Influence of scaling rules on loss of acoustic energy

The damping of acoustic oscillations in two different design concepts of rocket engine demonstrators is assessed. Both engines represent different scaling strategies for subscale demonstrators. In this context, one major question is which of the two designs are less prone to combustion instabilities. Different methods are presented to show the influence of geometry on the loss of acoustic energy. The decay coefficient is determined, which accounts for the overall loss of acoustic energy independent of the physical mechanism. The admittance at the intersection of the combustion chamber and nozzle is evaluated. The acoustic flux is calculated and divided into different kinds of contributions. In this regard, the importance of the relation between pure acoustic transport and convective transport is emphasized. It shows that the pure acoustic transport is directed into the combustion chamber. However, its fraction of acoustic energy is compensated by the convective transported acoustic energy, which is convected out of the thrust chamber. Thus, the overall acoustic energy in the system is decaying. The two designs show different damping characteristics for different modes. The photoscaled design is less stable for longitudinal modes but more stable for transverse modes. The first transverse mode in the Mach-scaled design is barely damped.