Effects of ambient pressure on cavitation in the nozzle and the discharged liquid jet Abstract Cavitation in the nozzle plays an important role in the atomization of the discharged liquid jet. Because of this, understanding in-nozzle cavitation is very important for the control of spray characteristics. However, in-nozzle cavitation is not fully understood at present, in part because it is affected by various factors, such as fluid properties, injector geometries, and ambient pressure. Although it is obvious that ambient pressure hinders cavitation development in the nozzle, the extent of the effect has not been quantitatively predicted yet. In the present study, visualization of cavitation in an enlarged two-dimensional (2D) transparent nozzle and the discharged liquid jet is carried out under various ambient pressure Pa (Pa = 0.1, 0.2, 0.3, 0.4, and 0.5 MPa). From the flow visualization result, an image analysis is carried out to obtain the experimental data on the effects of ambient pressure on cavitation length Lc, cavitation width Wc, and liquid jet angle θ. Finally, a set of correlations on Lc, modified cavitation number σc, jet angle θ, and the Weber number We at various ambient pressures are proposed, based on the image analysis results.

Two kinds of cavitation may occur in mini-sac type diesel injectors. The first is geometrical cavitation, which can usually be seen as a film-like structure in the nozzle. The second is the filament-like string cavitation. Both types of cavitation are known to affect fuel spray characteristics, although the effects of geometrical cavitation and that of string cavitation have not been individually clarified. Moreover, the mechanism behind string cavitation occurrence is still unclear. String cavitation usually occurs at low needle lift, which might indicate the existence of a vortex ring flow in the sac. However, because of the difficulty in precise flow measurement of the three-dimensional flow structure in the sac, the link between vortex ring flow and string cavitation occurrence in the sac has not been proven. In this study, high-speed imaging of string cavitation, geometrical cavitation, and discharged liquid jet of an enlarged three-hole mini-sac diesel fuel injector was conducted to individually clarify the effects of string cavitation and geometrical cavitation on the discharged liquid jet angle. Furthermore, tomographic–stereo particle image velocimetry was carried out on the sac. The experiments were conducted at two different needle lifts, to clarify the link between needle lifts and flow structure in the sac, as well as how it affects string cavitation occurrence and liquid jet angle. The results confirmed that at low needle lift, vortex ring flow forms in the sac, which may induce helical flow in the nozzle, resulting in a large jet angle. Vortex strength varies with time, and string cavitation occurs when the vortex is particularly strong. As a result, the magnitude of the jet angle increase at low needle lift is enhanced when string cavitation occurs. At high needle lift, flow pattern in the sac becomes relatively uniform, which makes it harder for string cavitation to form.

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