【作者】 徐敏；

【导师】 邓康耀； 崔毅；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2015， 博士

【摘要】 现有柴油机准维燃烧模型主要依据的是30多年前常规喷雾燃烧系统的喷射及油气混合理论与试验结果,受当时实验条件限制,关于柴油喷雾燃烧的许多现象无法观察到,燃烧模型的建模过程中采用了许多简化和假设,并且大多燃烧模型是根据单束燃油喷雾试验建立的,没有考虑多油束受限空间喷雾对空气卷吸的影响,在预测高功率密度柴油机缸内喷雾燃烧时存在较大偏差,无法满足高功率密度柴油机性能优化设计的需求。因此,本文从柴油喷雾的气液两相贯穿现象以及受限空间喷雾卷吸规律方面开展了研究,建立了气相两相贯穿模型以及受限空间射流浓度分布理论关系式,并依据气液两相贯穿模型和受限空间气相射流浓度分布规律,建立了基于气液两相贯穿分离现象并考虑受限空间喷雾的高功率密度柴油机准维燃烧模型。全文具体工作如下:首先,为了在高功率密度柴油机准维燃烧模型中采用浓度划分小区和计算卷吸方式并考虑燃油液相的影响,需要从燃油喷雾气液两相分离现象出发,对喷雾气相和液相区分别进行离散化。因此,针对气液两相贯穿分离开展相关理论推导工作,在理想喷雾假设基础上,以燃油喷雾液相区为控制对象,通过综合应用能量守恒、质量守恒和动量守恒等,并假设燃油蒸发受油气混合过程限制,即混合限制蒸发理论,建立了喷雾液相贯穿模型,模型能够预测燃油喷雾液相贯穿规律、液相长度以及液相长度处截面的饱和蒸气状态参数和当量比;进一步以喷雾中燃油完全蒸发后的气相区为控制对象,通过控制面的质量守恒和动量守恒,建立了喷雾气相贯穿模型,模型能够预测喷雾气相贯穿规律。通过结合气液两相贯穿模型,能够预测燃油喷雾中的气液两相贯穿规律,为高功率密度柴油机准维燃烧模型的贯穿计算和小区划分提供依据。其次,为了考虑高功率密度柴油机大油量、多油束和小空间受限空间对喷雾卷吸的影响,通过CFD仿真计算开展了受限空间气相射流研究,建立了受限空间气相射流CFD计算二维简化模型,考虑了实际柴油机气缸几何尺寸限制以及多油束限制,分析了空间受限对气相射流速度和燃油浓度分布及卷吸规律的影响。在CFD计算基础上,开展了受限空间气相射流截面浓度分布的定量分析,并定义了无量纲轴向距离?和无量纲射流夹角?,根据不同无量纲夹角下射流截面浓度分布系数随无量纲轴向距离变化具有相似性,建立了受限空间参数与射流截面浓度分布系数之间的关系式。针对环境气体压力、射流速度和油束间夹角等参数设计了验证方案,验证受限空间参数与浓度分布系数关系式预测其它条件射流浓度分布规律的准确性。验证结果表明:不同验证方案的分布系数均能与浓度分布关系式计算结果较好吻合,受限空间参数与射流截面浓度分布系数关系式预测不同缸内气体压力、射流速度以及油束夹角的射流截面燃油浓度分布规律是合理的。然后,以建立的柴油喷雾气液两相贯穿模型为子模型,来预测气液两相贯穿、最大液相长度及最大液相长度处饱和蒸气状态参数,并结合受限空间气相射流的相关研究,建立了基于气液两相贯穿和受限空间喷雾卷吸的高功率密度柴油机准维燃烧模型。模型从采用更符合高功率密度柴油机喷雾卷吸特性的燃油浓度划分小区方式并考虑燃油液相影响的角度出发,根据柴油喷雾气液两相分离现象,对喷雾气相和液相区分别进行离散化,液相区按时间步长划分小区,气相区根据浓度分布划分小区,实现了浓度划分小区方式与考虑燃油液相影响相结合,符合高功率密度柴油喷雾燃烧现象,对柴油机一般功率密度工况和高功率密度工况的缸内燃烧都有较好的预测效果和精度。最后,开展了高功率密度柴油机准维燃烧模的计算和试验验证工作。针对一台重载柴油机,应用参数试验和最优拉丁超立方设计相结合的方法设计燃烧模型计算和验证试验工况,验证燃烧模型对一般功率密度柴油机缸内燃烧以及NOx排放的预测能力;在此基础上进一步针对高功率密度柴油机,开展燃烧模型的高功率密度工况试验验证工作,验证燃烧模型对高功率密度工况燃烧过程的预测能力,并对比分析了不同功率密度下的传统油滴蒸发燃烧模型与高功率密度燃烧模型的计算结果。综合试验验证结果表明,高功率密度柴油机准维燃烧模型对一般功率密度柴油机的不同转速、负荷、喷油压力以及喷油正时等工况有较好的适应性;同时对高功率密度工况的预测效果明显优于广安模型,最高燃烧压力的相对误差较广安模型减小约5%,并且压缩过程和做功过程缸压均能较好的与试验结果吻合。更多还原

【Abstract】 The existing quasi-dimensional combustion models are based on old fuel spray and fuel-air mixing theory and experiment results. Due to the restriction of experiment condition, some phenomena on spray atomization and combustion process were not obseverd, such as: liquid and gas phase separation in spray and lift-off combustion, and the new phenomena are not taken into consideration in the old quasi-dimensional combustion models. In addition, most of the old models are based on the single spray experiment, and effect of confined space on entratinment of spray was not considered. Both of those leads to the large difference in predicting the combustion process of high power density diesel engine. So, it is necessary to built new combustion model based on liquid and gas phase separation and confined space spray for high power density diesel engine.Firstly, a theory investigation focusing on liquid penetration of diesel spray was presented. A one-dimensional model on liquid length is developed for the prediction of liquid length, based on the momentum flux and fuel mass flux conservation along the sprays’ axis, and energy conservation on the control volume. The 1D model on liquid length includes the parameters of ambient gas temperature and density, fuel injection pressure, nozzle diameter and fuel temperature. In addition, the heterogeneous distribution of velocity and fuel concentration over the cross-section of spray is taken into consideration. A Gaussian radial profile is assumed for the distribution of velocity and fuel volume fraction, and a distribution factor is introduced to describe the distribution profile of axial velocity and fuel volume fraction. Also, the model was validated by the measured data from Siebers’ experiment over a wide ambient gas temperature and density.Secondly, a study on gas jet in confined space was carried out through CFD simulation calculation in which actual diesel engine cylinder geometry and multi spray constraints was consided. Two-dimensional simplified CFD model for confined space gas jet was established, and effects of confined space on velocity and fuel mass fraction distributons of jet were analyzed. On the basis of CFD calculation, quantitative analyses on the concentration distributions of cross-section of jet in confined space were carried out. A dimensionless axial distance and jet angle were defined in the discussion, and an equation on the parameters of confined space and concentration distribution coefficient of jet were established. The verification results show that the distribution coefficient of different validation scheme can both agree well with the established formula on confined space parameters and fuel concentration distribution coefficient.Thirdly, a quasi-dimensional combustion model for high power density diesel engine was developed based on liquid and gas phase separation phenomenon and the study on entrainment of fuel spray in confined space. The model combines advantages of combustion model based on droplet evaporation and gas jet by the respective discretization of liquid and gas phase area of spray, is more identical with combustion characteristic of high power density diesel spray. Therefore, this combustion model is better in predcting in-cylinder combustion of common diesel engine and high power density diesel engine.Finally, calculation and experiment validations for turbocharging and high power density condition on the quasi-dimensional combustion model were developed. Based on a traditional turbochaging diesel engine, experiment condition are designed from design of experiment(DOE) method. Calculation and experiment validations on turbocharging engine are carried out based on the designed experiment condition to verify the prediction performance of combustion model on in-cylinder combustion and NOx emission. In addition, calculation and experiment validations on high power density diesel engine are further carried out to verify the prediction performance of combustion model on in-cylinder combustion at high power density condition. The experimental validations indicate that quasi-dimensional combustion model for high power density diesel engine is applicable to the prediction of in-cylinder combustion for common diesel engine, and is better than Hiroyasu’s model in prediting in-cylinder combustion of high power density engine. The relative error of the maximum in-cylinder pressure at high power density condition is lower than Hiroyasu’s model by 5%.更多还原