Flamelet Generated Manifold Strategies in Modeling of an Igniting Diesel Spray C. Bekdemir * , L.M.T. Somers, L.P.H. de Goey Eindhoven University of Technology, Department of Mechanical Engineering, The Netherlands www.combustion.tue.nl A study is presented on the modeling of fuel spray combustion in diesel engines. The objective is to model igniting diesel sprays with the detailed chemistry tabulation method FGM (Flamelet Generated Manifold). The emphasis is on the accurate prediction of auto-ignition as well as the steady combustion phase using one consistent approach. Introduction Due to ever increasing demands from emis- sion legislation (NO x and soot), fuel economy (CO 2 ) and fuel flexibility (bio-fuels) diesel engines be- come more and more complex. Therefore, conven- tional engine design approaches that rely on proto- type development become too time-consuming and expensive. The development of predictive and ef- ficient computational tools would represent a sig- nificant step forward in the ability to rapidly design high efficiency, low emission engines [1]. Modern diesel engine technology unequivocally applies liquid fuel injection with high pressure, that forms a non-homogeneous mixture leading to rela- tively high levels of soot. Modeling this process is extremely difficult due to the complex phenomena occurring during fuel injection and combustion. A direct approach (DNS) is unviable and advanced, so accurate and fast sub-models are needed to ap- ply it in engine design. Recently, efforts to accurately and efficiently model diesel spray formation resulted in a suitable model to compute the mixture formation [2]. And a first application of the FGM method to a diesel spray proved that experimentally observable phe- nomena like auto-ignition and flame lift-off can be successfully predicted with this method [3]. The objective of this study is to compare the spray igni- tion behavior for different manifold types. FGM Strategies The FGM approach combines the flamelet con- cept with a manifold method by using mixture frac- tion Z and progress variable PV to parameterize the combustion process [4]. Recently, the appli- cation of the FGM method proved to be able to predict auto-ignition and flame lift-off for a diesel spray simulation in engine-like conditions. How- ever, different ’generators’ can be chosen to fill the unsteady flamelet region, which may influence the final results in terms of for instance ignition delay time and combustion behavior. FGMs can be generated in many ways. For sta- tionary flames, there is a classical way with steady flamelets only, where a sequence of steady flames * Corresponding author: c.bekdemir@tue.nl Towards Clean Diesel Engines, TCDE 2009 with strain rates varying from a low value (close to equilibrium) to the quenching value is computed. An illustrative example of the ”accessible” space in Z -PV is shown in Figure 1, see the gray area be- tween the solution for the lowest strain rate and the solution at which the strain rate reached its maxi- mum before extinction. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Flamelet database generation Z [-] PV [-] igniting homogeneous reactors Lowest strainrate Steady solutions region Highest non-quenching strainrate Timedependently extinguishing or igniting flamelet Homogeneous reactors before ignition igniting flamelet extinguishing flamelet Figure 1: Ways to extend a stationary database Diesel combustion is characterized by auto- ignition, so to cover this aspect the table should also contain information in the area beneath the quenching strain rate solution. Several ways ex- ist to fill this gap in the Z -PV plane. One way is to solve a time-dependent flamelet with a higher strain rate than the highest possible non-quenching strain rate. In this way the flame is forced to extin- guish and in the mean time data are sampled to fill the gap. Another approach, that is more ap- propriate for this study, is solving time-dependent flamelets from a mixed, but non-reacting initial state. The ignition behavior is followed in time until a steady flame is reached. A third possibility is to reproduce ignition of mixtures covering the entire Z -space with homogeneous reactor auto-ignition calculations [5]. All three methods to fill the Z -PV gap are depicted schematically in Figure 1.