Lumping and Reduction of Detailed Kinetic Schemes: an Effective Coupling

Abstract

When it comes to handling large hydrocarbon molecules and describing the pyrolysis and combustion behavior of complex mixtures, the potential and limitations of detailed chemistry require a careful investigation. Indeed, as they involve a large number of species and reactions, detailed kinetic mechanisms often make the model predictions computationally expensive, thus strongly restricting their potential. Therefore, the automatic generation of detailed mechanisms with several thousands of molecular species and elementary reactions, very useful in many circumstances and a prerequisite to derive reduced models, may become useless from a more general application viewpoint. In order to overcome these limitations, in this paper a proper strategy to obtain suitable mechanisms for multidimensional computational fluid dynamics (CFD) applications is presented and discussed. It couples the advantages of two reduction techniques: chemical lumping of species and reactions and a flexible and reliable reduction technique aimed at eliminating unimportant species and reactions. It is shown that a central advantage of semidetailed kinetic models, already reduced with a chemical lumping, is their easier and more effective applicability to successive automatic reductions. Kinetic schemes of n-heptane and n-dodecane oxidation, reduced to 100 and 120 species, respectively, are obtained and compared with experimental data and with the complete original model. These dimensions easily allow the successive use of these reduced kinetic models particularly when the aim is to embody them within computational fluid dynamic models. In particular, simulations of steady-state, axisymmetric, laminar diffusion flames were performed comparing the original and the reduced kinetic mechanisms. The results demonstrated not only the reliability of the reduced mechanisms, but, more importantly, highlighted the great computational advantages, especially in terms of CPU times, of reduced kinetics for the simulation of multidimensional systems. Similar or even more apparent benefits are expected when reducing lumped kinetic schemes of combustion of real transportation fuels, such as gasoline, jet, and diesel fuels

Reduced Kinetic Schemes of Complex Reaction Systems: Fossil and Biomass‐Derived Transportation Fuels

“…The presence of inert species is forced in the mechanism, even if their fluxes are null. Previous experiences on the reduction of n-dodecane oxidation mechanism [62] clearly showed that the skeletal mechanism obtained by reducing a lumped mechanism involved about 120 species, whereas about 300 species were required by the reduced mechanism derived from a detailed scheme [42]. This fact is further confirmed by the recent work of Narayanaswamy et al [63], who reduced the detailed kinetics of n-dodecane to 295 species.…”

“…With the same accuracy, the skeletal model derived from a lumped scheme contains about one third of the species required by the skeletal model derived from the detailed mechanism, as already shown in Fig. and further discussed by Stagni et al …”

Study of the low-temperature reactivity of large n-alkanes through cool diffusion flame extinction

“…One method is to use fully detailed models as a base to derive semi-detailed lumped models [56] in which the isomers of large molecules are grouped together (for example, simplifying the three heptene 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 isomers into a general heptene species). These semi-detailed models can then be reduced appropriately [101]. Table 4 provides a survey of reduced n-dodecane and n-heptane models from the literature (as well as the 131-species NUI model from Table 2).…”

Ignition Characteristics in Spatially Zero-, One- and Two-Dimensional Laminar Ethylene Flames

“…In this study the oxidation of ethylene is modelled using the C1-C3 sub-mechanism from the July 2014 version of the detailed POLIMI mechanism for hydrocarbon combustion. 33 The full 106 species sub-mechanism was used for the zero-and one-dimensional calculations, whereas a 31 species reduced mechanism was generated using a combined chemical reduction approach, based on reacting flux analysis, 34 for the two-dimensional non-premixed simulations to minimise the required computational time. The cases investigated were chosen for qualitative comparison with previously published experimental 19 and turbulent DNS 29 studies of ethylene fuelled flames.Both studies feature ethylene based jets at Re = 10 000 issuing into hot, laminar coflows with bulk flow velocities approximately 10% of the mean jet exit velocity.…”