The acoustic transfer impedance of baffled injectors in three-dimensional combustion chambers

Baffled injectors are typical passive control devices to restrain combustion instabilities in liquid rocket engines. However, there still has been no appropriate model to describe the acoustic characteristics of baffled injectors. In this work, the baffled injector gap is modeled as a narrow passageway with a cross-sectional surface area gradient. An acoustic transfer impedance model is derived from the momentum conservation equation to relate the acoustic disturbances between the inlet and outlet of injector gaps. Validation is conducted by comparing the first tangential mode frequency of a three-dimensional combustion chamber without flame from the detailed structures model and that using the proposed acoustic transfer impedance model. The proposed impedance model is embedded in a numerical simulation model of a three-dimensional combustion chamber considering the unsteady heat release; the effects of baffled injectors geometry parameters on the first tangential acoustic mode are then investigated. Results show that the presence of baffled injectors not only damps the acoustic energy due to the viscothermal effect, but also can longitudinalise the original transverse mode.