【作者】 刘劲松；

【导师】 许沧粟；

【作者基本信息】 浙江大学 ， 车辆工程， 2010， 硕士

【摘要】 柴油机的喷雾特性影响着发动机的燃烧效率及排放,良好的雾化是高效、节能、环保的前提。对于柴油机的喷雾特性的研究,由于目前的实验测试存在着不能观察喷雾场内部结构的缺陷,CFD则在这方面有着其独特的优势。本文主要运用CFD软件Fluent对柴油机的喷雾过程进行模拟研究,分析其变化规律。结果表明,WAVE模型能在一定程度上较好地说明柴油机的喷雾近场雾化过程。通过分析可以看出,随着入射压力的增加或者背压的降低,喷嘴内外压差变大,其喷射初速度和能量就大,对后续的气流扰动会产生影响,可以增加喷雾锥角与贯穿距离,而表面不稳定波的波长由于气流扰动的加大会变短,雾化质量变好,但是由于背压变化范围有限,喷雾质量的提高不明显；孔径不变的条件下,随着长径比的增加,管内的阻力加大,贯穿距离减少,雾化质量变差。随后引入粗糙度的概念,粗糙度的存在使喷嘴内部的湍流发生了变化,有了初始的扰动,喷雾射流离开喷嘴后,就会迅速与气流产生扰动,增加了径向的喷雾场发展,喷雾锥角得到了增加,在一定程度上促进了雾化,同时由于在径向的作用,其在轴向的运动相对减弱,不同的粗糙度对于喷雾的锥角和贯穿距离影响各不相同,雾化质量有区别。模拟计算喷雾场中气流速度场的情况,可以看到在喷嘴出口处,由于扰动,气流被卷入喷雾射流中,但在喷雾射流的前端,气流被喷雾射流往外推,形成卷吸涡,而这种卷吸会随背压的降低有所减弱。随后运用混合模型,在考虑粗糙度的前提下,对柴油机喷嘴内的空穴进行模拟,可以看出,随着入射压力的减少,降低了内外压差,减低了静压力小于饱和蒸汽压的概率,空穴的区域随之减少；孔径不变的条件下,长径比的减少会增加空穴宽度,但是这个增加的范围有限；R型喷嘴中,r/D的增加减少了断面的收缩,空穴区域变小,且其值在一定临界值以上时,不会出现空穴现象；R型与S型相比,同样的外部条件下,空穴的出现区域要小,这与壁面的圆滑过渡有关。综合分析空穴现象及其后续的喷雾射流的雾化,可以看出,空穴对于柴油机的喷雾雾化有一定的促进作用,其在管内产生的初级扰动对于雾化的发展有很重要的影响。综合分析模拟研究整个柴油机喷雾过程,可以认为,喷孔内的空穴、管内的湍流扰动以及空气动力干扰是影响喷雾雾化的主要原因。更多还原

【Abstract】 Previous studies have shown that the emission and efficiency of diesel engines are closely tied to the fuel spray. The spray should be well done to achieve clean and effective combustion in diesel engine. The internal structure could not be measured in conventional experiment test, but the CFD has its own advantages on this point.In this paper, it presented the simulations of the diesel spray by Fluent which is one of CFD soft, discussed the relationship parameter of spray development. It is found that WAVE model could simulate the near field spray well. The pressure drop will get large while the injection pressure increased or backpressure dropped, then the velocity of injection and energy increased correspondingly, working with disturbance intensively, the spray cone angle and spray penetration increased, surface unstable wave will short with gas disturbance, the spray will improve, and with backpressure change is limited, via drop backpressure to improve spray is unobvious. The longer Ratio of length to diameter (L/D), the bigger resistance of nozzle, the shorter spray penetration, and the worse spray is. And, then roughness is introduced in the simulation, with roughness’s existence, the turbulence strength changed, initial disturbance is emerged in nozzle, spray injection has function with gas disturbance, develop the spray in radial way, spray cone angle increased, so roughness can improve the spray in some way, and different roughness has different effect of spray. Simulation the gas velocity of spray at the nozzle exit, it is found that with disturbance, gas is rolled in spray injection, but at the front of injection, gas is pushed out to form a volute, and it will weaken with backpressure drop.At the precondition of considering roughness, it simulated the cavitation phenomena in nozzle with mixed model. It is found that with injection pressure decreased, pressure drop dropped, the pressure lower than saturation vapor pressure, cavitation area decreased. The shorter L/D number, the larger cavitation area is. In R style nozzle, while the round corner radius increases, the cavitation area become smaller, until the cavitation phenomena disappears. Comparing the R style with S style, it is seen that, R style nozzle’s cavitation area is smaller at the same external conditions, which has something to do with smooth walls. Considering the cavitation and spray, it is found that cavitation is good at spray improvement with its initial disturbance.Take the whole spray simulation into account, it could be say that cavitation, turbulence in nozzle and air turbulence is the reason of spray development.更多还原