【作者】 沃恒洲；

【导师】 胡献国；

【作者基本信息】 合肥工业大学 ， 机械设计与理论， 2013， 博士

【摘要】 柴油机喷嘴内的空化对柴油机的性能会产生严重影响，一方面它能促进燃油的雾化，使得燃油在发动机内部能高效、充分的燃烧，降低了积炭产生的可能；另一方面空化能引起喷嘴内部的空蚀磨损，从而影响喷嘴的使用寿命与性能。但由于喷嘴的流体通道的尺寸非常小，流速又非常高，给直接观察与测量空化与空蚀磨损等研究带来困难。本学位论文通过计算流体力学，采用混合均相流模型加完全空化模型进行数值模拟,着重探讨了柴油发动机轴针式喷嘴和孔式喷嘴内部空化流动的影响规律和气蚀磨损特性，研究了生物质燃油在柴油发动机喷嘴内的积炭特性，以期为新型发动机代用燃油---生物质燃油在发动机上的应用打下一定的基础。对于轴针式喷嘴，研究了喷射压力、背压和针阀升程对空化流动的影响。结果表明：喷射压力会促进空化，当背压为定值时，针阀升程为0.2mm时，喷射压力增加到5MPa时，开始出现空化，10MPa时空化现象明显，以后随着喷射压力的增加，空化程度加深。另外，背压的增加会抑制空化。而针阀升程越大，越易于出现空化现象，如当喷射压力为定值10MPa时，针阀升程为0.1mm时，没有出现空化。针阀升程为0.2mm时,出现明显的空化，当针阀升程为0.3mm时，空化现象有明显的加剧。对于孔式喷嘴，研究了喷射压力、背压、入口圆角、长径比对空化的影响。喷射压力和背压对空化的影响与轴针式喷嘴相同。不同的喷射压力下的质量流量与压力差（喷射压力与饱和蒸气压）的平方根成正比。在空化初生前，随着喷射压力的增加，流量系数与空化数的平方根成正比；空化初生后，流量系数与空化数的平方根成反比。当直角喷嘴的L/D值为1/8时，在0.3MPa时发生空化。而喷嘴入口处圆角半径为40μm时，在喷射压力为1MPa时空化初生。当直角喷嘴的L/D值为1/16时，在0.4MPa时发生空化。对比柴油和生物质燃油，燃油的物性对空化产生较大影响，粘度越大的燃油，发生空化的临界空化压力越大，即越难发生空化。饱和蒸汽压越大的燃油，发生空化的临界空化压力越小，越容易发生空化。但燃油的表面张力对临界空化压力几乎没有影响。喷嘴内部出现的空化现象会导致气蚀磨损。本学位论文研究了喷嘴中针阀表面硬度和表面制造精度对气蚀磨损的影响。分别将S195柴油机的实际喷嘴（表面硬度840HV）和经过热处理后的喷嘴（表面硬度420HV），在S195柴油机上运转一段时间后，考察喷嘴的气蚀磨损特性。结果发现实际喷嘴出现了直径在10μm以下的空蚀坑，而经过热处理喷嘴的空蚀坑直径在20-30μm，极少量空蚀坑直径在30μm以上，表明硬度较低的针阀表面容易产生空蚀磨损。并且针阀密封表面上的原始凹坑缺陷由于位置的不同，造成了具有不同的磨损特性。处于针阀密封面上游位置的原始缺陷处，空蚀磨损程度较大，而处于针阀密封面下游位置处的原始缺陷处，没有明显的磨损，即空化强度大的区域空蚀磨损程度也较大。其磨损机理为：原始的表面凹坑缺陷倾向于汇聚游移空泡，造成空化泡易在原始缺陷区域的团聚，空化泡团聚到一定程度后，会发生溃灭。空化泡在高速流场中在极短的时间内溃灭，其溃灭会对壁面产生高的压力脉冲，从而对固壁表面造成破坏。通过自制的孔式喷嘴空蚀磨损试验装置，用紫铜代替孔式喷嘴中的部分喷孔，在喷孔直径200μm，燃油的喷射压力为10MPa，持续时间为0.5h的条件下，发现在喷孔入口区域有直径10μm以下的空蚀坑，且深度较深，呈针状和麻点状。利用表面活性剂制备了生物质油/柴油乳化混合油。分别用该混合油和柴油在S195柴油机上运转一段时间后，对比了两种燃油的积炭特性。结果表明：两种油使用后积炭物均为非晶态，但混合油的积炭物结晶状态，比柴油形成的积炭物结晶状态好。生物质油/柴油乳化混合油的积炭物与柴油积炭物相比，碳和氢元素的含量较高，氧元素的含量较低。更多还原

【Abstract】 Cavitation inside the nozzle has strong influence on performance of the diesel engine. On the onehand, it can promote fuel atomization, make fuel efficiently and fully combust in engine，reduce thecarbon deposition. On the other hand, it can cause cavitation erosion in the nozzle，and affect theperformance and service life of nozzle. Because the size of orifice in engine nozzle is very small andthe flow velocity of fuel in orifice is very high，it is difficult to research cavitation phenomenon inengine. This dissertation simulated the cavitation phenomenon inside the nozzle with ComputationalFluid Dynamics method. The model of numerical simulation was mixed homogeneous flow modelwith full cavitation model. Influence factors of cavitaion flowing and characteristics of cavitationerosion were emphatically discussed. Carbon deposition features of biomass fuel in diesel enginenozzle were studied in order to provide some basic information for the applications of biomass fuelas one of alternative engine fuel.For pintle-type nozzle, the influences of injection pressure，back pressure and needle lift oncavitation phenomenon were studied. Numerical simulation results showed that，for pintle-typenozzle, injection pressure would promote cavitation phenomenon. When back pressure was constantvalue, needle lift was0.2mm and injection pressure was5MPa, the cavitation emerged. Wheninjection pressure increased to10MPa, the cavitation was obvious. Then with the increase ofinjection pressure, the degree of cavitation was aggravated. The increasement of back pressurewould restrain cavitation. The higher the needle lift was, the cavitation more easily appeared. Wheninjection pressure was10MPa and needle lift was0.1mm, there was no cavitation in the nozzle.When needle lift was0.2mm, the cavitation was obvious. When needle lift increased to0.3mmm,cavitation was obviously aggravated.For hole-type nozzle，the influence of injection pressure, back pressure, L/D ratio and inlet roundradius on cavitation phenomenon were researched. The influences of the injection pressure and backpressure on cavitation are the same with pintle-type nozzle. Mass flow rate under different injectionpressures were direct proportion to the square root of differential pressure (injection pressure and thesaturated vapor pressure). Before the beginning of the cavitation, with the increase of injectionpressure, flow coefficient was direct proportion to the square root of the cavitation number. Aftercavitation inception, flow coefficient was inverse proportion to the square root of the cavitationnumber. When injection pressure was0.3MPa, cavitation would occur in rectangular nozzle whichL/D is1/8. In the nozzle which inlet radius is40μm, cavitation occurred when the injection pressurewas1MPa. In the rectangular nozzle which L/D is1/16, cavitation occurred at0.4MPa injectionpressure. Compared with diesel fuel and biomass fuel, fuel properties had impact on cavitation. Critical cavitation pressure was higher for fluid with higher viscosity and lower saturation vapourpressure. In one word, under the same condition, cavitation likely occurred for the fluid with lowerviscosity and higher saturation vapour pressure. However, the surface tension had no influence oncavitation.The cavitation phenomenon in the nozzle would cause cavitation erosion. This dissertationstudied the influence of the surface hardness of needle and surface defects on cavitation erosion.Two kinds of nozzle (one was obtained from the actual diesel with surface hardness of840HV, theother was after heat-treatmented with surface hardness of420HV) were run in S195diesel enginerespectively. After operation two kinds of needle sealing surfaces were observed by SEM. The resultshowed that there were cavitation pits which diameters were below10μm on the sealing surface ofactual nozzle. There were cavitation pits which diameters were20-30μm (little over30μm) onsealing surface of heat-treatmented nozzle. This result showed that sealing surface with lowerhardness easily produce cavitation erosion. Surface defects which located in different position ofneedle sealing surface had different wear characteristics. The defects located in upstream of needlesealing face had more serious damage, and the defects located in downstream of needle sealing facehad no obvious wear. In a word, the area where the cavitation intensity is higher more likely causesdamage.The wear mechanism is that original surface defect pits tend to collect bubble together,caused bubble easily reunited in original defect area. When the cavitation bubble reunited to acertain level, collapse happened. Cavitation bubble collapsed in a very short time, and high pressureimpulse to wall would be produced. The high pressure impulse caused damage to the wall. A set oftesting equipment was established in the present distertation. The method was cutting the head ofnozzle which was replaced by specimen with the orifice. The material of specimen is copper. Thetest was operated under the diameter of orifice was200μm, injection pressure10MPa, durationtime0.5h. The specimen was observed with SEM after experiment. The results showed that obviouscavitation pits located in entrance areas of orifices, which size was below10μm and appearedneedle shape.Emulsify biomass oil/diesel was prepared using the surface active agent. Emulsified biomassoil/diesel and diesel respectively was used in S195diesel engine in a period of time. Carbondeposition properties of two kinds of fuels were compared. The results showed that the carbondeposition produced by emulsified biomass-oil/diesel and diesel were amorphous, but crystallizationstate of carbon deposition which produced by emulsified biomass-oil/diesel was better than that ofcarbon deposition which produced by diesel. Particle diameter of Carbon deposition was about10-30μm which formed in the condition of diesel oil. But particle diameter of Carbon deposition was50μm which formed in the condition of emulsified biomass-oil/diesel and it appeared obvious reunion. In carbon deposition which formed in the condition of emulsified biomass-oil/diesel, elementcontent of carbon and hydrogen was higher, element of oxygen content was lower compared withdiesel.更多还原