• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.5
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• pp.79-86
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• 2005

The role of the injector as an acoustic resonator is studied for the high performance rocket engine adopting the gas-liquid scheme injector. Acoustic behavior in the combustor with single injector is investigated numerically adopting linear acoustic analysis for cold condition. Acoustic-damping effect of the injector is evaluated by damping factor as a function of the injector length. From the numerical results, it is found that the injector can play a significant role in acoustic damping and the optimum length of the injector corresponds to half of a full wavelength of the longitudinal mode with the acoustic frequency to be damped in the chamber. In baffled chamber, the optimum lengths of the injector are calculated as a function of baffle length for both cold and hot conditions.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.8
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• pp.1302-1307
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• 2001

This study identifies the major coupling path by examining the variation of the coupled acoustic modal frequencies and modes. A 1/2-size acryl compartment model is designed and manufactured for the measurement and analysis of coupled acoustic modes. Due to acoustic coupling, a new acoustic mode appears in a low frequency range. This coupling affects only longitudinal acoustic modes of compartment. Experimental result shows that the package tray holes contribute to the coupling much more than the back seat and hole size is an important design factor to control low frequency acoustic modes in the coupled system.

• Journal of the Korean Society of Propulsion Engineers
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• v.15 no.4
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• pp.48-56
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• 2011

A Possibility of combustion instability on longitudinal mode has a high level at large scale of L/D. Solid propellant has a metal particle and a grain of control to pressure oscillation. Solid rocket motor in slotted-tube grain controls pressure oscillation of longitudinal mode. Slotted-tube grain restrains longitudinal 1st pressure oscillation. But cavity volume of aft. insulation ablation amplifies 2nd pressure o scillation by vortext shedding. A study has suppressed combustion instability and vortex shedding by modified 4 slotted tube solid rocket motor design.

• Journal of the Korean Society of Combustion
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• v.21 no.2
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• pp.29-37
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• 2016

This study described the experimental investigations of combustion instability in a model gas turbine combustor. Strong coupling between pressure oscillations and unsteady heat release excites a self-sustained acoustic wave, which results in a loud and annoyed sound, and may also lead to a structural damage to the combustion system. In this study, in order to examine the combustion instability phenomenon of a dual swirling combustor configuration, the information of heat release and pressure fluctuation period with respect to the variation in both thermal power and combustor length was collected experimentally. As a result, the fundamental acoustic frequency turned out to increase with the increasing thermal power without respect to the combustor length. The frequency response to the combustor length was found to have two distinct regimes. In a higher power regime the frequency significantly decreases with the combustor length, as it is expected from the resonance of gas column. However, in a lower power regime it is almost insensitive to the combustor length. This insensitive response might be a result of the beating phenomenon between the interacting pilot and main flames with different periods.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.11a
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• pp.187-199
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• 2011

The main objective of this study was investigation of natural gas flames in a lean premixed swirl-stabilized dump combustor with an attention focused on the effect of the various fuel-air mixing section geometry on the combustion instability characteristics. The multi-channel dynamic pressure transducers were located on the combustor and inlet mixing section region to observe combustion pressure oscillation and difference phase at each dynamic pressure measurement results. Dynamic pressures were also measured to investigate characteristics of combustion at the same time. The combustor and mixing section length was varied in order to have different acoustic resonance characteristics from 800 to 1800 mm in combustor and 470, 550, 870 mm in mixing section. We observed two dominant instability frequencies in this study. Lower frequencies were obtained at lower equivalence ratio region and it was associated with a fundamental longitudinal mode of combustor length. Higher frequencies were observed in higher equivalence ratio conditions. It was related to secondary longitudinal mode of coupled with the combustor and mixing section. In this instability characteristics, pressure oscillation of mixing section part was larger than pressure oscillation of combustor. As a result, combustion instability was strongly affected by acoustic characteristics of combustor and mixing section geometry.

• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.4
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• pp.57-69
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• 2012

The main objective of this study was investigation of natural gas flames in a lean premixed swirl-stabilized dump combustor with an attention focused on the effect of the various fuel-air mixing section geometry on the combustion instability characteristics. The combustor and mixing section length was varied in order to have different acoustic resonance characteristics from 800 to 1800 mm in combustor and 470, 550, 870 mm in mixing section. We observed two dominant instability frequencies in this study. Lower frequencies were associated with a fundamental longitudinal mode of combustor length. Higher frequencies were related to secondary longitudinal mode of coupled with the combustor and mixing section. As a result, combustion instability was strongly affected by acoustic characteristics of combustor and mixing section geometry.

• Journal of Mechanical Science and Technology
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• v.18 no.7
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• pp.1246-1257
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• 2004

The objective of this paper is to characterize dynamic pressure traces measured at self-excited combustion instabilities occurring in two combustion systems of different hardware. One system is a model lean premixed gas turbine combustor and the other a fullscale bipropellant liquid rocket thrust chamber. It is commonly observed in both systems that low frequency waves at around 300㎐ are first excited at the onset of combustion instabilities and after a short duration, the instability mode becomes coupled to the resonant acoustic modes of the combustion chamber, the first longitudinal mode for the lean premixed combustor and the first tangential mode for the rocket thrust chamber. Low frequency waves seem to get excited at first since flame shows the higher heat release response on the lower frequency perturbations with the smaller phase differences between heat release and pressure fluctuations. Nonlinear time series analysis of pressure traces reveals that even stable combustion might have chaotic behavior with the positive maximum Lyapunov exponent. Also, pressure fluctuations under combustion instabilities reach a limit cycle or quasi-periodic oscillations at the very similar run conditions, which manifest that a self-excited high frequency instability has strong nonlinear characteristics.

• 한국연소학회:학술대회논문집
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• 2015.12a
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• pp.107-108
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• 2015

The acoustic optimization of a swirl coaxial jet injector mounted upstream a combustion chamber is investigated to tackle combustion instabilities. The least damped modes are extracted with the help of the dynamic mode decomposition (DMD). The sensitivity of the heat release perturbation to the velocity perturbation for the second longitudinal mode is investigated by combining the Crocco's equation and the inhomogeneous wave equation and computing the flame transfer function (FTF). DMD and FTF results agree in terms of the optimized injector length.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.714-717
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• 2017

Among the factors considered in the design stage of a space launch vehicle using liquid propellant, research has been focused out on the pogo phenomenon, longitudinal dynamic instability. The pogo phenomenon refers to the instability that the longitudinal vibration of the launch vehicle structure causes a change in the pressure and flow rate of the fluids in propulsion system, and this change re-excites the fuselage structure. This mechanism constitutes a closed system to gradually increase the vibration of the launch vehicle. This paper specifically focuses on the dynamic analysis of pressure and flow changes in the propulsion system. Based on the example study of the space shuttle, the acoustic modal analysis of the propulsion system is performed to predict the modes of the supply line causing instability of the fuselage.

• Journal of the Korean Society of Propulsion Engineers
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• v.24 no.4
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• pp.25-32
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• 2020

A numerical study on combustion instabilities in a model combustor was conducted with various momentum flux ratios. Five ratios are calculated based on an actual operating condition of rocket engine. As momentum flux ratio increases, the spreading angle on the injector outlet decreases. And, as increase of axial momentum flux, pressure fluctuation decreases inside the combustor. By using dynamic mode decomposition method, the acoustic modes inside the combustor are identified. Combustion stabilities are analyzed by comparing the damping coefficient of the 2^nd longitudinal mode.