【作者】 李从心；

【导师】 张欣；

【作者基本信息】 北京交通大学 ， 载运工具运用工程， 2010， 博士

【摘要】 摘要：近年来,由于全球石油资源日渐枯竭和生态环境日趋恶化,代用燃料.的研究成为发动机领域的重要研究方向,其中低热值气体燃料是现阶段正在受到广泛关注的代用气体燃料。本文以数值模拟和台架试验为主要研究手段,对发动机燃用低热值气体燃料的点火、缸内燃烧和排放特性进行了系统研究。研究工作为发动机高效低污染的燃用此类燃料提供理论依据和优化改进的方法,对理解低热值气体燃料燃烧特性具有重要的理论意义和工程实用价值。本文建立了基于离散粒子的发动机燃用低热值气体燃料的点火模型。模型由火花塞放电子模型、离散粒子发展速度子模型、组分燃烧率子模型和火花塞效应子模型组成。离散粒子发展速度子模型中加入了火花塞附近整体对流速度的影响,在组分燃烧率子模型中引入了分形概念,基于分形理论、以火焰折皱度因子来描述湍流对火核发展阶段瞬时燃烧率的影响,同时基于详细化学反应机理完成火核焰后化学反应放热的计算。在火焰传播阶段中,基于Damkohler判据建立了发动机燃用低热值气体燃料的湍流燃烧模型,该模型具有自适应功能,根据当前湍流和化学反应时间尺度的比较来确定该单元的组分燃烧率的计算方法。同时,在计算中引入了详细化学反应机理,并实现了与缸内流场的耦合计算。在开发的KIVA/CHEMKIN耦合计算软件性能分析的基础上,建立了基于MPI和OpenMP复合算法的并行计算系统,分别利用了消息传递模型和共享存储模型的优势使引入详细化学反应机理后的计算时间大幅降低,使得耦合化学反应动力学的工程应用成为可能。基于建立的发动机燃用低热值气体燃料的燃烧模型,运用开发的并行计算软件对低热值气体燃料燃烧性能进行了研究。结果发现,随着湍流脉动速度强度和涡流比的增加,火核生长速度加快,分形维数增大,表明火核表面折皱程度增大,燃烧更加剧烈。点火能量主要影响火核初期半径的大小,低热值气体燃料的惰性组分系数越大,火核生长的速度越慢,分形维数降低。当过量空气系数增大时(稀燃极限内),火核生长速度会明显降低,分形维数也随之下降。低热值气体燃料缸内主要污染物NOX、CO的分布具有明显的规律性,甲醛生成量相比常规排放物小两个数量级。HC排放主要由未燃燃料、醇类和醛类的CH3OH、CH2O,少量的烯烃和烷烃类的C2H4、C2H6组成,UHC主要分布在缸壁附近和壁面淬熄区。惰性组分系数的增加能减小同一曲轴转角下的高温分布区域,同时,OH根的浓度明显下降,火焰发展期增加,NO生成量会随之降低。增大发动机点火提前角,低热值气体发动机动力性能明显提高,但同时NO浓度值也大幅增加。本研究搭建了发动机测试台架,开展了发动机燃用低热值气体燃料的试验研究。结果发现低热值气体燃料中惰性组分会增加火焰发展期,而对快速燃烧期的影响相对较小,同时会导致发动机放热率曲线型心偏离上止点,燃烧等容度变差。低热值气体燃料的惰性组分系数增大会导致循环变动加强,而当低热值气体惰性组分系数小于20%时,平均指示压力与最大缸压具有强线性相关性,但当惰性组分系数超过30%,小负荷工况下燃烧循环变动明显增加。低热值气体燃料中掺入氢气降低了发动机工作循环的火焰发展期和快速燃烧期,尤其是降低了火焰发展期的持续时间。同时,燃料中掺氢降低了发动机的循环变动,随着掺氢比的增加,发动机平均指示压力的分布区域向其均值靠近,而且平均指示压力和最大缸压之间的相关性明显加强。发动机燃用低热值气体燃料的CO和HC排放随着惰性组分系数的增加而增加,NOx排放则随着惰性组分系数的增加迅速降低,尤其是当燃料中的惰性气体为CO2时尤为明显。在低热值气体燃料中掺入氢气能够有效降低CO和HC排放,但同时会大幅的增加NOx排放。更多还原

【Abstract】 ABSTRACT:In recent years, due to the shortage of the global petroleum resource and the serious pollution to living environment, the research on clean alternative fuels is becoming an important direction of engine techniques. Lower heating value (LHV) gas has been considered as an alternative gas fuel nowadays. In this study, the characteristics of ignition, combustion and emissions about the engine fuelled with LHV gas was investigated using fully coupled multi-dimensional CFD and detailed chemical kinetics model combined with experiments. It is useful to promote the effective and clean combustion in engine fuelled with LHV gas and the study has important theoretic and engineering practical value.Based on discrete particles method, the ignition model for engine fuelled with LHV gas is developed. The model is composed of four parts, Electric energy deposition sub-model, Discrete particle velocjty sub-model, Burn rate sub-model, spark plug protrusion and electrode heat transfer sub-model. In the discrete particle model, a convection velocity has been considered. Using fractal theory, a coefficient used to describe the wrinkling effect of turbulent combustion has been defined. In flame propagation period, a self-adapting turbulent combustion model based on Damkohler number was built. The model has been used to calculate the burn rate in proper way by comparison of turbulence and chemical kinetics time scale. At the same time, detailed chemical kinetics model has been combined with CFD.Based on the analysis of coupled simulation software consisting of KIVA and CHEMKIN, the parallel calculation system by MPI and OpenMP is built. The system combines the advantage of Message-passing model and Shared-memory model, so the time consuming is reduced markedly.Based on the parallel simulation system for engine fuelled with LHV gas, the characteristics of ignition and combustion was studied. The simulation results indicate that with the increase of turbulent intensity and swirl ratio, the velocity of flame kernel development increases. Meanwhile, fractal dimension inceases and the combustion process are enhanced. Ignition energy mainly influence the initial stage of fame kernel radius. With the increase of the volume fraction of inert gas in fuel, the velocity of fame kernel radius development and fractal dimension decrease. It shows the same behavior if the excess air ratio increases. The normal exhaust emissions like NOX, CO have clear distribution regularity in cylinder. The amount of CH2O is two order of magnitudes lower than that of normal emissions. UHC emissions mostly reside in the vicinity of cylinder wall, and the compositions are mainly composed of unburned CH4, CH2O, CH3OH, C2H4 and C2H6. Meanwhile, with the increase of the volume fraction of inert gas in fuel, the higher temperature area decreases at the same crank angle. At the same time, the flame development duration increase remarkably, but the amount of NO emission reduce. Advancing spark timing can increase the maximum pressure value, but the amount of NO emission also increase.The experimental bench for engine fuelled with LHV gas was built and the combustion characteristics of the engine has been tested. The results show that the inert gas in the LHV gas fuel has great effect on the flame development duration, but less effect on the rapid combustion duration. Meanwhile, the inert gas causes the center of the heat release curve to move apart from TDC and with the increase of inert gas volume fraction, the degree of departure is enhanced. The level of inert gas fraction has strong influence on the cyclic variations at low load operations. With the increase of inert gas fraction, cyclic variation of the engine fuelled with LHV gas is strengthened. When the level of inert gas volume fraction is lower than 20%, the combustion process has good stability under all tested load conditions and the indicated mean pressure has strong linear correlation with the maximum pressure. But if the level of inert gas fraction in blend is higher than 30%, the engine fuelled with LHV gas shows poor stability performance, especially in the case of the low load conditions. Hydrogen addition can decrease the flame development duration and rapid combustion duration, but hydrogen gives the larger influence on the flame development duration than on the rapid combustion duration. At the same time, hydrogen addition into the LHV gas decreases the cycle-by-cycle variation. For a specified inert gas fraction, the indicated mean pressure shows higher and concentrated value when hydrogen addition is introduced. This effectiveness becomes more remarkably at high hydrogen addition fraction. Strong independency between indicated mean pressure and peak pressure is presented with the increase of hydrogen fraction. With the increase of inert gas fraction, CO and HC emissions increase, but NOX emission decreases markedly especially with CO2 dilution gas. Hydrogen addition can decrease CO and HC emissions effectively, but the amount NOx emission increases obviously meanwhile.更多还原