蒸发性对燃油瞬态喷雾撞击壁面的动态传热影响

直喷发动机燃油喷雾撞击壁面形成油膜,导致燃烧效率降低,颗粒物排放增加。伴随撞壁的动态传热过程对油膜蒸发具有重要影响。本文针对正戊烷、甲醇、甲醇汽油混合燃料瞬态喷雾撞击壁面,研究了不同条件下蒸发性对燃油瞬态喷雾撞击壁面动态传热影响。结果表明,提高喷油温度可促进燃油雾化,增大喷油压力或降低喷油距离可提高液滴撞壁强度,缩短液膜存在时间。撞壁瞬态温度与热流密度动态变化特征受燃油蒸发性与喷雾条件联合影响。     

超高压燃油喷雾特性及诱导激波传播特性的试验研究

在现代柴油机的研究中,提高喷射压力对于改善射流雾化效果及燃烧性能具有重要意义。本文通过自主搭建的超高压燃油喷雾系统,对超高喷射压力条件下射流雾化宏观特性(喷雾贯穿距、喷雾锥角)进行研究。采用纹影系统和高速摄影仪相结合的实验系统,得到超高喷射压力条件下的喷雾图像,并在喷雾体前端捕捉到激波现象。本文主要研究在超高喷射压力下的激波传播特性,包括激波角、多重激波现象等。通过本实验研究,得到超高喷射压力下的激波传播特性,探索超高压对诱导激波的影响规律。     

喷雾夹角对柴油机性能影响的数值模拟

为研究喷雾夹角对柴油机性能的影响,应用STAR-CD程序对不同喷雾夹角的燃烧过程进行三维数值模拟.计算结果表明喷雾夹角决定了油束在燃烧室的空间分布和燃油与壁面的碰撞参数,进而影响到燃油的雾化与燃烧.当碰撞距离增加,燃油雾化时间增加.当撞壁入射角度减小,燃油壁面涂布能力增强,促进燃油蒸发雾化.当油束将燃烧室有效容积等分时,油气混合均匀,具有良好的燃烧效果.     

乙醇与汽油的喷雾及蒸发特性研究

随着汽车发动机的发展,缸内直喷汽油机(GDI)逐渐成为关注和研究的重点。液体燃料喷雾的结构和蒸发与直喷汽油机内燃烧的稳定性、发动机效率以及尾气排放有着密切联系。本文通过纹影测试技术对乙醇和汽油的直喷喷雾的雾化和蒸发进行测试,研究结果表明环境温度、环境压力和喷射压力对喷雾贯穿距有着显著的影响,而燃油温度在低于燃油沸点时对喷雾贯穿距影响很小。因此可以通过控制环境温度和压力,以及燃油的喷射压力和温度对喷雾贯穿距进行优化.     

在定容燃烧装置中进行柴油机喷雾特性的研究

本文报道了作者在自行设计的高温高压燃烧装置中,对柴油机的喷雾特性进行了研究。在实验中,用高速纹影摄影技术观察研究了燃烧室中的空气温度、空气压力、空气涡流以及喷孔直径对喷雾贯穿距离、喷雾锥角、喷雾形状和喷雾碰壁反溅现象的影响,并得出了相应的结论。     

缸内高压直喷柴油/天然气喷射混合过程分析

在天然气发动机中,天然气和柴油射流的喷射及混合过程对燃烧排放性能有重要影响。本文基于AVL FIRE软件,模拟了定容燃烧腔内高压天然气和柴油射流喷射过程,分析了不同喷孔高度、喷孔倾斜角以及喷孔交角下柴油和天然气射流的混合情况,研究结果表明喷孔高度,倾斜角,交角与喷雾贯穿距呈正比,柴油喷雾贯穿距是影响雾化质量的重要因素。喷孔相对位置中喷孔交角因素对射流的影响最大,对柴油喷雾的雾化效果提升明显。     

喷雾撞壁后近壁区域火焰发展及碳烟生成研究

本文基于多尺度液滴撞壁形态提出了新的考虑了不同飞溅状态的撞壁模型。结合容弹实验研究了近壁区域喷雾撞壁后火焰发展过程,对影响近壁区燃烧过程及碳烟生成规律的边界温度进行了探讨。结果表明,环境温度和壁面温度对喷雾撞壁后的液膜沉积量以及后续的燃烧过程影响显著。碳烟产生峰值直接和壁面燃油沉积量呈正相关,而较高的近壁区域温度可加速燃油蒸发并减少燃油沉积量。燃油沉积量的不同,其火焰发展形式不同:当沉积燃油较少时,近壁混合气形成较快,从而维持了稳定的燃烧过程;而当沉积燃油较多时,近壁区域的燃烧呈现不稳定的池火过程。随着环境温度和壁温的增加,油膜燃烧时刻提前并形成可燃混合气,从而加速了碳烟的生成和氧化。     

生物柴油高压共轨喷雾特性的试验研究

应用高速摄影系统与高压共轨系统,在高压定容器内研究了生物柴油的喷雾特性,并与柴油进行了对比。结果表明,生物柴油油束形态比柴油细且长,喷雾贯穿距离比柴油大,喷雾锥角比柴油小;高的运动黏度与表面张力导致生物柴油雾化性能比柴油差。随着喷油压力的升高,生物柴油喷雾贯穿距离变长,喷雾锥角增大;随着背压的升高,生物柴油喷雾贯穿距离...     

小型直喷式柴油机燃烧室内喷雾碰壁的试验研究

本文采用透射光衰减法研究了小型直喷式柴油机燃烧室内喷雾碰壁特性，并与大气开放条件下和定容燃烧模型室内的研究结果进行了比较。在喷雾碰壁前后，当量喷雾锥角突然增大，然后减小。在正常涡流比条件下，燃烧室内涡流显著地影响壁面油柬的扩展，但未碰壁油束的弯曲量很小。着火前，在燃烧室内喷嘴附近混合气最不均匀。     

含甲缩醛柴油喷雾和燃烧排放特性的试验研究

应用激光相位多普勒技术测量了含甲缩醛柴油喷雾的速度场和粒径场,在直喷式柴油机上研究了该含氧混合燃油的燃烧排放特性。结果表明,添加甲缩醛可改善柴油的雾化,增加喷雾轴线上的粒子速度,但减小喷雾锥角;同时以远大于其加入的比例降低柴油机排气烟度,但对氮氧化物的排放影响不大。柴油机采用甲缩醛作燃油添加剂时,需改造燃油系统。     

实心与空心锥形喷雾与热多孔介质相互作用的数值研究

在KIVA-3V中增加了油滴碰撞热多孔介质壁面的碰撞模型、传热模型及空心喷雾的线性不稳定性液膜破碎模型(LISA).在多孔介质结构简化描述的基础上,详细模拟了实心喷雾与空心锥形油雾与热多孔介质之间的碰撞过程.针对Senda等人的实验进行了数值计算,油束碰壁后油滴和油蒸汽分布的数值计算结果与实验结果吻合得很好.计算结果表明油雾在碰撞到热多孔介质后,油束会发生分裂,为油滴的快速蒸发和油蒸汽与空气充分混合创造了前提.油滴初始动能相同的条件下,空心喷雾的油滴穿越多孔介质的可能性比实心喷雾要小.     

超声速气流中液体横向射流雾化过程数值模拟

为了更加深入了解超燃冲压发动机燃烧室中的燃料雾化机理,对来流Mach数为1.94的超声速气流中液体横向射流的雾化过程进行了数值模拟研究.计算采用Euler-Lagrange方法,液滴二次破碎模型采用K-H/R-T模型.计算结果表明:考虑液滴二次破碎时,采用雾化锥模型获得的射流穿透深度以及液滴速度分布与实验结果符合得很好...     

IMPINGING JET ATOMIZATION AT ELEVATED AND SUPERCRITICAL AMBIENT TEMPERATURE AND PRESSURE CONDITIONS

A study of a spray formed by a pair of liquid nitrogen jets impinging on one another at elevated up to above-critical ambient temperature and pressure conditions has been conducted using double-pulse, two-reference-beam holography and high-speed photography. Qualitative observations as well as quantitative measurements on droplet size distribution and its dependence on pressure and temperature were obtained. Droplet size measurements showed that, at subcritical conditions, the increase of the ambient pressure initially makes the atomization quality deteriorate. Further increase in pressure (above 50% of the critical pressure) improves the atomization quality. The spray pattern changed drastically as the ambient pressure approached and exceeded the critical pressure in an environment already at supercritrical temperature. At supercritical ambient conditions, the usual mechanisms of droplet formation are no longer in place. The liquid nitrogen spray undergoes a mixing process with the nitrogen environment.     

Combustion of partially premixed spray jets

Gas turbines, liquid rocket motors, and oil-fired furnaces utilize the spray combustion of continuously injected liquid fuels. In most cases, the liquid spray is mixed with an oxidizer prior to combustion, and further oxidizer is supplied from the outside of the spray to complete diffusion combustion. This rich premixed spray is called “partially premixed spray.” Partially premixed sprays have not been studied systematically although they are of practical importance. In the present study, the burning behavior of partially premixed sprays was experimentally studied with a newly developed spray burner. A fuel spray and an oxidizer, diluted with nitrogen, was injected into the air. The overall equivalence ratio of the spray jet was set larger than unity to establish partially premixed spray combustion. In the present burner, the mean droplet diameter of the atomized liquid fuel could be varied without varying the overall equivalence ratio of the spray jet. Two combustion modes with and without an internal flame were observed. As the mean droplet diameter was increased or the overall equivalence ratio of the spray jet was decreased, the transition from spray combustion only with an external group flame to that with the internal premixed flame occurred. The results suggest that the internal flame was supported by flammable mixture through the vaporization of fine droplets, and the passage of droplet clusters deformed the internal flame and caused internal flame oscillation. The existence of the internal premixed flame enhanced the vaporization of droplets in the post-premixed-flame zone within the external diffusion flame.     

On the behaviour of organic gel multi-size spray diffusion flames

The recently reported, experimentally observed, unusual behaviour of organic gellant-based fuel droplets which, under appropriate ambient thermal conditions, evaporate and burn in an oscillatory fashion is incorporated in a phenomenological manner in a model of a two-dimensional arbitrary multi-size spray diffusion flame. Non-unity Lewis numbers are permitted for the fuel vapour and oxidant. A combined analytical/numerical solution of the governing equations is presented and used to investigate how a spray's initial polydispersity and the frequency of oscillatory evaporation influence the combustion field. It is demonstrated that the initial droplet size distribution and the frequency of evaporation of the burning gel droplets can have an acute impact both on the homogeneous diffusion flame shape, height and width and on the thermal field downstream of the flame front. Hot spots of individual (or clusters of) burning droplets can be created and under certain operating conditions can lead to hotter temperatures than experienced in the main homogeneous flame. The intensity of these hotspots, their number and location are sensitive to spray related parameters. In realistic combustion chambers there is a danger inherent in the existence of hotspots in undesirable regions as they can damage the structural integrity. Other computed results demonstrate that, in relation to the spray diffusion flames obtained using an equivalent purely liquid fuel spray, the use of a gel fuel spray can lead, under certain operating conditions, to a reduction in flame height and temperature. The latter effect is critical when considering flame extinction.     

LES study of diesel flame/wall interaction and mixing mechanisms at different wall distances

In this paper, the flame-wall interaction of reacting diesel spray under engine like conditions is investigated using large eddy simulations. The aim of this study is to understand the influence of the distance between the wall and the spray nozzle on the air entrainment rate, which is a key variable in formation/oxidation process of soot. Three experimental cases are investigated, a free jet case and two wall impingement cases with a distance from nozzle to wall of 30 mm and 50 mm, which are considered as characteristic wall impingement distances for light- and heavy-duty bores in diesel engines, respectively. The optical soot measurements imply a positive influence of wall on the rate of soot oxidation. Numerical simulations are employed to elucidate importance of different mechanisms for the air entrainment, i.e., air entrainment prior to flame lift-off position, enhanced mixing due to the wall impingement and enhanced mixing by the entrainment wave. The results show that oxidation process after the end of injection is driven by a different mixing mechanism depending on the distance to the wall. The 30 mm case resulted in a “mixing boost”, where the dominant mixing mechanism is the wall impingement vortex mixing, which gives rise to the fastest soot decay among the cases. The mixing in the 50 mm case is governed by a late wall impingement vortex mixing, giving rise to a low, but a constant air entrainment rate, i.e., a “mixing plateau”. The free jet case resulted in mixing governed by the entrainment wave mechanism. Both wall impingement cases have faster soot oxidation rate compared with the free jet case, but due to a different underlying mixing process. LES is shown to be able to replicate the line-of-sight measurements of natural OH* chemiluminescence and distribution of soot region from the optical soot diagnostics.     

LES-CMC of high-altitude relight in an RQL aeronautical combustor

Large-Eddy Simulations with the Conditional Moment Closure sub-grid combustion model and detailed chemistry for kerosene were performed for the ignition process in an Rich-Quench-Lean aviation gas turbine combustor at high-altitude conditions. The simulations used realistic boundary conditions for the flow inlet and spray droplet size distributions and velocity. Due to the large droplets, the Central Recirculation Zone (CRZ) is filled with fuel, mostly in liquid form. The first phase of the ignition process is critical and the results show that the spark kernel must provide enough energy to evaporate the spray and pyrolyse the fuel for the flame to grow and establish in the corner of the combustor. The second phase is characterised by the flame burning the mixture in the scorner and propagating around the Inner Shear Layer. This phase is also critical, as the flame needs the prevaporised fuel and smaller droplets in the corner to sufficiently increase the temperature and be able to propagate inside the CRZ, filled with liquid fuel and cold air. If this propagation inside the CRZ is achieved, phase three is accomplished and the burner is fully ignited. The simulations demonstrate the particular importance of detailed chemistry and proper boundary conditions for flame ignition simulations in high-altitude relight conditions.     

高温条件下甲醇喷雾的燃烧特性

高温条件下甲醇喷雾的燃烧特性杨长林，舒国才，刘月辉，王懿铭（天津大学内燃机燃烧学国家重点实验室天津３０Ｏ０７２）关键词高温环境，甲醇喷雾，燃烧特性１前言甲醇作为一种内燃机代用燃料，已经受到了人们的广泛重视，但由于甲醇的十六烷值低，着人性能差，因此在柴...