This paper investigates different types of longitudinal mode coupling in subscale rocket combustion chambers using experimental data and numerical simulations. Based on a one-dimensional planar wave acoustic model of coupled cavity resonators with two acoustic inlet boundary conditions, mode selection rules are derived, providing a simple way of predicting which injector and combustion chamber modes have matching frequencies. Longitudinal mode coupling of an injector with an acoustically open inlet boundary condition has been reported in the literature for the start-up transient of a research combustor experiment. In this experiment, every third injector mode couples to a corresponding chamber longitudinal mode, which is explained in terms of the selection rules derived in this paper. Numerical simulation results for a different combustor experiment show an unexpected mode coupling behavior when an acoustically closed injector inlet is used. Theoretical analysis by using the one-dimensional wave model and applying the derived selection rules shows that in this setup, the injector acoustic mode can accommodate two different acoustic boundary conditions at the injector-chamber interface simultaneously. This results in different acoustic mode shapes in the injector, explaining the unexpected behavior for the resonant coupling with an acoustically closed injector inlet.

Despite considerable research effort in the past 60 years, the occurrence of combustion instabilities in rocket engines is still not fully understood. While the physical mechanisms involved have been studied separately and are well understood in a controlled environment, the exact interaction of fluid dynamics, thermodynamics, chemical reactions, heat-release and acoustics, ultimately leading to instabilities, is not yet known. This paper focuses on the investigation of flame-acoustic interaction in a model combustion chamber using detached-eddy simulation (DES) methods. We present simulation results for a new load point of combustion chamber H from DLR Lampoldshausen and explore the flame response to resonant and non-resonant external excitation. In the first part of the paper, we use time-averaged results from a steady-state flow field without siren excitation to calculate the combustion chamber Helmholtz eigenmodes and compare them to the experimental results. The second part of the paper presents simulation results at a non-resonant excitation frequency. These results agree very well with the experimental results at the same condition, although the numerical simulation systematically overestimates the oscillation amplitudes. In the third part, we show that a simulation with resonant siren excitation can correctly reproduce the shift in eigenmode frequencies that is also seen in the experiments. Additionally, for this new load point, we confirm previous numerical results showing a strong influence of transversal excitation on the shape of the dense LOx cores. This work also proposes a bombing method for determining the resonant eigenmode frequencies based on an unexcited steady-state DES by simulating the decay of a strong artificial pressure pulse inside the combustion chamber.

Improved delayed detached eddy (IDDES) simulation results of a backward facing step flow are analyzed using dynamic mode decomposition (DMD). Different flow variables and the time-resolved skin friction coefficient are investigated and compared to a spectral analysis of the wall pressure fluctations. Although the flow field does not contain single dominant modes, two distinct flow features can be extracted and visualized using the DMD mode shapes. A low frequency flapping motion of the shear layer is found in the mode decomposition of the pressure, the wall-normal velocity and the skin friction coefficient. At higher frequencies, a wake mode similar to a von Karman vortex street is identified in the streamwise velocity, the pressure and the vorticity field

To replace highly toxic hydrazin-driven reaction control systems, a number of non-toxic alternatives are under development. These are usually referred to as “green propellants”. One candidate is the bi-propellant combination of nitrous oxide and hydrocarbons that combine good storability with a comparatively high specific impulse (${\mathrm{I}}_{\mathrm{SP}}$) at a medium to low system complexity level compared to existing hydrazine thrusters. This propellant combination was chosen because of experimentally available results with C₂H₄-N₂O thrusters as validation data. One advantage of this fuel/oxidizer combination is that both gases are self-pressurant and that they can be used as monopropellants at a lower specific impulse ${\mathrm{I}}_{\mathrm{SP}}$ with reduced model complexity. This helps the design of the propulsion system on satellites. A detailed numerical simulation of a representative thruster based on the fuel combination ethylene and nitrous oxide is presented. The numerical model is set up with a suitable kinetic reaction mechanism for the simulation of the reactive mixture in the combustion chamber. It is validated against experimental data available in the literature.