The effects of in-nozzle phase transition on injection characteristics of near- and supercritical kerosene jets

The escalating requirements for thermal management in advanced high-performance aviation engines necessitate the injection of aviation fuels under near- and supercritical conditions. The jet morphology both exterior and interior to the nozzle of aviation kerosene jets, injected from near-critical and supercritical states into a quasi-quiescent, near-atmospheric environment, was experimentally investigated, focusing on the effects of in-nozzle phase transitions. The in-nozzle flow behavior was characterized using shadowgraph imaging and pressure profile measurements, while downstream jet behavior was captured using both shadowgraph imaging and planar Mie scattering techniques. It is found that without observable in-nozzle phase transitions (OIPTs), the near-nozzle jet structure exhibits low sensitivity to injection temperature. However, under OIPT conditions, a reduction in injection temperature precipitates notable increases in jet expansion angle, jet diameter, and Mach disk diameter. Additionally, it initially provokes a downstream displacement of the Mach disk, subsequently reversing its direction upstream. Analysis using the surrogate fuel thermodynamic phase diagram indicates that without OIPT, jets injected from the supercritical and vapor states disintegrate similarly due to stages of isentropic expansion, compression, and isobaric mixing, leading to fuel condensation. Under OIPT conditions, crossing the Widom line or saturation line leads to an increase in the specific heat ratio of the fuel along the injection path and changes the near-nozzle shock structure from barrel-shaped to bowl-shaped, potentially triggering a compression shock wave in the ambient air. These findings are useful for the design and modification of supercritical fuel nozzles, fuel supply systems, and combustion chambers.