One-dimensional model on fuel penetration in diesel sprays with gas flow

In diesel engine, spray penetration is usually changed by in-cylinder gas flow. Accurate prediction on diesel spray with gas flow is important to the optimal design of diesel fuel injection system. This paper presents a theory investigation focusing on the penetration of diesel spray with gas flow. In order to understand the effect of gas flow on the penetration of diesel spray, a one-dimensional spray model is developed from an idealized diesel spray, which is able to predict the spray behavior under different gas flow conditions. The ambient gas flow is simplified as ideal flow that has only constant flow velocity along x-axial and y-axial directions of spray. The x-axial and y-axial directions are respectively defined as along and vertical spray directions. The main assumption is that the y-axial direction gas flow has no effect on the penetration of spray along x-axial direction. The principles of conservation of mass and momentum are used in the derivation. Momentum of in-cylinder air flow is also taken into consideration. Validation of the model at stable condition is achieved by comparing model predictions with experimental measurements of diesel spray without gas flow from Naber's experiments. Furthermore, CFD simulations on penetration of diesel spray with gas flow were performed with the commercial code AVL-fire. The onedimensional model is validated by the penetration results with gas flow from CFD calculation. Results show that a reasonable estimation of the spray evolution can be obtained for both with and without ambient gas flow conditions.     

Comparison of the Spray and the Spray/Wall Interaction of Two Gasoline Injectors

One important parameter influencing mixture formation and spray/wall interaction within engines is the geometry of the nozzle. In contrast to Diesel nozzles, the influence of the orifice geometry on spray formation has hardly be investigated for gasoline nozzles. In order to demonstrate the potential of adjusting the nozzle geometry of a modern GDI nozzle, we compare two six-hole, high-pressure nozzles with an identical structure, but different rounding radius of the orifice hole-inlet and different orifice hole-geometries: nozzle A with a rounded inlet and an orifice length to diameter ratio of 3/2 and nozzle B with a sharp inlet and an orifice length to diameter ratio of 1. In a first measurement campaign the spray formation is visualized using high-speed shadowgraphy imaging. The results show differences in spray angle and penetration depth. In a second measurement campaign we examine the spray/wall interaction and wall film formation by means of infrared thermography. The thermography measurements indicate that the geometry of nozzle B produces sprays with beneficial characteristics. This is very important for a clean combustion process and a decrease of soot emissions.     

Influence of nozzle hole diameter and orifice diameter on dme spray to get the similar heat value with diesel spray using the constant volume chamber

Low heating value (LHV) of di-methyl ether (DME) is lower than that of diesel. To get the similar heat value with diesel from the diesel engine operation, single injection quantity of DME should be increased. This investigation was tried to increase the injection quantity of DME by the modified diesel injector and investigated the penetration length and spray angle of DME spray. DME was injected by using three-type modified diesel injectors those nozzle-hole diameters (Injector 1: 1.66 mm, Injector 2 and 3: 0.25 mm) and orifice diameters were different (Injector 1 and 2: 0.6 mm, Injector 3: 1 mm). Spray characteristics of DME was investigated with a various ambient pressures (2.5, 5.0 MPa) in the constant volume chamber and a fuel was injected by varied injection pressure from 35 to 70 MPa by interval of 5 MPa using a DME common rail fuel injection system. The result shows that DME injection quantity by Injector 3 was 1.69 ~ 2.02 times larger than that of diesel injection quantity by Injector 1. In this case, DME spray got the similar heat value compared with diesel spray. The penetration speed of DME spray by Injector 3 was the fastest, thus when the spray development was end, the penetration length of DME spray by Injector 3 was the longest compared with the other cases. In case of the spray angle, Injector 2 and 3 had the similar spray angle and these were larger than that of diesel and DME sprays by Injector 1. As the result, Injector 3 was the solution for how to solve the low heating value of DME.     

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.     

Jet characteristics of CNG injector with MPI system

A theoretical model is developed that assumes the natural gas transient jet can be characterized as a spherical vortex interacting with a steady-state jet. Schlieren photographs revealed a low-pressure gas jet's starting and extending process from the nozzle.Comparison between the model and experimental results for the tip penetration of round jet, show that both have similar characteristics, and the results show that the tip penetration is proportional to the square root of time. Moreover, when per-time calorific values of the injecting fuel are the same, the tip penetration of the gas jet is longer than that of gasoline.     

Effects of injection parameters on the spray characteristics of swirl and slit injectors using the Mie-scattering method

We investigated the effects of injection parameters such as injection pressure, ambient pressure, and ambient temperature on spray characteristics. We calculated the turbulence occurring point (t _c ), defined as the time required to generate a vortex, and the deceleration point (t _b ), defined as the time when spray penetration begins to decelerate, to elucidate the breakup mechanism of the test injectors. The spray velocity coefficient (C_v) was obtained to evaluate the spray characteristics. As the ambient pressure increases in the case of a slit injector, C_v decreases. We investigated the effects of nozzle tip shape according to injection pressure, ambient pressure, and fuel properties on spray characteristics and provide a C_v value of 0.38 for the swirl injector with a spray angle of 60° and the slit injector under atmospheric conditions. The value of C_v in the case of a slit injector was reduced by increasing the ambient pressure. Our results suggest that C_v of a swirl injector is constant regardless of changes in ambient pressure, injection pressure, and fuel properties. On the other hand, C_v of a slit injector is altered by changes in ambient pressure.     

Experimental study on macroscopic spray characteristics after impingement in a slit-type GDI injector

In this paper, an experimental study on the wall-impinging spray of the slit-type GDI injector is presented. To examine the effects of various factors on the development of a spray impinging on the wall, experiments were conducted at various injection pressures, ambient pressures, wall distances from the injector tip, wall temperatures, and wall inclination angles. Behavior of the impinging spray was visualized using a planar laser scattering method. It is shown that the spray path penetration of the wall-impinging spray increases with increases in injection pressure, wall distance, wall temperature, or wall angle. On the other hand, the spray path penetration of the wall-impinging spray decreases with increases in ambient pressure. The predicted spray path penetration calculated by the empirical equation estimates the spray path penetration in all cases, and the empirical equation is optimized for the total injection pressure.     

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。     

Analysis of the transient atomization characteristics of diesel spray using time-resolved PDPA data

The transient atomization characteristics of a single-hole diesel spray were investigated to clarify the time-dependent droplet formation process of the spray through time-resolved analysis of the droplet size data acquired by using a 2-D PDPA (phase Doppler particle analyzer). Comparisons among the three single-hole diesel nozzles on the atomization characteristics were made to confirm the effects of the hole-diameter. The hole diameter of the single-hole diesel nozzles varied with d_n=0.22, 0.32 and 0.42 mm. The time-resolved diameter, SMD (Sauter mean diameter) and AMD (arithmetic mean diameter) of droplets in diesel spray injected into still ambient air were measured. The SMD and AMD decreased with decreasing nozzle hole diameter. The SMD distribution along the spray centerline steeply decreased with increasing axial distance before reaching a constant value. In the time-dependent analysis of the SMD of the whole flow field, the SMD gradually increased with time after the initiation of injection, reached a maximum value, and then decreased.     

Large-eddy simulation of air entrainment during diesel spray combustion with multi-dimensional CFD

Large-Eddy Simulation (LES) was used to perform computations of air entrainment and mixing during diesel spray combustion. The results of this simulation were compared with those of Reynolds Averaged Navier Stokes (RANS) simulations and an experiment. The effect of LES on non-vaporizing and vaporizing sprays was evaluated. The validity of the grid size used for the LES analysis was confirmed by determining the subgrid-scale (SGS) filter threshold on the turbulent energy spectrum plot, which separates a resolved range from a modeled one. The results showed that more air was entrained into the jet with decreasing ambient gas temperatures. The mass of the evaporated fuel increased with increasing ambient gas temperatures, as did the mixture fraction variance, showing a greater spread in the profile at an ambient gas temperature of 920 K than at 820 K. Flame lift-off length sensitivity was analyzed based on the location of the flame temperature iso-line. The results showed that for the flame temperature iso-line of 2000oC, the computed lift-off length values in RANS matched the experimental values well, whereas in LES, the computed lift-off length was slightly underpredicted. The apparent heat release rate (AHRR) computed by the LES approach showed good agreement with the experiment, and it provided an accurate prediction of the ignition delay; however, the ignition delay computed by the RANS was underpredicted. Finally, the relationships between the entrained air quantity and mixture fraction distribution as well as soot formation in the jet were observed. As more air was entrained into the jet, the amount of air-fuel premixing that occurred prior to the initial combustion zone increased, upstream of the lift-off length, and therefore, the soot formation downstream of the flame decreased.     

高强化柴油机喷雾射流对缸盖热负荷影响的研究

针对高强化柴油机缸盖鼻梁区热负荷过高的问题,研究了喷雾射流对缸盖热负荷的影响,提出了通过调整喷孔夹角、喷孔位置和喷雾锥角来改变喷雾射流,从热侧控制燃气的对流传热,以降低缸盖热负荷的方法。对高强化柴油机进行缸内燃烧和传热过程等三维仿真的结果表明,高强化柴油机中,喷孔夹角约为155°时对燃烧最有利,且不会显著增加缸盖热负荷;减小喷孔出口突出距离有利于改善燃烧效率,但同时会提高缸盖的热负荷;喷雾锥角增大则不利于缸盖热负荷的改善。     

单孔主动预燃室火焰射流机理研究

基于单孔主动预燃室，研究了火焰射流的形成机理。在圆形自由紊动射流理论的基础上，提出了单孔主动预燃室点火的火焰射流贯穿距随时间变化的经验公式。该经验公式可以通过射流锥角、预燃室喷口直径和射流在预燃室喷口处的初速度预测射流贯穿距。通过主动预燃室的定容燃烧弹试验和CONVERGE仿真验证，该经验公式所预测的射流贯穿距和反向求得的射流初速度与实测值吻合，最大相对误差分别为 6.92%和 5.82%。认为此公式能够正确预测单喷孔主动预燃室的射流贯穿距，对于优化预燃室结构以实现更高效地点火，继而提升内燃机的理论热效率具有参考意义。     

Breakup modeling of a liquid jet in cross flow

We propose a novel breakup model to simulate the catastrophic breakup regime in a supersonic cross flow. A developed model has been extended from an existing Kelvin-Helmholtz/Rayleigh-Taylor (K-H/R-T) hybrid model. A new mass reduction rate equation, which has critical effects on overall spray structure, is successfully adopted, and the breakup length, which is an important parameter in existing model, is replaced by the breakup initiation time. Measured data from the supersonic wind tunnel with a dimension of 762×152×127 mm was employed to validate the newly developed breakup model. A nonaerated injector with an orifice diameter of 0.5 mm is used to inject water into a supersonic flow prescribed by the momentum flux ratio of the liquid jet to free stream air, q ₀ . The conservation-element and solution-element (CE/SE) method, a novel numerical framework for the general conservation law, is applied to simulate the supersonic compressible flow. The spray penetration height and average droplet size along with a spray penetration axis are quantitatively compared with data. The shock train flow structures induced by the presence of a liquid jet are further discussed.     

Instantaneous 2-D visualization of spray combustion and flame luminosity of GTL and GTL-biodiesel fuel blend under quiescent ambient conditions

An experimental study has been performed on spray combustion and two-dimensional soot concentration in diesel (ULSD), GTL and GTL-biodiesel fuel jets under high-pressure, high-temperature quiescent conditions. Instantaneous images of the fuel jets were obtained with a high-speed camera. It was confirmed that by blending GTL with 20% rapeseed biodiesel, certain fuel properties such as kinematic viscosity, density, surface tension, volatility, lower heating value and others may be designed and improved to be more like those of conventional diesel fuel but with considerable decrease in the amount of sulfur, PAH, cold filter plugging point, etc. The results showed that the spray tip penetration increased and the spray cone angle decreased when 20% biodiesel fuel was added to GTL fuel. Autoignition of the GTL-biodiesel blend occurred slightly earlier than that of diesel fuel. Experiments under high-pressure, high-temperature conditions showed that higher injection pressure induced a lower soot formation rate. The integrated flame luminosity, which serves as an indicator of soot concentration in the fuel jet, was slightly higher for the GTL-biodiesel blend than for pure GTL fuel due to the slightly higher sulfur content of pure biodiesel fuel.     

双层分区燃烧系统对高强化柴油机性能的影响

柴油机的燃烧系统是混合气形成质量的关键。为改善某高强化柴油机的燃烧和排放性能,在保证原机压缩比不变的条件下,设计了一种双层双弧脊分区燃烧系统——双层燃烧室匹配双排喷孔,并基于计算流体力学软件Converge进行数值模拟,研究不同上下排喷孔油束夹角对缸内燃烧和排放的影响。研究结果表明:新设计的燃烧系统的燃烧和排放性能均优于原机,上下排喷孔油束夹角会影响燃油在上下层弧脊处的分配,较大的上排喷孔油束夹角有利于对燃烧室顶隙空间的利用和上层弧脊下侧混合气的形成,较小的下排喷孔油束夹角有利于燃烧室底部凹坑附近空气利用率的提高和混合气分布范围的增加。因此,需要对上下排喷孔油束夹角进行合理的选择和匹配,使得发动机的整体燃烧和排放性能达到最优。     

Numerical investigation of a DI diesel spray flame using a detailed chemical reaction mechanism

Emission standards have grown increasingly stricter, consequently triggering greater interest in issues surrounding environmental pollution. In particular, soot and NOx released from DI diesel vehicles is considered to be the main source of air pollution in urban environments. However, the mechanics of fuel spray formation and the influence of the operating parameters on the resulting spray flame are not yet fully understood. In this study, the original KIVA code was modified to incorporate a detailed chemical reaction mechanism involving various species and multiple reaction steps to better understand the spray characteristics. n-Heptane, C₇H₁₆, was used as the representative fuel for diesel fuel, and the reaction mechanism for this fuel was composed of 66 species and 274 elementary reaction steps. The accuracy of the predicted results was demonstrated primarily by a comparison with experimental results. The numerical prediction of a specific operating condition for the parametric investigation correlates well with the experimental results.     

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.     

多孔引燃柴油喷射器瞬态柴油初始喷雾过程模拟计算

采用模拟计算的方法对多孔引燃柴油喷射器瞬态柴油初始喷雾过程进行了研究,并利用纹影试验结果对计算模型进行了验证。研究结果表明,KHRT模型对多孔引燃柴油喷射器柴油初始喷雾的贯穿距和喷雾形状的预测与试验结果均较为吻合;柴油初始喷雾贯穿距随背压升高而减小,随环境温度的升高而增大;索特平均直径随背压的升高而降低,随环境温度的升高而增加;喷雾锥角随背压的升高而增大,随环境温度的升高而减小;因此背压较小,环境温度较低时柴油的引燃可靠性较高。     

不同喷射工况下的GDI喷雾模拟标定研究

准确模拟喷雾是提高缸内模拟准确性的关键,为了使模拟喷雾与试验喷雾更加一致,需要根据试验喷雾贯穿距及粒径对模拟喷雾进行标定。使用AVL FIRE软件建立定容弹及喷雾数值模型,对处于不同喷射工况(喷射压力、环境压力、油温)下的喷雾进行数值模拟,根据试验喷雾的贯穿距及喷嘴下方30 mm平面处SMD对模拟喷雾进行标定,并对不同喷射工况的标定参数选择进行探讨。结果表明:为了同时满足贯穿距和粒径的标定要求,需要根据工况参数对标定参数进行调整;对于本研究中的GDI喷油器,喷射压力10 MPa时使用KH-RT模型的模拟结果与试验值匹配较好,5 MPa时使用Huh-Gosman模型匹配更好;在喷射压力、环境压力和环境温度相同的条件下,高油温和低油温工况可以用同一套参数满足标定要求。     

烟火式气体发生器燃烧室喷嘴的设计与研究

应用火药燃烧定律和喷嘴的流量速度公式可以计算和设计气体发生器燃烧室喷嘴的孔径和孔数。分别对气体发生器中不同燃烧室喷嘴尺寸进行引燃试验,可以看出,当计算出的燃烧室喷嘴的孔径和孔数为试验所采用时,该气体发生器燃烧室的压力与预先设定的数据基本吻合。此气体发生器产生的压力和时间能满足安全气囊的技术要求。