【作者】 刘宏升；

【导师】 解茂昭；

【作者基本信息】 大连理工大学 ， 动力机械与工程， 2008， 博士

【摘要】 随着世界经济的飞速发展,能源及环境成为当今世界首要关注的两大热点问题,而作为能源消耗和环境污染的主要来源的汽车发动机已得到了全世界研究者的广泛关注,如何实现高效和清洁燃烧已成为国际发动机界的一个最紧迫的课题。在众多的关于发动机的技术革新中,多孔介质发动机作为一种基于全新理念的新型发动机,以其独特的燃烧方式、低排放、低噪音等优越性而受到越来越多的关注。本文通过理论分析和数值模拟的方法对多孔介质发动机工作循环及燃烧特性进行研究,在机理实验的基础上考察液体燃料在多孔介质内的燃烧特性。旨在通过理论与实验的研究,深入地了解多孔介质发动机的工作特性,以期推动多孔介质发动机的实用化。首先,将已有多孔介质燃烧器中得到的结论应用于多孔介质发动机,建立多孔介质发动机热力学模型,对多孔介质发动机理想循环和不可逆循环中的功效特性及可用能损失等加以分析。(1)以经典热力学理论为指导,分别对开式、闭式两种多孔介质发动机的工作循环进行系统的热力学分析。从理论上分析多孔介质发动机的工作过程,讨论理想循环的性能参数如压缩比、极限温度等对发动机效率、循环功的影响,分析循环可用能损失,并将多孔介质回热循环与传统发动机的Otto循环、Diesel循环进行比较。结果表明多孔介质发动机的整体性能水平优于传统发动机,具有效率高、循环功大的特点。(2)用有限时间热力学方法分析了闭式多孔介质发动机内多孔介质回热循环的功效特性。在分别考虑热损失和活塞摩擦损失的条件下,推导出循环功与效率的关系及最大功输出时的效率界限等性能关系,并给出了较详尽的数值计算结果,讨论了与燃烧、摩擦相关的参数对功效特性的影响,使得热力学循环分析更为接近实际过程。这对实际多孔介质发动机性能的评估和改进具有重要意义。其次,基于目前国际流行并已得到验证的HCCI燃烧模型中单区、多区模型,针对多孔介质发动机的特性,建立适用于多孔介质发动机模拟的单区模型和双区模型。(1)以热力学第一定律为基础,应用CHEMKIN化学动力学软件包中的SENKIN模块结合发动机燃烧的零维单区模型,模拟了正庚烷在多孔介质发动机中的燃烧过程。通过修改SENKIN程序,加入Woschni传热模型、多孔介质换热模型和燃烧放热率模型,建立了多孔介质发动机的能量方程。燃烧放热率模型中分别采用代用燃烧规律和正庚烷详细氧化机理两种方式,在正庚烷详细氧化机理中加入了氮氧化物的生成机理。计算了多种工况参数下多孔介质发动机缸内温度、压力变化规律,分别讨论了压缩比、多孔介质温度、体换热系数和过量空气系数等参数对多孔介质发动机燃烧过程的影响。通过比较多孔介质发动机与传统发动机温度、压力的变化规律,证明多孔介质使缸内温度和压力的变化趋于平缓。(2)在考虑各区间质量分布和交换、壁面传热、区间质量交换等因素的基础上,结合多孔介质换热模型,建立了开式、闭式多孔介质发动机的准维双区模型,对其燃烧过程进行模拟。程序中耦合了化学反应动力学计算软件包CHEMKINⅢ,两区之间的质量交换基于Komninos等人的多区模型并加以简化,壁面传热模型为针对HCCI发动机而设计的改进的Woschni模型,以异辛烷为燃料,采用为HCCI发动机定制的骨架反应机理模拟燃烧过程。着重讨论进气温度和压力、压缩比、过量空气系数及多孔介质初始温度等运行参数对多孔介质发动机性能的影响。计算结果表明,由于多孔介质的高温及其对混合气的预热作用,促进了液体燃料汽化和燃烧反应发生,多孔介质初始温度和压缩比是决定发动机的压燃着火的重要因素。开式PM发动机,燃油的供油方式及汽化过程对缸内燃料的质量分布有较大的影响;闭式PM发动机,液体燃料的汽化过程完全在多孔介质室内进行,不受喷油时刻、载荷等运行参数的影响,阀门开启的时间决定蒸汽与空气的混合,是决定多孔介质中能否着火的重要因素。最后,通过机理实验研究液体燃料在多孔介质内的燃烧特性。鉴于发动机高速瞬态工况不适合用于原理性研究的观察和测量,本文设计了使用液体燃料的多孔介质燃烧器,对多孔介质中的“渗孔液雾自均匀化”和超绝热燃烧进行原理性实验研究。自行设计和制作的实验台包括燃烧器(石英玻璃管)、气体供给系统、燃油供给系统以及测量系统等。该实验台具有研究多孔介质中气体燃烧波传播规律及液体燃烧特性的双重功能。通过气体燃料在多孔介质内的燃烧对多孔介质进行预热,然后将液体燃料喷入多孔介质燃烧器,汽化后燃烧。通过热电偶测量燃烧区的温度分布,讨论混合气流量和当量比对填充床内燃烧波的传播速度和最高温度的影响。更多还原

【Abstract】 With the rapid development of global economy, the problems of energy crisis and environment pollution have become two focuses of attention. As the main source of petroleum consumption and emissions to atmosphere, automotive engines have received increased attentions, and how to realize high efficiency and clean combustion has become a urgent objective pursued by the entire engine community. Porous medium (PM) engine, as a newborn thing, is characterized with a number of advantages, such as low-pollution, higher-efficiency and extended limits of flammability, and, hence, is receiving more and more attentions.This thesis presents a theoretical analysis of cycle characteristic and numerical study on the ignition and combustion process of PM engines, and presents the investigation on combustion of liquid fuel in prous media based on the mechanism experimental. The principal aim of this work has been to have a primary recognition about the working characteristics of PM engine, and resolve the key problem of liquid fuel evaporation and combustion in porous medium, with the purpose of providing some theoretical foundations for its application.Firstly, thermodynamic models of porous medium engine are set up based on the knowledge gained from previous studies on porous medium burners, and characteristics of work and efficiency as well as the availability analysis of both ideal cycle and irreversible cycle in porous medium engine are discussed.(1) Based on the classical thermodynamics theory, systematic analysis of theworking cycle is conducted for two types of porous medium engine, i.e. one with closed PM chamber and other with open PM chamber. The general performances of the porous medium engines are theoretically analyzed, and the influences of compression ratio, limit temperatures and volume expansion ratio etc on net work and efficiency of the ideal heat regenerative cycle in the PM engines are discussed, and availabilities in the working process are derived. Comparison of the heat regenerative cycle of the PM engine with the Otto cycle and Diesel cycle shows that performance of the PM engine is prior to the conventional engine, the PM engine can improve net work output at a little expense of thermal efficiency.(2) Finite-time thermodynamic analysis is applied to evaluate the thermodynamic performance of the irreversible heat regenerative cycle in the PM-engine with closed PM chamber. The irreversibility of heat transfer between the working fluid and cylinder wall, and the friction loss due to piston movement are taken into account in the real PM heat regenerative cycle. The relationship between the net work output and thermal efficiency, and corresponding limit conditions for the PM engine are derived, in which the effect of heat losses through the cylinder wall is took into account. According to detailed numerical computations, effects of combustion and heat transfer on the cycle are also discussed. The results obtained here could provide significant guidance for the performance evaluation and improvement of practical PM-engines.Secondly, available combustion models, which are suitable for HCCI engines, are modified and applied to investigate the ignition and combustion characteristics of the PM engine. A simulation system including a single-zone model and a two-zone model for porous medium engine is developed, which could rapidly bring accurate information for the prediction of ignition point.(1) Based on the First thermodynamic Law, the SENKIN code of the CHEMKIN chemical kinetics package, combined with a zero-dimensional single-zone model of engine combustion, was used to simulate the combustion process of a PM engines fueled by n-heptane. The code has been modified to incorporate the Woschni heat transfer correlation, a model of heat transfer within porous medium and heat release rate models to build the energy balance equation. A substitutional combustion rate model and a detailed chemical kinetics mechanism with detailed chemistry mechanism of NO_x formation are used to calculate the heat release rate, separately. Evolutions of pressure and temperature in the PM engine are calculated under various working conditions. Influences of operating parameters, e.g. compression ratio, the initial temperature and volumetric heat transfer coefficient of the porous medium, the excess air ratio etc on the combustion process of the PM engine are discussed. Comparison of the PM engine with conventional engines shows that PM can relax the evolution of the in-cylinder temperature and pressure.(2) The combustion processes of both permanent and periodical contact PM engines were simulated by a quasi-dimensional two-zone model, considering the influences of the mass distribution, heat transfer from the cylinder wall, mass exchange between zones and the heat transfer in porous medium. A computer program was developed and coupled with the chemical kinetics package Chemkin III. The mass exchange model is adopted and simplified from the model of Komninos based on the assumption of uniform pressure throughout both zones. Wall heat losses were predicted with an improved Woschni model for the HCCI process by Chang. The e PM enginewas fueled with iso-octane and a skeletal kinetic mechanism for iso-octane oxidation was used for the chemistry simulation. Influences of operating parameters, e.g. intake temperature and pressure, the initial temperature of PM, compression ratio, the excess air ratio etc on the performance of the PM engine were emphatically discussed. It is found out that the porous medium, acting as a heat recuperator, can preheat the mixture and significantly enhance the evaporation of liquid fuel, which promotes the ignition and combustion in the cylinder; and that the initial PM temperature and the compression ratio are critical factors controlling the compression ignition of the mixture.For the permanent contact PM engine, the modes of fuel supply and the progress of fuel evaporation influence the mass distribution in the cylinder greatly. However, for the periodical contact PM engine, the evaporation process of liquid fuel occurs in the PM chamber, which is decoupled from the cylinder and is almost independent of other operating factors such as spray timing, power output etc. The mixing of fuel vapor and air is controlled by the timing of the PM valve opening, which is the critical factor determining ignitiontiming.Finally, the dissertation presents an experimental study of evaporation enhancementand combustion characteristics of liquid fuel spray aided by porous media. A set of experimental system for combustion of liquid fuel in porous media was built up to study the mechanism of the "in-pore spray" and the superadiabatic combustion in porous medium. The combustion system consists of a combustor (quartz glass tube), a gas supply system, a fuel supply system, a measurement system and so on. The expermental system can be employed for studying the combustion characteristics of both gaseous and of liquid fuels.The PM was preheated by combustion of gaseous fuel for a short time, and then liquid fuel (diesel) was sprayed into the porous medium combustor, where evaporation and combustiton happened. Temperature distributions in the combustion zone are measured with thermocouples, furthermore, the influences of mixture speed and equivalence ratio on the combustion wave speed and the maximum combustion temperature in the packed bed are discussed.更多还原