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683ad1f9edbce4b05398af4bd42b2ab0ec33.pdf - Influence of...

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International Journal of Automotive Engineering Vol. 5, Number 1, March 2015 Influence of nozzle geometry and injection conditions on the cavitation flow inside a diesel injector A. Zandi 1 , S.Sohrabi 2 , M. Shams 3 1,2-MSc Graduated 3Associated Professor, Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Tehran, Iran Abstract Cavitation and turbulence in a diesel injector nozzle has a great effect on the development and primary breakup of spray. However, the mechanism of the cavitation flow inside the nozzle and its influence on spray characteristics have not been clearly known yet because of the internal nozzle flow complexities. In this paper, a comprehensive numerical simulation is carried out to study the internal flow of nozzle and the cavitation phenomenon. The internal cavitation flow of the nozzle is simulated using the Eulerian-Eulerian two-fluid model. In this approach, the diesel liquid and the diesel vapor are considered as two continuous phases, and the governing equations of each phase are solved separately. Simulation method is validated by comparing the numerical results with experimental data and good correspondence is achieved. The effective parameters on the nozzle flow are investigated, including injection pressure, back pressure, inlet curvature radius of orifice, orifice iconicity and its length. Results clearly show the importance of nozzle geometrical characteristics and dynamic parameters on the internal nozzle flow. Discharge coefficient of nozzle and cavitation distribution in the nozzle are extremely dependent on these parameters, so the effect of cavitation on the primary breakup is not negligible. Keywords : Diesel injector, cavitation, turbulence, multi-fluid model, nozzle geometry. 1. Introduction Formation and activity of bubbles in a liquid is called cavitation. This phenomenon starts from voids or tiny bubbles containing gas or vapor. If these bubbles are exposed to a pressure less than the vapor pressure, they rapidly grow. As the surrounding pressure increases to values higher than the vapor pressure, these bubbles become unstable and collapse [1]. Increasing the injection pressure in modern diesel engines produces higher pressure gradients and shear stresses in the injector nozzle [2]. With a sudden decrease in the cross section of the nozzle orifice inlet, a low-pressure region in the inlet is created, and as the flow accelerates, the local pressure drops to values lower than the vapor pressure, so bubbles appear. Cavitation is undesirable in industrial devices such as valves, pumps, etc.; because it decreases the efficiency and causes surface damages in these systems. In diesel injector nozzles, cavitation can be useful because it can improve the fuel spray atomization. The purpose of fuel spray is to enhance the surface area of the liquid, and consequently, to increase heat and mass transfer rate, since it causes the air and fuel to mix better [3]. Experimental and numerical studies have shown that spray atomization
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