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The response characteristics of the bi-swirl coaxial injector under oscillating backpressure were investigated numerically based on coupled level-set and volume of fluid model. The backpressure oscillation frequency was set ranging from 1000 to 5000 Hz. The frequency and amplitude response characteristics of liquid films were analyzed under different injection conditions. The amplitude of the pressure response inside the injector was determined by the acoustic damping of the system. Due to the central gas core inside the inner injector, the inner injector manifold was found to be more sensitive to the backpressure oscillation. The frequency characteristic of the liquid film response indicated the presence of the characteristic frequency where the response amplitude was maximum. This characteristic frequency was linearly positively correlated with the mass flow rate of the inner injector, while the flow in the outer injector could suppress the response amplitude of liquid films. Meanwhile, the nonlinearity of the response was dominated by the inner liquid film and was also the strongest at the frequency with the highest response amplitude. The simulated flow fields showed that the oscillation of the ambient airflow periodically hindered the liquid film, leading to Klystron effect. Finally, it was suggested that the characteristic frequency was related to the axial velocity of the inner liquid film at the injector outlet and the acoustic length of the gas core. In addition, the response amplitude of liquid films was found to be controlled by the spray cone angle and the total mass flow rate of propellants. [ABSTRACT FROM AUTHOR]

A methane/oxygen mixture is considered to be an appropriate propellant for many future rocket engines due to its practicality and low cost. To better understand the combustion instability in methane/oxygen-fed rocket engines, the spontaneous transverse combustion instability in a rectangular multi-element combustor (RMC) was analyzed both experimentally and numerically. Severe combustion instabilities occurred in the RMC during repeatable hot-fire tests. The physical mechanisms were systematically investigated through numerical simulations based on the stress-blended eddy simulation and flamelet-generated manifolds method with detailed chemical mechanisms (GRI Mech 3.0). The numerical results for the frequency spectrum and spatial modes agree well with the theoretical analysis and experimental data. The driven regions of the combustion instability were identified on both sides of the combustion chamber through a Rayleigh index analysis. The longitudinal pressure oscillations in the oxidizer post were found to be coupled with the transverse pressure waves in the combustion chamber and led to periodic oscillations of the mass flow rate of propellant. Moreover, the mixing was highly enhanced when the pressure wave interacted with walls of the combustion chamber. Therefore, a sudden release of heat occurred. The pressure oscillations were enhanced by pulsated heat release. A closed-loop system with positive feedback associated with periodic oscillations mass flow rate of the propellant, and sudden heat release, was believed to account for the present combustion instability. [ABSTRACT FROM AUTHOR]

Self-excited combustion instabilities of transverse modes were experimentally investigated in a rectangular multi-injector model combustor, operating with the bipropellants O2/CH4. The propellants were injected through a linear array of five oxidizer-centered shear coaxial injectors into the combustor. High-amplitude limit cycles obtained in hot-fire tests were analyzed in detail. Different combustion instability modes, including first and second width modes, were observed in cases with three different injection distribution schemes. Hence, the injection distribution strongly determined the combustion dynamics. One insight can be gained that the stable combustion could be achieved by properly designing the propellants' injection distributions. [ABSTRACT FROM AUTHOR]

Rocket engines are prone to suffer from transverse combustion instabilities, manifesting as periodic oscillations in pressure and heat release rate. Self-excited transverse instabilities in a rectangle multi-injector rocket combustor powered by n-C 12 H 26 and O 2 , are investigated experimentally and numerically in this paper. Transverse combustion instabilities are captured during repeatable experiments, while dynamic characteristics in the combustion chamber and driving mechanisms of instabilities are analyzed via numerical simulation based on stress-blended eddy simulation. Dynamic mode decomposition analysis reveals coupling behaviors between the transverse mode in the combustion chamber and longitudinal mode in the oxidizer post, resulting in propellant mass flow rate oscillations. Substantial evidence demonstrates that the intensities of heat release rate near the end walls are higher than that detected around the chamber center due to its intense coupling to the local first transverse (1W) acoustic mode. Flow field animations indicate strong deflection of oxygen posts and liquid particles by a transverse moving gas mixture leading to flame interactions and collision between adjacent share injectors. The propellant injected into the chamber does not burn immediately and is transported to the combustor end wall. Propellant mixing performance is significantly enhanced during interactions between acoustic pressure and end wall, causing a sudden heat release near 1W pressure anti-node, thereby strengthening pressure oscillations. Finally, a combustion instability driving mechanism associated with propellant mass flow rate oscillations, and interactions between unsteady acoustic pressure and heat release rate is proposed. The insights obtained from this work can aid comprehension of transverse combustion instabilities encountered by rocket engines. • Both liquid fuel and gaseous oxygen are highly deflected by transverse moving gas mixture when transverse combustion instability appearing. • Large flame interactions and collision structures appear inside the combustion chamber. • Combustion instability is driven by pulsated heat release rate near the end walls and periodic supply of propellant mass flow rate. [ABSTRACT FROM AUTHOR]

Self-excited longitudinal combustion instabilities were analyzed in a sub-scale combustor operating with a propellant combination of O 2 /CH 4. Couplings between acoustics, hydrodynamics, and heat release were investigated numerically based on an experimental setup. The numerical results were in good agreement with the experimental data. Various post-processing methods have been adopted to explain the thermoacoustic behavior and reveal the underlying unstable combustion mechanisms. The classical longitudinal acoustic characteristics of the combustion chamber were identified by the pressure mode shape and dynamic mode decomposition (DMD) analysis. It was found that the incoming streams along the injector were periodically hindered by the traveling pressure wave originating from the chamber, thus, resulting in an oscillatory propellant supply and a pulsating heat release. This, in turn, feeds energy into the acoustic pressure oscillations, and maintains the combustion instability. Meanwhile, the coherent vortex structures in the reacting mixing-shear layer were found to be the principal reason for sustaining combustion instabilities. Besides, a positive feedback closed-loop system associated with periodic vortex shedding and merging is believed to sustain combustion instabilities. The plot of the Rayleigh index further confirmed that the shear layer region drives thermoacoustic instabilities. [ABSTRACT FROM AUTHOR]