Using a statistical approach based on the unsteady Reynolds-averaged Navier-Stokes equations (URANS) and ensemble averaging, pressure evaluation of spray-induced flow is conducted by means of stereo-particle image velocimetry. The method allows the determination of the full velocity gradient tensor in order to characterize the governing equations. The spray-induced flow of a two-hole gasoline direct injection (GDI) research sample is investigated at 100 bar injection pressure, 1 bar ambient gas pressure, 25 °C fuel and ambient gas temperature. Low-and high-pressure regions are observed representing entrainment, displacement and recirculation flow. In reference to the ambient pressure, the mean pressure field indicates small pressure differences in the order of 0.1 mbar. For the assessment of pressure evaluation, a comparative measurement with a pressure sensor is carried out, which shows good agreement of the temporal pressure course in the intermediate spray region. The propagation of instantaneous velocity uncertainties to the pressure field indicates low pressure uncertainties for an ensemble average of 50 measurement samples. As a result of scale analysis, the pressure gradient is mainly described by the local acceleration, whereas the contribution of convective acceleration and Reynolds stresses focuses in the spray proximity. The analysis shows negligible effects of viscosity and gravity. The sprayinduced flow is regarded as incompressible in case of low mass and heat transfer. The observations justify a simplification of the governing equations minimizing measurement and computational expense.
The development of the injector nozzle is a dynamic area in regard of several technical aspects. At first, the internal flow influences the near-field spray characteristics via various phenomena such as cavitation and turbulence. However, these phenomena are not fully understood due to their extremely fast, complex and multiscale nature. Furthermore, it governs the spray targeting inside the combustion chamber. High-speed X-ray imaging of GDI injector nozzles is performed in this study. The experimental results presented are related to the internal flow and primary breakup of discharged liquid jets. The injectors used are equipped with nozzles made of aluminum which have been specially developed for these investigations to enhance optical accessibility. The visualization of the needle motion, in-nozzle flow and the primary breakup region provides several exciting observations. First, the needle lift tracking exhibits short overshooting right before the steady-state of the injection phase. This event leads to a short-term, however, significant change in the associated performance of the breakup. This phenomenon is found to be a consequence of the transient behavior of the in-nozzle flow. It is shown that under some circumstances hydraulic flip may occur during this overshooting period. The primary jet breakup region is visualized and evaluated by means of image processing. Thus, the transient behavior of liquid jet expansion is quantified in the vicinity of the nozzle. It is observed that the liquid jet direction deviates from the hole axis already at the nozzle outlet, which is caused by internal flow characteristics.
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