Ignition delay and low-frequency coupling in hybrid rocket combustion

Hybrid rocket engines (HREs) require a post chamber to allow additional enthalpy production of unburned mixture, which may be expelled without participating in combustion. Many studies have confirmed that the post chamber is responsible for generating low-frequency instability (LFI). This study performed various tests with increasing post-chamber lengths. The test results show that post-chamber length is an important factor that determines the occurrence of the LFI. While the LFI persists, there is an observed positive coupling between pressure oscillations (p ') and heat release oscillations (q '), which share the same high-frequency band (400-500 Hz). But a phase difference changes periodically over time, it forms pressure beats of approximately 15-20 Hz. Video images (with LFI) show that combustion flows bounce back upstream after impinging on the nozzle wall. The bounced-back flow then collides with the incoming flow and forms a counter flow in the post chamber. In addition, re-ignitions with delays are observed in the counter-flow region, which is discontinuous and periodic. This study revealed that the periodic positive coupling of p ' and q ' is one of the manifestations of thermoacoustic instability. A modified kick oscillator model is used to account for the ignition delay and successfully predicted the pressure beats.