Combustion Oscillations Research Articles

Reset Control of Combustion Oscillation Model

Combustion oscillations occur themselves as pressure and heat release oscillations in lean premixed combustors, and may cause some unexpected damage. This paper proposes a design method of a reset controller to eliminate a combustion oscillation in a lean premixed combustor model. We consider a plant model consisting of a linear system with a nonlinear feedback element and a reset controller using a first-order lag element, which is called a first-order reset element (FORE). We show parameter conditions of the reset controller for which limit cycles are eliminated, using a describing function of the nonlinear element. We apply the proposed design method of the reset controller and a flame describing function to eliminate the combustion oscillation in the lean premixed combustor model and show its effectiveness appropriately.

Effect of Helmholtz Resonator on Experiment of Mitigating Combustion Oscillation

Effect of Helmholtz Resonator on Experiment of Mitigating Combustion Oscillation

Examination of oscillation frequency for combustion oscillation considering acoustic boundary conditions

Examination of oscillation frequency for combustion oscillation considering acoustic boundary conditions

Detection of frequency-mode-shift during thermoacoustic combustion oscillations in a staged aircraft engine model combustor

We conduct an experimental study using time series analysis based on symbolic dynamics to detect a precursor of frequency-mode-shift during thermoacoustic combustion oscillations in a staged aircraft engine model combustor. With increasing amount of the main fuel, a significant shift in the dominant frequency-mode occurs in noisy periodic dynamics, leading to a notable increase in oscillation amplitudes. The sustainment of noisy periodic dynamics during thermoacoustic combustion oscillations is clearly shown by the multiscale complexity-entropy causality plane in terms of statistical complexity. A modified version of the permutation entropy allows us to detect a precursor of the frequency-mode-shift before the amplification of pressure fluctuations.

Convection characteristics in a closed vessel in the presence of exothermic combustion and ambient temperature oscillations

The influences of ambient temperature oscillations on the convection characteristics generated by an exothermic reaction in a closed vessel have been investigated. The left, right and top walls of the vessel are maintained at a constant temperature while the temperature of the bottom wall is assumed to be oscillating about a constant mean temperature. The governing equations are solved using finite difference method. A demarcation line between the explosion and the oscillatory convection is presented in terms of the Rayleigh and Frank-Kamenetskii numbers. It reveals that when the heat generation is large the convection of heat to the side walls must be higher to circumvent the thermal explosion. When the frequency of oscillation and the amplitude of oscillation of the ambient temperature are increased, the intensity of the stream function and the temperature is found to increase in a certain period of time. After that, the reverse characteristics are seen in the next period of time and these occur periodically. For a certain period of time, a cooler region is recognized near the bottom wall. Results also show that for higher values of the Frank-Kamenetskii number the oscillatory convection tends to chaotic and when it exceeds a certain value there occurs thermal explosion. Moreover, the system encounters a rapid explosion with the increase of Frank-Kamenetskii number and Rayleigh number.

Combustion Oscillation Characteristics and Flame Structures in a Lean Premixed Prevaporized Combustor

The combustion oscillation characteristics in the lean premixed prevaporized (LPP) combustor at different equivalence ratios and inlet velocities of the combustor were experimentally investigated. Acoustic modal analysis was carried out to identify the acoustic eigenmodes in the LPP combustor. The flame structures under different operating conditions were investigated. The evolution laws of flame structures, coordinates and velocities of the flame centers, and heat release rates throughout a combustion oscillation cycle were investigated by phase-locked OH planar laser-induced fluorescence (PLIF). The result shows that there is a periodical process of flame roll-up for the flame close to the LPP combustor axis and a periodical process of separation and consolidation for the flame far from the LPP combustor axis. The spatial distribution of the normalized Rayleigh index in the LPP combustor was calculated to determine the driving zones of the combustion oscillation. The mechanism of combustion oscillation ...

Chaotic and Linear Statistics Analysis in Thermoacoustic Instability Detection

Recent experiments reveal that even simple thermoacoustic systems may exhibit nonlinear behavior, far more elaborate than period-1 limit cycle oscillations. Consequently, a new approach based on nonlinear dynamics is emerging besides conventional linear analysis to characterize unstable regimes in gas turbine combustors. This approach reduces or avoids the risk of misunderstanding the deterministic chaos, hidden in the measured signals also during stable combustion regimes, as stochastic noise. The gained information will be available to analytically formulate an index acting as the earliest warning signal of an impending oscillatory combustion instability. In light of this, we first apply the chaotic analisys to an unsteady thermoacoustic time series coming from a typical industrial combustor operated in a stability-to-humming/back-to-stability transition. Then, observing that the Rayleigh and extended Chu indices describe the instability as a linear-interaction-induced synchronization between heat release and flow disturbances, to account for nonlinear interactions, the chaotic synchronization theory is applied. In particular, the interdependence index , a nonlinear causality detector, with no previous applications in combustion instability sensing, is proven to be more effective than linear analysis detectors, both in the early perception of self-sustained (chaotic or not) thermoacoustic oscillations and in gaining meaningful insight on unsteady combustor physics.

Oscillatory combustion of propene during in situ mechanical activation of solid catalysts

Mechanochemical activation of solids can lead to a strong increase in their activity as catalysts in heterogeneously catalyzed reactions. In the following, we report on the effects of solid catalyst activation during ball milling that lead to oscillatory behavior in CO and CO2 formation during propene oxidation. The oscillations arise under in situ ball milling conditions over chromium(III) oxide (Cr2O3) and cerium(IV) oxide (CeO2), respectively. The experiments were conducted under continuous gas flow at ambient pressure and temperature, using both a modified steel and a tungsten carbide milling vessel. Abrasion of particles from the steel milling vessel could be eliminated as the sole cause for the oscillations through substitution by a tungsten carbide milling vessel. The intensity and frequency of oscillations are shown to be dependent on the propene-to-oxygen ratio, the milling frequency, milling ball size and metal oxide used. Overall, Cr2O3 shows higher activity for oscillatory propene combustion under in situ mechanical activation than CeO2.

Vented Methane-air Explosion Overpressure Calculation—A simplified approach based on CFD

Abstract This paper presents new correlations developed through numerical simulations to estimate peak overpressures for vented methane-air explosions in cylindrical enclosures. A series of experimental tests are carried out first and the results are used to validate the numerical models developed with the commercial CFD software FLACS. More than 350 simulations consisting of 16 enclosure scales, 12 vent area to enclosure roof area ratios, 8 gas equivalence ratios and 9 vent activation pressures are then carried out to develop the Vented Methane-air Explosion Overpressure Calculation (VMEOC) correlations. Parameters associated with burning velocity and turbulence generation, oscillatory combustion and flame instabilities in vented gas explosion are taken into account in the development of new correlations. Comparing to CFD simulations, the VMEOC correlations provide a faster way to estimate the peak overpressure of a vented explosion. Additionally, it is proved in this study that the VMEOC correlations are easier to use and more accurate than the equations given in the up-to-date industrial standard- NFPA-68 2013 edition.

The influence of cavity parameters on the combustion oscillation in a single-side expansion scramjet combustor

Cavity has been validated to be efficient flameholders for scramjet combustors, but the influence of its parameters on the combustion oscillation in scramjet combustor has barely been studied. In the present work, a series of experiments focusing on this issue have been carried out. The influence of flameholding cavity position, its length to depth ratio L/D and aft wall angle θ and number on ethylene combustion oscillation characteristics in scramjet combustor has been researched. The obtained experimental results show that, as the premixing distance between ethylene injector and flameholding cavity varies, the ethylene combustion flame will take on two distinct forms, small-amplitude high frequency fluctuation, and large-amplitude low frequency oscillation. The dominant frequency of the large-amplitude combustion oscillation is in inverse proportion to the pre-mixing distance. Moreover, the influence of cavity length to depth ratio and the aft wall angleθexists diversity when the flameholding cavity position is different and can be recognized as unnoticeable compared to the impact of the premixing distance. In addition, we also find that, when the premixing distance is identical and sufficient, increasing the number of tandem flameholding cavities can change the dominant frequency of combustion oscillation hardly, let alone avoid the combustion oscillation. It is believed that the present investigation will provide a useful reference for the design of the scramjet combustor.

Analysis of operator splitting errors for near-limit flame simulations

High-fidelity simulations of ignition, extinction and oscillatory combustion processes are of practical interest in a broad range of combustion applications. Splitting schemes, widely employed in reactive flow simulations, could fail for stiff reaction–diffusion systems exhibiting near-limit flame phenomena. The present work first employs a model perfectly stirred reactor (PSR) problem with an Arrhenius reaction term and a linear mixing term to study the effects of splitting errors on the near-limit combustion phenomena. Analysis shows that the errors induced by decoupling of the fractional steps may result in unphysical extinction or ignition. The analysis is then extended to the prediction of ignition, extinction and oscillatory combustion in unsteady PSRs of various fuel/air mixtures with a 9-species detailed mechanism for hydrogen oxidation and an 88-species skeletal mechanism for n-heptane oxidation, together with a Jacobian-based analysis for the time scales. The tested schemes include the Strang splitting, the balanced splitting, and a newly developed semi-implicit midpoint method. Results show that the semi-implicit midpoint method can accurately reproduce the dynamics of the near-limit flame phenomena and it is second-order accurate over a wide range of time step size. For the extinction and ignition processes, both the balanced splitting and midpoint method can yield accurate predictions, whereas the Strang splitting can lead to significant shifts on the ignition/extinction processes or even unphysical results. With an enriched H radical source in the inflow stream, a delay of the ignition process and the deviation on the equilibrium temperature are observed for the Strang splitting. On the contrary, the midpoint method that solves reaction and diffusion together matches the fully implicit accurate solution. The balanced splitting predicts the temperature rise correctly but with an over-predicted peak. For the sustainable and decaying oscillatory combustion from cool flames, both the Strang splitting and the midpoint method can successfully capture the dynamic behavior, whereas the balanced splitting scheme results in significant errors.

Simulations of combustion oscillation and flame dynamics in a strut-based supersonic combustor

This paper numerically investigated the dynamic characteristics of combustion in a model scramjet. Three-dimensional compressible large eddy simulation was performed on a hydrogen fueled combustor and pressure fluctuations were recorded. The analysis of pressure data showed that the combustion processes are intrinsically unstable under supersonic air inflow conditions. Flame dynamics were convinced by the fluctuations in flame lift-off distance away from the strut base. Combined with the corresponding time interval, instantaneous flame speed was calculated. Results indicated that pressure oscillations at different locations show difference in amplitude, frequency, and the underlying control mechanism. Flame front oscillation analysis showed that the flame–shock interaction in the strut recirculation zone was responsible for the combustion instability. Flame dynamics were compared with low-speed turbulent lifted flames. The transition between flame propagation just after the strut and shock-induced combustion in the subsonic bubble at the intersection of two wall-reflected oblique shocks made for the flame stabilization.

Radial / Tangential Modes of Resonant Oscillatory Combustion in Tubular Flame Burners

Radial / Tangential Modes of Resonant Oscillatory Combustion in Tubular Flame Burners

ピントル型噴射器における燃焼振動挙動の光学計測

ピントル型噴射器における燃焼振動挙動の光学計測

Examination of oscillating frequency for combustion oscillation considering acoustic boundary conditions

Examination of oscillating frequency for combustion oscillation considering acoustic boundary conditions

Experimental Mitigation of Combustion Oscillation by Using Helmholtz Resonator

Experimental Mitigation of Combustion Oscillation by Using Helmholtz Resonator

ヘルムホルツ共鳴器を用いた燃焼振動の制振に関する実験的検討

ヘルムホルツ共鳴器を用いた燃焼振動の制振に関する実験的検討

Performance and Regression Rate Characteristics of 5-kN Swirling-Oxidizer-Flow-Type Hybrid Rocket Engine

Burning tests were carried out for the 5-kN-thrust swirling-oxidizer-flow-type hybrid rocket engine to establish the engine technology and to obtain the fuel regression characteristics. The engine attained stable burning without any combustion oscillation and efficiencies of more than 98% under the 5-kN-thrust conditions. The maximum time-averaged thrust realized was 4.4 kN, which was lower than 5 kN. This was attributed to the regression rate being lower than the value estimated by regression rate correlation. The fuel regression behavior of this engine was characterized as the flow-development region near the grain leading edge and the fully developed turbulent flow region. The axial length of the flow-development region varied from that observed in the laboratory-scale engine, and it depended on the oxidizer mass flow rate, grain initial port diameter, and swirl strength. The controlling parameter of the regression rate in the fully developed flow region was modified to correlate with the grain port diameter. By effecting these modifications, the accuracy of the regression rate prediction was improved. The range of the applicable controlling parameter of the modified regression rate correlations was extended for use for larger-scale engines.

Dynamical Properties of Combustion Instability in a Laboratory-Scale Gas-Turbine Model Combustor

We present an experimental study on the nonlinear dynamics of combustion instability in a lean premixed gas-turbine model combustor with a swirl-stabilized turbulent flame. Intermittent combustion oscillations switching irregularly back and forth between burst and pseudo-periodic oscillations exhibit the deterministic nature of chaos. This is clearly demonstrated by considering two nonlinear forecasting methods: an extended version (Gotoda et al., 2015, “Nonlinear Forecasting of the Generalized Kuramoto-Sivashinsky Equation,” Int. J. Bifurcation Chaos, 25, p. 1530015) of the Sugihara and May algorithm (Sugihara and May, 1990, “Nonlinear Forecasting as a Way of Distinguishing Chaos From Measurement Error in Time Series,” Nature, 344, pp. 734–741) as a local predictor, and a generalized radial basis function (GRBF) network as a global predictor (Gotoda et al., 2012, “Characterization of Complexities in Combustion Instability in a Lean Premixed Gas-Turbine Model Combustor,” Chaos, 22, p. 043128; Gotoda et al., 2016 (unpublished)). The former enables us to extract the short-term predictability and long-term unpredictability of chaos, while the latter can produce surrogate data to test for determinism by a free-running approach. The permutation entropy based on a symbolic sequence approach is estimated for the surrogate data to test for determinism and is also used as an online detector to prevent lean blowout.

Disturbance energy budget of turbulent swirling premixed flame in a cuboid combustor

In order to clarify combustion oscillation based on the exact disturbance energy budget, three-dimensional direct numerical simulations (DNS) of hydrogen–air turbulent swirling premixed flames are conducted in this study, considering a detailed kinetic mechanism and temperature dependencies of transport and thermal properties. Stoichiometric and lean conditions under two swirl numbers (S=0.6 and 1.2) are investigated. The exact disturbance energy budget is closed to more than 92% for all computational conditions. Investigation of source term contributions reveals that the swirl number affects the order of contribution levels of some sources, while the equivalence ratio influences the fluctuation amplitude of the time derivative value of disturbance energy. It is shown that source terms related to fluctuations of entropy and heat sources are major contributors to disturbance energy growth and decay, respectively. In addition to these major terms, source terms related to non-equilibrium combustion chemistry are also determined to be influential to the time evolution of disturbance energy. The relation between disturbance energy generation and flame characteristics is discussed, based on the time–averaged distributions of some of the abovementioned important sources.