Nonlinear analysis of low-frequency combustion instabilities in liquid rocket engines

Abstract

Low-frequency combustion instabilities are here studied taking advantage of the software EcosimPro. A specific module has been implemented based on the double time lag model and the coupling of combustion chamber and feed line oscillations were investigated by using a complete set of nonlinear equations. The characteristic time lags have been identified following two approaches: (i) a constant time lag approach; and (ii) a variable time lag approach based on correlations available in open literature. To prove the module capabilities, an experimental setup was reproduced and a stability map was generated, comparing the obtained results with literature data from both experiments and a linear double time lag model. The stability boundaries obtained with the chugging module are in good agreement with those obtained in open literature and the first characteristic frequency of the engine is well predicted. Furthermore, the model proves its capability in reconstructing the reversal in the slope of the stability boundary at low fuel injector pressure drops and in detecting the high-frequency content typically observed in presence of multimode oscillations. However, in the calculations, the higher frequency does not dominate the instabilities, that is, in the unstable regime, the model diverges with a frequency equal to the first characteristic frequency. In the last part of the paper, the variable time lag approach is used to investigate a portion of the aforementioned stability map. Thanks to the semiempirical correlations, the present authors managed to improve the prediction of the first characteristic frequency, whereas the stability boundary does not change significantly and remains comparable with the one predicted by the constant double time lag approach.