Effects of Fuel Species on Soot Formation in Laminar Counterflow: Comparison Between Diesel Fuel and Palm Methyl Ester as an Alternative Fuel

Spray combustion of liquid fuel is utilized in many combustion systems. There are, however, still remained-problems. One of the most important problems is to clarify the combustion characteristics, especially soot formation process. At the same time, the liquid fuel of biomass is getting a lot of attention as an alternative fuel in recent years from the viewpoint of the environmental issues and the exhaustion of fossil fuels. In this study, we focused on the palm methyl ester (PME), which has a large production capacity and oxygen content in its molecular structure. The aim of this study is to clarify the combustion characteristics and soot formation characteristics of PME spray flame for the effective utilization to the conventional combustion systems as the alternative fuel. In order to clarify the soot formation characteristics of PME spray flame, measurements of Sauter mean diameter (SMD) and droplet size distribution by using phase Doppler anemometry (PDA) and measurement of two dimensional soot formation characteristics by using Laser Induced Incandescence technique (LII) are conducted in laminar counterflow field. In addition, since the PME has a normal chain structure and oxygen content in its molecular structure, it needs to clarify effect of the oxygen content and normal chain structure. The comparison with diesel fuel and n-dodecane are also conducted. Results of LII measurements show that the PME and the n-dodecane spray have similar spray flame structures, time-averaged soot volume fraction and instantaneous structure of soot formation while PME has the oxygen content in the molecular structure. On the other hand, the time-averaged soot volume fraction and soot formation area of PME are smaller than those of diesel fuel. It is because the diesel fuel has some components with the aromatic ring. These results indicate that it is not the oxygen content but the normal chain structure in the PME play an essential role in influencing soot formation characteristics.