Physical Science: New Insights and Developments Vol. 5

Ignition in cryogenic space rocket engines is preceded by a short transient phase during which the propellant feed system is swept with an inert gas before and, in some cases, during the initial admission of the cryogenic oxidiser. This chapter examines the influence of helium-sweep management on flow behaviour during the pre-ignition transient of a cryogenic rocket-engine injector. Experiments were conducted using a full-scale injector mock-up representative of the liquid-oxygen supply, with liquid nitrogen used as the simulant fluid under cryogenic conditions. The transient period was limited to 300 ms after the liquid-nitrogen valve was opened. Two operating conditions were compared: liquid-nitrogen injection after the helium sweep was stopped at valve opening, and liquid-nitrogen injection with the helium sweep continued for the first 150 ms. Pressure, impact force and two-phase density measurements were used to estimate the mass flow rate and analyse flow behaviour. The results showed unstable two-phase flow under both conditions, with oscillatory behaviour in mass flow rate, pressure drop and density. Flow-regime interpretation indicated annular or mist-type flow during the transient. Frequency analysis identified dominant oscillation modes near 13 Hz, 25 Hz and 50 Hz, consistent with low-frequency thermoacoustic oscillations. This indicates that thermoacoustic oscillations begin in the engine unit before ignition inside the combustion chamber, with possible coupling to oscillations occurring after ignition that could increase instability. Continued helium sweeping reduced the influence of the secondary 50 Hz mode and decreased the amplitude of the dominant 13 Hz mode during the later part of the transient. These observations suggest that helium sweeping can contribute to more stable injector flow before ignition.