多成分燃料高压射流喷雾局部浓度及蒸发率的瞬时测试方法研究

Liquid fuels for combustion engines are mostly comprised of multiple components with a variety of boiling points. To date, however, quantitative measurement methods of concentration in the evaporating spray of multi-component fuels are rarely reported, though the concentration information is very important for development and calibration of simulation models and controls of combustion and pollutant formation. The objective of this study is to establish a quantitative measuring method of local concentration and vapor fraction of multi-component fuel spray, based on the laser breakdown spectroscopy (LIBS) technique. The intensity ratio of light emissions between atomic H in the fuel and atomic N in the ambient gas is utilized to measure the concentration in the multi-component fuel spray. For oxygenated fuel spray, the intensity ratio of light emissions between atomic O in the fuel and atomic N in the ambient gas is used to characterize the concentration of oxygenated fuel in the spray. When liquid droplets exist in the measuring field, the vapor fraction is obtained based on the analysis of variations in the full width at half maximum (FWHM) due to the Doppler effect of liquid emission lines. The calibration system and spray test rig for the LIBS technique are designed and set up. Based on the theoretical analysis and experiments, the effects of Doppler broading, Lorentz broading with variations in the ambient temperature and pressure as well as the effect of bottom plasma emissions on the measuring accuracy are examined, and the calibration method for these effects is proposed. The reliability of the LIBS measurement is testified to by comparing the measuring results with those by the two-wavelength absorption and scattering technique and laser induced exciplex fluorescence techniques. Finally, the concentration and vapor fraction in the sprays of multi-component fuels with different boiling points and oxygenated fuels including methanol gasoline blends and dimethyl ether (DME) are measured by using the method developed in this study. These experimental results are believed to be an important and rare database for calibration and development of numerical spray combustion models with a higher predictive accuracy to real fuels in combustion engines.

发动机用液体燃料通常含有多种成分，但目前多成分燃料喷雾蒸发特性的测量方法却极为罕见。本研究旨在建立一种基于激光励起原子发光光谱分析的喷雾局部浓度及蒸发率的测试方法。利用燃料中氢原子与氛围气中氮原子发光强度之比实现多组分碳氢燃料浓度测试；利用燃料中氧原子与氛围气中氮原子发光强度之比定量分析含氧燃料的局部浓度；利用液滴“微爆”多普勒效应引起的原子发光谱线半峰宽度变化得出局部蒸发率。建立本方法的标定与实验测试系统，结合理论与实验分析探究由环境温度变化引起的原子发光谱线多普勒展宽、压力变化引起的劳伦兹展宽以及基底效应等现象对测试结果的影响并提出修正方法。最后，利用本方法对沸点不同的多组分燃料以及甲醇汽油、DME等含氧燃料喷雾局部浓度及蒸发率进行测试；阐明单组分、双组分及多组分燃料喷雾蒸发特性的不同，揭示机理。研究结果将为新测试方法的开发与实际燃料喷雾仿真模型的构建提供重要而稀缺的实验数据支持。

实际发动机燃料通常含有多种组分，由于燃料中不同组分的蒸发和混合特性不尽相同，导致单组分替代燃料与实际燃料的喷雾特性有明显差异。由于缺乏多组分燃料喷雾蒸发特性的定量测试技术，目前的喷雾模型基本是基于单组分燃料建立的，然而这些模型很难精确预测实际发动机中喷雾及混合气形成过程。本项目研究旨在利用LIBS技术建立一套能够实现多组分燃料喷雾局部浓度与蒸发率瞬时定量测量的方法与流程，从而阐明多组分燃料喷雾的蒸发和混合特性，并为新测试方法的开发和喷雾仿真模型的标定提供数据支撑。首先完善了LIBS技术应用于射流燃油浓度测量中的理论基础并构建了完整的LIBS射流浓度的标定和测量系统；利用高速纹影法和米氏散射法对气体和液体射流进行成像，分析了射流的气液相宏观特性；详细调查了环境温度压力变化和激光能量对LIBS测试的影响，然后测量了氨掺氢、氨气、甲烷和氢气射流的燃油浓度分布并对比了不同气体射流的混合特性；利用高速液滴显微成像结合光谱诊断建立了多组分燃料喷雾蒸发率的标定策略，并探索了环境和喷射条件对正己烷-甲醇混合燃料射流的浓度和蒸发率的影响。基于LIBS理论和试验结果提出最小稳定激光能量的概念，试验结果表明环境压力和温度变化均显著影响LIBS测试，因此开发了射流浓度和气体温度同时测量的方法以修正温度影响。氢氮和氧氮原子谱线比与其相应原子浓度比之间均存在良好的线性关系并且标定曲线不依赖燃油种类，因此利用两个谱线比可以定量含氧多组分燃料的组分浓度；C2谱线可以表征液相燃油，利用二者之间的相关性可以定量喷雾中液相燃油浓度，从而建立起多组分燃料射流浓度和蒸发率定量测量方法。结果表明正己烷和甲醇在时间和空间上的分布均存在差异，混合燃料中甲醇比例的提高会显著降低喷雾蒸发混合效果。结合系统和随机误差，评估浓度测量不确定度为6%。这些结果对多组分燃料喷雾混合气形成过程仿真模型的构建与验证以及先进燃烧系统开发和优化具有重要意义。