The contribution describes the flow field inside modern gasoline direct-injection nozzles and sprays. Starting from the internal nozzle flow, results from transparent real-size nozzles are shown, where a significant vapor fraction even for cold fuel conditions is proven. Based on vapor fraction inside the nozzle, evidence for (super-)sonic flow conditions inside the nozzle is shown. The nozzle outlet velocity is determined by means of X-ray structure tracking velocimetry, which is a very powerful measurement technique to gain access to the very dense spray at the nozzle outlet. The X-ray velocities are compared to values that are determined by means of optical-phase Doppler anemometry/laser Doppler anemometry and Schlieren imaging-measurement techniques. By extrapolating the maximum droplet velocities found by laser Doppler anemometry in the more downstream regions of the spray to the nozzle outlet region, very similar velocities to the one derived from the X-ray measurements close to Bernoulli velocity are evaluated for typical gasoline direct-injection engine conditions. A third access to the nozzle outlet velocity is given by the derivation of penetration curves. The combination of vapor fractions and outlet velocities provides a measure for the initial spray momentum.
This paper describes the adaptation of the laser-induced fluorescence measurement technique for the investigation of the primary breakup of modern diesel and gasoline direct injection sprays. To investigate the primary breakup, a microscopic technique is required, and with the help of special tracer dyes, a high fluorescence signal can be achieved in the visible range of the electromagnetic spectrum, resulting in good image quality with a nonintensified camera. Besides the optimization of the optical setup for the microscopic field of view, different tracer dyes are compared, and their solubility and fluorescence are tested in the desired surrogate and real-world fuels. As a tracer, the phenoxazine dye Nile Red was found to provide sufficient solubility in alkanes as well as suitable emission and excitation spectrum for the use of the second-harmonic frequency of a Nd:YAG laser (532 nm). The good quantum efficiency delivered by Nile Red also meant that single-shot images clearly showing spray structures in regions measuring up to 3 mm by 3 mm around the nozzle outlet could be recorded. Compared to relatively easy shadowgraph techniques and complex and costly x-ray synchrotron measurements, light sheet fluorescence microscopic imaging is not overly complex yet delivers excellent data on spray structures as well as qualitative fuel distribution.
In modern gasoline direct injection engines, the fuel is (partially) superheated for a significant proportion of the time during operation. This means that the vapour pressure of the fuel, or at least of many of its components, is higher than the ambient pressure inside the engine during injection. If the excess fuel enthalpy cannot be removed by evaporation at the free surface of the spray, the liquid phase boiling creates new surfaces. This phenomenon is known as flash boiling. Flash-boiling atomization produces smaller droplets and can therefore be beneficial as an additional atomization mechanism. Furthermore, it can reduce the penetration depth of a spray, although it also decreases the stability of fuel sprays. This is manifested in undesired targeting changes, that is, spray contraction due to jet-to-jet interaction. In extreme cases, a complete spray collapse can occur, where a multi-hole or hollow-cone spray contracts towards the spray axis and forms a jet-like structure that increases penetration depth. To understand the relationship between flash-boiling atomization and targeting changes, flash boiling was investigated with a single-hole generic injector without jet-to-jet interaction. In addition to macroscopic spray parameters, this study also focused on the flow field of the spray itself measured using laser Doppler anemometry, as well as the spray-induced flow field of the surrounding gas phase measured using fluorescent particle image velocimetry. The results show a strong radial expansion of the jet directly after nozzle exit, caused by internal flash boiling. It is shown that this expansion is caused by a zone of expanding fuel vapour in the centre of the spray. As a result, the displacement of air after injector opening as well as at the front of the spray is significantly increased, causing a decrease in spray front velocity and penetration depth. The stationary air entrainment, however, is only moderately increased as is the total amount of captured air, since the fuel vapour displaces air in the spray.
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