闪急沸腾喷雾研究的现状与展望

阐述了闪急沸腾喷雾的基本原理,通过比较分析闪急沸腾喷雾与传统喷雾的差异,指出闪急沸腾喷雾是一种极具潜力的燃油喷射方式;论述了近年来国内外对过热燃油和溶气燃油闪急沸腾喷雾研究的进展与现状,介绍了过热液体闪急沸腾喷雾的数学模型及其发展,并从理论上分析了影响闪急沸腾喷雾的因素;展望了闪急沸腾喷雾的应用前景以及进一步研究的发展方向。     

柴油喷雾与LPG/柴油混合喷雾的对比研究

以ZH1105W柴油机为模型,采用Jaynes等人提出的雾化液滴尺寸和速度联合数目分布函数,对柴油喷雾和LPG/柴油混合喷雾的燃料液滴尺寸和速度数目分布进行了对比研究;采用液滴蒸发和碰撞模型对液滴尺寸理论数目分布与试验结果进行了对比研究。液滴尺寸和速度联合数目分布的研究结果表明,随着液体喷射速度的增大,液滴尺寸数目分布曲线向小颗粒偏移。随着液滴尺寸的增大,较大颗粒的液滴速度数目分布曲线的峰值先明显增大,到达中等尺寸后又明显减小。由于LPG/柴油混合燃料的闪急沸腾喷雾对燃料液滴具有突爆的加速作用,L30速度分布曲线的峰值速度比柴油的明显增大。累积的液滴尺寸分布和速度数目分布研究表明,由于混合燃料的闪急沸腾喷雾,L30尺寸数目分布曲线峰值相对于柴油向小颗粒方向偏移,L30的峰值大于柴油的峰值。说明L30喷雾所产生的小颗粒液滴多于柴油,雾化质量提高,碳烟排放得以大幅度降低。L30速度分布曲线的峰值比柴油略小一些,且略微向大速度方向偏移。说明与柴油相比,L30燃料较大速度液滴的数目略有增加。液滴蒸发和碰撞模型对于石油液体燃料的研究是可行的,对于液化气体燃料与石油液体燃料的混合燃料的研究,模型还有待于进一步改进。     

柴油掺混二甲醚喷雾粒子尺寸分布特性试验研究

为确定二甲醚掺入柴油后对燃料雾化性能的改善效果,利用阴影成像与数字图像处理技术,对不同掺混比、喷射压力以及喷孔直径等条件下的二甲醚-柴油混合燃料喷雾粒子尺寸分布特性进行了对比试验研究.结果发现:由于柴油中二甲醚的闪急沸腾作用,随着二甲醚掺混比的增加,混合燃料粒子尺寸分布曲线整体向小颗粒方向偏移,较大粒子数目较柴油明显减少,有助于降低发动机炭烟排放;喷孔直径、喷射压力等喷射参数对混合燃料雾化粒子分布有较大影响,减小喷孔直径使燃油粒子更加细化,降低喷射压力则使混合燃料雾化效果有变差的趋势.     

转子发动机喷雾特性试验研究

利用定容弹、纹影仪以及高速相机等装置,就柴油转子发动机工况下的喷雾过程进行了试验研究,重点分析了喷雾环境背压和喷射压力对喷雾特性的影响。结果表明:在转子发动机喷雾过程中,喷雾扩散速度先快速增大后逐渐减小;喷雾锥角在初次雾化阶段内急剧减小,然后在二次雾化作用下保持相对稳定。喷雾环境背压的增大,有效减小了喷雾贯穿距离,增大了喷雾锥角,说明喷雾环境背压的增大对喷雾贯穿距离和锥角都有显著的影响,从而为转子发动机喷油正时的优化提供了试验数据支持;随着喷射压力提高,喷雾贯穿距离和喷雾锥角都增大,并且增大喷射压力加强了燃油的初次雾化和二次雾化,有利于提高转子发动机喷雾质量,为优化柴油转子发动机油气混合状态创造了条件。     

柴油机SCR系统车用尿素喷雾特性仿真与试验研究北大核心

通过建立喷射系统CFD三维仿真模型,研究了柴油机SCR系统在空气辅助雾化条件下尿素喷射的喷雾速度流场、雾化粒径、喷雾形态和贯穿距等变化规律,并通过试验对模型进行了验证;根据喷射模型对SCR系统的喷孔直径等结构参数进行了优化设计,通过发动机ESC,ETC试验对还原剂雾化性能进行试验验证。试验结果表明:采用改进后的柴油机空气辅助喷射还原剂供给系统后,柴油机NOx排放和NH3泄漏等指标均满足国Ⅴ阶段排放法规要求,验证了所采用的喷雾特性仿真优化设计方法有效可行。     

柴油-甲缩醛混合燃料喷雾的粒子动态分析研究

为研究代用燃料甲缩醛的喷雾机理,采用基于相位多普勒原理的粒子动态分析仪(PDA),研究了不同百分比的柴油-甲缩醛混合燃料喷雾的粒度场、速度场。试验结果表明,柴油添加甲缩醛后,雾化得到改善,喷雾轴线上的粒子轴向速度提高,但三者径向速度相差不大。     

喷雾液体射流模型研究及应用

喷雾雾化是液体通过喷嘴进入到气体环境中,经过气体与液体的互相作用使之碎裂成液滴的过程,其中包含着复杂的能量交换、动量交换以及传热过程。燃油雾化是内燃机燃烧过程中的一个重要环节,良好的雾化效果可以促进燃油粒子与空气进行充分混合,从而使燃烧热效率得到提高,进而使内燃机的动力性、经济性以及排放性皆有所改善。由于喷雾液体通过喷嘴的物理结构不同,故所形成的射流形状也不同。主要的射流形式有平面射流、圆柱射流以及环状射流等。文章将对这三种射流形式进行展开研究,从射流的碎裂机理进行深入的分析,通过引入物理方程以及射流表面波理论,将液体射流现象转变为数学模型,从而探究出影响射流雾化效果的关键因素,进而促进喷雾学科的发展。     

双组分燃油蒸馏及喷雾闪沸特性研究

在较高燃油温度及较低环境背压下,直喷喷雾易发生闪沸。闪沸喷雾具有贯穿距小、易于雾化等优势,能够有效降低发动机排放、提高燃油经济性,在直喷发动机中具有较好应用前景。然而随着涡轮增压的使用,进气压力的提高导致喷雾较难达到闪沸状态。针对这种现象,在研究混合燃油蒸馏特性后提出了通过向高沸点燃油中添加低沸点燃油的方式来促进喷雾的闪沸。试验燃料为一系列不同比例低沸点燃油(异戊烷)与高沸点燃油(正十一烷)组成的混合燃油,通过蒸馏测试探究了不同组分燃料的蒸馏特性;通过在定容弹内拍摄不同燃油温度、环境背压下的燃油喷雾的米氏散射图,研究了混合燃油形态变化规律。结果表明,不同于单一油品仅在沸点时沸腾产生馏出物,混合燃油是在一定的温度范围内均会沸腾产生蒸馏物,表现为3段式蒸馏曲线;混合燃油蒸馏前期大部分馏出物为低沸点燃油,表明混合燃油受热时,低沸点燃油会优先蒸发;对喷雾宏观形态的测量结果表明,向高沸点燃料中添加低沸点燃油能够使喷雾在高背压下达到闪沸状态,且40%体积分数是最佳的添加量;另外,油温越高添加低沸点燃油对喷雾闪沸的促进效果越好。     

正丁醇-柴油混合燃料喷雾特性的模拟研究

根据喷雾可视化试验的结果,利用三维CFD软件建立了正丁醇柴油喷雾的仿真模型,并验证了模型的准确性。利用该模型研究正丁醇柴油混合燃料的喷雾特性,模拟了喷雾的发展过程,计算了不同掺混比对喷雾贯穿距、液滴索特平均直径、速度场、密度场的影响。结果表明:模拟计算与可视化试验结果基本一致;随着正丁醇掺混比的增加,混合燃料的喷雾贯穿距和索特平均直径均降低,喷雾锥角变大,雾化特性变好。     

直喷发动机燃油喷雾特性经验公式定量研究

基于对节能减排的需求,直喷发动机喷雾特性受到研究人员广泛关注.大量试验与数值仿真方法被应用于对直喷喷雾特性的测量与预测中.其中,基于试验的研究以获取描述喷雾特性的经验公式为目的展开.此类研究可获得大量数据,而从中提炼的经验公式对于实际喷雾系统的优化具有指导意义.然而经验公式法本身存在较大局限性,多数经验公式只适用于特定或若干工况,且其假设的先决条件往往不完全符合实际燃油雾化环境.此外,不同经验公式获取的结果由于其参数选取、试验条件不同,彼此间存在一定的差异.因此,如何评价喷雾特性经验公式的普适性是领域内亟需解决的问题.本研究以独立试验为基准,定量比较了现有文献中主流经验公式的预测结果,并根据特定工况预测结果偏差对各经验公式的局限性进行了阐述,对于下一代汽油机设计与燃油雾化系统优化具有一定的指导价值.     

Measurement of fuel vapor concentration in flash boiling spray by infrared extinction/scattering technique

The MPI (multi-point injection) technique is widely used for gasoline engines due to its high responsiveness and high controllability. The pressure in the intake manifold becomes lower than the saturated vapor pressure of some components of gasoline in some operating conditions, and this causes the flash boiling phenomenon of the components. This phenomenon has great influence on spray atomization and vapor formation.In this study, the distribution of fuel vapor concentration of n-pentane was observed by applying the Infrared Extinction/Scattering method. The fuel vapor concentration can be calculated from the intensities of transmitted infrared light, under various spray conditions including the flash boiling spray.     

Modeling on atomization and vaporization process in flash boiling spray

This paper presents the model analysis on atomization and vaporization processes in a flash boiling spray based on experimental results obtained from an injection system in the suction manifold of a gasoline engine. Two kinds of liquid fuel, n-Pentane and n-Hexane, are injected into quiescent gaseous atmosphere at room-temperature with low pressure through a pintle type injector. Fuel spray is observed, by taking photographs, with variation of the ambient back pressure. Then, in the flash boiling spray region where the back pressure was below the saturated vapor pressure of fuel, the bubble nucleation process due to flash boiling was modeled by both experimental results and the nucleation rate equation as a parameter of the pressure difference between back pressure and vapor pressure. Further, the fuel vaporization process was assessed by considering growth calculation of cavitation bubbles and fuel evaporation from the film surface due to heat transfer at the gas-liquid interface. Accordingly, we could estimate quantitatively the transient changes in the bubble growth and the vapor mass fraction inside the spray for each back pressure condition.     

喷嘴孔数及其布置对汽油直喷喷嘴闪沸喷雾-环境气体相互作用影响的研究

为了区分火花塞点火式缸内直喷(SIDI)发动机喷雾和环境气体两相流场,将优化后的高速双色PIV(Particle Image Velocimetry)技术应用于多孔直喷喷油器的喷雾和环境气体速度的测试.此双色PIV系统由一个特殊的示踪和滤波系统组成,可同时对燃油喷雾及其环境气体的速度场进行测量.本研究采用该双色PIV方法研究不同环境压力和燃油温度的条件下,喷嘴孔数及其布置情况对燃油喷雾和环境气体的相互作用的影响.在此研究中,对3个汽油直喷喷嘴做了详细的研究,包括1个6孔喷嘴,1个3孔喷嘴及1个2孔喷嘴.研究结果表明,随着燃油温度的提高或者环境气体压力的降低,喷雾雾化增强,燃油颗粒粒径减小,导致喷雾油束变宽,喷雾与环境气体接触面积变大,喷雾和环境气体的两相流场的作用变强.不同孔数和布置的喷油器在冷态及闪沸条件下油束间干扰作用的强度不同,导致喷雾传递给环境气体的动能不同.较强的油束间的干扰作用加强了燃油喷雾与环境气体之间的动量交换过程,进而增强了环境气体的动能.     

车用歧管式催化器内部流速及压力分析

通过数值模拟和试验验证,分析了歧管式催化器内部气流的速度场、压力场和气流分布状态,并与常规的底盘下催化器进行了比较.结果表明:随入口流量的增大,歧管式催化器内部气流流速增大、压力损失增大、径向分布均匀性降低;气流的径向分布不同于常规催化器集中于轴线的轴对称形式,而是在载体前端面呈比较分散的状态,其中管板复合型歧管式催化...     

Low temperature premixed combustion within a small bore high speed direct injection (HSDI) optically accessible diesel engine using a retarded single injection

An optically accessible single-cylinder high speed direct-injection (HSDI) Diesel engine equipped with a Bosch common rail injection system was used to study low temperature Modulated Kinetics (MK) combustion with a retarded single main injection. High-speed liquid fuel Mie-scattering was employed to investigate the liquid distribution and evolution. By carefully setting up the optics, three-dimensional images of fuel spray were obtained from both the bottom of the piston and the side window. The NOx emissions were measured in the exhaust pipe. The influence of injection pressure and injection timing on liquid fuel evolution and combustion characteristics was studied under similar fuel quantities. Interesting spray development was seen from the side window images. Liquid impingement was found for all of the cases due to the small diameter of the piston bowl. The liquid fuel tip hits the bowl wall obliquely and spreads as a wall jet in the radial direction of the spray. Due to the bowl geometry, the fuel film moves back into the central part of the bowl, which enhances the air-fuel mixing process and prepares a more homogeneous air-fuel mixture. Stronger impingement was seen for high injection pressures. Injection timing had little effect on fuel impingement. No liquid fuel was seen before ignition, indicating premixed combustion for all the cases. High-speed combustion video was taken using the same frame rate. Ignition was seen to occur on or near the bowl wall in the vicinity of the spray tip, with the ignition delay being noticeably longer for lower injection pressure and later injection timing. The majority of the flame was confined to the bowl region throughout the combustion event. A more homogeneous and weaker flame was observed for higher injection pressures and later injection timing. The combustion structure also proves the mixing enhancement effect of the liquid fuel impingement. The results show that ultra-low sooting combustion is feasible in an HSDI diesel engine with a higher injection pressure, a higher EGR rate, or later injection timing, with little penalty on power output. It was also found that injection timing has more influence on HCCI-like combustion using a single main injection than the other two factors studied. Compared with the base cases, simultaneous reductions of soot and NOx were obtained by increasing EGR rate and retarding injection timing. By increasing injection pressure, NOx emissions were increased due to leaner and faster combustion with better air-fuel mixing. However, smoke emissions were significantly reduced with increased injection pressure.     

Numerical modeling of hollow-cone fuel atomization,vaporization and wall impingement processes under high ambient temperatures

In the following paper, a numerical study of the atomization, vaporization and wall impingement processes of hollow-cone fuel spray from high-pressure swirl injectors under various ambient temperature conditions was carried out. Also, the availability of applied models and the effect of ambient temperature on spray characteristics is discussed. The Linearized Instability Sheet Atomization (LISA) model combined with the Aerodynamically Progressed Taylor Analogy Breakup (APTAB) model, the improved Abramzon model and the Gosman model are used to calculate the atomization, vaporization and wall impingement processes of hollow-cone fuel spray, respectively. Spray models are implemented with the modified KIVA code. The calculation results of the spray characteristics under two ambient temperatures, including spray tip penetration, spray structure and radial distance after spray-wall impingement are compared to the experimental results obtained by the Laser Induced Exciplex Fluorescence (LIEF) technique. The droplet size distribution, ambient gas velocity field, vapor phase distribution and fuel film mass generated by spray-wall impingement, measurements which are generally difficult to obtain by experimental methods, are also calculated and discussed. Quantitative discussions on the effect of the ambient temperature on the spray development process are conducted. It is shown that the applied models are applicable even in the high ambient temperature condition.