Large eddy simulation of effects of oxidizer inlet temperatures on the transition routes before and after thermoacoustic instability in a subcritical hydrogen peroxide/kerosene liquid rocket engine

This paper presents the first numerical evidence of the intermittency routes that exist before and after the occurrence of thermoacoustic instability in a subcritical single-element liquid rocket engine burning liquid kerosene and decomposed hydrogen peroxide with increasing oxidizer inlet temperatures (T). Three-dimensional compressible large eddy simulation algorithms, combined with Euler–Lagrangian frameworks, are employed to model the spray turbulent combustion process in a high-pressure rocket combustor where a one-equation eddy viscosity sub-grid turbulence model and a PaSR sub-grid combustion model are used based on OpenFOAM. After verifying the numerical framework and achieving grid independence, we focus on (i) dynamical transition routes before and after the thermoacoustic regime, (ii) frequency-locking phenomena between acoustic perturbations, vortex dynamics, and combustion heat release, and (iii) the underlying physical mechanisms associated with different dynamical states. The results show that as we increase the oxidizer inlet temperature (700 K ≤ T ≤ 900 K), the system dynamics undergo a transition from a state of combustion noise to a period-1 limit cycle via intermittency. Furthermore, by further increasing the oxidizer inlet temperature (950 K ≤ T ≤ 1450 K), a second bifurcation occurs, causing a transition from a limit cycle state back to a combustion noise state also via intermittency. These bifurcation phenomena are attributed to frequency-locking interactions among pressure fluctuations, combustion heat release, and vortex dynamics. Evidence from combustion flow subsystems including mixture fraction, burning modes, and flame-induced vorticity sources provides additional insights into the complex instability mechanism.