A Reduced Mechanism for High-Temperature Oxidation of Biodiesel Surrogates

Abstract

A skeletal mechanism with 118 species and 837 reactions was developed from a detailed LLNL mechanism that consisted of 3329 species and 10806 reactions for a tricomponent surrogate mixture, consisting of methyl decanoate, methy-9-decenoate, and n-heptane, which is suitable for combustion modeling of biodiesel derived from various feedstocks. The method of directed relation graph (DRG) for skeletal mechanism reduction was improved for mechanisms with large numbers of isomers. The improved DRG together with isomer lumping and DRG-aided sensitivity analysis (DRGASA) were subsequently applied to obtain a minimal skeletal mechanism from the detailed mechanism for the given error tolerance. The reduction was performed within a parameter range of pressure from 1 to 100 atm, equivalence ratio from 0.5 to 2, and temperature higher than 1000 K in autoignition and perfect stirred reactors (PSR). Although reduced in size almost by a factor of 30, the skeletal mechanism features high accuracy for high-temperature applications both in predicting the global system parameters, such as ignition delay and extinction time, and detailed profiles of species concentrations. Furthermore, numerical simulations of jet stirred reactors were compared with experimental measurements for rapeseed oil methyl esters. The temperature and species profiles in one-dimensional atmospheric counterflow diffusion flames were well predicted as well compared with experimental data in the literature.

A reduced mechanism for biodiesel surrogates for compression ignition engine applications

“…The worst case error tolerance was set to be 30%. Such an error tolerance is comparable to the overall uncertainty of the detailed mechanism, as such there is no significant loss in chemical fidelity through the skeletal reduction [32]. It will be further shown in the next section that the resulting skeletal mechanism obtained with such an error tolerance performs well in predicting experimental data in most cases compared with the detailed mechanism.…”

“…The method of DRG [32] was first applied to eliminate unimportant species and reactions from the detailed mechanism, and a 664-species skeletal mechanism was obtained. Isomer lumping [25] was subsequently applied to the 664-species skeletal mechanism to further reduce the mechanism size.…”

“…However, this skeletal mechanism consists of more than 600 species thus is not suitable for 3-D engine simulations. Skeletal mechanisms for the biodiesel surrogate were then developed using DRG-based methods and lumping [31][32][33]. The surrogate mixture consists of MD, methyl-9-decanoate (MD9D) and n-heptane, which allows the flexibility in matching the physical and important combustion properties of real biodiesel from different feed-stocks.…”

“…The skeletal mechanism in Ref. [32] consists of 118 species and 837 reactions. It however does not consist of the low temperature chemistry thus is not suitable for CI engine applications.…”

Development of a skeletal oxidation mechanism for biodiesel surrogate

“…The recent investigations indicate that a biodiesel surrogate fuel containing large-molecule components could better reproduce the ignition and combustion behavior of actual biodiesel fuel [16,18,19]. However, only a few reduced mechanisms with long-chain esters are available at present, and all of these mechanisms have been obtained by reducing the detailed mechanism of the biodiesel surrogate of Ref [15] that composes n-heptane, MD, and MD9D.…”