Thrust chamber acoustics is a critical aspect of rocket engine design, as acoustic oscillations can significantly impact the combustion process and potentially lead to catastrophic failures. Researchers like Dr. Justin Hardi and his team at the DLR Institute of Space Propulsion have been pushing the boundaries of understanding thermoacoustic processes in liquid rocket engines for over one and a half decades.
Interaction between Acoustics and Combustion
One key aspect of thrust chamber acoustics is the interaction between acoustics and combustion. Acoustic oscillations can drive the combustion process, leading to high amplitudes that increase heat transfer to the inner surfaces of the combustion chamber. This can cause the engine to burn from the inside out, resulting in disaster. Researchers use experimental research combustors to study these interactions, observing the response of flames to acoustic disturbances under controlled conditions that mimic real rocket engines, with pressures up to 80 atmospheres and temperatures up to 3600 K.
Acoustic Modes and Flame Dynamics
The team’s findings include the elucidation of mechanisms responsible for combustion instabilities, the influence of injection conditions on acoustic modes in the combustion chamber, and the coupling of combustion chamber and injector acoustics with flame dynamics. These findings have a significant impact, allowing the motion and size of the flame to be predicted for a wide range of excitation amplitudes.
| Acoustic Mode | Frequency Range |
|---|---|
| Longitudinal | 1000 – 5000 Hz |
| Transverse | 5000 – 20000 Hz |
| Tangential | 10000 – 40000 Hz |
The interaction between these acoustic modes and the flame dynamics can lead to combustion instabilities, which can be further exacerbated by the coupling between the combustion chamber and the injector acoustics.
Combustion Instability
Combustion instability is a high-priority problem in rocket engine construction. High-frequency (HF) combustion instabilities can lead to massive damage to the combustion chamber and surrounding components. The Aestus engines faced this issue during their later stages of development and testing, contributing to the rise of the Franco-German research programme Rocket Engine Stability iniTiative (REST). Dr. Hardi and his team, participating in the REST programme, investigate thermoacoustic phenomena in liquid propellant rocket engines, focusing on visualising combustion instabilities under realistic conditions.
Quantifying Combustion-Acoustic Instabilities
In terms of quantifiable data, the review of combustion-acoustic instabilities provides valuable information on admittances, unsteady heat release rates, and boundary conditions.
Admittance
The admittances of a combustion chamber can be measured in terms of the pressure drop across the chamber, the volume flow rate through the chamber, and the acoustic velocity in the chamber. The admittance is a critical parameter in predicting the stability of the combustion process.
Unsteady Heat Release Rate
The unsteady heat release rate can be determined using perturbations in the fuel delivery system or by analyzing hydrodynamic processes in ramjet dump combustors. This parameter is crucial in understanding the coupling between acoustics and combustion.
Boundary Conditions
Boundary conditions, such as the pressure and temperature at the combustion chamber inlet, can also be measured and analyzed to predict and quantify combustion-acoustic stabilities accurately. These boundary conditions play a significant role in the overall stability of the thrust chamber.
Conclusion
Thrust chamber acoustics is a complex and critical field in rocket engine design, with researchers like Dr. Justin Hardi and his team at the DLR Institute of Space Propulsion making significant contributions to the understanding of thermoacoustic processes in liquid rocket engines. By studying the interaction between acoustics and combustion, the influence of injection conditions on acoustic modes, and the coupling of combustion chamber and injector acoustics with flame dynamics, these researchers have provided valuable insights that can help mitigate the risks of combustion instability and ensure the reliable performance of rocket engines.
References:
– Interaction of Acoustics and Combustion in Rocket Engines
– Combustion Instabilities in Liquid Rocket Engines
– Combustion Instability in Liquid Rocket Engines
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