Experimental and numerical studies of biodiesel combustion mechanisms using a laminar counterflow spray premixed flame

Biodiesel is a mixture of long chain fatty acids such as methyl esters and is mainly used in diesel engines. Its fundamental properties and combustion pathways still need to be analyzed and validated. The present study concerns the creation and development of new data for the combustion of rapeseed methyl ester biodiesel (RME) and methyl decanoate as a surrogate fuel (MD). Experimental and numerical studies are conducted on a laminar counterflow premixed flame configuration where spray biodiesel/air (or MD/air) is injected against methane/air mixture at atmospheric pressure for different strain rates and equivalence ratio conditions. As chemical schemes for methane/air reactions are enough well known, this configuration is suitable to perform validations of chemical schemes for biodiesel/air (or MD/air) combustion, by taking methane/air flame as a reference. Planar Laser-Induced Fluorescence (PLIF) of OH as well as visible and UV chemiluminescence measurements of the excited radicals CH∗(A2Δ) and OH∗(A2Σ+) are employed to experimentally analyze the biodiesel and MD flame structure. The counterflow spray MD flame is simulated by choosing a skeletal reaction mechanism to which we add CH∗ and OH∗ reactions. In the case of biodiesel flame simulations, a new surrogate kinetics is developed by combining two existing skeletal kinetics schemes. The new scheme guarantees not only a good prediction of measured radicals but also a good methane/air flame speed which is necessary to well predict the flame front position in the counterflow configuration. CH∗ and OH∗ sub-mechanisms are also added to this kinetic scheme. The numerical predictions of the CH∗ concentration are very close to the experimental profiles along the central axis, for both biodiesel and MD kinetic schemes. However the numerical and experimental results show differences in the OH∗ production routes between MD and methane flames.