| The role of the fuel injection system in kinds of new concept combustion is paramount and well recognized as a means of controlling diesel engine performance and meeting the ever more stringent emission regulations. In this case, the precise understanding and prediction of microcosmic flow in diesel injector nozzle is crucial, as it is a key component that connecting the upstream fuel injection and downstream spray. In recent years, the tapered multi-hole nozzle attracts more and more attention from the researchers and business community, but reports concerning about its internal flow characteristics is rather poor. In this paper, a lot of visualization experimental research about this tapered hole nozzle was carried out, the research point mainly centered around cavitation generation mechanism, influencing factor, string cavitation and effects of string cavitation on spray characteristics. The main content and innovative research results are as flows:(1) A nozzle flow and spray visualization test rig equipped with a fuel heating system was designed and set up. The scaled-up nozzle tip was manufactured by an innovative use of the 3D printing and finish machining. Measurements show that the dimensional accuracy of the nozzle tip reached to 0.02 mm, which can assure the similarity of internal flow passage between the scaled-up nozzle tip and real nozzle.(2) Comparative study of flow inside normal cylindrical hole, tapered hole and divergent hole was carried out. Results show that the divergent hole is more prone to cavitate than the cylindrical hole, while the tapered hole shows a strong inhibitory effects on cavitation. Among the influencing factors of nozzle spray primary break up, Cavitation is the biggest impact factors. Shedding cavitation collapse near the nozzle exit and release energy will have a significant impact on spray cone angle.(3) The mechanism of hydraulic flip phenomenon were also discussed. It occurs because once the cavitation bubbles filled separated boundary layer reaches the end of the nozzle hole, surrounding air at the nozzle exit pressed into the recirculation region. The result is cavitation disappeared immediately, and the spray consists entirely of liquid fuel and is laminar in appearance.(4) The main physical parameters of diesel in 10~90℃ was systematically and comprehensively measured for the first time. Comparative study of internal nozzle flow under 15℃, 45℃ and 75℃ was carried out. Results show that high temperature fuel cavitation incept at a small injection pressure. In the cavitating flow stage, under the same injection pressure, high temperature fuel corresponding to a larger cavitation area in the hole than low temperature fuel. As higher temperature make the cavitation in nozzle hole varying fast, mass flow and discharge coefficient of high temperature fuel will reduce much quicker than that of low temperature fuel.(5) String cavitation in multi-hole nozzle was carefully investigated by using a two-hole tapered nozzle for the first time. It was found that string cavitation are not present continuously, but disappearing and reappearing in the nozzle hole and nozzle sac, it is closely associated with vortex flow structure. The string cavitation has two origins, one is emerged at hole inlet and developed to hole outlet, the other is origin from outside air and developed to hole inlet and nozzle sac. The appearance of string cavitation mainly influenced by needle lift position and injection pressure. Low needle lift is good for the happening and existence time of string cavitation, while high needle lift inhibited the appearance of string cavitation. Under the same needle lift position, high injection pressure will increase the happening and existence time of string cavitation.(6) String cavitation have great influence on nozzle internal flow and spray. In multi-hole nozzle, string cavitation in different holes will interact with each other and form a hole to hole string cavitation. String cavitation have a certain cause and promoting effect on geometric-induced cavitation. The emergence of string cavitation will increase the spray cone angle. |
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