Optical Measurements of In-Flame Soot in Compression-Ignited Methyl Ester Flames

Abstract

This study investigates in-ame sooting characteristics of biodiesel surrogates in compressionignited spray ames. The aim of the study is to produce reliable experimental data on in-ame soot for validation of kinetic mechanisms and soot models. A rapeseed oil biodiesel (RME) was compared to neat methyl oleate (MO) and methyl decanoate (MD). In addition, neat nheptane was chosen as a baseline fuel and the eect of blending n-heptane and MD on soot production was investigated. The study was performed in a single cylinder research engine with optical access to a single spray for varying ambient gas temperatures (825 K to 990 K).The in-ame soot was measured by a high speed diuse back-illuminated extinction imaging system, and the ame lift-o length (FLOL) and the corresponding estimated equivalence ratio at FLOL was measured from high speed OH* chemiluminescence imaging. Results show that RME has the highest tendency to soot, closely followed by MO, independent of the FLOL equivalence ratio. Having almost identical fuel properties, this is likely due to the higher degree of unsaturation in RME compared to MO.When comparing MD to n-heptane, a much lower in-ame soot production rate is observed despite the fact that MD has a higher number of carbon-carbon bonds, concluding that the high fuel oxygen ratio in MD is eectively reducing in-ame soot production. Generally, for FLOL equivalence ratio leaner than ∼2, no in-ame soot was produced for all ambient gas temperatures. The in-ame soot production rate also showed a clear ambient gas temperature dependence for constant FLOL equivalence ratios, where the soot production rate increased with increased gas temperatures.

Mini Review of Current Combustion Research Progress of Biodiesel and Model Compounds for Gas Turbine Application

“…In the numerical investigation of Lin et al, 30 remarkable production of CO and CO 2 was predicted in methyl butanoate oxidation due to the fuel-bound oxygen in the methyl ester moiety. 30,33,34 Compared to n-butane oxidation, the formation of oxygenated species preferred in methyl butanoate oxidation simultaneously reduces the production of acetylene and ethylene which can be deemed as soot precursors. 30 Therefore, the existence of the ester group can lead to the reduction of soot precursors, which is consistent with the observed difference in the yield sooting index (YSI) between methyl esters (e.g., methyl pentanoate with a YSI value of −12.6) and n-alkanes (e.g., n-pentane and n-hexane with YSI values of −8.9 and 0.0, respectively) measured by McEnally and Pfefferle.…”