【作者】 赵鹏；

【导师】 李国岫；

【作者基本信息】 北京交通大学 ， 载运工具运用工程， 2013， 博士

【摘要】 对于直喷式柴油机来说,燃油的喷射、雾化、蒸发和燃烧过程非常重要,燃油液滴的蒸发过程和燃油蒸气的空间分布很大程度上决定了直喷式柴油机的燃烧特性,从而进一步影响其总体性能。因此有必要开展燃油液滴蒸发过程传热传质机理的研究。本文以直喷式柴油机燃油液滴的蒸发过程为研究背景,将燃油液滴蒸发过程中传热传质机理作为研究重点,采用数值模拟、理论分析与实验方法进行了研究。由于实验条件和数值模拟方法的限制,目前对于燃油液滴蒸发过程传热传质机理的认识并不完善。对于存在液滴蒸发过程等工程应用中,多采用经验公式进行研究。然而现有的经验公式并不能准确地模拟液滴的蒸发过程,而且实验条件也很难观测到液滴内部流场等分布。对于液滴蒸发过程的数值模拟研究,多采用气液相分开建模的方法。因此有必要采用气液两相流数值模拟方法对燃油液滴传热传质机理进行研究。本文首先建立了基于气液两相流VOF方法的燃油液滴蒸发大涡模拟数学模型。实现了燃油液滴蒸发过程的气液两相流联合模拟,弥补了常规数值模拟方法无法考虑液滴形状变化以及气相与液相分开模拟等缺陷,为采用数值方法模拟液滴的蒸发过程提供了一种研究方法。针对Marangoni效应普遍存在于两相流中的现象,在基于气液两相流VOF方法的燃油液滴蒸发数值模拟中,考虑了由表面张力梯度引起的Marangoni效应。将Marangoni效应简化为一种相界面处的体积力,得出了基于VOF方法并考虑表面张力与温度关系的Marangoni效应作用力计算公式,并将Marangoni效应添加到气液两相流液滴蒸发数学模型中。研究了Marangoni效应对燃油液滴蒸发过程传热传质特性的影响。由于在直喷式柴油机的缸内环境中,燃油液滴的蒸发过程受到温度、压力、湍流等环境因素的影响。为了深入地了解燃油液滴蒸发过程中的传热传质机理,研究了环境温度、环境压力、雷诺数和湍流脉动等不同环境因素对液相传热和气液相传质的影响,探讨了不同环境因素对液滴传热传质特性的影响关系。现有的燃油液滴蒸发速率模型在液滴蒸发相关计算中广泛使用,如喷雾燃烧等工程应用中。此模型的基本假设为液滴始终保持圆球的形状,因此其面积或体积等相关计算都按照球体的计算公式获得。然而在实际情况中,燃油液滴在蒸发过程中形状不断地发生变化。本文通过分析得出了影响液滴变形的关键因素和液滴表面积的变化规律,应用回归分析方法得出了液滴的表面积变化公式,并对燃油液滴蒸发速率模型进行了修正。在整个计算周期内,修正模型的结果都比原模型的结果更加接近于实验值。这说明基于液滴变形的燃油蒸发速率修正模型,能够更加准确地预测实验值,比原蒸发速率模型有一定的改进。通过分析柴油机喷雾场中燃油液滴的分布情况,采用液滴阵列来近似喷雾场中的液滴分布情况,研究了液滴阵列蒸发过程中前液滴对后液滴传热传质的影响,不同液滴数阵列以及液滴间距对液滴传热传质的影响,对单液滴的努塞尔数进行改进,提出基于液滴阵列的努塞尔数改进公式。通过对比喷雾场中的燃油蒸气分布的数值模拟结果与实验结果,对基于液滴阵列的努塞尔数改进公式进行了验证。建立了能够进行温度闭环控制的高温环境液滴蒸发实验系统。系统地分析了不同组分燃油液滴随环境温度变化的蒸发特性,不同组分燃油液滴蒸发特性的区别,以及不同组分混合液滴的蒸发特性。并利用燃油液滴蒸发实验得到的结果,进一步对燃油液滴蒸发的数值模型进行了验证。更多还原

【Abstract】 The process of fuel spray, atomization, evaporation and combustion is very important to the direct inject diesel engine. The fuel evaporation process and spatial distribution largely determines the characteristics of the diesel combustion process. Therefore, it is necessary to carry out the study of the heat and mass transfer mechanism of fuel droplet evaporation process in direct injection diesel engine. Based on the fuel droplet evaporation in direct injection diesel engine, the study of heat and mass transfer mechanism of droplet evaporation was carried out using numerical simulation, theoretical analysis and experimental methods.Due to the limitations of experimental conditions and method of numerical simulation, the current understanding of the heat and mass transfer mechanism of fuel droplet evaporation process is not perfect. The researcher used the empirical formula to study droplet evaporation for engineering applications. However, the existing empirical formula can not accurately simulate the droplet evaporation process and experimental conditions can not be observed in the distribution of droplet internal flow field. The methods of separate gas and liquid phase modeling are widely used in numerical simulation of droplet evaporation process. It is necessary to adopt a numerical simulation method of gas-liquid two-phase flow of fuel droplet heat and mass transfer.The mathematical model of fuel droplet evaporation process based on large eddy simulation VOF method of gas-liquid two-phase flow is established. This model can describe the gas-liquid two-phase flow field, the temperature gradient, the concentration gradient and the deformation of fuel droplets. Formula based on Fick’s law of diffusion concentration gradient principle, be applied to the numerical simulation of fuel droplet evaporation process. The numerical simulation of the fuel droplet evaporation process will be introduced by the Marangoni effect caused by surface tension gradient in the phase boundary. The Marangoni effect is simplified to the volumn force, which considers the effect of the temperature. The effect of the heat and mass transfer mechanism of fuel droplet evaporation process caused by Marangoni effect is studied.The fuel droplet evaporation process is influenced by the role of environmental factors in the temperature, pressure and turbulence in direct injection diesel engine. For in-depth understanding of the heat and mass transfer mechanism of the fuel droplet evaporation process, the various environmental factors on the liquid phase heat transfer and gas-liquid phase mass transfer is studied.Existing fuel droplet evaporation rate model widely used in the correlation calculation in the droplet evaporation, such as spray combustion applications. This model is assumed that the droplets always maintain the shape of the ball, and therefore the correlation calculation of the area or volume is obtained in accordance with the formula of the sphere. For practical terms, the shape of fuel droplets in the evaporation process continues changing. By analyzing the obtained variation of the surface area and the key factors affecting droplet deformation, the formula of droplet surface area is obtained by regression analysis, and fuel droplet evaporation rate model was successfully corrected. For the entire compute cycles, the results of the modified model are closer to the experimental value than that of the original model. This shows that droplet deformation correction model can accurately predict the experimental values.Through the analysis of the distribution of the fuel droplets in the field of diesel spray, the effect of different droplet number array and the droplet spacing on heat and mass transfer are studied. The Nusselt number of the droplet array is improved.The improved Nusselt number of the droplet array is validated.The high-temperature environment experimental system based on temperature closed-loop controlled is established. A systematic analysis of the fuel droplets evaporation characteristics of the different components with the environmental temperature changing is carried out. The fuel droplet evaporation characteristic of different components is also carried out. The numerical model is validated by the experimental results of fuel droplet evaporation.更多还原