Dynamic Characteristics of a Coaxial Jet–Swirl Injector with External Excitation

Combustion instability is caused by the coupling of acoustics, hydrodynamics, and flame dynamics. Therefore, understanding the dynamic characteristics of the injector ejecting fuel and oxidizer into the combustion chamber is very important in understanding the combustion instability. This study measures the injector transfer function (ITF) to investigate the dynamics of a gas–gas coaxial jet–swirl injector capable of simulating a full-flow staged combustion cycle. The ITF peaks were generated at the frequency corresponding to the resonant frequency of the tube through which the excited fluid flows. The Kelvin–Helmholtz (K–H) instability was strong when the velocity difference between two flows (Δ V ) was large; hence, the ITF was large. The K–H instability weakened and the ITF was small when Δ V ≈ 0, but the Holmboe instability corresponding to the second mode of the K–H instability occurred, such that the perturbation corresponding to twice the excitation frequency became bigger. The results of the dynamic characteristics of the gas–gas coaxial jet–swirl injector in this study will be helpful in understanding the injector dynamics and in solving the combustion instability, which applies to the full-flow staged combustion cycle.