeling, the question what model fuel should be used is discussed and in the past, a mixture of n-decane and α-methylnaphthalene, denoted as IDEA fuel, was found to be a good surrogate fuel for Diesel for the conventional Diesel combustion mode. New combustion designs such as PCCI (premixed charged compression ignition) are a possible solution for the strict upcoming emission limits. Due to a shift to lower temperatures and better homogenization, less NOx and soot is formed. To model these combustion designs, a re-evaluation of the model fuel that is to be used is required when the benefit of a detailed chemical reaction mechanism is favored in the combustion modeling.

Representative Interactive Flamelets (RIF) have proven successful in predicting Diesel engine combustion. The RIF concept is based on the assumption that chemistry is fast compared to the smallest turbulent time scales, associated with the turnover time of a Kolmogorov eddy. The assumption of fast chemistry may become questionable with respect to the prediction of pollutant formation; the formation of NOx, for example, is a rather slow process. For this reason, three different approaches to account for NOx emissions within the flamelet approach are presented and discussed in this study. This includes taking the pollutant mass fractions directly from the flamelet equations, a technique based on a three-dimensional transport equation as well as the extended Zeldovich mechanism. Combustion and pollutant emissions in a Common-Rail DI Diesel engine are numerically investigated using the RIF concept. Special emphasis is put on NOx emissions.

Quenching of premixed flames at cold walls is investigated to study the importance of the model fuel choice for combustion modeling. Detailed chemical mechanisms for two different fuels, namely the low-molecular-weight fuel methane, and the more complex fuel iso-octane are employed. For both fuels the response of the flame to the very rapid heat loss at the cold wall is studied. The most important and significant difference between methane and iso-octane for this problem is the postquench oxidation of unburned hydrocarbons. Methane shows fast oxidation of unburned fuel and intermediate hydrocarbons whereas postquench oxidation for iso-octane is slow especially for the intermediate hydrocarbons. Furthermore, the Soret effect which is usually considered to be of minor importance appears to be important in modeling the rate limiting diffusion process. This is caused by different directions of the thermal diffusive transport for certain species.

The non-premixed turbulent combustion and emission formation in a modern DI diesel engine relies mostly on the mixture formation process induced by the diesel fuel spray. Therefore the numerical simulation of this process has to incorporate accurate spray modeling which captures the physics of the spray formation, propagation and vaporization. A widely used framework for spray modeling is the Discrete Droplet Model (DDM) which also is applied in the present work. In the DDM framework, separate submodels account for droplet breakup, droplet-droplet interaction and evaporation. Due to the empirical nature of these submodels (particularly droplet breakup and collision) necessitated by an incomplete representation of the physics, and by the inability to isolate each process under diesel engine relevant conditions, some of the constants controlling the outcomes of these submodels require calibration.

Recently, a consistent mixing model for the two-way coupling of a CFD code and a zero-dimensional multi-zone code was developed. This work allowed for building an interactively coupled CFD-multi-zone approach that can be used to model HCCI combustion. In this study, the interactively coupled CFD-multi-zone approach is applied to PCCI combustion in a 1.9l FIAT GM Diesel engine. The physical domain in the CFD code is subdivided into multiple zones based on one phase variable (fuel mixture fraction). The fuel mixture fraction is the dominant quantity for the description of nonpremixed combustion. Each zone in the CFD code is represented by a corresponding zone in the zero-dimensional multi-zone code. The zero-dimensional multi-zone code solves the chemistry for each zone, and the heat release is fed back into the CFD code. The thermodynamic state of each zone, and thereby the phase variable, changes in time due to mixing and source terms (e.g., vaporization of fuel, wall heat transfer).

Subject of this work is the recently introduced extended Representative Interactive Flamelet (RIF) model for multiple injections. First, the two-dimensional laminar flamelet equations, which can describe the transfer of heat and mass between two-interacting mixture fields, are presented. This is followed by a description of the various mixture fraction and mixture fraction variance equations that are required for the RIF model extension accounting for multiple injection events. Finally, the modeling strategy for multiple injection events is described: Different phases of combustion and interaction between the mixture fields resulting from different injections are identified. Based on this, the extension of the RIF model to describe any number of injections is explained. Simulation results using the extended RIF model are compared against experimental data for a Common-Rail DI Diesel engine that was operated with three injection pulses.

Combustion and pollutant formation in a new recently introduced Common-Rail DI Diesel engine concept with roof-shaped combustion chamber and tumble charge motion are numerically investigated using the Representative Interactive Flamelet concept (RIF). A reference case with a cup shaped piston bowl for full load operating conditions is considered in detail. In addition to the reference case, three more cases are investigated with a variation of start of injection (SOI). A surrogate fuel consisting of n-decane (70% liquid volume fraction) and α-methylnaphthalene (30% liquid volume fraction) is used in the simulation. The underlying complete reaction mechanism comprises 506 elementary reactions and 118 chemical species. Special emphasis is put on pollutant formation, in particular on the formation of NOx, where a new technique based on a three-dimensional transport equation within the flamelet framework is applied.

A Flamelet formulation for premixed turbulent combustion has been developed based on a scalar field equation. Local effects like flame stretch and flame front curvature were introduced into the equation. Direct numerical simulations of a flame propagating in a turbulent flow field revealed characteristic flame structures as they are experimentally observed. The calculated turbulent flame speed is in good agreement with correlations of experimental data. Experimental verification of the model has been carried out by Laser tomographic experiments in a VW transparent engine. Flame front structures were visualized resolving the highest possible length scale range. The spectral properties of the flame structures were investigated and compared with the model predictions. A good agreement was found between the characteristic power spectrum of the spatial flame front fluctuations and the scalar fluctuations in the model.

Subject of this work is a simulation model for PCCI combustion that can be used in closed-loop control development. A detailed multi-zone chemistry model for the high-pressure part of the engine cycle is extended by a mean value model accounting for the gas exchange losses. The resulting model is capable of describing PCCI combustion with stationary excactness. It is at the same time very economic with respect to computational costs. The model is further extended by identified system dynamics influencing the stationary inputs. For this, a Wiener model is set up that uses the stationary model as a nonlinear system representation. In this way, a dynamic nonlinear model for the representation of the controlled plant Diesel engine is created.