柴油机喷嘴内小桐子油流动特性仿真分析

为对燃用小桐子油柴油机的燃油喷射及雾化进行系统研究，建立了考虑燃油黏度、密度及气泡数密度随喷射条件变化的三维气液两相流空穴模型，在对所建数学模型验证的基础上，进行了喷油压力、喷油背压及燃油温度对喷嘴内小桐子油空穴流动特性影响的数值模拟，并与喷嘴内柴油的流动特性进行了对比分析。结果表明：应用小桐子油作燃油时，随着喷油压力的增加，喷孔出口燃油的平均流速及质量流量逐渐增加，流量系数逐渐降低，但空化效应变化并不明显；喷油背压的变化对喷嘴内小桐子油的流动特性影响较小；随着燃油温度的升高，喷孔出口平均流速、质量流量及流量系数均大幅提高，空化效应显著增强。相同喷射条件下，小桐子油的空化效应及喷孔出口平均流速均低于柴油，但其质量流量及流量系数是否低于柴油，还受到燃油温度的影响。该研究可为燃用小桐子油柴油机工作过程的深入分析提供参考。

Simulation analysis of flow characteristics of Jatropha curcas oil in diesel injector

Abstract: The formation and development process of fuel spray has a great influence on fuel atomization, fuel evaporation, fuel-air mixing, and combustion, which can further influence the properties of a diesel engine. The atomization process and its quality are affected not only by the mutual friction between the fuel spray and ambient air, but also by the flow behavior in the nozzle orifice such as the cavitation effect and the turbulent flow among which the flow disturbance induced by the cavitation is the first important factor effecting the atomization of fuel spray. A lot of research studies on an alternative fuel, Jatropha curcas oil (JCO), for diesel fuel have been carried out by both domestic and international scholars with the focus mainly on the preparation methods, and physical and chemical properties as well as the performance and the pollutant emissions of a diesel engine fueled with JCO. Nevertheless, the issues related to a systematic study of the cavitation flow and the spray outflow characteristics, which have a great influence on the injection and atomization of JCO, are seldom reported. In the present paper, a three-dimensional gas-liquid two-phase model of cavitation flow was developed, which took the influence of injection conditions on fuel viscosity, fuel density, and bubble number density into consideration. After the validity of the model was verified, a three-dimensional numerical simulation that was about the influences of injection pressure, injection back pressure and fuel temperature on the cavitation flow of JCO in a diesel nozzle was carried out. The results showed that: the mean flow velocity at outlet (MFVO) and the mass flow rate is enhanced gradually with the increment of injection pressure, and the discharge coefficient decreases simultaneously. However, the cavitation effect does not change significantly. The injection backpressure has little influence on the flow characteristics of JCO in the diesel nozzle. MFVO, mass flow rate, and discharge coefficient raise remarkably with the increment of JCO temperature, and at the same time, the cavitation effect is significantly enhanced. The cavitation effect and MFVO of JCO are lower than those of diesel fuel under the same injection condition, but whether the mass flow rate and the discharge coefficient of JCO are higher than those of diesel fuel also depends on the fuel temperature. The results obtained indicate that the increment of JCO temperature will remarkably enhance the cavitation effect, strengthen the flow disturbance, and improve MFVO and the cycle fuel injection quantity, which are beneficial for the fuel injection, atomization, and the performance of a diesel engine. The research can provide references for in-depth analysis on the working process of diesel engine fueled with JCO.