柴油/甲醇二元燃料发动机缸内燃烧数值模拟

为研究柴油/甲醇二元燃料的缸内燃烧过程,基于对二元燃料燃烧特征的分析,发展了湍流耦合反应动力学的柴油/甲醇二元燃料缸内燃烧机理和燃烧模型.基于一个已有的甲醇/正庚烷二元燃料燃烧机理,进一步提高了机理的预测精度,燃烧模型通过计算混合时间尺度和化学反应时间尺度来衡量燃烧的受控因素,其中化学反应时间尺度以熵增率衡量.通过发动机试验对模型进行了标定和验证,结果表明:该燃烧机理和燃烧模型能够很好地对纯柴油和柴油/甲醇二元燃料燃烧过程进行预测,包括随着甲醇比例的增加,滞燃期延长,甲醇火焰传播预混燃烧放热峰值逐渐明显.采用直接求解化学反应而不考虑湍流的燃烧模型,对燃烧进程的预测结果则随着甲醇量的增加而逐渐高于试验值.     

基于详细化学反应机理的增压二甲醚发动机燃烧与NO_x排放数值研究

使用KIVA-3V对增压柴油机和二甲醚发动机标定功率点的缸内燃烧过程与NOx排放进行了数值模拟研究.研究结果表明:计算所得的气缸压力和放热率曲线与实测值吻合较好.对缸内燃烧的温度分布计算表明:柴油燃烧滞燃期为2.5 °CA左右,二甲醚为1.5 °CA.柴油燃料着火始于喷雾前端两侧,在燃烧初期,其高温区分布在喷雾前端一侧,且在燃烧室内气流作用下沿垂直于喷雾方向扩散;二甲醚的着火点位于喷嘴附近,随喷雾的进行,其燃烧高温区从喷嘴附近一直延伸到喷雾前端,呈现狭长的高温带.在扩散燃烧后期,与柴油相比,二甲醚燃烧温度分布较均匀,且最高温度比柴油低.选用的9步NOx生成机理可较好地预测发动机实际运行中NOx排放水平.     

汽油和乙醇预混对柴油机燃烧与排放影响的试验

基于一台单缸柴油机,分别采用进气道喷射汽油和乙醇的预混方式,通过调节不同负荷下喷射汽油/柴油和乙醇/柴油燃料比例,以研究其对发动机燃烧与排放的影响.结果表明,低负荷下采用汽油或者乙醇预混,缸内最高燃烧压力降低,实现较为缓和的燃烧放热,均可以同时降低碳烟(Soot)和NOx,但HC和CO排放增加.较高负荷下随着预混比例增大,放热更加剧烈,缸内最大爆压升高.滞燃期随燃油预混比例增大而增大,但增大幅度随负荷增大而逐渐减小.与汽油预混方式相比,采用乙醇预混的方式滞燃期增大更加明显,且燃烧温度较低,在较高负荷下仍能实现较为缓和的燃烧放热,表明采用乙醇预混的方式有利于向高负荷拓展.     

汽油、正丁醇掺混柴油对部分预混压燃的燃烧和排放影响

通过一台高压共轨重型柴油机,使用汽油/柴油和正丁醇/柴油掺混燃料,掺混比例为40%、60%和80%(体积分数),研究了平均有效压力(BMEP)为0.48 MPa和0.95 MPa工况下汽油和正丁醇燃料掺混对柴油部分预混压燃(PPCI)模式的燃烧和排放影响.结果表明:随着汽油和正丁醇掺混比例的提高,滞燃期明显延长,更大程度地将喷油与燃烧过程分离,实现高比例预混燃烧.在BMEP为0.48 MPa工况下,各比例掺混燃料均易实现高比例预混燃烧,掺混比例为40%结合EGR即可满足欧Ⅵ排放限值,而掺混比例为80%时燃烧则受到压力升高率极限和燃烧效率恶化的约束.随BMEP升至0.95 MPa,各燃料滞燃期缩短、预混燃烧比例明显降低,掺混比例为40%和60%时,各掺混燃料均呈明显的扩散燃烧过程.相比于汽油,正丁醇掺混燃料在较低掺混比例可获得更低的有效燃油消耗率(BSFC)和soot排放,正丁醇以高掺混比例(80%)结合中等EGR率实现了87%的预混燃烧比例,NOx以及颗粒物排放分别为0.4 g/(kW·h)和0.001 5 g/(kW·h).     

燃油喷雾过程的大涡模拟研究

将大涡模拟湍流模型应用于定容弹中的柴油喷雾过程的数值研究,同时以试验结果作为参照,与RANS(RNG κ-ε)模型进行了对比分析,考察r网格密度对计算结果的影响.结果表明:使用RANS模型,网格疏密对喷雾贯穿距影响很小;而大涡模拟计算的贯穿距随着网格的加密而减小,且更接近于试验值.另一方面,液滴索特直径对湍流模型以及网格的密度均不敏感.大涡模拟预测的燃油喷雾的各项特性均优于RANS模型,而且大涡模拟可以捕捉到湍涡的小尺度随机结构,从而更精确地预测湍流特性.     

利用特征时间燃烧模型研究燃料分层对柴油PCCI燃烧过程的影响

针对柴油可控的预混燃烧方式,提出了一种基于化学动力学反应和湍流混合特征时间尺度的改进燃烧模型.介绍了一个燃料混合不均匀性程度函数H(φ),由此导出了模型中的湍流混合时间尺度系数f,用来表征燃料混合不均匀性程度与湍流混合时间尺度的关系.利用该模型对调制多脉冲喷油模式下柴油可控预混充量压燃PCCI燃烧进行了数值模拟,计算结果与试验数据符合良好.调制多脉冲喷油模式下柴油可控预混充量压燃PCCI燃烧的着火过程是分步容积式的,并受燃料分层的强烈影响.由于缸内存在浓度与温度的分层,降低了高浓度区域的温度,有助于减少NOx,此外"V"型当量比分布有利于推迟高温反应.     

柴油机燃用柴油/二甲氧基甲烷混合燃料的燃烧特性研究

开展了柴油机燃用柴油／二甲氧基甲烷混合燃料的燃烧特性研究,为含氧燃料的使用和研究提供理论与试验依据。研究结果表明,在相同平均有效压力(pRMEP)和转速下,随着燃料中二甲氧基甲烷掺混比例(含氧量)的增加,放热率峰值增加,放热率曲线型心向上止点偏移,预混燃烧比例增加而扩散燃烧比例减小,燃烧过程等容度提高,发动机当量柴油的有效燃油消耗率降低,缸内气体最高平均温度无明显升高。柴油中添加DMM对主燃烧时间影响不大,但总燃烧时间变短。     

二阶矩亚网格燃烧模型对射流火焰的大涡模拟

用二阶矩亚网格燃烧模型对美国Sandia国家实验室测量甲烷／空气射流火焰进行了大涡模拟(LES),与实验数据和用二阶矩输运方程湍流燃烧模型的雷诺平均(RANS)模拟结果进行了对比．LES得到时间平均的温度, 甲烷浓度以及温度脉动均方根值和实验值符合很好．LES时均值和RANS模拟结果接近,LES脉动均方根值优于 RANS模拟结果．LES瞬态结果显示了有燃烧时的拟序结构比无燃烧时的强,同时拟序结构强化了燃烧,湍流射流火焰呈皱折火焰面的状态．     

柴油机燃用甲醇/生物柴油着火过程

提出了甲醇/生物柴油混合燃料在柴油机缸内着火的评价指标,并建立了186,F柴油机燃用甲醇/生物柴油混合燃料燃烧过程的三维计算模型.通过对缸内温度、燃料浓度、氧浓度的分析,考察了甲醇掺混比例对混合燃料着火过程的影响.结果表明:当甲醇掺混比例由10%,增加到50%,时,缸内最大爆发压力由7.80,MPa降低到7.13,MPa,瞬时放热率峰值呈上升趋势.着火位置的分析表明,随着甲醇掺混比例的增加,缸内生物柴油的浓度降低,生物柴油的着火时刻推迟,缸内甲醇的浓度升高,逐渐接近甲醇可以着火的浓度;生物柴油的着火区域从油束的边缘逐渐转移到油束的中心、从油束的头部逐渐转移到喷油嘴附近.当甲醇的掺混比例小于或等于30%,时,生物柴油先于甲醇着火;当甲醇的掺混比例大于30%,时,甲醇先于生物柴油着火.     

MULINBUMP复合燃烧系统多次脉冲喷油控制参数的优化研究Ⅰ:发动机参数优化模拟平台的建立

在MULINBUMP复合燃烧系统多次脉冲喷油控制参数的优化过程中,利用一种高效全局智能寻优方法--微种群遗传算法,并结合发动机三维数值模拟程序KIVA,建立起一个发动机多参数优化模拟平台.为提高模拟精度对标准的KIVA程序进行了改进,引入KH-RT雾化模型、层流和湍流多莺特征时间尺度燃烧模型、Hart和Reitz传热模型以及Hiroyaau和Nagle碳烟排放模型来模拟柴油机多次脉冲喷雾、燃烧、壁面传热及排放过程,同时增添改进的Shell自燃模型来模拟着火过程.计算结果表明,采用改进的KIVA程序计算的喷雾贯穿距、缸内压力、着火时刻、燃烧放热率以及NOx和碳烟排放更接近试验结果;用改进的KIVA程序能够较好地模拟柴油机多次脉冲喷雾和燃烧过程,为进一步利用三维数值模拟计算来指导多脉冲喷油控制参数的优化奠定了基础.     

天然气/柴油双燃料发动机燃料喷射及着火特性

基于计算流体动力学(CFD)软件CONVERGE模拟了缸内高压直喷式柴油微引燃液化天然气(LNG)发动机的燃料喷射混合以及着火过程,校核和验证了湍流模型对模拟结果的影响,分析了天然气喷射正时、天然气喷射持续期及柴油与天然气射流中心轴线的夹角对缸内柴油和天然气射流发展、混合和着火的影响.结果表明:Smagorinsky大涡模拟湍流模型更适用于缸内高压直喷式LNG发动机的模拟;柴油与天然气射流中心轴线夹角越大,火核倾向于在天然气喷束的两侧形成从而点燃天然气;天然气喷射正时提前,天然气喷入缸内时与柴油的距离更近,利于天然气被引燃;天然气循环喷射量不变,天然气喷射持续期越短,天然气喷孔处压力更大,对柴油油束的冲击更强,促进了柴油的燃烧从而有利于天然气射流的着火.     

喷射时刻对柴油和汽油/柴油混合燃料低温燃烧的影响

使用纯柴油和一种汽油/柴油混合燃料在一台单缸柴油机上进行低温燃烧试验.通过调节燃油喷射时刻,研究了不同燃料的燃烧与排放性能.结果表明,这两种燃料的低温燃烧具有两阶段放热.在固定燃油喷射时刻的情况下,两种燃料具有类似的低温放热开始时刻,然而汽油/柴油混合燃料的高温放热开始时刻要迟于纯柴油燃料.主燃烧阶段的各个燃烧相位是线...     

多孔介质发动机流动与燃烧特性的大涡模拟初探

对多孔介质发动机的燃烧特性采用大涡模拟进行了初步分析.首先计算了考虑多孔介质随机结构特性的定容燃烧室内气体燃料喷射过程,并与自由空间中的喷射过程进行了对比.然后采用大涡模型对两种结构形式的多孔介质发动机的燃烧过程进行了初步的计算分析.多孔介质的存在增强了湍流涡团的小尺度结构,明显改变了燃料的空间分布,而采用大涡模拟(L...     

Numerical study on flash‐boiling spray of multicomponent fuel

Flash‐boiling occurs when a fuel is injected into a combustion chamber where the ambient pressure is lower than the saturation pressure of the fuel. It has been known that flashing is a favorable mechanism for atomizing liquid fuels. On the other hand, alternative fuels, such as gaseous fuels and oxygenated fuels, are used to achieve low exhaust emissions in recent years. In general, most of these alternative fuels have high volatility and flash‐boiling takes place easily in the fuel spray when injected into the combustion chamber of an internal combustion engine under high pressure. In addition the multicomponent mixture of high‐ and low‐volatility fuels has been proposed in the previous study in order to control the spray and combustion processes in an internal combustion engine. It was found that the multicomponent fuel produces flash‐boiling with an increase in the initial fuel temperature. Therefore, it is important to investigate these flash‐boiling processes in fuel spray. In the present study, the submodels of a flash‐boiling spray are constructed. These submodels consider the bubble nucleation, growth, and disruption in the nozzle orifice and injected fuel droplets. The model is implemented in KIVA3V and the spray characteristics of multicomponent fuel with and without flashing are numerically investigated. In addition, these numerical results are compared with experimental data obtained in the previous study using a constant volume vessel. The flashing spray characteristics from numerical simulation qualitatively show good agreement with the experimental results. In particular, it is confirmed from both the numerical and experimental data that flash‐boiling effectively accelerates the atomization and vaporization of fuel droplets. This means that a lean homogeneous mixture can be quickly formed using flash‐boiling in the combustion chamber. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 369–385, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20117     

喷雾锥角对柴油机燃烧过程影响的数值模拟

柴油机燃烧过程中喷雾内部的物理化学过程非常复杂,传热、蒸发、扩散、流动等物理过程控制着化学反应,影响着火和燃烧过程,进而决定着发动机的动力性、经济性以及排放性能。利用CFD分析软件FIRE对一台直列6缸增压柴油机的喷雾与燃烧过程进行模拟。研究了喷雾锥角对燃烧过程的影响规律以及喷雾锥角对碳烟和NOx生成的影响。     

Experimental investigation concerning the influence of fuel type and properties on the injection and atomization of liquid biofuels in an optical combustion chamber

Due to the scarcity of fossil fuels and the future stringent emission limits, there is an increasing interest for the use of renewable biofuels in compression ignition engines. However, these fuels have different physical, chemical and thermodynamic properties affecting atomization, spray development and combustion processes. The results reported in this paper have been obtained by experimentation with a constant volume combustion chamber. The influences of physical fuel properties on injections under non-evaporating conditions are studied, using a pump-line-nozzle system from a medium speed diesel engine with injection pressures up to 1200 bar, by changing the fuel type and temperature. Experiments were conducted for diesel, biodiesel, straight vegetable oils and animal fats. Injection pressure and needle lift measurements were analyzed. A high speed camera was used to visualize the spray, which enabled us to study the spray penetration and spray angle. Our results show that the fuel temperature is an important parameter to control because it significantly affects the fuel properties. Both the injection timing and injection duration are affected by the fuel properties. The influences of these properties on the spray development were less pronounced. At low temperatures, a strongly deteriorated atomization of oils and fats was observed.     

汽油/柴油掺混燃油两段喷射喷雾特性试验研究

基于定容燃烧弹与超高速数码相机搭建的LED-Mie散射喷雾试验台,研究了不同参数对柴油、汽油质量占比20%的柴汽混合油(记为G20)单段与两段喷射主喷液相喷雾特性的影响。结果表明,单段喷射喷雾贯穿距与喷雾锥角随喷射压力的增大而增大,G20喷雾贯穿距略小于柴油喷雾贯穿距,G20喷雾锥角略大于柴油喷雾锥角。将环境温度由300K升高到850K,喷雾贯穿距变小且喷雾很快达到稳定。冷态环境下(300K),两段喷射主喷喷雾贯穿距起始阶段与单段喷射喷雾贯穿距基本一致,但随着喷雾发展200μs左右后,两段喷射主喷喷雾贯穿距变得略小于单段喷射喷雾贯穿距。两段喷射主喷喷雾锥角略大于单段喷射喷雾锥角,预主喷间隔时间对喷雾锥角影响较小。     

Spray and combustion characterization for internal combustion engines using optical measuring techniques - A review

The number of studies on spray and combustion characteristics in IC (internal combustion) engines using optical techniques has rapidly increased in the past few years due to an increase in the number of alternative fuels and stricter emission standard regulations. This study investigates better ways of controlling the combustion process, thus ensuring optimum performance and minimum emission levels produced during the combustion process. Alternative fuels such as CNG (compressed natural gas), biodiesel and hydrogen have been studied by many researchers due to their relative low environmental impact. Meanwhile, for conventional fuels such as gasoline and diesel, studies have focused on spray characteristics to provide better air/fuel mixtures in order to produce a cleaner combustion process. Few experimental works have investigated the effects of modifications to the injector itself, for example, varying the injection rate, injection pressure, etc. In order to provide a better understanding of spray and combustion characteristics, researchers have studied macroscopic and microscopic parameters using optical techniques. This paper presents the significance of spray and combustion study with optical techniques that have been reported by previous researchers in order to provide in depth knowledge as assistance to readers interested in this research area.     

Theoretical and experimental investigations on the performance of dual fuel diesel engine with hydrogen and LPG as secondary fuels

The mathematical models to predict pressure, net heat release rate, mean gas temperature, and brake thermal efficiency for dual fuel diesel engine operated on hydrogen, LPG and mixture of LPG and hydrogen as secondary fuels are developed. In these models emphasis have been given on spray mixing characteristics, flame propagation, equilibrium combustion products and in-cylinder processes, which were computed using empirical equations and compared with experimental results. This combustion model predicts results which are in close agreement with the results of experiments conducted on a multi cylinder turbocharged, intercooled gen-set diesel engine. The predictions are also in close agreement with the results on single cylinder diesel engine obtained by other researchers. A reasonable agreement between the predicted and experimental results reveals that the presented model gives quantitatively and qualitatively realistic prediction of in-cylinder processes and engine performances during combustion.