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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.

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.

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.