Cryogenic flow from a coaxial injector under low-pressure in-space conditions

The effect of near-vacuum operating pressure on the evolutionary behavior of cryogenic fluid injected from a single-element coaxial injector has been examined experimentally. The investigated injector simulates one element of the injector array of cryogenic rocket engines under realistic operating conditions. Actual rocket-engine propellants, liquid oxygen and gaseous hydrogen, are simulated by chemically inert and safe to operate liquid nitrogen and helium. This work focuses specifically on the transient evolution of liquid nitrogen jet under in-space conditions. The diagnostic techniques utilized here are high-speed Mie-scattering and Schlieren imaging. The results showed that the behavior of liquid nitrogen jet was significantly affected by the low-pressure conditions as well as the strong heat shielding effect of coaxial helium jet. The cryogenic jet undergoes extraordinary jet expansion at sub-atmospheric pressures. Initial freezing of liquid nitrogen droplets and ligaments was observed close to the injector exit. The heat transfer from the warm surroundings to cold liquid nitrogen jet was reduced significantly in the presence of coaxial gaseous jet, which confined the cold liquid nitrogen jet and prevented its expansion to some extent.