High-speed X-ray phase-contrast imaging of the cavitating flow developing within an axisymmetric throttle orifice has been conducted using high-flux synchrotron radiation. A white X-ray beam with energy of 6 keV was utilized to visualize the highly turbulent flow at 67 890 frames per second with an exposure time of 347 ns. The working medium employed was commercial diesel fuel at flow conditions characterized by Reynolds and cavitation numbers in the range of 18 000–35 500 and 1.6–7.7, respectively. Appropriate post-processing of the obtained side-view radiographs enabled the detailed illustration of the interface topology of the arising vortical cavity. In addition, the visualization temporal and spatial resolution allowed the correlation of the prevailing flow conditions to the periodicity of cavitation onset and collapse, to the magnitude of the underlying vortical motion, as well as to the local turbulence intensity.

The present investigation illustrates the temporally-resolved, phase-contrast visualization of the cavitating flow within an enlarged injector replica conducted at the ANL Advanced Photon Source. The flow was captured through side-view, x-ray radiographies at 67890 frames per second with an exposure time of 347ns. The orifice employed for the experiments has an internal diameter of 1.5mm and length equal to 5mm. A parametric investigation was conducted considering various combinations of the Reynolds and cavitation numbers, which designate the extent of in-nozzle cavitation. Proper post-processing of the obtained radiographies enabled the extraction of information regarding the shape and dynamical behaviour of cavitating strings. The average string extent along with its standard deviation was calculated for the entire range of conditions examined (Re=18000-36000, CN=1.6-7.7). Furthermore, the effect of the prevailing flow conditions on quantities indicative of the string dynamic behaviour such as the breakup frequency and lifetime was characterized and the local velocity field in the string region was obtained. KeywordsFuel injection, synchrotron radiation, nozzle flow, high-speed radiography, velocimetry IntroductionModern fuel injectors operate at extreme pressures of the order of 3000 bars, as it has been demonstrated that this degree of pressurization leads to increased engine performance and reduced pollutant emissions [1]. The turbulent nature of injector flows gives rise to highly-transient cavitation phenomena emanating due to the geometrical layout and/or the emerging secondary flow pattern [2]. The onset of vortical (string) cavitation within the injector holes has been demonstrated to lead to atomization enhancement and spray cone angle increase [3]. In contrast, as demonstrated in [4], the collapse of unstable vapour clouds appears to be more aggressive compared to attached cavities in terms of cavitation-induced erosion. Referring to two-phase flow visualization, optical techniques, e.g. shadowgraphy, Schlieren or LIF/LIEF imaging, have been traditionally employed and thus a transparent, durable material such as Perspex or sapphire must be utilized for the manufacturing of the nozzles. Several experimental investigations dealing with the characterization of the cavitating flow arising within fuel injectors have been performed on enlarged nozzle replicas [5], since the lower pressure of injection along with the larger geometrical length scale compared to real-size injectors facilitate the attainment of higher temporal and spatial resolution and, thus, allow a more thorough elucidation of the flow phenomena setting in. Concurrent studies having been conducted on real-size injectors with transparent tips have verified the main conclusions established by the investigations on replicas with regard to the flow topology and main features [6]. Flow similarity between the different examinations performed on varying length scales is ensured by the adjustment of the non-dimensional quantities that desi...