Impact of injector mass flow fluctuations on combustion dynamics in liquid engines

Reynolds-averaged Navier-Stokes simulations were carried out to study the influence of acoustically driven injector mass flow fluctuations on combustion dynamics in a liquid propellant rocket engine. First, a framework of transfer functions is introduced, showing the links between pressure fluctuations in the chamber and resulting heat release modulation on the basis of the subprocesses involved. An analytical model for the injector admittance is evaluated, and computational-fluid-dynamics results for a single-injector configuration are obtained. The impact of mass flow fluctuations on evaporation dynamics and on heat release modulation is studied for four load points. Results show that the combustion dynamics are controlled by the droplet evaporation time. Dynamic mode decomposition results explain the convective transport of the droplets and give a detailed insight into the fluctuating fields. A weak mutual influence of the fuel evaporation is observed, whereas the fluctuations of the heat release rate are governed by the oxidizer evaporation rate. Evaporation rates and heat release rates are found to be linearly dependent on the mass flow excitation amplitude allowing for the superposition of transfer functions. Finally, a flame transfer function in the form of a low-pass filter is derived, providing a linear feedback model as input for low-order combustion instability assessment tools.