Ultrafast-time-gated ballistic-photon imaging and shadowgraphy in optically dense rocket sprays
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Understanding the process of liquid-core breakup in optically dense sprays is critical for the development of predictive models in a variety of combustion engine applications. One of the limitations of current optical techniques is the inability to extract information on internal spray dynamics within regions shrouded by a dense cloud of droplets. Similar difficulties are faced with the use of any technique based on path-integrated absorption, such as X-ray radiography. Recently, a technique referred to as ballistic imaging, has been shown to improve the visualization of liquid-core breakup in dense sprays by employing an ultrafast time gate to discriminate against diffuse light. The goal of this work is to improve on the existing imaging system and investigate the breakup mechanisms of the liquid core immediately following injection. By reducing the contribution from photons that exit the spray after undergoing multiple scattering events, it is possible to emphasize the contribution from photons that undergo little to no scattering (ballistic photons). In the current work, the ballistic imaging technique is used in optically dense rocket sprays with light attenuation levels of 97% to 99%. The images collected in this manner reveal a variety of breakup mechanisms, with coherent liquid-core structures dominating the near-nozzle region. Through comparisons with conventional (non-time-gated) shadowgraph images, it is shown that these coherent structures are surrounded by a dense cloud of droplets. Tests were conducted for a variety of nozzle geometries and a range of flow conditions. In addition to investigating trends in the liquid breakup process, it is also observed that the internal nozzle flow within the injector can have a significant effect on the subsequent spray structure under certain flow conditions.