柴油/碳酸二甲酯燃烧颗粒的微观结构和分形特征

为降低柴油机颗粒物排放,探讨了柴油机燃用柴油/碳酸二甲酯混合燃料燃烧颗粒微观结构和分形特征的变化规律. 采用扫描电镜、透射电镜和SAXS(同步辐射小角X射线散射)相结合的方法,针对DMC(碳酸二甲酯)添加量(以w计)对柴油机燃烧颗粒微观结构和分形特征参数的影响规律进行了研究. 结果表明:与柴油燃烧颗粒相比,D10〔w(DMC)为10%〕、D20〔w(DMC)为10%〕燃烧颗粒的碳粒子平均层面间距分别增加了5.3%、15.1%,弯曲度平均值分别增加了4.0%和10.3%,表明燃烧颗粒的氧化活性增加；与柴油相比,D10、D20燃烧颗粒的质量分形维数分别增加了0.44和0.52,下限均升高了0.2,SAXS与电镜图像对燃烧颗粒的质量分形维数的分析结果相一致,即质量分形维数随DMC添加量的增加而升高,表明燃烧颗粒的团聚程度提高；与柴油燃烧颗粒相比,D10、D20燃烧颗粒表面分形维数的下限分别升高了0.1和0.2,表明其燃烧颗粒表面的粗糙程度和不规则程度提高；与柴油燃烧颗粒相比,D10、D20燃烧颗粒的活化能分别降低了3.9和7.9 kJ/mol,表明燃烧颗粒的氧化活性随着DMC添加量的增加而增强,验证了燃烧颗粒微观尺寸结构和分形特征的研究结果. 研究显示,柴油中添加DMC能够提高燃烧颗粒的氧化活性,燃烧颗粒的氧化活性越强,其在后处理过程中就越易被氧化,有助于降低柴油机的颗粒物排放. 

Microstructure and Fractal Characteristic Analysis of Particulate Matter from Diesel/DMC Combustion

Abstract: Particulate matter (PM) is generated by incomplete combustion of fossil fuels. In diesel engines, the poor mixing of fuel and air causes more rich-fuel zones. It promotes the formation of PM at areas of high temperature. Normally, PM can be reduced by burning oxygenated fuels. The formation processes and characteristics of PM are affected by the conditions of combustion and the properties of the fuel. The oxidization process and the microstructure and fractal characteristics of PM could be affected by the fuel. The combustion particulates morphology of dimethyl carbonate blending with diesel (diesel/DMC) was measured by scanning electron microscope and transmission electron microscopy. Comparing the elementary combustion particulates formed by diesel and fuel blended with 10% and 20% (by weight) DMC, the average fringe separation distances between elementary particulates were increased by 5.3% and 15.1%, and the average tortuosity of elementary particulates was enhanced by 4.0% and 10.3%, respectively. The results indicate that the combustion particulates formed by diesel/DMC were easier to be oxidized. The combustion particulates fractal feature of diesel/DMC was also measured by small angle X-ray scattering. The results show that after blending with 10% and 20% DMC, the mass fractal dimensions measured by electron microscope were increased by 0.44 and 0.52, respectively, and the minimum mass fractal dimensions measured by small angle X-ray scattering were increased by 0.2. The mass fractal dimensions were increased with the increasing of DMC blending ratio; this indicated an easier aggregation of combustion particulates. The minimum surface fractal dimensions were increased by 0.1 and 0.2 while the blending ratio of DMC increased from 0 to 10% and 20%, respectively, indicating that the surface roughness and the oxidative activity were enhanced by DMC. Also, the combustion particulates oxidation characteristic could be conducted by thermo gravimetric analysis (TGA). The activation energies of the combustion particulates were decreased by 3.9 kJ/mol and 7.9 kJ/mol while blending with 10% and 20% DMC. This means that with the increasing of DMC blending ratio, the combustion particulates activation energy was reduced, and oxidation activity was increased accordingly. In conclusion, oxygenated fuels could reduce emissions of PM.