Experimental and Numerical Combustion Instability Modes in Asymmetric Baffled Chambers

Abstract

To date, the manner in which baffles damp combustion instabilities has not been fully explained. In an effort to develop such understanding, recent experimental studies of liquid rocket engine (LRE) combustion instabilities at Georgia Institute of Technology have shown that a single, short baffle can completely damp the first tangential mode instability. Also observed during these studies, the frequency of this mode decreases as the length of the baffle increases, reaching a minimum value when the baffle is fully inserted into the LRE, essentially dividing the circular transverse plane near the injector plate into two parts. This paper describes the results of a numerical study whose objective is to explain these observations. It applies the finite element method to solve the homogeneous Helmholtz equation that describes the transverse oscillations in the experimentally investigated LRE. The solutions provide the eigenvalues and eigenvectors of the LRE acoustic modes, which are the natural acoustic mode frequencies squared and the mode shapes, respectively. Analysis of the model predictions shows that the calculated frequency for the first tangential mode of the unbaffled LRE agrees with the analytical value to within 0.3%. The predictions of the code are also compared with measured data. This comparison shows that the code predictions match the experimental trends of the effect of the length of a single baffle on the first tangential mode frequency within approximately 4%, showing that the frequency of the first tangential mode steadily decreases from approximately 5000 Hz for an unbaffled chamber to 3000 Hz when the single baffle was fully inserted into the LRE. The developed code is then utilized to study the effects of baffle thickness upon the LRE acoustics. This study shows for the experimental LRE arrangement, not accounting for the baffle thickness would alter the natural frequency of the first tangential mode by 3.5%.