Analysis On Internal Flow Characteristics Of Each Nozzle Hole Of Valve Covered Orifice Diesel Injector

Not only can the secondary breakup process(because of the aerodynamic forces) affect the quality of atomized fuel, the primary breakup process which is due to the cavitation, turbulence and external aerodynamical action can affect it as well. The internal flow characteristics of injection nozzle hole are important parameters affecting fuel sprays exiting nozzle hole outlet. They play a crucial role in improving spray quality, optimization of the combustion process and decreasing pollutant emissions. This necessitates the investigations of each nozzle hole internal flow, since it can contribute significantly in improving the design of multi-hole fuel injection system and engine performances.In this study, a three-dimension model of valve covered orifice(VCO) multi-hole injector used in a two-valve diesel engine was designed. Numerical simulations of internal flow process were based on the two-fluid model and the cavitation model.Also, based on the spray momentum theorem experimental test rig, experiments were designed to measure the spray impact forces of each nozzle hole. From the law of conservation of momentum, the spray impact forces of each nozzle hole could be used to calculate the injection rate of each hole. The calculated and measured fuel injection rate from each nozzle hole were compared and analyzed for validation purposes. The validation showed a strong correlation between experimental and simulated results with differences within acceptable limits. Meaning, the model could be used to study the internal flow characteristics of the various nozzle hole.The transient flow characteristic of each nozzle hole was obtained from numerical simulation with the boundary conditions being experimental injection pressure and back-pressure. The internal flow characteristics of each nozzle hole differ at different injection stages. That is, at the opening stages of needle lift, the injection pressure increases with increment in cam rotation angle, the growing rate of cavitation zone within each nozzle hole is different. Increasing pressure increases the fuel velocity. At maximum needle lift position, continually changes in injection pressure per cam rotation angle renders the internal cavitations of each hole unsteady, thereby influencing the injection rate at the outlet. The fuel velocity is also changing continuously. At the closing stages of needle lift, the injection pressure and velocity decreases with minimal or no change in cavitation morphology. The cavitation zone becomes a little bigger when the needle is almostly at the closed position.The difference in angle between the needle axis and the nozzle axis of each hole(nozzle angle) results in different cavitation development and distributions. The growth rate of the cavitation zones within hole 3 and hole 4(with higher nozzle angle) are relatively faster resulting in larger cavitation zones through the nozzle outlets, which is beneficial to the spray exiting the nozzle. However, the injection rates from hole 3 and hole 4 are decreased. For nozzle hole 1, the growth rates of the internal cavitation zone is slow through the nozzle outlets with corresponding increment in cam angle. However, the injection rate from hole 1 is increased. The spray characteristics and the injection rate should be comprehensively consided when designing and installing the two-valve multi-hole nozzle to ensure optimum mixture formations, combustion and emissions reductionCompared numerical results from VCO and SAC nozzle, the internal cavitation morphology of VCO nozzle is more pronounced. That is with high degree of cavitation development, the flow velocity from each hole is higher due to the presence of relatively larger cavitation region. Although this is beneficial to the spray exiting the nozzle, the bigger cavitation zone results in the lower injection rate. Meaning for the VCO, the flow velocity is higher and the injection rate is lower as compared to the SAC nozzle. For the SAC, the internal cavitation zones are smaller and distributed at the top of the nozzle hole.