Investigation of Feed System Coupled Low Frequency Combustion Instabilities in Hybrid Rockets

The paper discusses the possible interaction between combustion instabilities and induction times of droplets (and sprays) to autoignition. It is shown that acoustic pressure/temperature oscillations significantly affect the induction times of n-heptane droplets. This may play an additional role in low frequency dynamics and might be the main driver of high frequency dynamics. Experiments on single droplets in an acoustic field were used to validate numerical simulations on the autoignition of large n-heptane droplets. The simulations were then extended towards technical droplet sizes and a gas turbine typical pressure range of 17 bar. It was found that the acoustic-scale changes of the pressure and temperature result in significant changes of the ignition delay. Applying numerical calculations to micro-sized droplets enabled to study the thermo-acoustic effects under conditions approximating real gas-turbines. The findings reveal the importance of thermo-acoustic effects on ignition processes in the instability-driving mechanisms of combustion and indicate that “acoustics-ignition”-interactions must be taken into account for low-frequency as well as for high-frequency dynamics; this in addition to the flow and mixture perturbations which are well known to drive combustion instabilities in gas-turbines.

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Combining LOFAR and Apertif Data for Understanding the Life Cycle of Radio Galaxies

Galaxies ◽

10.3390/galaxies9040088 ◽

2021 ◽

Vol 9 (4) ◽

pp. 88

Author(s):

Raffaella Morganti ◽

Nika Jurlin ◽

Tom Oosterloo ◽

Marisa Brienza ◽

Emanuela Orrú ◽

...

Keyword(s):

Active Galactic Nuclei ◽

Phased Array ◽

Low Frequency ◽

Galactic Nuclei ◽

Radio Galaxies ◽

Host Galaxy ◽

Evolutionary Stage ◽

Spectral Indices ◽

Feed System ◽

Lockman Hole

Active galactic nuclei (AGN) at the centres of galaxies can cycle between periods of activity and of quiescence. Characterising the duty-cycle of AGN is crucial for understanding their impact on the evolution of the host galaxy. For radio AGN, their evolutionary stage can be identified from a combination of morphological and spectral properties. We summarise the results we have obtained in the last few years by studying radio galaxies in various crucial phases of their lives, such as remnant and restarted sources. We used morphological information derived from LOw Frequency ARray (LOFAR) images at 150 MHz, combined with resolved spectral indices maps, obtained using recently released images at 1400 MHz from the APERture Tile In Focus (Apertif) phased-array feed system installed on the Westerbork Synthesis Radio Telescope. Our study, limited so far to the Lockman Hole region, has identified radio galaxies in the dying and restarted phases. We found large varieties in their properties, relevant for understanding their evolutionary stage. We started by quantifying their occurrences, the duration of the ‘on’ (active) and ‘off’ (dying) phase, and we compared the results with models of the evolution of radio galaxies. In addition to these extreme phases, the resolved spectral index images can also reveal interesting secrets about the evolution of apparently normal radio galaxies. The spectral information can be connected with, and used to improve, the Fanaroff–Riley classification, and we present one example of this, illustrating what the combination of the LOFAR and Apertif surveys now allow us to do routinely.

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Effect of Port Gas Injection on the Combustion Instabilities in a Spark-Ignition Lean-Burn Natural Gas Engine

International Journal of Bifurcation and Chaos ◽

10.1142/s0218127418501249 ◽

2018 ◽

Vol 28 (10) ◽

pp. 1850124

Author(s):

Li-Yuan Wang ◽

Li-Ping Yang ◽

En-Zhe Song ◽

Chong Yao ◽

Xiu-Zhen Ma

Keyword(s):

Natural Gas ◽

High Frequency ◽

Gas Injection ◽

Low Frequency ◽

Combustion System ◽

Combustion Instabilities ◽

Lean Burn ◽

Engine Load ◽

Gas Engine ◽

Natural Gas Engine

The combustion instabilities in a lean-burn natural gas engine have been studied. Using statistical analysis, phase-space reconstruction, and wavelet transforms, the effect of port gas injection on the dynamics of the indicated mean effective pressure (IMEP) fluctuations have been examined at a speed of 800[Formula: see text]rpm and engine load rates of 25% and 50%. The excessive air coefficient is 1.6 for each engine load, and the port gas injection timing (PGIT) ranges from 1 to 120 degrees of crankshaft angle ([Formula: see text]CA) after top dead center (ATDC) of the intake process. The results show that the PGIT has a significant effect on cyclic combustion fluctuations and the dynamics of the combustion system for all studied engine loads. An unreasonable PGIT leads to increased combustion fluctuations, and loosened and bifurcated structures of combustion system attractors. Furthermore, for both low and medium engine loads, the IMEP time series at earlier gas injections ([Formula: see text]CA and [Formula: see text]CA ATDC) undergoes low-frequency fluctuation together with high-frequency fluctuations in an intermittent fashion. For other PGITs, high-frequency intermittent fluctuations become persistent combined with weak low-frequency oscillations. Our results can be used to understand the oscillation characteristics and the complex dynamics of combustion system in a lean-burn natural gas engine. In addition, they may also be beneficial to the development of more sophisticated engine control strategies.

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Low-Frequency Feed-System-Coupled Combustion Instability in Hybrid Rocket Motors

Journal of Thermal Science and Technology ◽

10.1299/jtst.8.380 ◽

2013 ◽

Vol 8 (2) ◽

pp. 380-394 ◽

Cited By ~ 1

Author(s):

Takakazu MORITA ◽

Saburo YUASA ◽

Koki KITAGAWA ◽

Toru SHIMADA ◽

Shigeru YAMAGUCHI

Keyword(s):

Combustion Instability ◽

Low Frequency ◽

Hybrid Rocket ◽

Feed System ◽

Rocket Motors

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Low-frequency combustion instabilities of an airblast swirl injector in a liquid-fuel combustor

Combustion and Flame ◽

10.1016/j.combustflame.2018.06.031 ◽

2018 ◽

Vol 196 ◽

pp. 424-438 ◽

Cited By ~ 12

Author(s):

Byeonguk Ahn ◽

Jeongjae Lee ◽

Seungchai Jung ◽

Kyu Tae Kim

Keyword(s):

Liquid Fuel ◽

Low Frequency ◽

Combustion Instabilities ◽

Swirl Injector

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Role of Precombustion Chamber Design in Feed-System Coupled Instabilities of Hybrid Rockets

Journal of Propulsion and Power ◽

10.2514/1.b37706 ◽

2020 ◽

Vol 36 (6) ◽

pp. 796-805

Author(s):

Jungpyo Lee ◽

Artur Elias De Morais Bertoldi ◽

Artem Andrianov ◽

Renato Alves Borges ◽

Carlos Alberto Gurgel Veras ◽

...

Keyword(s):

Feed System ◽

Precombustion Chamber ◽

Chamber Design ◽

Hybrid Rockets

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Vortex-driven acoustically coupled combustion instabilities

Journal of Fluid Mechanics ◽

10.1017/s0022112087000958 ◽

1987 ◽

Vol 177 ◽

pp. 265-292 ◽

Cited By ~ 379

Author(s):

Thierry J. Poinsot ◽

Arnaud C. Trouve ◽

Denis P. Veynante ◽

Sebastien M. Candel ◽

Emile J. Esposito

Keyword(s):

Heat Release ◽

Vortex Shedding ◽

Combustion Instability ◽

Low Frequency ◽

Combustion Instabilities ◽

Rayleigh Criterion ◽

Phase Average ◽

Steady Heat

Combustion instability is investigated in the case of a multiple inlet combustor with dump. It is shown that low-frequency instabilities are acoustically coupled and occur at the eigenfrequencies of the system. Using spark-schlieren and a special phase-average imaging of the C2-radical emission, the fluid-mechanical processes involved in a vortex-driven mode of instability are investigated. The phase-average images provide maps of the local non-steady heat release. From the data collected on the combustor the processes of vortex shedding, growth, interactions and burning are described. The phases between the pressure, velocity and heat-release fluctuations are determined. The implications of the global Rayleigh criterion are verified and a mechanism for low-frequency vortex-driven instabilities is proposed.

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