| 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. |
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