Impact of Injector Mass Flow Fluctuations on Combustion Dynamics in Liquid Engines

Abstract

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.

Thermo-acoustic instabilities driven by fuel droplet lifetime oscillations

“…In numerical simulations of liquid propellant rocket engines, transverse acoustic oscillations were shown to lead to shorter flames due to acceleration of mixing and evaporation of the propellant injected as liquid droplets [10] , a phenomenon which is also observed in experiments and simulations conducted at transcritical conditions [11,12] . Longitudinal high-amplitude acoustic oscillations were also found to reduce the length of liquid fuel sprays [13] .…”

“…where δ denotes the Dirac distribution. This equation is reminiscent of expressions of heat release disturbances associated to high frequency transverse flow disturbances [10,15] . The next step is to link the disturbances of the boundary y c to disturbances of the droplet fuel diameter D , which depends itself on the gas velocity u .…”

“…These disturbances are shown to constitute an additional thermo-acoustic source of destabilization of the combustor. The mechanism is reminiscent of the displacement of a flame interface in an acoustic field as observed in some high frequency instabilities coupled to transverse modes [10,15] .…”

Influence of atomization quality modulation on flame dynamics in a hypergolic rocket engine

“…On the other hand, the real part of the FTF is significantly smaller than its amplitude, because the phase of the FTF is near 90 . This indicates that the driving potential is relatively low and that other types of feedback like direct pressure 2 or velocity 11 coupling or injector mass flow fluctuations 23 may dominate the feedback of the flame to the acoustic field in the type of hypergolic rocket engines investigated in the study.…”

“…An additional aspect to be considered is that the oscillating diameter distribution automatically modulates the total number of droplets introduced into the computational domain per time step if the mass flow rate is kept constant. This assumption will be made in the following to study the effects originating from fluctuating atomization independent of the direct dynamic effects from the mass flow fluctuation investigated for the same configuration in Sattelmayer et al 11,23 It is important to note that it is not per se justified to assume that the dynamics of the evaporation of MMH and NTO are independent of each other.…”