含水乙醇乳化燃料高压喷雾燃烧时消烟作用的机理研究

Soot emissions in diesel engines can be greatly reduced through using aqueous ethanol emulsified fuels. However, the mechanism of the soot reduction is not well understood. The possible effects of aqueous ethanol addition are: 1)changes in the fuel physical properties including kinematic viscosity, surface tension,and distillation temperature may help to improve spray atomization, accelerarate droplet evaporation, and enhance mixture formation processes; 2)extended ignition delay allows more time for fuel-air mixing; 3) increased oxygenate component in the fuel helps to reduce soot emissions; 4) lowered flame temperature affacts soot formation and oxidation. In this study,the spray angle, spray tip penetration, and droplet size distribution of the non-evaporating and evaporating spray (both liquid and vapor phases)in a high-pressure, constant-volume vessel are studied by using marco/micro high speed imaging techniques. Further,a simplified model of spray mixture development based the law of momentum conservation is established and calibrated with the experimental data. Then, with the model the quantity of entrained ambient air and the fuel concentration distribution in the mixture are analyzed,and the physical effects of the aqueous ethanol addition are clarified. To understand the chemical effects of aqueous ethanol addition, experiments with spray combustion are conducted with the standard fuel (a mixture of hexadecane and 2,2,4,4,6,8,8-heptamethylnonane) as baseline fuel and the aqueous ethanol as additives. To isolate the effects of extended ignition delay due to aqueous ethanol addition, the fuel ignitability is changed with varing the ratio of the two components in the standard fuel. Two-color method is employed to visualize the distributions of soot (KL factor) and flame temperature. Flame lift-off lengths of different fuels are characterized by imaging the OH radical distribution. Finally, combining the experimental resutls with the analysis based on reaction kinetics calcualtion for soot formation and oxidation, the effects of extended ignition delay, increased oxygenate coponents in the fuel, and the lowered flame temperature are clarified.The results of this study will provide important information for the fuel design and combustion system design when using aqueous ethanol emulsified fuels.

利用含水乙醇乳化燃料可以大幅降低柴油机碳烟排放，但其机理有待解明。含水乙醇乳化混入的影响有：燃料微观结构与粘度等物性的变化及"微爆"现象引起的液体射流雾化机理的改变对混合气形成特性的影响；燃料自着火性、含氧量等化学组分结构的变化对滞燃期、火炎温度等燃烧特性的影响；喷雾火炎几何形态的变化对碳烟的影响。本研究利用宏观与微观高速摄影技术，对定容容器内的非蒸发与蒸发喷雾气液两相的锥角、贯穿距及粒径分布进行系统地分析并建立简化数学模型，分析喷雾内卷入当量空气量分布，阐明含水乙醇混入的物理影响。采用标准燃料为基础燃料，通过改变其自着火性的策略灵活控制滞燃期，利用两色法测量火炎温度与碳烟浓度(KL值)的分布，结合反应动力学计算，阐明燃料化学组分结构的变化对碳烟的影响。最后，进行火炎lift-off特性分析，阐明喷雾火炎几何结构特征对碳烟特性的影响。本研究结果将为燃料设计及相关燃烧系统的设计提供重要信息。

在柴油机中燃用含水乙醇乳化燃料不仅可以提升生物乙醇全生命循环利用效率，还可以大幅降低柴油机氮氧化物与碳烟等有害物排放。然而，关于含水乙醇柴油乳化燃料喷雾燃烧特性的基础研究较少，特别是关于含水乙醇乳化燃料燃烧时消烟作用的机理方面还存在诸多疑点亟待阐明。本项目研究旨在利用多种光学测试技术研究含水乙醇柴油乳化燃料喷雾燃烧特性，阐明其消烟作用的机理。首先，课题组自主设计研制了预混合燃烧式高温高压定容燃烧容器并搭建了高压共轨燃油喷射系统；开发了含水乙醇柴油乳化燃料的稳定混合方法并对混合燃料的蒸馏特性、密度、动粘度，表面张力、十六烷值及含氧量等物理化学特性进行了实验测试与计算；利用高速阴影摄影技术对乳化燃料在不同喷射压力、不同环境密度、温度等惰性条件下非蒸发喷雾与蒸发喷雾锥角与贯穿距离等宏观特性进行了观测与分析；利用米氏散射技术测试了表征燃料液滴蒸发快慢的最大液相贯穿距离；最后，开发了基于黑体辐射理论的两色法并对其测试不确定性进行充分分析后，进行了不同氛围氧浓度条件下的喷雾燃烧实验，对含水乙醇柴油乳化燃料喷雾燃烧温度场与碳烟浓度(KL值)二维分布的时间演变特性进行了测试与分析。研究结果表明，虽然随着含水乙醇添加量的增加，乳化燃料密度、动粘度及表面张力等物性有一定变化，特别是蒸馏温度有显著降低，但是无论室温条件下的非蒸发喷雾还是高环境温度条件下的蒸发喷雾锥角与贯穿距离等宏观特性没有明显差异；而蒸发喷雾中表征蒸发快慢的最大液相贯穿距离显著增加，表明蒸发混合速率降低。然而，各氛围氧浓度下，随着含水乙醇量的增加，滞燃期明显增长，喷雾火焰中碳烟浓度显著降低。这些结果表明，含水乙醇乳化燃料消烟作用可主要归因于滞燃期的增长与燃料含氧量的增加。在本实验条件下，无论蒸发喷雾还是喷雾燃烧实验中都没能观测到具有说服力的“微爆”现象。分离滞燃期增长与含氧量增加的影响以及“微爆”现象的观测与证实需进一步研究。