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This work presents a state-of-the-art review of micro-combustion flame dynamics and micro propulsion systems. In the initial section, we focus in on the different challenges of micro-combustion, investigating the typical length and time scales involved in micro-combustion and some critical phenomena such as flammability limits and the quenching diameter.We present an extensive collection of studies on the principal types of micro-flame dynamics, including flashback, blow-off, steady versus non-steady flames, mild combustion, stable flames, flames with repetitive extinction, and ignition and pulsatory flame burst. In the final part of this review, we focus on micropropulsion systems, their performance metrics, conventional manufacturing methods, and the advancements in Micro-Electro-Mechanical Systems manufacturing. [ABSTRACT FROM AUTHOR]

To further study the gas-coal dust explosion hazards under complex conditions. Experiments were conducted in pipe-line systems containing explosive pipe-line and pressure relief pipe-line. The effect of coal dust concentration and particle size on the flame dynamics of gas-coal dust explosion was investigated. The flame structure in the explosion pipe-line is divided into two-stages: spherical and round-finger. In the pressure relief pipe-line, the flame structure e varies at working conditions. Flame front position increases with time, showing a class exponential growth trend. The flame front velocity increases in the explosion pipe-line, has a short drop during the entry into the pressure relief pipe-line, and then increases. With the increase of coal dust concentration, flame propagation time has been increasing in the pipe-line system and the explosion pipe-line, with the shortest time at 25 g/m³. Flame propagation time first increases and then decreases in the pressure relief pipe-line, the shortest time at 50 g/m³. The maximum flame front velocity first increases and then decreases. The maximum flame front velocity is maximized at 50 g/m³. With the increase of coal dust particle size, flame propagation time has been increasing in the pipe-line system, and in the explosion pipe-line, flame propagation times have been decreasing in the pressure relief pipe-line. The maximum flame front velocity first increases, then decrease, and increases again. The maximum flame front velocity is minimal at 45 µm. [ABSTRACT FROM AUTHOR]

In this study, the combustion instability characteristics of gaseous methane–gaseous oxygen atmospheric flame were investigated. A swirl coaxial injector was used under external modulation with a central methane flow and a loudspeaker. The flame transfer function with a photomultiplier tube was examined for possible errors, and an injector transfer function was defined to study the influence of the input perturbation on the cold gas flow without combustion. Both the results of flame transfer function and injector transfer function were affected by acoustic resonance. However, a higher mode of resonance did not appear in the flame transfer function. To analyze this difference, the flame movement was visualized using a high-speed camera by capturing the OH* chemiluminescence signal. Furthermore, a local flame transfer function was defined to examine the flame response zone-by-zone through a light signal. A higher-mode reaction appeared again in the local flame transfer function. It was found that flame separation due to modulation was the cause of the difference among the flame transfer function, injector transfer function, and local flame transfer function. By capturing the entire flame, its periodicity could be lost; hence, the local flame transfer function could prevent misinterpretation of the flame fluctuation. • A difference between dynamic response of cold gas flow and hot firing test is found. • In the flame transfer function results, the higher mode of resonance disappeared. • Using divided flame signal image, the response at high frequency is found again. • Separated flame clusters emit flame transfer function signals individually. • With interaction of each cluster signals, periodicity of flame could be missed. [ABSTRACT FROM AUTHOR]

This paper presents experimental investigations of the polyurethane foam influence on the combustion dynamics of hydrogen-air flames propagating in a channel with a sudden change in cross-section (i.e. expansion). The channel is open at both ends. Porous media of various lengths and pore size are considered. The porous inserts are placed downstream of the sudden expansion, inside the diagnostic section of dimensions 20 × 40 mm. A Schlieren visualization technique is used to monitor flame shape and propagation dynamics. Various equivalence ratios ranging from 0.3 to 1.0 are tested. The results show that depending on the equivalence ratio, porous length and pore size, the mixture can either propagate throughout the foam or be quenched. In propagating regime, it is found that the output velocity just behind the foam increases linearly with porous matrix length, indicating that the tortuous flow within the foam plays a significant role in the propagation of the flame. These results could be used both to increase the efficiency of gaseous combustion and to ensure the explosion safety of the gas equipment. [ABSTRACT FROM AUTHOR]

Thickened flame models are prolific in the literature and offer an effective method of resolving flame dynamics on coarse LES meshes. The current state of the art relies heavily on the use of efficiency functions to compensate for impaired wrinkling of the thickened flame. However in practice these functions can involve parameters that are difficult to determine, perform poorly outside of certain ranges or require a posteriori analysis to evaluate performance. An alternative based on a generalised thickening is evaluated across a range of canonical configurations. The approach is demonstrated to perform well across a large range of thickening factors in capturing phenomena such as localised quenching and pinch off as well as generation of flame surface. Including good performance even in the case of large flame dynamics under acoustic forcing where the model has a clear advantage over DNS in achieving grid independence. Finally the approach is unified into an Large Eddy Simulation/Adaptive Mesh Refinement framework and applied to a turbulent Bunsen flame. The results show that even if the internal flame structure is poorly resolved on the original mesh, the global system behaviour is well predicted and compares favourably with other approaches. [ABSTRACT FROM AUTHOR]

This work presents a state-of-the-art review of micro-combustion flame dynamics and micro propulsion systems. In the initial section, we focus in on the different challenges of micro-combustion, investigating the typical length and time scales involved in micro-combustion and some critical phenomena such as flammability limits and the quenching diameter.We present an extensive collection of studies on the principal types of micro-flame dynamics, including flashback, blow-off, steady versus non-steady flames, mild combustion, stable flames, flames with repetitive extinction, and ignition and pulsatory flame burst. In the final part of this review, we focus on micropropulsion systems, their performance metrics, conventional manufacturing methods, and the advancements in Micro-Electro-Mechanical Systems manufacturing.

This paper studies the effects of the inner and outer pilot swirlers on the flame structures and thermo-acoustic instabilities in the triple-swirler staged combustors under various inner and outer pilot swirl numbers. We obtain energetic modes in the combustor's velocity and the OH radical mass fraction fields, utilizing the proper orthogonal decomposition (POD) technique. With the increment of the pilot inner swirl number, the vortex shedding mode and flame radial propagation mode for the flame fluctuation is enhanced, and the axial propagation mode is weakened. Nevertheless, the increased pilot outer swirl number primarily impacts the vortex shedding mode's enhancement while positively influencing the other flame fluctuation modes. Specifically, the vortex shedding in the inner shear layer is the leading factor affecting the thermo-acoustic instability in the combustion chamber. Nevertheless, coupling this mode and the pressure oscillation weakens the area downstream of the central recirculation zone. Thus, due to the comprehensive impacts of the vortex shedding mode, spatial distribution, and the premixed flame area proportion, the impact of this mode's energy ratio on the thermo-acoustic instability increases non-linearly with the increased pilot swirl number.

Combustion instability constitutes the primary loss source of combustion chambers, gas turbines, and aero engines, and it affects combustion performance or results in a sudden local oscillation. Therefore, this study investigated the factors affecting flame fluctuation on unsteady combustion flow fields through large-eddy simulations. The effects of primary and secondary holes in a triple swirler staged combustor on flame propagation and pressure fluctuation in a combustion field were studied. Moreover, the energy oscillations and dominant frequencies in the combustion field were obtained using the power spectral density technique. The results revealed a variation in the vortex structure and Kelvin–Helmholtz instability in the combustion field, along with a variation in the pressure pulsation during flame propagation under the influence of the primary and secondary hole structures. Additionally, the spatial distributions of pressure oscillation and heat release rate amplitude were obtained, revealing that the foregoing increased owing to the primary and secondary holes in the combustion field, reaching a peak in the shear layer and vortex structure regions. [ABSTRACT FROM AUTHOR]

The impinging jet flame is studied experimentally and numerically accounting for the complex flame-wall interactions in practical combustion devices. Flame dynamics and heat transfer with the effect of impinging wall are analyzed. The 3-D large eddy simulation coupled with detailed chemical reaction mechanism and particle image velocimetry experiment based on cross-correlation measurement principle are performed for verification and further analysis. Results show that vortices are generated due to the Kelvin-Helmholtz instability originated from velocity gradient. The 3-D vortex interactions involving vortex rings and spirals are also indicated by vorticity and the convection of streamwise vorticity is responsible for the effect of vortex spirals associated with turbulent flow transition. In addition, results calculated from four wall thermal conditions are compared and analyzed. Dirichlet condition is inferred to be more suitable for the case of wall materials with higher thermal conductivity. It is indicated that wall thermal condition mainly affects the heat transfer in the near-wall region, but has little effect on the momentum transfer. This study provides references for the adoption of wall conditions in numerical simulation and near-wall treatment in combustion systems. [ABSTRACT FROM AUTHOR]

The use of flame spread models as a tool in fire safety engineering practice is subject to uncertainties arising from the assumptions inherent in the models and the input parameters required to implement them. Research on flame spread continues to add complexity to existing models, which places a greater onus on suitable input parameters to provide the knowledge needed to implement them. This approach is appropriate when experimental datasets are available for the validation of models for research purposes. However, it can be detrimental for design applications when there is limited knowledge about the underlying parameters for model verification in a broad range of design applications. Therefore, increasing complexity in flame spread models has the potential to reduce the strength of any such decisions made. In this article we provide a comprehensive review of the complexity of different flame spread models. This review classifies model approaches based on their inherent assumptions and sub-models to describe the complexity of the problem. It shows that, while complex modelling approaches may be necessary to adequately represent complex phenomena such as flame spread, the impact of uncertainties associated with the additional inputs is not always quantified or justified. [ABSTRACT FROM AUTHOR]

Micro-flow reactors and porous media burners are prone to instability in the form of flame with repetitive extinction and ignition (FREI) near the region of a temperature gradient. Although significant progress has been made in this field, there is still a lack of understanding regarding the nature of such stable-to-unstable transition. Some studies reported a smooth and continuous instability transition in microchannels, interpreting this phenomenon as a supercritical bifurcation. Meanwhile, others have observed that the transition is subcritical, with rapid, stepwise evolution of the stable flame to FREI. This study aims to rigorously determine the transition character theoretically and numerically using a two-dimensional model with a precise resolution of the bifurcation parameter (flow velocity) near the critical point and gradual wall temperature ramp. The results indicate that the transition is a subcritical bifurcation with strong hysteresis. However, the amplitude of the limit cycle born in the bifurcation point could be small compared to FREI-like oscillations. Moreover, further development of the instability depends significantly on the channel width. In relatively wide channels, the stable regime transforms immediately into the extinction/ignition one with rapid growth in amplitude. In moderate channels, complex transitional dynamics were observed with an evident pulsating regime, which evolved into FREI through mixed-mode oscillations. In narrow channels, the amplitude of transitional pulsating flame becomes comparable to the fully-developed extinction/ignition cycles, and the bifurcation diagram has a continuous character with no significant jumps or discontinuities. Such qualitative variation of transition character provides a possible explanation for contradictory interpretations presented in the literature. Novelty and Significance Statement Currently, there is no consensus about the character of transition between the stable and extinction/ignition regimes in microchannels. Some studies have reported a continuous instability transition with an evident pulsating regime, interpreting it as a supercritical bifurcation, while others have observed subcritical bifurcation with a rapid transition. The novelty of this research lies in the rigorous determination of the bifurcation type and further instability development in microchannels of different widths based on an accurate simulation of flame dynamics near the critical point with very small steps of the flow velocity variation, along with the bifurcation theory. The study contributes to understanding the instability transition nature and may explain the different interpretations of the transitional phenomenon found in the literature. The results offer valuable insights for designing micro-scale and porous media combustion devices. [ABSTRACT FROM AUTHOR]

Owing to the promotion of DME (dimethyl ether) liquefaction transportation technology and in-station hydrogen production technology, the explosion risk of DME/H2 blended gas caused by unintended or uncontrollable leakage has become the focus of attention. To assess the explosion risk, the explosion dynamics and chain mechanism of DME/H2 blended gas at typical fuel-lean/rich concentrations were investigated with experiments and numerical simulations. The results show that the maximum explosion overpressure Pmax increases and then decreases with increasing equivalence ratio for Φ = 0.6–2.0, and Pmax is maximal for Φ = 1.0 (the maximum increase is 20.3%). In the zone away 2.85 m from the ignition point, the effect of fuel-rich concentrations on flame temperature lift and explosion risk enhancement gradually emerges, which is because that the fuel concentration here is diluted to near stoichiometric concentration. The fuel-rich concentration caused a secondary flame acceleration and further flame propagation in the outfield zone, and the flame propagation distance is increased by 22.6% compared to the fuel-lean concentration. The high temperature caused by the incorporation of hydrogen makes the pyrolysis reaction CH3OCH3(+M)CH3O + CH3(+M) promotes the explosion most effectively. The sensitivity coefficients of most elementary reactions reach maximum at the stoichiometric concentration, thereby promoting or inhibiting explosions most effectively. At Φ = 1.0–1.1, the equilibrium molar fractions of H and OH radicals are maximal, the chemical reactivity gets highest, and the explosion risk is greatest.

This paper presents an experimental study on dynamic response of a forced low-swirl methane/air premixed flame with external acoustic excitation over a wide range of driving frequency. Global flame response in terms of gain and phase delay between flame intensity and incoming velocity perturbation is determined. Local flame response is investigated in detail at three typical frequencies: 55 Hz, 105 Hz and 155 Hz. The effect of swirl number on the flame response is also discussed. Proper orthogonal decomposition is applied to identify the large coherent structures in the forced flame. Experimental results show flame response gain exhibits a successive of valleys and peaks which is dependent on swirl number. Time delay decreases as swirl number is increased. The low-swirl flame oscillates back and forth mainly in the axial direction at low excitation frequency and it turns into radially dominated direction at high frequency. Flame intensity fluctuation is mainly dominated by the tail of the flame at 55 Hz and 155 Hz while the flame response is controlled by a combined effect of the base and tail region at 105 Hz. Further POD analysis shows symmetric, anti-symmetric and helical modes in the flame. The most energetic modes (mode 1 and mode 2) feature a symmetric wave-like structure at low excitation frequency while it tends to be in antisymmetric modes at high frequency. [ABSTRACT FROM AUTHOR]

Combustion instability constitutes the primary loss source of combustion chambers, gas turbines, and aero engines, and it affects combustion performance or results in a sudden local oscillation. Therefore, this study investigated the factors affecting flame fluctuation on unsteady combustion flow fields through large-eddy simulations. The effects of primary and secondary holes in a triple swirler staged combustor on flame propagation and pressure fluctuation in a combustion field were studied. Moreover, the energy oscillations and dominant frequencies in the combustion field were obtained using the power spectral density technique. The results revealed a variation in the vortex structure and Kelvin–Helmholtz instability in the combustion field, along with a variation in the pressure pulsation during flame propagation under the influence of the primary and secondary hole structures. Additionally, the spatial distributions of pressure oscillation and heat release rate amplitude were obtained, revealing that the foregoing increased owing to the primary and secondary holes in the combustion field, reaching a peak in the shear layer and vortex structure regions.

The replacement of hydrocarbon fuels by ammonia in industrial systems is challenging due to its low burning velocity, its narrow flammability range, and a large production of nitric oxide and nitrogen dioxide when burned close to stoichiometric conditions. Cracking a fraction of ammonia into hydrogen and nitrogen prior to injection in the combustion chamber is considered a promising strategy to overcome these issues. This paper focuses on evaluating how different levels of ammonia cracking affect the overall burning velocity, the lean blow-off limit, the concentration of nitric oxide and nitrogen dioxide, and the flame response to acoustic perturbations. Swirl stabilized premixed flames of pure ammonia–air and ammonia–hydrogen–nitrogen–air mixtures mimicking 10%, 20%, and 28% of cracking are experimentally investigated. The results show that even though ammonia cracking is beneficial for enhancing the lean blow-off limit and the overall burning velocity, its impact on pollutant emissions and flame stability is detrimental for a percentage of cracking as low as 20%. Based on an analysis of the flame dynamics, reasons for these results are proposed.

The present work introduces an annular combustion chamber operated at intermediate pressures. The combustor is operated with CH 4 -H 2 blends leading to a variety of azimuthal combustion instabilities. The influence of the hydrogen content, the air mass flow rate and the equivalence ratio on the instabilities is investigated over a wide range of operating conditions with mean chamber pressures from 1.5 to 3.3 bar. This leads to a range of exit boundary conditions, from partially to fully reflecting. It is found that pure methane and methane-hydrogen mixtures with low hydrogen contents result in stable combustion. However, when the hydrogen content reaches 25% by volume high-amplitude instabilities are excited, which exhibit higher order harmonics with significant pressure amplitude contributions. Such harmonic response was not previously observed in atmospheric annular combustors. The amplitudes decrease slightly when the H 2 content is increased further. The harmonic response is found to be amplitude dependent with fewer significant harmonic contributions occurring at low-amplitudes and a cut-on amplitude of the fundamental mode at which higher harmonics become significant. The interaction between the harmonic components of the pressure amplitudes is shown to follow a quadratic relationship. The modal response was analyzed and it was found that all high-amplitude instabilities feature clockwise spinning modes whereas lower-amplitude instabilities feature counter clockwise spinning modes. Finally, a low- and high-amplitude case were investigated in detail and phase-averaged images are discussed. The low-amplitude instabilities result in flame dynamics similar to those observed in atmospheric combustors previously whereas the high-amplitude instabilities exhibit large oscillations in the flame height and intensity. A characterization of the boundary conditions is also provided for numerical simulations which includes temperature measurements, acoustic characterization and cold flow velocity profiles. [ABSTRACT FROM AUTHOR]

• Impacts of H 2 -enrichment and pilot flames on CH4/H2/air flames in a swirl burner are investigated. • Simultaneous PIV and OH-PLIF measurements are performed to characterize the flame and flow structures. • H 2 -enrichment and pilot flames increase flame front wrinkling and enhance flame stability. • Close-to-LBO, H 2 /CH 4 flames exhibit significant local extinction. • Close-to-LBO, the reaction zones are in the inner recirculation zone (IRZ). This work investigates the impact of pilot flame and fuel composition on the structures and stabilization of swirling turbulent premixed methane/hydrogen/air flames in a lab-scale gas turbine model combustor. Simultaneous measurements of the velocity field and OH radicals distribution in the combustor were conducted using particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) methods, respectively. Flames under stable and close to lean blow-off (LBO) conditions were studied for two fuel mixtures, with a hydrogen mole ratio of 0 and 50 % in the hydrogen/methane mixture, respectively. The studied flames were at a constant Reynolds number of 20,000 with different equivalence ratios. Two pilot-to-global fuel ratios were investigated (2 % and 6 %) while keeping the pilot-to-global air ratio constant at 2 %. Data for non-piloted flames were also acquired for comparison. The pilot flames were shown to extend the operability range. The LBO equivalence ratio of the main flame decreased with increasing fuel mass flow rate in the pilot flames due to the increased amount of hot gases with high concentrations of OH radicals in the outer recirculation zone (ORZ), which significantly enhanced the stabilization of the main flame. The stable flame reaction zone was in the high-speed shear layer between the ORZ and the inner recirculation zone (IRZ). When approaching LBO, the reaction zone was pushed downstream to the IRZ and subsequently decreased the size of IRZ, indicating a strong flow/flame interaction. Hydrogen enrichment was shown to reduce the LBO equivalence ratio. When close to LBO, the OH radicals in the hydrogen-enriched flames were observed in isolated pockets due to differential diffusion, which enhanced resilience to LBO. The flame front curvature, mean progress variable, and flame surface density were calculated from the acquired OH-PLIF data to quantify the impact of fuel composition and pilot flames on the flame structures. [ABSTRACT FROM AUTHOR]

Convective velocity perturbations (CVPs) are known to play an important role in the response of flames to acoustic perturbations and in thermoacoustic combustion instabilities. In order to elucidate the flow-physical origin of CVPs, the present study models the response of laminar premixed slit flames to low amplitude perturbations of the upstream flow velocity with a reduced order flow decomposition approach: A linearized G-equation represents the shape and heat release rate of the perturbed flame, while the velocity perturbation field is decomposed into irrotational and solenoidal contributions. The former are determined with a conformal mapping from geometry and boundary conditions, whereas the latter are governed by flame front curvature and flow expansion across the flame, which generates baroclinic vorticity. High-resolution CFD analysis provides values of model parameters and confirms the plausibility of model results. This flow decomposition approach makes it possible to explicitly evaluate and analyze the respective contributions of irrotational and solenoidal flows to the flame response, and conversely the effect of flame perturbations on the flow. The use of the popular ad hoc hypothesis of convected velocity perturbation is avoided. It is found that convected velocity perturbations do not result from immediate acoustic-to-hydrodynamic mode conversion, but are generated by flame-flow feedback. In this sense, models for flame dynamics that rely on ad-hoc models for CVPs do not respect causality. Furthermore, analysis of the flame impulse response reveals that for the configuration investigated, flame-flow feedback is also responsible for “excess gain” of the flame response, that is, the magnitude of the flame frequency response above unity.

Front propagation speeds from fully resolved unsteady one dimensional simulations with dimethyl-ether (DME)/methane (CH 4)/air mixtures under engine relevant conditions are presented using complex kinetics and transport. Different time-scales of monochromatic inhomogeneities in DME concentration with varying DME/CH 4 blending ratios are simulated to unravel the fundamental aspects of auto-ignition and flame propagation under the influence of reactivity stratification. To understand the influence of different stratification time-scales on the flame-ignition interaction, two sets of conditions are simulated such that low temperature chemistry is present in only one of them. For a given amplitude of stratification, it is found that the instantaneous propagation speed is significantly affected by the level of CH 4 concentration in the binary fuel blend. Specifically, for cases with low temperature chemistry, at relatively smaller time-scales, the overall fluctuation in the instantaneous propagation speed is found to subside as the level of CH 4 concentration in the mixture is increased. However, for both sets of conditions, at comparatively larger time-scales, a rapid change in the instantaneous propagation speed is observed with an increase in the level of CH 4 concentration in the mixture. The intrinsic effects of stratification time-scales on the low temperature chemistry and the high temperature chemistry are further examined to assess the flame-ignition interaction. A displacement speed analysis is also carried out to elucidate the underlying combustion modes that are responsible for such a variation in flame response. [ABSTRACT FROM AUTHOR]

Thermal runaway propagation (TRP) of lithium-ion batteries (LIBs) is one of the important causes of safety accidents in new energy vehicles. In order to investigate the theoretical system of TRP of Li(Ni 0.8 Co 0.1 Mn 0.1)O 2 (NCM811) cell/module under dual heat sources, an experimental platform of coupled thermal abuse stimulation was built to study the TRP and combustion behavior of NCM811 module in different states-of-charges (SOCs). The results show that the time to reach thermal runaway (TR) of LIB under dual heat sources is shortened by 952.3 s on average, and the possibility of TRP of the cell module is greatly enhanced. With the increase of SOC from 25% to 100%, the TR trigger time of the cell is shortened from 2773 s to 2311 s, the TR trigger temperature is reduced from 263.7 °C to 175.7 °C. The TRP of the cell module under dual heat sources induction can occur at different SOCs, and the TRP duration increases from 57 s at 25% SOC to 174 s at 100% SOC. In addition, the cell eruption and combustion behaviors induced by dual heat sources become more intense with the increase of SOC, the cell mass loss rate also increases rapidly with the increase of eruption and combustion behaviors. • Dual heat sources increase the risk of thermal runaway propagation • The mechanism of thermal runaway spread of lithium battery module is proposed • The heat transfer model of dual heat sources thermal runaway module is proposed • The thermal runaway eruption behavior of lithium battery is revealed • The effect of SOC on the thermal runaway behavior of lithium battery is revealed [ABSTRACT FROM AUTHOR]

Abstract In many combustion devices, strong self-excited flow oscillations can arise from feedback between unsteady heat release and acoustics, resulting in increased vibration and pollutant emissions. Open-loop acoustic forcing has been shown to be effective in weakening such thermoacoustic oscillations, but current implementations of this control strategy require the forcing to be continuously applied. In this proof-of-concept study, we experimentally demonstrate an alternative method of weakening thermoacoustic oscillations in a self-excited combustion system – a laminar premixed flame in a double open-ended tube. Unlike existing methods, the proposed method combines the use of transient forcing with hysteresis and mode switching, thus avoiding the need to continuously supply energy to the control system. Control is achieved by exploiting the fact that most combustors have a multitude of natural thermoacoustic modes, some of which are linearly unstable but some are nonlinearly unstable. By applying open-loop acoustic forcing at an off-resonance frequency and at an amplitude higher than that required for synchronization, we find that the combustor can switch to one of the nonlinearly unstable natural modes (f 2) and remain there, even after the forcing is removed. Dynamic mode decomposition of high-speed chemiluminescence videos shows that this mode switching occurs because the flame structure at f 2 is more robust than that at the original linearly unstable natural mode. The final unforced state has a thermoacoustic amplitude of just half that of the initial unforced state, even though the Rayleigh index of the former is higher than that of the latter. Although this 50% reduction in thermoacoustic amplitude is not as large as the 95% reduction achieved with asynchronous quenching, it is achieved without the use of continuous forcing. This is a distinct advantage over existing control strategies as it allows the complexity and power requirements of the control system to be reduced. With further development and testing, particularly on turbulent swirling combustors, the proposed control strategy could pave the way for a new class of open-loop control techniques based on transient forcing rather than continuous forcing. [ABSTRACT FROM AUTHOR]

Turbulent combustion models for Large Eddy Simulation (LES) aims at predicting the flame dynamics. So far, they have been proven to predict correctly the mean flow and flame properties in a wide range of configurations. A way to challenge these models in unsteady situations is to test their ability to recover turbulent flames submitted to harmonic flow modulations. In this study, the Flame Transfer Function (FTF) of a CH4/H2/air premixed swirled-stabilized flame submitted to harmonic flowrate modulations in a non-adiabatic combustor is compared to the response computed using the Filtered TAbulated Chemistry for LES (F-TACLES) formalism. Phase averaged analysis of the perturbed flow field and flame response reveal that the velocity field determined with Particle Image Velocimetry measurements, the heat release distribution inferred from OH* images and the probability of presence of burnt gases deduced from OH-Planar Laser Induced Fluorescence measurements are qualitatively well reproduced by the simulations. However, noticeable differences between experiments and simulations are also observed in a narrow frequency range. A detailed close-up view of the flow field highlight differences in experimental OH* and numerical volumetric heat release rate distributions which are at the origin of the differences observed between the numerical and experimental FTF. These differences mainly originate from the outer shear layer of the swirling jet where a residual reaction layer takes place in the simulations which is absent in the experiments. Consequences for turbulent combustion modeling are suggested by examining the evolution of the perturbed flame brush envelope along the downstream distance of the perturbed flames. It is shown that changing the grid resolution and the flame subgrid scale wrinkling factor in these regions does not alter the numerical results. It is finally concluded that the combined effects of strain rate and enthalpy defect due to heat losses are the main factors leading to small but sizable differences of the flame response to coherent structures synchronized by the acoustic forcing in the outer shear layer of the swirling flow. These small differences in flame response lead in turn to a misprediction of the FTF at specific forcing frequencies. [ABSTRACT FROM AUTHOR]

Abstract In the current study, the effect of external thermal pattern on the dynamics and characteristics of methane-oxygen and methane-oxygen-carbon dioxide premixed flames in non-adiabatic meso-scale cylindrical reactors is investigated experimentally. In this regard, two different external thermal patterns were imposed on the outer surface of the reactors. The results showed that imposing method/direction and also temperature level of the external thermal pattern have impressive effect on flame dynamics and chemiluminescence in the non-adiabatic meso-scale reactors. Also, it was shown that increasing the temperature level of the external thermal pattern could significantly extend the flame stability and its presence range in the meso-scale reactors, especially for the vitiated mixtures (methane-oxygen-carbon dioxide). Moreover, the results demonstrated that decreasing the inner diameter of a meso-scale reactor, which was subjected to an external thermal pattern, could increase the flame controllability and its presence range in the non-adiabatic meso-scale reactors. Highlights • Imposing external heat source affects the flame dynamics in small scale reactors. • External heat source and its imposing direction affect asymmetric flame shape. • Flame dynamics may be accompanied by variation in the flame chemiluminescence. • Effect of external heat source on stability of the vitiated flames is more obvious. • Decreasing the diameter of a heated meso reactor extends flame controllability. [ABSTRACT FROM AUTHOR]

Abstract The response of a premixed laminar flame, stabilised in a narrow channel (ℓ i = 5 mm), is investigated for an amount of heat supplied upstream through the wall. The first objective is to determine the set of canonical problems that a reduced chemistry must accurately reproduce to simulate the flame/wall coupling under such circumstances. The second objective is to identify the major energy transport mechanisms at play, i.e. convective heat transfer in the gas or conductive in the solid wall. The stoichiometric methane-air flame numerical simulations performed include complex molecular transport and the fully coupled solving of heat transfer at and within the wall. Initially, the flame is stabilised by an inlet mass flow rate matching the flame burning velocity. A reference simulation is first performed with a comprehensive methane-air chemistry. Then, two chemistry reduction strategies targeting a different set of canonical problems are applied and the results compared to conclude on the necessary constraints for chemistry reduction under such flow configuration. Various heating intensities are considered and a minimum heating supply is found necessary to initiate a complete upstream flame translation. A specific attention is paid on the relative contributions of heat convection in the flow and heat conduction in the solid. The heat transfer mechanism triggering the flame movement is revealed to be mainly convective. The flame translation is found to be organised in two stages, with first a downstream movement due to fresh gases expansion because of heating by the wall, followed by the upstream propagation due to the enhancement of the burning rate by the preheating of the mixture. The shape, the flash back speed, and the final position of the flame vary with the amplitude of the heat flux brought to the external surface of the wall. [ABSTRACT FROM AUTHOR]

This study mainly investigates the flame and flow characteristics under combustion instability in a centrally-staged swirl spray combustor, using 5 kHz CH* chemiluminescence (CL) and 15 Hz particle image velocimetry (PIV). The inlet air total temperature and total pressure are up to 600 K and 1 MPa, respectively. A dynamic pressure sensor with an acquisition frequency of 20 kHz is located downstream of the combustor casing. The pressure signal shows large-amplitude limit cycle oscillations when the fuel split ratio of the pilot stage decrease. The phase-averaged CH* CL images indicate that the periodic ignition/extinction of the main stage flame triggers combustion instability. A proper orthogonal decomposition (POD) analysis based on the reacting flow field further explains this phenomenon. The first two POD modes containing three coherent vortex pairs are associated with the extinction and ignition of the main stage flame, respectively. The vortex pairs located at the central shear layer (CSL) generated by Kelvin-Helmholtz (K-H) instability affect the primary recirculation zone (PRZ). The change in the size of the PRZ influences the main stage flame dynamics. In a cycle of oscillation, ignition happens when the size of PRZ increases and the coupling between the main and pilot stages enhances. Extinction takes place when the opposite happens. The flow-flame interaction mechanism demonstrated in this study provides strong experimental support for understanding combustion instability in aero-engine combustors. [ABSTRACT FROM AUTHOR]

This paper reports on the effect of pressure on the response of methane–air and propane–air swirl flames to acoustic excitation of the flow. These effects are analyzed on the basis of the flame transfer function (FTF) formalism, experimentally determined from velocity and global OH* chemiluminescence measurements at pressures up to 5 bar. In parallel, phase-locked images of OH* chemiluminescence are collected and analyzed in order to determine the associated flame dynamics. Flame transfer functions and visual flame dynamics at atmospheric pressure are found to be similar to previous studies with comparable experimental conditions. Regardless of pressure, propane flames exhibit a much larger FTF gain than methane flames. For both fuels, the effect of pressure primarily is to modify the gain response at the local maximum of the FTF, at a Strouhal number around 0.5 (176 Hz). For methane flames, this gain maximum increases monotonically with pressure, while for propane flames it increases from 1 to 3 bar and decreases from 3 to 5 bar. At this frequency and regardless of pressure, the flame motion is driven by flame vortex roll-up, suggesting that pressure affects the FTF by modifying the interaction of the flame with the vortex detached from the injector rim during a forcing period. The complex heat transfer, fluid dynamics, and combustion coupling in this configuration does not allow keeping the vortex properties constant when pressure is increased. However, the different trends of the FTF gain observed for methane and propane fuels with increasing pressure imply that intrinsic flame properties and fuel chemistry, and their variation with pressure, play an important role in controlling the response of these flames to acoustic forcing. [ABSTRACT FROM AUTHOR]

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 (CH 4 ) 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. [ABSTRACT FROM AUTHOR]

The combustion process in the scramjet engines generally takes place in unsteady conditions. The unsteadiness of the flame can be present locally or globally in supersonic combustion, for instance, reactant mixing, ignition, flame stabilization, and blow-off. Thus, unsteady supersonic combustion is an urgent problem that needs to be solved for real scramjet engine applications. This work helps to advance the knowledge of dual-mode scramjet combustion operating with different equivalence ratios by conducting quantitative analyses that characterize flame dynamics under flow separation. A hydrogen-fueled direct-injection scheme was computed using blastFoam code via large-eddy simulation of a scramjet test rig at a stagnation temperature of 950 K and a pressure of 0.82 MPa. The computational results were in good agreement with the experimental data, and two typical flame structures (cavity shear-layer and cavity assisted jet-wake stabilized flame) corresponding to the scram/ram operating modes were reproduced. The local flame structure and flow regime were investigated using a modified flame index and filter functions. A particular focus was set on the continuous flame flashback during mode transition, which could be divided into three stages involving jet/shear-layer interaction, unsteady heat release, and the establishment of a large-area separation zone, with the decay of the cavity recirculation zone. Additionally, the unsteady jet-wake ram combustion presented three oscillation modes, giving rise to intermittent flameout of varying levels. The coherent-structures motion caused by the combined effects of low-speed separated flow and the fuel-jet was the main cause for this kind of unsteady combustion. [ABSTRACT FROM AUTHOR]

In the present work, the discrete flame model developed by Shoshin et al. (Doklady Akademii Nauk SSSR, 1986) and further scrutinized by Tang et al. (Combustion Theory and Modelling, 2009) is augmented by introducing the thermal inertia of particles in the preheating zone. The effect of particle thermal inertia on the speeds, propagation limits, and near-limits dynamics of one-dimensional discrete combustion waves is studied using the new model. It is found that, with the increase of particle thermal inertia, the propagation velocity of the discrete flame decreases due to the smaller heating rate of the particles. Besides, particle thermal inertia extends the propagation limits compared to the prediction of the old model. Furthermore, it is mathematically proven that the nonphysical branch of the solutions for the discrete flame speeds, found using the old discrete model, is a set of solutions for the propagation limits of steady-state discrete flames with particle thermal inertia included. The flame speed predicted using the new model is also compared with that determined analytically using a continuum model considering the thermal inertia of the condensed phase, attributed to Goroshin et al. (Combustion and Flame, 1996). We find that the discrete flame speeds predicted by the both models become closer to each other with increasing particle thermal inertia. Finally, the two models converge regardless of the discrete nature of the heat sources when particle thermal inertia is large enough so that can limit the flame propagation. The particle thermal inertia controlled flames could be regarded as a new kind of combustion regime. [ABSTRACT FROM AUTHOR]

The flames inside porous burners with low filtration velocity are prone to oscillatory behavior similar to unstable combustion in narrow channels but accompanied by complicated dynamics due to the irregularity of porous structure, flow field, and thermal gradient formed as a result of heat recuperation. This work is a numerical study within the framework of the pore-scale simulation approach devoted to investigating the flame oscillation mechanisms and characteristics in a two-dimensional pseudo-irregular packed bed of particles generated using the Voronoi tessellation algorithm with constant pore size. The results demonstrate that unstable flame oscillations like flames with repetitive extinction and ignition (FREI) lead to the velocity pulsation in adjacent pore channels, where the stable flame fragments are exposed to the hydrodynamically-driven fluctuations. In the case of the flow rate close to the stability threshold, the flow pulses can reach a magnitude enough to shift the stable flame in the upstream quenching zone, promoting its oscillations with repetitive extinction and ignition. Results of parametric simulation with a variation of the particle diameter and flow velocity were summarized in the regime diagram revealing the transition between stable normal flame, FREI, and stable weak flame with the velocity reduction. The pulsating transitional regime was detected between the stable normal flame and the FREI. The amplitude and intensity of such pulsations increase monotonically with the flow rate reduction in a narrow range. Oscillations of the unstable flame and hydrodynamically-driven flame pulsations are similar in dynamics but different in their physical mechanisms. However, in irregularly packed beds of particles, both these phenomena unavoidably coexist and interfere significantly with each other due to the connectivity of the pore channels. [ABSTRACT FROM AUTHOR]

This paper analyzes the dynamics of harmonically forced, non-premixed flames, both experimentally and computationally. Prior theory has made a number of predictions on how wrinkles on non-premixed flamelets are excited by flow disturbances, convect axially, and evolve in disturbance magnitude, including setting up interference patterns in wrinkle disturbance magnitude. The objective of this study was to obtain measurements from forced flames to determine if these features are present, and to compare the gain/phase of these wrinkles with predictions using the measured velocity field as inputs. High speed PIV data was taken on a coflowing methane–air diffusion flame, equipped with speakers for harmonic forcing, over a variety of flow velocities, forcing frequencies, and forcing amplitudes. These measured velocity fields were used as inputs to a Z -equation solver, and the resulting space-time dynamics of iso- Z surfaces were extracted from the Z field solutions. Both experimental and numerical results show that flame wrinkles propagate axially at the mean flow velocity, a result consistent with previous analytical findings. These wrinkles start with near zero magnitude at the fuel tube lip and grow with downstream distance, until peaking at some axial location. Further downstream, the wrinkle magnitude modulates, indicative of interference effects which have been previously predicted in analytical studies. [ABSTRACT FROM AUTHOR]

This study reports the combustion oscillation features in a three-dimensional (3D) rocket-based combined-cycle (RBCC) engine combustor under flight Mach number ( M flight ) 3.0 conditions both experimentally and numerically. Experiment is performed on a direct-connect ground test facility, which measures the wall pressure along the flow-path. High-speed imaging of the flame luminosity and schlieren is carried out at exit of the primary rocket. Compressible reactive large eddy simulation (LES) with reduced chemical kinetics of a surrogate model for kerosene is performed to further understand the combustion oscillation mechanisms in the combustor. LES results are validated with experimental data by the time-averaged and root mean square (RMS) pressure values, and show acceptable agreement. Effects of the primary rocket jet on pressure oscillation in the combustor are analyzed. Relation of the high speed rocket jet oscillation, which is thought to among the most probable sources of combustion oscillation, with the RBCC combustor is recognized. Results reveal that the unsteady over-expanded rocket jet has significant impacts on the combustion oscillation feature of the RBCC combustor, which is different from a thermo-acoustics type oscillation. The rocket jet/air inflow physical interactions under different rocket jet expansion degrees are experimentally studied. [ABSTRACT FROM AUTHOR]

This research is motivated by the issue associated with high frequency combustion instability. Large eddy simulation was performed to investigate spontaneous combustion instability in an air/LO 2 /C 2 H 5 OH tripropellant air heater. The simulation predicts self-excited transverse oscillations. Overall behavior of combustion instability including pressure time histories, mode shapes, Rayleigh index and unsteady response of the injector were studied in detail. Special emphasis was given to the flame behavior, droplet trajectories, pressure evolutions, and formation of large-scale vortical structures during combustion instability in present air heater. Furthermore, in contrast to previous investigations, a new process is identified in the simulation that may feed energy into the acoustic mode and drive combustion instability. [ABSTRACT FROM AUTHOR]

The primary aim of this paper is to establish the effectiveness of microwave plasma discharges to improve combustor flame dynamics and stability through minimizing heat release fluctuations. A continuous, volumetric, direct coupled, nonequilibrium, atmospheric microwave plasma discharge was applied to a swirl-stabilized premixed methane-air flame to minimize combustion instabilities. Proper orthogonal decomposition (POD) is used to postprocess data and extract information on flame dynamics that are usually lost through classical statistical approaches. POD analysis carried out on OH planar laser-induced fluorescence images reveals that even at coupled plasma powers corresponding to less than 5% of the thermal power output, significant improvement in mean energy content of flames (~23%) was observed. The corresponding decrease in heat release fluctuations resulted in improved combustor flame dynamics and flame stability, which was found to be in good agreement with acoustic pressure measurements. In the presence of plasma discharge, an effective decoupling between the flame oscillations and the fluid unsteadiness was established due to the differences in flame stabilization mechanisms resulting in up to 47% reduction in root-mean-square pressure fluctuations. Thus, effective fluid-acoustic decoupling in addition to the accelerated combustion chemistry due to the nonthermal effects of plasma led to significantly improved combustor dynamics, namely, decreased heat release and pressure fluctuations. [ABSTRACT FROM AUTHOR]

Flame pulsation has a significant effect on combustion, and understanding its oscillatory behavior is important to the combustion community. An experiment was performed to analyze the pulsation characteristics of a swirl non-premixed flame under various parameters. The results showed that as fuel mass flow rate increased, the puffing frequency increased due to the decreased flame radiation fraction, and the puffing amplitude became smaller resulting in a more stable flame. With an increase in combustor pressure, the flickering frequency declined because of the increasing soot radiation, while the flickering amplitude uniformly increased, leading to more deteriorative flame stability. With an increment in mass flow rate of primary air, the puffing frequency decreased due to the enhanced mixing between fuel and primary air. Also, the puffing amplitude had an oscillating relationship with the mass flow rate of primary air. When the exit velocity of the injector was increased, the flickering frequency diminished nearly linearly because of the improving swirl intensity, and the flickering amplitude was approximately unaffected by injector exit velocity. Moreover, the measured puffing frequencies summarized over all cases varied within the range of 3–22 Hz, the predicted values from theoretical models based on non-swirl flame also fell within this range. The puffing frequency of swirl combustion was more sensitive to the variation in operating conditions than that of non-swirl combustion. Additionally, the obtained correlations indicated that the Strouhal number St was proportional to Fr−1.4 (the Froude number) and Re−2.9 (the Reynolds number), respectively.

The structure and dynamics of laminar nonpremixed dimethyl ether (DME)/air coflow flames were investigated at elevated temperatures and pressures. Computations with detailed chemistry were performed for DME and heated coflow air at 30 atm with uniform but sinusoidally oscillating inlet velocities. These unsteady cases were compared with the steady results from Deng et al. (2015)[14] to elucidate the effect of oscillation frequency on the flame dynamics. To benchmark the unsteady cases, a normalized displacement velocity was defined to differentiate flame propagation from autoignition, and this definition was validated against the steady cases. In the oscillating reacting flow, transition between a multibrachial autoignition front and a tribrachial flame occurs periodically. However, unlike the harmonic velocity oscillation, the combustion mode transition is hysteretic. The oscillation cycle starts with the largest inlet velocity, with the multibrachial thermal structure, located downstream, being governed by autoignition chemistry. As flow velocity decreases, the autoignition front moves upstream and transitions to a tribrachial flame near the lower velocity limit, similar to the steady flow, as autoignition chemistry becomes weaker with decreasing upstream residence time. As the flow velocity increases again, the tribrachial flame is convected downstream, and, ultimately, due to the radical and heat accumulation in time, autoignition eventually occurs and becomes the dominant pathway. The finite induction time for autoignition results in the hysteretic behavior during the decreasing- and increasing-velocity cycles, which diminishes at lower oscillation frequency as there is more time for chemistry to respond to the hydrodynamic changes and consequently approach steady state. At the relatively low oscillation frequencies investigated in the current study, first-stage NTC chemistry is less affected by flow dynamics with only second-stage autoignition and flame chemistry, which accounts for the majority heat release, coupled with the flow oscillation. [ABSTRACT FROM AUTHOR]

We report on the linear stability of micro-combustion in pipe, where two instabilities manifest at high and low flow-rates. The combustion of a stoichiometric methane/air premixed mixture has been numerically investigated within a 3D reduced model. This model reproduces decently the flame dynamics in the range of speed between 5–100 cm/s. The flame position, the stability thresholds of the Flame with Repetitive Extinction and Ignition (FREI) and the flame shape are in accordance with the experiments. Furthermore, an analysis of the integral values of all mechanisms involved in the flame evolution has been carried out near the two stability thresholds. The phase shift between the reaction term and the radial diffusion has been identified as the source of instability in both cases. The global behavior has been then investigated with a linear stability analysis. The 2D and 3D temperature and concentration perturbations have been found by solving the eigenvalue problem obtained by linearizing the model around the basic state. Only one unstable axisymmetric mode has been found. This is in agreement with the direct numerical simulation of the model. [ABSTRACT FROM AUTHOR]