汽油缸内直喷喷雾在不同壁温和二次喷射下的附壁油膜特性研究

采用激光诱导荧光(laser induced fluorescence,LIF)法测量缸内直喷喷油器喷雾附壁油膜的厚度分布,研究了附壁油膜质量和平均厚度随不同的壁面温度和喷射策略的变化。结果表明:常温壁面下油膜呈"波浪状",热壁面下油膜的边缘轮廓呈"带状"凸起,在壁面温度为413K时,还出现了聚集的小液滴。保持总喷油量不变,随着壁面温度的增大,单次喷射的附壁油膜质量逐渐减小,二次喷射的附壁油膜质量先减小后增大,单/二次喷射的附壁油膜平均厚度都逐渐增大;相对于单次喷射,相同热壁面温度下的二次喷射附壁油膜平均厚度都较小,在壁面温度同为413K时最多减小了42%。仅增加第二次喷射的喷油脉宽,壁面温度为298K和373K时的附壁油膜质量先增大后减小,壁面温度为413K时附壁油膜质量逐渐增大;相同喷油脉宽下,壁面温度为413K时附壁油膜平均厚度最大,而壁面温度为298K和373K时附壁油膜平均厚度与喷油间隔相关。     

汽油机冷起动附壁油膜研究

利用高速摄影和激光荧光法研究了汽油油束碰撞平板后壁面上油膜的发展过程.分析了喷射高度、喷射角度及喷油脉宽对油膜面积及油膜最大厚度的影响,定量地得到了不同参数下的最大油膜厚度和油膜面积.试验结果表明,喷射高度增大时,最大油膜厚度和油膜面积都减小,附壁燃油量减少;喷射角度减小时,最大油膜厚度减小,而油膜面积却增大,有利于燃油挥发;喷油脉宽增大时,最大油膜厚度和油膜面积都增大,附壁燃油量也增加.利用无量纲时间,对油膜面积的变化速度研究发现,无量纲时间在1.5之后,油膜面积基本不再变化.     

壁面温度对喷射油膜附壁厚度的影响

研究了喷射油膜厚度随壁面温度的变化规律.设计了PID闭环加热平板,以及二维可调节移动平台,使用激光诱导荧光法(LIF)定量测量了在垂直情况下,喷射燃油到平板上形成的附壁油膜厚度随温度变化的规律,并通过高速CCD摄像机拍摄照片进行比较.研究结果表明:随着壁面温度从26℃增加到120℃,附壁油膜的厚度减小,油膜最厚处的厚度从80μm下降到35μm,同时油膜覆盖面积从5.66cm2减小到1.55cm2;试验还测量了油膜厚度与油膜形成时间之间的关系,得出在喷油结束40ms时,油膜厚度达到最大值.     

喷雾撞壁油膜流动的研究

对燃油喷雾撞壁形成的油膜运动进行了三维数值模拟计算 ,在应用边界层理论建立的油膜流动模型中将油膜视为极薄湍流边界层 ,考虑了雾滴与壁面油膜的相互作用、油膜蒸发、卷吸、与气体间的传热传质等物理过程 ,通过对已有试验数据的比较 ,分析了平均油膜厚度、速度、喷雾粘附比、卷吸量等变化及其影响因素 ,预测结果与试验结果相符 ,计算结果表明喷射撞壁后液滴的质量、动量和能量分布是决定油膜动力学特性的主要因素     

小型汽油机进气道燃油雾化和蒸发特性研究

由喷射燃油撞壁产生的进气道内油膜是导致过渡工况空燃比控制偏差的主要原因。本文对小型汽油机进气道内燃油雾化和蒸发模型进行了分析,总结出了影响油滴油膜形成和蒸发的主要因素。     

喷嘴距离和喷嘴角度对油膜附壁的影响

为研究进气道燃油喷射汽油机内喷嘴距离和喷嘴角度对燃油碰壁形成的附壁油膜的影响,采用激光诱导荧光法,研究了不同喷嘴距离和喷嘴角度下的附壁油膜厚度二维分布规律.结果表明,喷嘴角度越小,附壁油膜厚度较大的区域越向下游移动,油膜较厚区域的范围越大,油膜的最大厚度越小.喷嘴距离越大,附壁油膜的厚度越薄.     

汽油机起动工况附壁油膜挥发过程的计算分析

建立了附壁油膜计算模型,研究了进气道喷射摩托车汽油机燃油落点位置、燃油喷射距离及燃油喷射量等参数对起动工况循环内附壁油膜挥发量的影响.分布在进气阀背面上部与进气道交界处附壁油膜挥发速度较大,如果喷射距离过长,部分燃油会分布到温度较低的进气道壁面.导致附壁油膜挥发速度降低.燃油循环喷射量存在一限值:喷射量低于该限值时,循环内附壁油膜挥发量随喷射量的增加而增加;高于该限值时,附壁油膜挥发量随喷射量的增加而减少.     

空气流速对附壁油膜分布影响的试验研究

采用激光诱导荧光技术研究了空气流速对燃油碰壁形成的附壁油膜厚度二维分布的影响.同时,采用CCD相机记录了油膜的形状、长度和面积变化.结果表明,随着空气流速的增加,油膜形状从圆形变成椭圆形,同时,油膜厚度减小.随着空气流速的增加,垂直于空气流动方向上的油膜长度减小.当空气流速低于10 m/s时,随着空气流速的增加,沿着空气流动方向上的油膜长度减小,油膜面积减小.但当空气流速继续增加到10 m/s以上时,沿着空气流动方向上的油膜长度和油膜面积将会随着空气流速的增加而增加.     

电控汽油机进气道附壁油膜模拟研究

电控汽油机进气道多点喷射的附壁油膜现象是导致燃油动态传输迟滞的根本原因,为此笔者从喷雾碰壁出发,研究附壁油膜形成过程以及附壁油膜形状和质量分布的影响因素。通过Fluent软件对喷雾碰壁多影响因素的模拟及自定义函数（UDF）对空间颗粒统计分析,从本质上得出影响附壁油膜形成、燃油动态传输迟滞以及混合气形成品质的主要因素,为进气道多点喷射的燃油精确控制提供了较为有力的理论依据。     

撞击平板壁面的柴油喷雾的燃油分布研究

利用复合激光诱导荧光(PLIEF)技术,在一个高温高压定容弹内研究了柴油喷雾撞击平板后的燃油分布。研究结果表明:撞壁对燃油分布有重要影响。撞壁喷雾可以分为:(1)气液相喷雾同时撞壁,在近壁面形成近壁浓区,瞬时最大燃空当量比(Φmax)和最低温度(Tmin)存在于撞壁点附近;(2)气相喷雾撞壁液相喷雾不撞壁,近壁面当量比相对较高,Φmax和Tmin存在于撞壁点上方的自由射流段中;(3)气液相喷雾都不与壁面接触,相当于自由喷射。随着撞壁距离的增加,液相燃油质量分数增加,Φ2的过浓区间的气相燃油质量分数减小,1Φ≤2的较浓区间的气相燃油质量分数增加到一定数量并保持变化不大,0Φ≤1的稀混合气区间的气相燃油质量分数逐渐增加。     

Improvement of fuel economy of an indirect injection (IDI) diesel engine with two-stage injection

This paper focuses on the effects of early stage injection and two-stage injection on the combustion characteristics and engine performances of an indirect injection (IDI) diesel engine. In a direct injection (DI) diesel engine, HC emission increases with early stage injection because some of the fuel spray adheres to the cylinder wall and burns in the gap between the piston and the cylinder. On the other hand, since the fuel spray of early stage injection in an IDI diesel engine is injected into an auxiliary combustion chamber such as a swirl chamber, the IDI diesel engine could reduced the HC emission produced from the gap compared with a DI diesel engine. In a two-stage injection IDI diesel engine, NO and smoke emissions are improved when the amount of fuel in the first stage injection is small and the first stage injection timing is advanced over −80° TDC. And 20% improvement in fuel consumption is achieved when the first stage injection timing is advanced over −80° TDC. Conversely, HC and CO emissions of two-stage injection increases compared with that of conventional injection of an IDI diesel engine. However, CO emission can be improved a little when the first stage injection timing is advanced over −100° TDC and the second stage injection timing is retarded over TDC.     

Three-dimensional numerical investigation on wall film formation and evaporation in port fuel injection engines

ABSTRACT

Wall film formation and evaporation were studied on a flat wall inside a constant-volume vessel using a three-dimensional numerical method. The computation was based on the discrete phase model (DPM) of spray dispersion, a spray–wall interaction model coupled with an enhanced wall film evaporation sub-model, in which the operating conditions of cold wall are considered for port fuel injection (PFI) engines. The influence of impacting parameters including injection pressure, the impingement distance from the injector and the impinged wall, injection duration, impingement angle, and wall temperature was discussed.     

Measurement and simulation of wall-wetted fuel film thickness

In gasoline engines, the quality of fuel mixture preparation in the intake has a strong influence on the performance and exhaust emission. This paper deals with the basic research on the fuel mixture preparation process, and reports the experimental and numerical investigations on characteristics of the wall-wetted fuel film. In the experiments, iso-octane mixed with 3-pentanone was injected against a flat wall. The film thickness on the wall was measured by using laser induced fluorescence (LIF) technique. The film area was measured with CCD camera. Influences of the injection duration, the impingement distance, and the impingement angle on the film thickness, film length, and film area are discussed. In the numerical simulation, the commercial computational fluid dynamics (CFD) software FLUENT was used. The results show that, in the radial direction, the film thickness increases to a peak around the impingement center and then decreases eventually reaching zero at the external edge. Enlarging the injection duration could expand the film area and make the fuel film thicker. As the impingement distance gets farther, the fuel film becomes thinner, and the film area becomes smaller. Minishing the impingement angle could expand the area of the thick part of the fuel film and meanwhile make the maximum film thickness smaller.     

基于多组分蒸发模型的汽油机冷起动数值模拟

建立了汽油多组分油膜蒸发模型,并运用该模型对进气道喷射式汽油机冷起动首循环的气流运动、喷雾过程、油膜形成和发展过程、混合气形成过程及缸内燃烧进行了三维数值模拟,详细解析了油滴和油膜中每种组分的蒸发规律,给出了每种组分的浓度分布.模拟结果表明:冷起动时轻组分的蒸发主导首循环混合气的形成质量,中、重组分的蒸发情况仅起辅助作用;汽油的轻组分越多,燃料蒸发量越大,有利于形成稳定可靠的可燃混合气,改善缸内燃烧过程,提高起动稳定性;计算所得的缸压曲线与试验结果吻合良好,说明开发的多组分汽油蒸发模型具有较高的模拟精度.     

BUMP燃烧室的稀扩散燃烧机理

开发了BUMP燃烧室并进行了对比实验,发现BUMP燃烧室中可以形成稀的扩散燃烧氛围,使NOx和碳烟排放同时降低．喷射定时3°CA ATDC时,烟度排放降低了约70％,NOx排放与对比燃烧室相当．CFD模拟研究表明,不同的喷射定时下,燃油到达燃烧室壁面时的混合和燃烧状态不同,只有油束在滞燃期内到达燃烧室壁面,由限流沿(BUMP环)扰动形成的二次空间射流才能充分形成,稀扩散燃烧才能明显发生．     

直喷式发动机喷雾模型研究进展

雾化是在点火之前喷射燃油与环境因素发生复杂的物理作用的结果。本文从雾化过程喷嘴内流动、射流破裂、油团破裂、油滴蒸发四个阶段着手,在考虑到雾化过程中所伴随的减速、变形、扩散、缸壁油膜形成、油滴之间碰撞以及油滴与缸壁碰撞等物理现象基础上,综合对比了国际上近年来用实验分析方法所建立的各种相应计算模型的特点、应用情况及不足。分析了选择子模型组合并集成在计算模型(KIVA)中对缸内喷射对象的雾化过程模拟的可行性。最后结合计算机技术的发展指出了雾化过程模拟发展趋势。     

Optimization of south window area and wall thickness in cold climates (LEH)

The net savings (cost of fuel saved minus the cost of additional thickness of wall and the singly glazed window of the south wall) by the use of a thicker wall and glazed window on the south side have been evaluated for the cold desert climate in India (typified by Leh in Ladakh); the traditional construction implies mud walls and room temperature maintained at 25°C or 17°C by burning Kerosene oil. The thickness of the south wall and the size of window on the south wall (of a 6 × 4 × 2.7 m³ room) have been optimized for maximum savings, corresponding to room temperatures of 25°C or 17°C and the South wall made of mud or concrete; three outer finishes of the walls viz. bare natural, blackened and unglazed and blackened and glazed have been considered. Optimum insulation on the roof and the other three walls has been assumed.