The frequency characteristics of the shear layer oscillation in hybrid rocket post-chamber

Visualizing flame images revealed the occurrence of LFI (Low Frequency Instability <20 Hz) in hybrid rocket combustion is significantly related with pressure and combustion fluctuations of around 500 Hz in the post chamber. In this study, numerical calculations for cases with different ER (Expansion Ratio) and the wall blowing effect were conducted to investigate which factor has a more influence on pressure fluctuation of 500 Hz band. In addition, the flow structure change in each case was examined and compared with the baseline. Results show that both wall blowing and the increment of ER increases the turbulent energy of the time-averaged flow and pushes the reattachment point to downward. And it seems that the wall blowing effect considerably increases the recirculation length by modifying the turbulent flow structures. Moreover, results indicate that the wall blowing effect promotes the vortices generation from the inlet and supplies the vortices with more energy to appear in the wake. In the spectrum analysis, also, the wall blowing effect was found to oscillate re-attachment point, resulting in the appearance of additional oscillatory characteristics of St = O (0.3) in the downstream otherwise completely dissipated. Note that the oscillation frequency of St = O (0.3) is corresponding to a dimensional frequency of 490 Hz, which approximately coincides with a measured frequency band of 500 Hz. Therefore, the pressure oscillation in about 500 Hz band in the experiment is strongly related with temporal characteristic of the modified flow structure by wall blowing.