Low-frequency Combustion Instability Research Articles

Reactive Flow Dynamics of Low-Frequency Instability in a Scramjet Combustor

This study numerically investigated the combustion instability and characteristics of a laboratory-scale gaseous hydrogen-fueled scramjet combustor. For this purpose, a numerical simulation with an improved detached eddy simulation and a detailed hydrogen/oxygen reaction mechanism was performed. The numerical framework used high-resolution schemes with high-order accuracy to ensure high resolution and fidelity. A total of five fuel injection pressures were considered to characterize the combustion instability as a function of the equivalence ratio. A sampling time of up to 100 ms was considered to sufficiently accumulate several cycles of low-frequency combustion instability dynamics with a period in the order of 100 Hz. Numerical results revealed the repetitive formation/dissipation dynamics of the upstream-traveling shock wave, and it acts as a key factor of combustion instability. The period and derived principal frequency of these upstream-traveling shock waves is several ms. The frequency analysis showed that the instability frequency increased in the low-frequency range as the combustion mode transitioned from the cavity shear-layer to the jet-wake type. This characteristic was derived from the transition in combustion mode at the same equivalence ratio. Therefore, it suggests that the instability frequency shifting is governed by the combustion mode rather than the equivalence ratio. These comprehensive numerical results demonstrated not only the effect of the equivalence ratio but also the important role of the combustion mode on the low-frequency combustion instability.

Performance Comparison of Paraffin/Ethanol Fuel Blends in a Laboratory-Scale Hybrid Rocket Motor

This paper discusses the performance characteristics of a paraffin-based blend of liquid ethanol with paraffin as compared to pure paraffin in a hybrid rocket motor. Since the disclosure of the high regression rates of liquefying fuels as compared to classic fuels such as hydroxyl-terminated polybutadiene (HTPB), many studies using paraffin have been reported in the literature. Although pure paraffin regresses three to four times faster than HTPB, it is not an ideal fuel for launcher applications for the following reasons: it does not provide the optimum mechanical strength, it may suffer from combustion instability, and it offers low combustion efficiency. The proposed blend is biphasic, with drops of liquid ethanol trapped in a paraffin binder; and a nonionic surfactant was employed to emulsify the ethanol into paraffin wax. The results indicated that at a mean prefiring of 0.6 and a of 60, both the P95E05 and P90E10 fuels demonstrated no significant statistical difference compared to pure paraffin in terms of thrust, specific impulse, fuel mass flow rate, characteristic velocity, and combustion efficiency. However, the P95E05 and P90E10 fuels did show damping in the pressure oscillations relative to paraffin, indicating a reduction in the low-frequency combustion instability observed in the ballistic responses of paraffin.

Effect of Counter- and Co-Swirl on Low-Frequency Combustion Instabilities of Jet A-1 Spray Flames

Abstract The present article experimentally investigates the influence of pilot swirling directions on low-frequency combustion instabilities of pilot diffusion flames in a laboratory-scale combustor with jet A-1 fuel and air at atmospheric pressure. Airblast atomization nozzles with either counter-rotating (CTR) or corotating (COR) pilot swirl flows were examined using nonlinear time-series analyses and high-speed flame imaging measurements under idle and subidle operating conditions. We show that while the amplitude and frequency of limit cycle oscillations are observed to be similar for both cases, detailed examinations of measured experimental data reveal marked differences in stabilization mechanisms and pressure-heat release coupling processes. The spray flame dynamics subjected to counter-rotating swirl flows are governed by large-amplitude pressure oscillations, even under the influence of destructive pressure-heat release rate interference. The mechanism of destructive interference is closely related to the interactions between a spiral diffusion flame and a periodically detached reaction zone. Nonpremixed liquid-fueled flames involving corotating swirl, on the other hand, feature a more compact and intense reaction zone.

Flame stabilization of liquid oxygen/kerosene bi-swirl injector at elevated pressure

Throttleable liquid-propellant rocket engines have wide range and economical applications. However, low frequency combustion instabilities were often excited in the oxygen-rich preburners of liquid oxygen (LOx) and kerosene rocket engines as the engines throttling. To understand the mechanisms and eliminate these instabilities, the detail features of the instabilities need to be described. Bi-swirl injectors with deep recess are used in the oxygen-rich preburners to spray propellants. This paper presents the non-premixed flame dynamic of a LOx/kerosene bi-swirl injector under high-pressure conditions. High-speed cameras captured OH* and CH* chemiluminescence images of the oxygen-rich flame for the first time. Results indicate that a stable flame was anchored inside the recess region and featured by a V-shaped, swirl flame. The flame-spreading angle under supercritical conditions was slightly larger than the liquid spray angle. Intense low-frequency combustion instabilities at 16–37 Hz were developed when the LOx/kerosene mixing ratio (ROF) was greater than threshold value. The flame oscillation process was coupled to the oscillations of the mass flow rate and the ROF. As the oxygen-rich level decreased, the flame oscillation pattern gradually shifted from flame blow-out/auto-ignition mode to flame length oscillation mode. Both modes were characterized by violent flashback and blow-off movement of the flame and large pressure variations. The instability was one type of “lean-limit” instability. A discussion of the oscillation periods qualitatively explains why the oscillation periods were found to decrease as the ROF decreases. The axial velocity of the kerosene is a controlling parameter of the oscillation period.

Influence of rotational asymmetry on thermoacoustic instabilities in a can-annular lean-premixed combustor

Low-frequency combustion instabilities, and ultimately, engine-level dynamics in can-annular gas turbine combustion systems, are driven by acoustic interactions between adjacent combustors through an annular cross-talk section upstream of first stage turbine nozzles. This is an area of substantial technological challenges, because experimental and numerical complexities scale exponentially with the number of combustors, restricting most investigations to a single combustor or a coupled two-combustor configuration. We recently reported that in a can-annular combustion system consisting of four identical lean-premixed combustors pressure oscillations occur in many combinations of characteristic patterns and frequencies, including symmetric (push–push) and asymmetric (push–pull) interactions, when the system is subjected to rotationally symmetric operating conditions. The impact of rotational asymmetry on can-annular thermoacoustics is still a largely open subject; here we use the same experimental setup to perform multiple independent analyses of the effect of broken rotational symmetry on large-scale pattern formations and modal dynamics of multiple eigenmodes. We demonstrate that the presence of rotational asymmetry can lead to fundamentally different dynamic states, including pairwise push-pull modes, spinning azimuthal instabilities, superposition modes, and strong mode localization, which are totally absent under rotationally symmetric conditions. Of particular importance is the experimental observation of spinning azimuthal instabilities in the annular cross-talk section, which are acoustically separate from flame-dynamics-driven standing wave motion in the flame tube sections. This leads to the coexistence of traveling and standing waves in the can-annular system. In conjunction with FEM-based Helmholtz simulations, we determine the origin of the azimuthal modes to be the simultaneous excitation of two different pairwise push-pull interaction modes at the same frequency – a phenomenon known as degenerate eigenmodes.

Nonlinear mode transition mechanisms of a self-excited Jet A-1 spray flame

Low-frequency combustion instabilities arise in an aero-engine gas turbine combustor under idle/sub-idle conditions, particularly when pilot-mode non-premixed combustion is sustained. Despite the fundamental importance of such instabilities, little is known about how they are initiated and grow in the system. Here, we present nonlinear mode transition processes using several analysis methods, including Fourier/Hilbert transforms, phase portraits, spectrograms, low-order analytic modeling, and high-speed flame visualization methods. Our results demonstrate that a non-premixed Jet A-1 spray flame yields an intermediate-amplitude, quasi-periodic L1 mode oscillation at 50 Hz for a low pilot equivalence ratio, and the frequency gradually increases with increasing fuel flowrates. Subsequent to a critical point (103 Hz), the system undergoes a discontinuous mode transition, giving rise to the formation of an extremely large pressure oscillation with an L2 mode structure at 244 Hz. Several key triggers were found to induce the mode shift: (i) generation of a high intensity pulse in the flame's heat release rate due to the spontaneous ignition of unburned reactant mixtures, (ii) emergence of a large-scale vortical structure and its interaction with a partially premixed flame front, and (iii) development of self-sustained limit cycle oscillations driven by periodic convection of a hot spot – a mechanism known as entropy wave propagation. Our study identifies the occurrence of a large-amplitude peak followed by a local minimum intensity, analogous to the activation energy concept, as an essential step for entry into a new state with large-amplitude limit cycle oscillations.

Oscillatory Pressure Effect on Mean Burning Rates of Solid Propellant Combustion at Low Frequency Conditions

AbstractThe investigation on acoustic pressure oscillation effect on the mean burning rate of solid propellant combustion under quasi and unsteady conditions has been carried out at elevated pressures typical of rocket operation. Experiments were conducted in the window bomb test facility to measure mean burning rate for the mean chamber pressure ranging from 1 to 7 MPa. This analysis facilitates current understanding regarding the mean burning rate variation due to the acoustic wave interaction with propellant flame complexes. The present work is focused on low frequency combustion instability and characterization of the long rocket motor configurations. The window bomb is connected with an exclusively designed rotating valve for the generation of acoustic pressure oscillations at selected mean chamber pressures and the corresponding frequency of operations. Two kinds of ammonium perchlorate/hydroxyl terminated poly butadiene (AP/HTPB) based composite propellants, one with aluminium, and the other without aluminium were considered. Experiments were conducted at the excited frequency of 140 Hz with different excited acoustic pressure amplitudes. This experimental setup facility has the provision of visual observation during propellant combustion using optical methods. The performance of the rotating valve was competent over the tested conditions. These studies reveal that the acoustic pressure amplitudes significantly enhance the mean burning rate of the solid propellants. The response of the aluminized propellants to the excited acoustic oscillations was relatively prominent as compared to that of its non‐aluminized counterparts. The results obtained are extremely useful in combustion instability characteristics of large scale rocket motors.

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

ABSTRACT Low-frequency combustion dynamics of a prefilming airblast injector were experimentally investigated in a laboratory-scale liquid-fuel combustor operated with Jet A-1 fuel and air at atmospheric pressure and elevated temperatures. Our measurements reveal that multiple modes – ranging from 45 to 292 Hz – can be excited in the non-premixed combustion system, including the Helmholtz, longitudinal, and hydrodynamic instabilities. The system's propensity for mode selection depends strongly on combustor length and pilot equivalence ratio. A strong Helmholtz mode with a peak-to-peak amplitude of ∼14 kPa occurs, provided that the combustor length is relatively short and the pilot equivalence ratio is high. A high-amplitude, intermittent burst at approximately 12 Hz always develops when the system is on the verge of transition to the Helmholtz instability. With a longer combustor length, on the other hand, the system transitions to the ¾-wave longitudinal mode via a discontinuous mode-hopping process. The system undergoes well-defined, limit cycle oscillations with an extremely large pressure amplitude of ∼30 kPa, but the global OH* fluctuation amplitude is limited to merely 8.3%. Phase-resolved flame imaging measurements demonstrate that a ring-like partially-premixed flame emerges periodically in the dump region, when the non-premixed flame wrapping around the central recirculation zone is lifted off the fuel nozzle. The mutual interaction between the partially-premixed and non-premixed reaction zones is the dominant mechanism for intense sound generation. Under certain conditions, superposition of the Helmholtz and longitudinal modes occurs, leading to nonlinear interactions manifested by additional spectral peaks at the sum and difference of the two frequencies. In contrast to the other two cases, this superimposed tone is not characterized by limit cycle behavior, but by a noise-driven unsteadiness.

Low-Frequency Combustion Instabilities Caused by Velocity Oscillations in Hybrid Rocket Motors

Low-Frequency Combustion Instabilities Caused by Velocity Oscillations in Hybrid Rocket Motors

Lean methane flame stability in a premixed generic swirl burner: Isothermal flow and atmospheric combustion characterization

Gas turbine combustors operating in lean premixed mode are known to be susceptible to flame blowoff due to competing influences of increasing chemical timescales and decreasing flow time scales under these conditions. In this study, combustion stability and the onset of flame blowoff in particular, are characterized in a new swirl burner operated with fully premixed methane (CH4) and air at thermal power of 55 kW, atmospheric combustor inlet pressure, and ambient (∼290 K) combustor inlet temperature. The onset of flame blowoff was shown repeatedly to exhibit high amplitude, low frequency combustion instabilities as a result of periodic flame extinction and reignition events. In addition to detailed isothermal characterization of the burner velocity field using particle image velocimetry, a combination of dynamic pressure sensing and optical combustion diagnostics, including OH∗ chemiluminescence and OH planar laser induced fluorescence, give indication of the combustion rig acoustic response and changes in flame acoustic response, heat release, and flame anchoring location related to the onset and occurrence of blowoff. This analysis shows that the onset of this instability was preceded by a marked reduction in dominant frequency and amplitude until frequency collapse and high amplitudes were observed throughout the burner inlet mixing plenum, burner pilot, combustion chamber, and exhaust ducting. Acoustic and optical signal analysis show potential viability for use in practical applications for precursor indications of lean blowoff. The flame anchoring location within the combustion chamber was shown to detach from the burner exit nozzle and stabilize within the outer and central recirculation zones near the lean blowoff limit, providing evidence of changes to both chemical and flow time scales. Chemical kinetic modelling is used in support of the empirical studies, in particular highlighting the relationship between maximum heat release rate and OH∗ chemiluminescence intensity.

A methodology to study the possible occurrence of chugging in liquid rocket engines during transient start-up

An investigation on the low frequency combustion instabilities due to the interaction of combustion chamber and feed line dynamics in a liquid rocket engine is carried out implementing a specific module in the system analysis software EcosimPro. The properties of the selected double time lag model are identified according to the two classical assumptions of constant and variable time lag. Module capabilities are evaluated on a literature experimental set up consisting of a combustion chamber decoupled from the upstream feed lines. The computed stability map results to be in good agreement with both experimental data and analytical models. Moreover, the first characteristic frequency of the engine is correctly predicted, giving confidence on the use of the module for the analysis of chugging instabilities. As an example of application, a study is carried out on the influence of the feed lines on the system stability, correctly capturing that the lines extend the stable regime of the combustion chamber and that the propellant domes play a key role in coupling the dynamics of combustion chamber and feed lines. A further example is presented to discuss on the role of pressure growth rate and of the combustion chamber properties on the possible occurrence of chug instability during engine start-up and on the conditions that lead to its damping or growth.

Effect of the fuel/air mixture concentration distribution on the dynamics of a low-emission combustor

An investigation of the low-emission premixed combustion in a conventional combustor is presented. The main problem encountered is the pressure fluctuations induced under certain operating conditions of the combustor. Low-emission operation of the combustor was studied numerically and experimentally. The effect of the concentration distribution at the outlet from the mixing zone on the position and macrostructure of the flame and the combustion stability was investigated at various excess air factors corresponding various GTU loads. It is demonstrated that, for a given excess air factor, there exists the concentration profile such that the interaction of the flame front with dominating flow structures results in excitation of the low-frequency combustion instability. The factors responsible for high-amplitude pressure fluctuations are examined. It is shown that the combustion stability can be estimated using a calculated criterion. Its direct relationship with pressure fluctuation amplitudes is described. The effect of the air pressure in a combustor on the flame macrostructure and the combustion stability was studied. It is shown that an increase in the combustor pressure has no considerable effect on the processes in the combustor. However, a change in the chemical reaction rates affects the stable combustion boundary. In this case, the combustion stability is achieved with higher nonuniformity of the fuel-air mixture entering the combustion zone. The experimental boundaries of stable combustion envelope at an air pressure of 350 and 1500 kPa are presented.

Effects of kerosene heating on dynamic characteristics of GOx/kerosene combustor

The objective of this study was to observe low-frequency instabilities caused by heating of kerosene under supercritical operating conditions. Gaseous oxygen and liquid kerosene were injected using a shear-coaxial injector. Under specific heating conditions, the fuel heating system induced an extremely low frequency pressure fluctuation ranging from 9.9 to 11.4Hz. When pressure oscillation occurred in the heating system, low-frequency combustion instability was subsequently induced in the range of 30–200Hz. To understand the effects of the fuel heating temperature on the combustion instability, the dynamic pressure and OH* chemiluminescence intensity were measured in a combustion chamber at high speed. Further, the reacting spray of the combustion was visualized by a shadowgraph technique. In this experiment, an approximate fuel pressure of 3.0MPa was employed in order to attain a supercritical condition of kerosene. The measured dynamic pressure and chemiluminescence intensity in the time domain were converted to frequency-domain spectra by fast Fourier transform. Analysis of the dynamic pressure and chemiluminescence intensity measurements confirmed that the low-frequency pressure oscillation in the heating system had an influence on the combustion instability. From the visualization data, it was also revealed that there existed varying amplitude levels of flow rate fluctuation. This fluctuation in turn caused a periodic injection of kerosene at a frequency similar to both the combustion instability frequency and the OH* chemiluminescence intensity frequency.

Mixed acoustic–entropy combustion instabilities in gas turbines

AbstractA combustion instability in a combustor terminated by a nozzle is analysed and modelled based on a low-order Helmholtz solver. A large eddy simulation (LES) of the corresponding turbulent, compressible and reacting flow is first performed and analysed based on dynamic mode decomposition (DMD). The mode with the highest amplitude shares the same frequency of oscillation as the experiment (approximately 320 Hz) and shows the presence of large entropy spots generated within the combustion chamber and convected down to the exit nozzle. The lowest purely acoustic mode being in the range 700–750 Hz, it is postulated that the instability observed around 320 Hz stems from a mixed entropy–acoustic mode, where the acoustic generation associated with entropy spots being convected throughout the choked nozzle plays a key role. The DMD analysis allows one to extract from the LES results a low-order model that confirms that the mechanism of the low-frequency combustion instability indeed involves both acoustic and convected entropy waves. The delayed entropy coupled boundary condition (DECBC) (Motheau, Selle & Nicoud, J. Sound Vib., vol. 333, 2014, pp. 246–262) is implemented into a numerical Helmholtz solver where the baseline flow is assumed at rest. When fed with appropriate transfer functions to model the entropy generation and convection from the flame to the exit, the Helmholtz/DECBC solver predicts the presence of an unstable mode around 320 Hz, in agreement with both LES and experiments.

Low-Frequency Combustion Instability Induced by the Combustion Time Lag of Liquid Oxidizer in Hybrid Rocket Motors

This paper deals with a theoretical analysis of the low-frequency combustion instability induced by the combustion time lag of liquid oxidizer in small-scale hybrid rocket motors. We obtained the determined linear stability limit using the following parameters: the combustion time delay of liquid oxidizer, the residence time of a combustion chamber, injector pressure, chamber pressure, mass flux exponent, O/F, and the polytropic exponent of mixture gas in a combustion chamber. Kitagawa and Yuasa sometimes observed low-frequency oscillations, such as chugging, in their swirling-oxidizer-flow-type hybrid rocket engine. The obtained theoretical stability limit was compared with these experimental data.

Lean-limit combustion instabilities of a lean premixed prevaporized gas turbine combustor

One type of low-frequency combustion instability was studied in a lean premixed prevaporized (LPP) gas turbine combustor experiment that was operated at elevated pressures using liquid Jet-A fuel. This intense low-frequency (15 Hz) instability is called a “lean-limit” instability, while in other papers it has been called a “cold tone”, an “incipient blowout” instability or an “entropy wave”. High-speed movies identified the physical mechanism. At first the flame enters the shear layer, is locally extinguished and lifts off. The extinguished region fills with unburned reactants. Finally the premixed flame flashes back into the region of unburned reactants, leading to a large pressure rise which triggers the next cycle. The observations are in general agreement with LES computations published by Huang and Yang. Measurements quantify the time lag between the flame luminosity buildup and subsequent pressure rise as well as the correlation between these two quantities and their spectra. The frequency of this “lean-limit” instability is found to be related to the flow velocity, flame velocity, and the combustor length, as is predicted for an entropy wave. A discussion is presented to qualitatively explain the measured frequencies, which increase as the fuel equivalence ratio is raised. The experiment includes the following realistic conditions: Jet-A liquid fuel, preheated air, elevated pressure and realistic sources of instabilities, including a commercial fuel injector/mixer that creates flame–flame interactions between a non-premixed Pilot and premixed Main flame that are necessary for aircraft applications.

Low-Order Modeling of Low-Frequency Combustion Instabilities in AeroEngines

Rumble is a combustion-induced instability occurring in aeroengines during start-up. Characteristically, low-limit-cycle frequencies between 50 and 150 Hz are obtained. Two basic feedback mechanisms are susceptible to promote rumble: entropy waves being reflected as pressure waves at the (nearly) choked combustor outlet and the purely thermoacoustic mode, originating from an in-phase oscillation of the heat release and the combustor pressure. Prior experiments on a generic rich-quench-lean (RQL) combustor have shown that the thermoacoustic mode determines the instability behavior over a wide range of operating conditions. A low-order model is developed for the staged RQL combustor to replicate theoretically the experimental findings and to investigate further the interaction of entropy waves and the purely thermoacoustic mode. The model accounts for dispersion and incorporates a simple flame transfer function, developed for spray combustion with negligible prevaporization. The model results confirm the experimental findings indicating a dominating thermoacoustic mode in the spectrum. The characteristically low-limit frequencies are a result of the characteristic timescales of droplet transport and combustion in the primary zone. It is further shown that the prevailing instability mode is strongly dependent on the dispersive properties of the primary zone and in the dilution sector of the combustor.

Transient analysis of hybrid rocket combustion by the zeldovich-novozhilov method

Hybrid rocket combustion has a manifestation of stable response to the perturbations compared to solid propellant combustion. Recently, it has revealed that the low frequency combustion instability about 10 Hz was occurred mainly due to thermal inertia of solid fuel. In this paper. the combustion response function was theoretically derived by use of ZN (Zeldovich-Novozhilov) method. The result with HTPB/LOX combination showed a quite good agreement in response function with previous works and could predict the low frequency oscillations with a peak around 10 Hz which was observed experimentally. Also, it was found that the amplification region in the frequency domain is independent of the regression rate exponentn but showed the dependence of activation energy. Moreover, the response function has shown that the hybrid combustion system was stable due to negative heat release of solid fuel for vaporization, even though the addition of energetic ingredients such as AP and Al could lead to increase heat release at the fuel surface.

The influence of the mache effect on combustion stability in a solid rocket motor

A model of intrachamber combustion that takes into account the change in the temperature distribution in the fuel-combustion products when the pressure in the combustion chamber changes (Mache effect) is proposed. The influence of the Mache effect on the combustion stability in a solid rocket motor is investigated. It is shown that the Mache effect facilitates the appearance of low-frequency combustion instability and substantially reduces the range of parameters for stable operation of the rocket motor. A numerical study of the Mache effect is performed for typical values of the solid-propellant parameters.

Low-frequency combustion instability mechanisms in a side-dump combustor

This article describes a study of a two-dimensional two-inlet side-dump combustor fed with a mixture of air and propane. The present results concern symmetric operating conditions with respect to the two inlets. Stable and unstable regimes which depend on the inlet velocity and the equivalence ratio have been identified. Schlieren visualization, radical imaging with an intensified CCD camera, and simultaneous pressure, inlet velocity and C 2 emission light measurements, have been used to characterize the combustor behavior. Imaging of the flowfield has provided an insight on the flame structure and its interaction with the entering jets. The geometry of the flowfield inside the combustion chamber with or without instability was symmetric with respect to the combustor centerline. For stable combustion, the flowfield was characterized by the presence of two zones of intense heat release located on both sides of the jet impingement region and were distributed along the combustor centerline. Two low-frequency unstable modes (a fuel-rich regime and a fuel-lean regime with an instability frequency around 500 Hz) were studied using a conditional imaging technique. These instabilities were characterized by the excitation of the quarter-wave mode of the combustor and were associated with a complex evolution of the jets and the flame. Jet oscillations were due to the kinematic superposition of the lateral entering jets and longitudinal velocity fluctuations generated by heat release oscillations in the dome region. It was found that unsteady heat release occurs in two different ways: pulsating combustion in the dome region and convection of reaction zones downstream of the jet-impingement region. Flame oscillations were induced by a periodic impingement of the jets on the centerplane of the chamber. Pressure fluctuations in the test section were roughly in phase with the global C 2 emission, indicating that the instabilities were sustained by energy addition to the acoustic field. A two-dimensional distribution of the Rayleigh index computed for each unstable mode indicated that the fuel-lean mode was driven by the unsteady heat release in the dome region whereas the fuel-rich mode was driven by the flame oscillations downstream of the jet-impingement region. The transition from the fuel-lean to the fuel-rich instability featured a shift of driving mechanism. This study shows that even in our idealized geometry the coupling mechanisms leading to low-frequency combustion instabilities are not unique and illustrates the difficulty of devising predictive models.