Two case studies on acoustic characteristics in combustion chamber using Helmholtz-type acoustic cavity: FASTRAC and KSR-III combustors

This paper presents two case studies to evaluate passive stabilization in combustion chambers by quantifying damping capacity characteristics of acoustic cavities in combustion chambers using a three-dimensional linear acoustic analysis that has been applied in a previous study. A feasible interpretation was performed for a FASTRAC thrust chamber, where both successful and failed cases were reported. Regarding the temperature and sonic velocity in an acoustic cavity, the possibility of fine-tuning is evaluated in terms of acoustic absorption and conductance characteristics using the acoustic impedance concept. For a given acoustic cavity, the sonic velocity in the cavity required to achieve an optimal tuning is determined, which agrees with previous experimental results. The results show that the adopted cavities are not tuned appropriately to the frequency of combustion instabilities. Hence, it can be concluded that using an acoustic cavity with a small volume or an orifice having a large cross-sectional area can result in sufficient acoustic damping and the suppression of instabilities. In addition, the feasible estimation of sonic velocity in the acoustic cavity is proposed to ensure sufficient acoustic absorption. The results show similar trends without significant deviations, which can be applied in the design and verification of an acoustic cavity. From the present results, a preliminary fine-tuning of the acoustic cavity can be performed using this pure acoustic analysis.