Local Flame Research Articles

Response of n-Heptane and Dodecane Swirl-Stabilized Spray Flames to Transverse Forcing Characterized By Flame Describing Functions Based on Downstream Acoustic Pressure or Velocity Signals

Abstract Predicting thermoacoustic instabilities in annular combustors requires knowledge of the impact of acoustic oscillations on heat release rate oscillations. Flame Describing Functions (FDF) measured at the burner exit using acoustic forcing are key elements of thermoacoustic instability analyses. FDFs based on acoustic pressure measurements, FP' or on axial velocity measurements FU', are compared here. This study is done on the TACC-Spray bench, an original linear array of spray flames stabilized by a strong swirling flow, representing an unfolded sector of a self-unstable annular combustor. Acoustic forcing of a standing transverse chamber mode is applied downstream of the injectors. Experiments are conducted with liquid n-heptane or dodecane, with the flames placed at a pressure or an intensity antinode of the transverse mode. FP' does not depend on the measurement location for acoustically-compact flames provided that this location remains in the flame vicinity. FU' can lead to significant discrepancies, as swirling flows present strong velocity gradients, which can be minimized by carefully choosing the measurement location. The injector admittance linking the two FDFs is shown to be quasi-independent of the forcing amplitude here. Consequently, both FDFs show a similar dependence on the forcing amplitude. FP' indicates constructive combustion-acoustics interference whatever the flame location in the acoustic field and the fuel, consistent with self-sustained instabilities observed in the annular combustor. An analysis using the Rayleigh criterion corroborates the results derived from the FDFs. So, FP' appears as a powerful and practical tool for characterizing combustion dynamics.

Study of chemiluminescence of methane–air flame stabilized on a flat porous burner

In this work, the spatial distribution and spectral characteristics of the chemiluminescence of chemically excited species, OH∗ and CH∗, are experimentally and numerically studied by using a stationary premixed methane–air flame stabilized on the surface of a flat porous burner for various equivalence ratio and normal pressure. Numerical simulations are carried out using detailed reaction mechanisms, and the experimental study includes high-resolution spatial and spectral optical measurements. Despite the data reported in the literature, it is found that (i) the rotational degrees of freedom of OH∗ and CH∗ are not in thermal equilibrium with the surrounding gas and therefore cannot be used to measure flame temperature; (ii) there is no direct correlation between the heat release rate and the distribution of OH∗ and CH∗; (iii) the detailed reaction mechanisms not only quantitatively, and also qualitatively differ in description of the OH∗ and CH∗ concentrations. Since the chemically excited species are well localized in a direction normal to the flame surface, they are demonstrated to be a very accurate markers of flame location. The shape of the combustion front can be reconstructed and resolved up to the accuracy of tens of microns, which is very important for estimation of blow-off critical parameters and measurement of the laminar burning velocity.Novelty and significance statementCurrently, there is a growing interest in the development of sensors for combustion control systems, including active control and suppression of instabilities, in combustion chambers of various devices and engines based on chemiluminescence of excited reaction species. The possibility of non-invasive determination of parameters such as flame temperature, stoichiometry, heat release rate location, etc. using this technique is discussed. We have found that most of these parameters cannot be estimated either due to fundamental limitations or insufficient knowledge of the reaction kinetics involved in the production of these species. Nevertheless, since OH* and CH* are well localized in the direction normal to the flame surface, they can be used as very accurate markers of flame shape and position, allowing us to reconstruct the flame surface to within tens of microns resolution, which is very important for estimating blow-off critical parameters and measuring laminar burning velocity.

Fire emergency management of large shopping malls: IoT-based evacuee tracking and dynamic path optimization

As urbanization accelerates and buildings become more complex, fire emergency evacuation has become increasingly challenging. Traditional evacuation plans often struggle with slow response times and suboptimal path planning in real-time dynamic and complex fire scenarios. To address these issues, this study proposes the IoT-based DWM-Evac model for fire emergency evacuation path planning. The model leverages IoT technology by using various sensors placed inside buildings to monitor fire incidents and spread in real-time, collecting critical data such as temperature, smoke concentration, and flame location. It integrates Dynamic Graph Neural Networks (DGNN), Whale Optimization Algorithm (WOA), and Markov Decision Process (MDP) to enhance path efficiency and safety. Experimental results indicate that the DWM-Evac model achieves an average evacuation time of 315 s in a virtual mall environment, 25 s shorter than traditional plans, with an average path length of 255 meters and a path safety score of 0.92, higher than the traditional plan’s 0.88. The application of IoT in fire emergency management not only improves response speed but also optimizes path planning, significantly enhancing personnel safety.

Role of secondary hydrogen injection on flame stabilization of ammonia/air swirling flames

Ammonia is widely recognized as one of the most advanced hydrogen carriers and can be utilized as a fuel in gas turbines. Premixing hydrogen into the ammonia carbon free fuel system can substantially enhance stable limits, but it significantly promotes cross-reactions, leading to increased NO production. Recent findings related to other fuels suggest that the alternative approach for mitigating combustion instabilities involves introducing minimal pure hydrogen at the chamber inlet in a non-premixed mode. This may introduce a novel approach to stabilize ammonia/air swirl flames by extending the stability through a minimal hydrogen via secondary injection, with minimal impact on increasing nitrogen oxide emissions. In the present study, the flame stabilization mechanism and nitrogen oxide emission behaviors of swirl ammonia/air flames by a secondary hydrogen injection were compared with the premixed ammonia/ hydrogen/air flames. OH-/NO-PLIF and PIV techniques were applied to reveal the lean blow-off characteristics and the reacting flow features. The NOx emissions were measured by the FTIR gas analyzer. The large eddy simulation method with a developed dynamic thickened flame model was employed to further reveal the experimental findings. Experimental results show that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. The secondary hydrogen injection exhibits the stronger enhancement ability for the lean/rich blow-off limits, primarily due to the local diffusion hydrogen flame at the flame root providing more active radicals and higher temperature gases. The NO emission shows an increase with the addition of premixed hydrogen, while in the secondary hydrogen injection, NO emission deteriorates due to higher temperatures, with the NO emission increasing by less than 10% compared with the ammonia flame. In the large eddy simulation analyses, the physical effects of the secondary hydrogen injection enhance the flame stability by increasing the resistance of the flame root to extinction. The heat release rate and the mass fractions of NH and H near the flame root are significantly increased by 2% secondary hydrogen injection. The enhancement of the ammonia decomposition process is stronger with secondary hydrogen injection than with fully premixed hydrogen addition. For 2% secondary hydrogen injection case, the local hydrogen injection to form a non-premixed combustion mode can provide much higher capability to increase the local heat release rate compared to the fully premixed mode.Novelty and significance statement: Introducing small amount of hydrogen to ammonia flame can effectively improving the flame stabilization in a swirl combustor. In this study, it is found that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. Comparing the way of premixing hydrogen in the unburned mixture, a larger effect on blow-off limits extension was found when introducing a separate non-premixed hydrogen at the chamber inlet, which is mentioned as the secondary hydrogen injection. The lean blow-off limits can be extended from about 0.67 to 0.59 when only introducing 2% secondary hydrogen injection by heat value. The NO emission shows in the secondary hydrogen injection, NO emission deteriorates by less than 10% compared with the ammonia flame. A thickened flame model for non-premixed combustion was established and verified adequate with velocity field and flame structure. The mechanisms of enhancing flame stabilization by hydrogen introduction including physical and chemical effects for premixing and injection cases were revealed. Furthermore, the difference in the mechanisms of the two cases was emphasized.

Challenges and solutions for local flame development during spark ignition in a kerosene-fueled scramjet combustor

This study experimentally investigates the challenges and solutions related to the development of local flame into global flame during kerosene spark ignition in a scramjet combustor operating at Mach 4 flight conditions. The ignition and intensity of local flame are explored with different injection pressures. Two potential solutions have been proposed to facilitate the development. The results show that injection pressure plays a critical role in controlling fuel transport into the ignition cavity T1, affecting the local equivalence ratio and local flame formation. Higher injection pressures lead to less fuel transported into cavity T1, resulting in fuel-lean local equivalence ratios and potential ignition failure. Extending the duration of ignition and injection improves ignition reliability. The suppressive effect of dense spray on local flame is the main cause of the local flame development problem. A higher injection pressure can reduce the suppressive effect and increase the intensity of downstream cavity flames. When the downstream cavity flames reach a critical intensity, the flashback of downstream cavity flame will occur, achieving global flames. The dense spray can be thinned out by very low upstream injection pressure, which can also result in global flames.

Numerical study on flame acceleration and deflagration-to-detonation transition affected by the solid obstacles with different shapes

To investigate which shape of solid obstacles is most optimal in terms of detonation-assisting capability within a closed channel, this study utilizes the Large Eddy Simulation (LES) method, based on the OpenFOAM platform, to conduct a detailed numerical study on the effects of different shaped obstacles (rectangular, rhombus, trapezoidal, circular, triangular) on flame acceleration and detonation initiation processes. The results show that the obstacles arranged in the middle of the combustion chamber can create more passages and local flames. Meanwhile, changes in the shape of obstacles can also produce different vortex structures and recirculation zones. These differences affect the flame surface area and the total combustion heat release rate, thereby affecting the flame acceleration effect. Judging from the results, triangular obstacles have the best flame acceleration effect, followed by rhombus, trapezoid, rectangle, and circular obstacles. In terms of detonation initiation, the obstacles with a slope can induce vortex to detach earlier, which produce a better vortex-flame interaction. Simultaneously, the slope can reflect the shock wave more frequently, creating more favourable conditions for the formation of Mach reflection and Mach stem. Based on this, it is found that trapezoidal and triangular obstacles have better detonation-assisting capability. Additionally, it is found that the stronger the leading shock wave and the shorter the distance between it and the flame front, the better the detonation initiation effect. In general, the types of detonation initiation in this study all belong to the shock detonation transition (SDT). However, the detonation initiation process can be further classified into two categories: (I) Detonation induced by shock wave reflection; (II) Detonation triggered by shock wave focusing.

Influence of AM Generated Burner Surface Roughness on NOx Emissions and Operability of Hydrogen-Rich Fuels

ABSTRACT The additive manufacturing (AM) technique enables the fabrication of advanced burner components to enhance the hydrogen capability of the existing gas turbines (GTs) and reduce the carbon footprints of the power generation sector. This technique produces rough surfaces that may require post-processing to maintain the desired functionality of a burner, particularly for hydrogen fuel with unique thermo-physical properties. This study, therefore, compared the stability of three swirlers of variable surface roughness manufactured using AM and traditional machining methods, with one of the AM swirler post-processed by grit-blasting. The comparison included a conventional benchmark (100% CH4), low carbon (23%volCH4/77%volH2) and zero carbon (100% H2) fuels across a range of equivalence ratios. Additionally, the study quantified the flame topology and emissions performance of the fuel blends for each swirler using high-speed OH* chemiluminescence and exhaust gas emissions measurements, respectively. The experimental investigation concluded that the AM-generated surface roughness within the considered range does not detrimentally impact NOX emissions and the stability of the fuel mix. However, the flame location was observed to be influenced by surface roughness and shifted more toward the vertical centerline of the burner with increased roughness. From the practical perspective, the results showed that post-manufacturing surface finishing offers negligible performance advantages, indicating potential cost reductions. It is recommended that further studies should investigate the influence of increased surface roughness on burner performance, as well as numerical modeling techniques which could provide an insight into when AM surfaces are likely to be more influential.

Numerical analysis of laminar velocity-forced premixed slit flames using modal decomposition techniques

The relation between Proper Orthogonal Decomposition (POD), Dynamic Mode Decomposition (DMD) and Flame Transfer Functions (FTF) is explored to gain further insight into the dynamics of two-dimensional laminar slit flames externally forced by velocity perturbations. The application of POD to the heat release rate fields suggests that the resultant modes can be split into two groups: the ones that are related to the displacement of the reactive sheet in the normal direction with regards to the flame front, and the ones that contain the local flame front distortions. The latter modes show preferential frequencies associated to the phase values of π, 2π and 3π of the FTFs, which can be related to the maximum gain value depending on the case. Furthermore, the results of the modal analysis seem to support that the flame tip dynamics can be conceptually modelled as a set of standing waves whose joint response can reconstruct a propagation in the normal flame front direction, generating also the temporal fluctuations of the spatially-integrated heat release in the domain. The DMD analysis shows the existence of an interaction between the flame and the flow, and illustrates the fundamental role played by the velocity perturbations at the base for the motion of the reactive sheet. Thus, the analysis shows how data-based decomposition methods can be used to identify complex physical phenomena contained in the FTF graphs with different levels of detail, and extend the modal analysis to the physical space.Novelty and significance statementThis paper proves the potential of modal decomposition techniques like POD and DMD to infer the underlying physics of canonical flames and their relation to the FTF, validating also the hypotheses and methodology of other reduced-order models in the literature. A canonical configuration featuring a two-dimensional slit flame has been selected to analyse the flame dynamics using these methods. Hence, this work aims to set the foundations to elaborate data-driven reduced-order models based on modal decomposition techniques to support the analysis and the study of flame dynamics.

Comparative study on the local hybrid combustion characteristics of the CH4-NH3 laminar premixed flames with/without the wall effects

The CH4-NH3 (50%:50%) premixed flame with/without the wall effects are simulated and compared at the stoichiometric condition in this study. The effects of cold wall and local flame dynamics on the flame temperature, heat release rate (HRR), CH4/NH3 oxidations and local CO/NO formations are analyzed quantitatively. Results show that the flame temperature and HRR are decreased and increased respectively along the flame front towards the axis of free flame, but they are both declined significantly towards the wall owing to suppressed chemical kinetics by the heat loss. The negative flame stretch and differential diffusion at the flame tip suppress the CH4 oxidation and the fast path (NH3→NH2→NH→N2) of NH3 oxidation effectively, but affect the slow path (NH3→NH2→HNO→NO→N2) of NH3 oxidation moderately. This contributes to suppressed CH4/NH3 oxidations and improved NO production at the flame tip of free flame. Based on NRR variations and ROP analysis, compared with CH4 oxidation, wall heat loss exerts stronger suppressions on the fast path of NH3 oxidation, while it decelerates the slow path of NH3 oxidation less evidently. Since the fast path is the primary pathway of the NH3 oxidation, the NH3 oxidation suffers more effective suppression of the wall heat loss than the CH4 oxidation. Furthermore, considering the significance of the slow path to the NO formation, the relatively enhanced importance of the slow path to the NH3 oxidation in the near-wall region predominates the comparatively effective NO formation in the CH4-NH3 flame compared with the pure CH4 flame under the influence of wall heat loss. For the CO/NO formations, the local CO production/oxidation in the wall-impinging CH4-NH3 flame are decelerated simultaneously due to the strong heat loss, while the local NO production/destruction are inhibited/improved by the wall heat loss.

Evolution of Displacement Speed Statistics During Head-On Flame-Wall Interaction within Turbulent Boundary Layers

ABSTRACT The statistical behaviours of the density-weighted displacement speed and its curvature and strain rate dependence during head-on interaction of statistically planar premixed flames have been analysed based on Direct Numerical Simulations. The analysis has been conducted within turbulent boundary layers featuring inert walls, under both isothermal and adiabatic wall boundary conditions. The flame quenches due to heat loss when it reaches close to the wall in the case of isothermal wall boundary condition. By contrast, the flame can propagate all the way to the wall before extinguishing due to the consumption of reactants in the case of adiabatic wall boundary conditions. Thus, the effects of thermal expansion, quantified by dilatation rate, and flame normal acceleration during flame-wall interaction in the case of the isothermal wall are weaker than that in the case of the adiabatic wall case due to flame quenching. This gives rise to significant differences in the curvature and strain rate dependences of the reactive scalar gradient magnitude, which affects the statistical behaviours of the reaction and normal diffusion components of density-weighted displacement speed including their mean values, widths of their probability density functions, and their local strain rate and curvature dependences. It has been found that the interaction of near-wall vortical structures with flame surface increases the range of curvature variation during head-on interaction, which acts to widen the probability density function of the density-weighted displacement speed. This trend is relatively stronger for the isothermal wall boundary condition because of the larger variation of the reaction rate component of the density-weighted displacement speed due to local flame quenching, which also acts to widen the range of the density-weighted displacement speed variation in comparison to that in the case of adiabatic wall boundary conditions. Although the qualitative nature of curvature, strain rate, and stretch rate dependences of the density-weighted displacement speed remain unaffected by the wall boundary condition, the strength of the correlation changes with the progress of head-on interaction. It has been found that the negative correlation between the density-weighted displacement speed and flame curvature weakens with the progress of head-on interaction, which gives rise to a reduction in the strength of the correlation between the density-weighted displacement speed and flame stretch rate. Similarly, the correlation between the tangential strain rate and the density-weighted displacement speed weakens with the progress of head-on interaction. Detailed physical explanations are provided for these behaviours, and their modeling implications are indicated.

The stability and morphology of laminar premixed hydrogen/air flames under various gravity conditions

In the present work, direct numerical simulations (DNS) were employed to understand the stability and morphology of laminar lean premixed hydrogen/air flames under various gravity conditions. The combined effect of Darrieus–Landau (DL) instability, thermal-diffusive (TD) instability and Rayleigh–Taylor (RT) instability was studied quantitatively for both the initial linear regime and the long-term nonlinear regime, with a particular focus on the effect of RT instability. In the linear regime, it was found that the RT instability has a negligible effect on the linear growth rate of flames with an equivalence ratio of 0.4, but the effect is significant when the equivalence ratio decreases to 0.3. The performance of various models for the dispersion relation was discussed, including a theoretical model considering the effects of all instabilities, which captures well the trend of the simulated results in the cases with unity Lewis numbers. The hydrogen/air flames with an equivalence ratio of 0.3 was investigated comprehensively in the nonlinear regime. For the cases with unity Lewis numbers, the RT-unstable flame exhibits a similar single-cusp structure as the RT-neutral flame, but with a deeper cusp and larger consumption speed as a result of the destabilizing effect of RT instability. The RT-stable flame gradually returns to a planar flame shape owing to the stabilizing effect of RT instability. For the cases with non-unity Lewis numbers, the effect of domain size on the flame evolution was investigated, showing a domain width of 200δT is enough to obtain a domain-independent result, where δT is the laminar flame thickness. It was found that the cases initialized with multimode perturbations reach the quasi-steady state faster, while the flame statistics are consistent with those of the cases initialized with single mode perturbations. The effect of RT instability was explored by comparing the results from various gravity conditions. The flame consumption speed is the largest in RT-unstable flames, and the smallest in RT-stable flames. The variation in flame consumption speed is due to the difference in the flame surface area, while the local reactivity and flame structure are not influenced by the RT instability. Further fractal analysis reveals that although the RT instability causes significant differences in flame surface area, the characteristic fractal dimension remains constant for flames under different gravity conditions.Novelty and significanceIn the present work, direct numerical simulations were employed to understand the stability and morphology of RT-unstable, RT-neutral and RT-stable laminar lean premixed hydrogen/air flames. For the first time, the effect of DL, TD and RT instabilities was quantitatively examined for both the linear regime and the nonlinear regime. A theoretical model considering various instabilities was compared with the simulated dispersion relations in the linear regime. In the non-linear regime, the flame surface area and corresponding flame consumption speed are influenced by the RT instability, while the flame stretch factor remains unaffected. A fractal analysis was conducted in laminar premixed hydrogen/air flames under the influence of various instabilities. Remarkably, it was discovered that the characteristic fractal dimension remains constant for the cases with and without the RT instability.

Influence of Lateral Explosion Relief Conduit Inclination on Methane/Air-Premixed Flame Propagation in Pipelines

ABSTRACT To study the influence of explosion relief conduit angle on the explosion characteristics of methane in the pipeline, in the 2000 mm square pipeline to carry out conduit explosion relief experiments, to explore the effect of different inclination angle of the conduit relief to the square pipeline as the object of study, and combined with the k-ε turbulence model to simulate the localized flame of the Explosion relief conduit. The results show that: the explosion relief conduit can effectively reduce the pressure in the pipeline, tilted at an acute angle of the explosion relief conduit in the flame propagation of turbulence is smaller, and more conducive to the release of pressure. When the angle of the explosion relief conduit is 45°, the intensity of turbulence generated at the mouth of the branch pipe is the smallest, the peak pressure in the pipeline compared to the no lateral conditions is reduced by 37.47%, the flame propagation time is extended by 41.44%, the explosion relief effect is related to the angle of the explosion relief conduit and the pipeline.

High-frequency hydrogen combustion dynamics driven by local flame displacement and multidimensional thermoacoustic interactions

Knowledge of the underlying physical mechanisms responsible for the triggering of high-frequency transverse combustion dynamics is of fundamental importance in the development of heavy-duty gas turbine combustors, aircraft engine afterburners, and bipropellant liquid rocket engines. Detailed information about three-dimensional thermoacoustic interactions and local flame dynamics, however, remains largely unknown and unanticipated, mainly because high-amplitude transverse mode instabilities are challenging to excite and detect in well-controlled sub-scale laboratory environments. To overcome this impasse, here we exploit a spatially tailored rectangular injector assembly consisting of ten equidistant horizontal slit nozzles to eliminate the complications of out-of-plane flame dynamics characterization. A total of 56 datasets of self-induced instabilities were acquired over a wide range of operating conditions to understand spatiotemporal phase dynamics and important mode shapes, in conjunction with 2D Rayleigh angle reconstruction and phase-resolved OH PLIF-based local flame front identification. Experimentally, we show that high-frequency transverse instabilities are excited only under high temperature and high thermal power conditions, manifested as non-evanescent pressure fluctuations at 6.50 kHz strongly coupled to the second-order tangential mode of the rectangular combustion chamber. Two vertically-oriented pressure nodal planes and the characteristic phase transition perpendicular to the horizontal slit injector direction are accurately measured and reconfirmed by Helmholtz simulations in terms of their interpositions and spatial orientation. Remarkably, the periodic formation of co-propagating coherent structures and concomitant local flame displacement/pinch-off are revealed to play an important role in driving the high-frequency hydrogen combustion dynamics.

Choice of reaction progress variable under preferential diffusion effects in turbulent syngas combustion based on detailed chemistry direct numerical simulations

The combustion of hydrogen and carbon-monoxide mixtures, so-called syngas, plays an increasingly important role in the safety context of non-fossil energy generation, more specifically in the risk management of incidents in process engineering plants for ammonia synthesis and in nuclear power plants. In order to characterize and simulate syngas/air combustion on industrially relevant scales, subgrid modelling is required, which is often based on a reaction progress variable. To understand the influence of different fuel compositions, turbulence intensities and flame topologies on different possible definitions of reaction progress variable, detailed chemistry direct numerical simulations data of premixed, lean hydrogen/air and syngas/air flames has been considered. A reaction progress variable based on normalized molecular oxygen mass fraction has been found not to capture the augmentation of the normalized burning rate per unit flame surface area in comparison to the corresponding 1D unstretched premixed flame due to preferential diffusion effects. By contrast, reaction progress variables based on other individual species, such as hydrogen, can capture the augmentation of the rate of burning well, but exhibit a pronounced sensitivity to preferential diffusion effects, especially in response to flame curvatures. However, a reaction progress variable based on the linear combination of the main products can accurately represent the temperature evolution of the flame for different mixtures, turbulence intensities and varying local flame topology, while effectively capturing the augmentation of burning rate due to preferential diffusion effects. However, its tendency to assume values larger than 1.0 in the regions of super-adiabatic temperatures poses challenges for future modeling approaches, whereas the reaction progress variable based on hydrogen mass fraction remains bound between 0.0 and 1.0 despite showing deviations in comparison to corresponding variations obtained from the unstretched laminar flame depending on flame curvature variations.

Effects of wall confinement on flame topologies and lean blowout characteristics of partially premixed DME/air flames in a gas turbine model combustor

Dimethyl ether (DME) is a promising alternative fuel for gas turbines, considering its renewable nature and potential for large-scale synthesis from CO2 feedstock. However, there are limited studies on the swirl combustion characteristics of DME, particularly regarding the stability performance. This work investigates the effects of wall confinement on flow, mixing, and lean blowout (LBO) characteristics of partially premixed DME/air swirl flames in a gas turbine model combustor. Flow and flame macrostructures are captured using particle image velocimetry and planar laser-induced fluorescence (PLIF) measurements, respectively. The results show remarkable differences in flame topologies between unconfined and confined flames, especially the formation of inner recirculation zone and outer recirculation zone. The LBO limits are measured over various Reynolds numbers and flame dynamics approaching LBO are captured using 10 kHz OH* chemiluminescence imaging. Unconfined flames have unexpectively lower LBO limits (∼0.4) than the LBO limits of confined flames (over 0.5). Approaching LBO, unconfined flames display a quiet blowout mode featuring swirling spiral blow-off behaviors. In contrast, confined flames exhibit a violent blowout mode featuring large-scale quasi-instability behaviors, including periodic events of local extinction, flame lift-off, and re-ignition. Based on the numerical simulations and simultaneous OH- and CH2O-PLIF measurements, the large discrepancy in the measured LBO limits between the two configurations is found to result from different re-ignition processes related to the low-temperature ignition before the final blowout. These findings highlight the crucial roles of wall confinement in determining the stable flame topologies, LBO characteristics, and mechanisms driving the LBO physics for partially premixed DME/air swirl flames.

The Structure Characteristics of Laminar Premixed Flames of Gasoline-like Fuel Under CI Engine-Relevant Conditions.

Gasoline compression ignition characterized by partially premixed and long ignition delays typically features complex flame structures such as deflagration or spontaneous ignition fronts. In this study, the flame structure and propagation characteristics of PRF90/air mixtures under compression ignition engine-relevant conditions are investigated numerically. Similar to other types of fuels, under such conditions, the propagation speed of PRF90 laminar premixed flames depends not only on the unburnt mixture properties but also on the residence time, and the transition of the flame regime depends only on the residence time. Nevertheless, due to the temperature-dependent autoignition chemistry of PRF90, flames with excessively high unburnt temperatures show different combustion behaviors after the transition from deflagration to autoignition-assisted flames. Sensitivity analysis showed that, the dominant chain branching reactions in the deflagration mode are H + O2 = OH + O and CO + OH = CO2 + H, and that in the autoignition-assisted flames with lower unburnt temperature are H2O2(+M) = 2OH(+M) and IC8H18 + HO2 = AC8H17 + H2O2, while for higher unburnt temperatures, the reactions C3H5 + HO2 = C2H3 + CH2O + OH and IC8H18 = IC4H9 + TC4H9 are more important than the fuel low-temperature oxidation reactions. In addition, a criterion based on chemical explosive mode analysis is used to analyze the local combustion mode. The results show that the difference in diffusion/chemical structure at the crossover progress variables C 0 and crossover temperature allows both C 0 and to be used as a flame location for distinguishing propagation modes in premixed flame. However, the effects of the equivalence ratio on C 0 are different from that on , which means that the selection of C 0 and may lead to different discriminant results for stratified mixtures. Comparing the applicability of C 0-based and -based locations in three-dimensional gasoline compression ignition flame, it is found that the flame location based on the value of C 0 at ϕ = 1.0 can more completely reflect the flame development characteristics in stratified premixed combustion.

Richtmyer-Meshkov instability when a shock wave encounters with a premixed flame from the burned gas

We present a linear stability analysis of the Richtmyer-Meshkov instability that develops when a shock wave reaches a sinusoidally perturbed premixed flame from behind. In the hydrodynamic regime, when acoustic contributions dominate the flame growth rate, the problem is analytically addressed by the direct integration of the sound wave equations at both sides of the flame, which are bounded by the reflected and transmitted shock waves and the flame front that acts as a contact surface in the hydrodynamic limit. The resolution involves: a hyperbolic change of variables to modify the triangular spatio-temporal domain, a transformation in the Laplace variable, the resolution of the functional equations in the frequency domain, and the final inverse Laplace transform. The latter involves a novel resolution method that is proven beneficial for long-time dynamics. Asymptotic analysis is also carried out to describe the early time and late time hydrodynamic response. The nonuniform flow field resulting from distorted oscillating shocks is characterized by acoustic, rotational, and entropic disturbances, each of which exerts a substantial influence on flame dynamics. These disturbances contribute to the intricate interplay of factors shaping the behavior of the flame in response to the nonuniform flow. In particular, the sensitivity of local flame propagation to temperature disturbances is investigated. This work contributes to a deeper understanding of Richtmyer-Meshkov instability dynamics and offers insights into instability reduction through the modulation of temperature disturbances generated by the transmitted shock.

Experimental research on dynamic response characteristics of lean premixed flame excited by acoustic standing wave

The compact combustor holds great potential in the aerospace field. However, it faces challenges in terms of interaction between acoustic waves and flames, which affects combustion efficiency and intensity. In this study, the dynamic response of lean premixed flames under the excitation of a transverse standing wave acoustic field with different frequencies and amplitudes. The results indicate that the flame's response frequency is consistent with the excitation frequency. Decreasing the sound wave frequency and increasing the sound amplitude both lead to an increase in flame area oscillation, thereby enhancing the heat release rate. However, excessively high sound pressure values can enhance fuel diffusion, resulting in a reduction in flame area or even local flame extinction, which significantly decreases flame intensity. This research provides valuable insights into the behavior of lean premixed flames under acoustic excitation, contributing to the optimization of combustion processes and improvement of efficiency and performance in compact combustors.

Investigation of differential diffusion in lean, premixed, hydrogen-enriched swirl flames

Hydrogen combustion is a promising alternative to fossil fuel combustion in an effort to reduce our carbon footprint. However, hydrogen combustion is prone to thermodiffusive instabilities largely dependent on differential diffusion, a phenomenon that can lead to higher probabilities of flashback in industrial burners, given hydrogen’s high reactivity and diffusivity. This paper evaluates low-swirl flames of methane and air enriched with hydrogen to highlight the onset of differential diffusion. Testing was conducted in a fully controllable swirl burner, where bulk velocity Uav = 13 m/s and swirl number S = 0.6 were kept constant for each hydrogen–methane blend to isolate increases in flame surface area from increases in turbulence intensity. Furthermore, each fuel blend of hydrogen and methane is evaluated at the same laminar flame speed of SLo = 0.267 m/s to isolate flame stretch effects on the turbulent burning rate. Combined hydroxyl (OH) PLIF and stereoscopic PIV at the National Research Council of Canada were used to analyze the OH fluorescence in a 2D-3C velocity field for each flame condition. High-speed PIV at McGill University was used to resolve local flame phenomena, such as local flame displacement velocity and flame stretch rate. Using these techniques, it can be observed that the flame displaces axially in response to turbulent flame speed while exhibiting increases in flamefront wrinkling. This increased corrugation due to flame stretch is highlighted in the PDFs of local curvature and κSf and is further evidenced by a shift towards positive curvatures (κ> 0) for increasing H2 volume fraction. This trend suggests that there is a strong correlation with increases in turbulent burning rate and positive curvature as a result of differential diffusion, but it is not necessarily a control mechanism of the most forward propagating points proposed by the theory of leading points.

Fire behavior and post-fire residual tensile strength prediction of carbon fiber/phthalonitrile composite laminates

Carbon fiber/phthalonitrile (Cf/PN) composite has a high potential for applications in the aerospace sector because of the outstanding heat and flame resistance of phthalonitrile resin. Here we aim to evaluate the fire behavior of Cf/PN laminate under localized one-sided flame heating as well as the residual tensile performance. The residual tensile strength after quasi-isothermal pyrolysis in an inert environment decreases linearly with the increase in the pyrolysis degree. The temperature response and pyrolysis behavior are analyzed through a combination of experiments and finite element simulations, and the damage mode of the laminate structure is concluded through morphological observation after flame exposure and the surface axial strain field using digital image correlation. Eventually, the residual tensile strength of laminates with different thicknesses after different times of flame exposure was effectively predicted based on the numerical simulated pyrolysis degree distribution. This research supplies fundamental test data on the mechanical properties of Cf/PN laminates and is expected to provide guidelines for the engineering application of Cf/PN composites in thermal protection/load-bearing all-in-one structures.