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With increasingly stringent emissions regulations, the trend toward using low-carbon fuels in internal combustion engines is unstoppable. Achieving higher rates of methane substitution is a crucial direction for future diesel-methane engines. However, the details of the combustion process involving the auto-ignition of micro pilot diesel fuel and ignition of methane main fuel to form premixed flames are still not clear. Therefore, the study employed visualization technique using a rapid compression machine and numerical study to analyze the process of igniting methane main fuel using various injection strategies for micro pilot diesel fuel. The results indicate that before the onset of the second diesel injection, a pale blue premixed flame had already formed within the cylinder, and the ignition delay of the second diesel injection was influenced by the presence of this premixed flame. The ignition delay of the diesel droplets from the second injection decreases gradually as SOI1 advances. Compared to the single injection strategy, the split injection strategy forms smaller high-temperature regions during the combustion process, thereby displaying a discernible trend toward reducing NOx emissions. Since the energy contribution of the second diesel injection is only 2%, the turbulence and diffusion flames formed by the second diesel injection have a relatively low impact on the propagation of the methane premixed flame. The flame front for single injection strategy is closer to the thickened flames region, suggesting a thicker flame surface due to the concentrated distribution of diesel. [ABSTRACT FROM AUTHOR]

This work compares the effects of adding water vapor and CO2 as diluent to fuel for a diffusion flame at different ambient pressures. The approach of this research is to study numerically a methane diffusion flame with various amounts of water vapor and CO2 added to the fuel, and to compare the flame behaviors with various percentages of water vapor and CO2 content (0% to 65%). This work uses a coflow burner in the PeleLM numerical framework for simulating the flame and computing the combustion behavior. The flame was simulated with pressures of 1.0, 1.4, 5.7, and 11.1 atm. The results extracted and analyzed include temperature profiles at fixed heights and various species mole fractions compared with their values at an equilibrium state condition. The results show that most of the diluent impact does not involve changes in flame chemistry but primarily changes to heat release and heat capacity in the flames. The results also show that the diluent amount has a much larger impact on flame behavior than do changes in pressure. [ABSTRACT FROM AUTHOR]

In this work, we investigated non-equilibrium plasma produced by nanosecond repetitively pulsed glow discharges applied across a lean premixed methane-air flame. The flame is stationary, axisymmetric, and laminar. The discharges are applied on the symmetry axis crossing the reactant gases, flame front, and product gases, allowing phase-locked averaged measurements and comparisons with axisymmetric numerical simulations. The thermal effect and methyl radical production are quantified in the discharge in the reactant gas region. One-dimensional, two-beam, hybrid, femtosecond-picosecond, coherent anti-Stokes Raman scattering is used to acquire spatial and temporal profiles of temperature and oxygen-to-nitrogen concentration ratio. Photo-fragmentation laser-induced fluorescence is used to acquire quantitative two-dimensional profiles of methyl radicals in the discharge providing the first quantitative imaging of methyl produced ahead of a flame by plasma-induced methane dissociation. The spatial profiles of temperature and oxygen-to-nitrogen concentration ratio are in steady state, indicating that individual discharges have an insignificant heating effect. Upper and lower bounds of the produced mole fraction of methyl radicals in the plasma are obtained due to uncertainties in the collisional quenching rates of excited state methylidyne radicals in the plasma. The discharges produce a maximum of 600–1100 ppm of methyl radicals upstream of the flame front within 25 ns. This amount is similar to the predicted methyl mole fraction for the flame without plasma and thus represents a significant chemical perturbation to the reactants upstream of the flame front. The produced methyl follows an exponential decay in the first microsecond after the discharge with a decay constant of 8 µs close to the flame, and 0.8 µs further from the flame. The decay then deviates from the exponential curve and the methyl persists for tens of microseconds. The results suggest that for the tested configuration, the thermal effect of individual discharges through fast gas heating is negligible, while active chemical species are produced in large quantities in the reactant gases, upstream of the flame front. [ABSTRACT FROM AUTHOR]

Utilising ammonia, which is one of the most promising carbon-free fuels, comes with the challenge of stable combustion due to its low reactivity. Therefore, advanced strategies are needed, including blending ammonia with highly reactive fuels or implementing innovative combustion techniques. For this reason, the present study proposes an ammonia-methane dual-fuel co-firing combustion mode. The second focus is to analyse the effect of preheating the main swirl air on NO emissions and examine the chemical kinetic reactions. Experimentally, chemiluminescence imaging was conducted to analyse OH* and NH 2 * emissions to evaluate the flame structure and reaction zones. Numerically, simulations were performed using a chemical multi-reactor network model to examine kinetics. By comparing the fully premixed combustion to the dual-flame co-burning mode, the NO emissions decreased by 80% in the latter case at T = 473K and ϕ = 0.6. The NH 3 reaction pathway analysis shows a significantly higher percentage reduction of NO and NH 2 reactions in dual flame mode compared to premixed flames. Preheating the main air impacts NO reactions in the dual flame mode, allowing for lower NO emissions. • Pilot premixed NH 3 jet flame surrounded by swirl CH 4 flame is an alternative fuel injection strategy. • Lower emissions of NO can be attained with dual flame without compromising flame stability. • Higher NO x emissions are attained with increasing temperature for premixed flame. • NH 2 +NO ↔ NNH + OH is strengthened with increasing preheating temperature for dual flame. • Dual-fuel flame with preheating is a viable combustion method to lower NO x emissions. [ABSTRACT FROM AUTHOR]

Sustainable technologies to replace current fossil solutions are essential to meet future CO2 emission reduction targets. Therefore, this paper compares key performance indicators of a hydrogen- and a methane-flame-fired crucible furnace with computational fluid dynamics simulations at identical firing powers, aiming to fully decarbonize the process. Validated numerical models from the literature were used to compare temperatures, radiation fields, radiation parameters and heat transfer characteristics. As a result, we observed higher combustion temperatures and a 19.0% higher fuel utilization rate in the hydrogen case, indicating more efficient operating modes, which could be related to the increased radiant heat flux and temperature ranges above 1750 K. Furthermore, higher scattering of the heat flux distribution on the crucible surface could be determined indicating more uneven melt bath temperatures. Further research could focus on quantifying the total fuel consumption required for the heating up of the furnace, for which a transient numerical model could be developed. [ABSTRACT FROM AUTHOR]

The purpose of this research is to develop a combustion model that can be applied uniformly to laminar and turbulent premixed flames while considering the effect of the Lewis number (Le). The model considers the effect of Le on the transport equations of the reaction progress, which varies with the chemical species and temperature. The distribution of the reaction progress variable is approximated by a hyperbolic tangent function, while the other distribution of the reaction progress variable is estimated using the approximated distribution and transport equation of the reaction progress variable considering the Le. The validity of the model was evaluated under the conditions of propane and iso-octane with Le ≠ 1 and methane with Le = 1 (equivalence ratios of 0.5 and 1). The estimated results were found to be in good agreement with those of previous studies under all conditions. A method of introducing a turbulence model into this model is also described. the validity of the model is confirmed by a comparison with the experimental results of a turbulent methane flame. It was confirmed that the model is in good agreement with experimental results and other turbulence models, and represents approximately a conventional turbulence model. [ABSTRACT FROM AUTHOR]

Flame temperature and spectral emissivity were the important parameters characterizing the sufficient degree of fuel combustion and the particle radiative characteristics in the Rocket Based Combined Cycle (RBCC) combustor. To investigate the combustion characteristics of the complex supersonic flame in the RBCC combustor, a new radiation thermometry combined with Levenberg-Marquardt (LM) algorithm and the least squares method was proposed to measure the temperature, emissivity and spectral radiative properties based on the flame emission spectrum. In-situ measurements of the flame temperature, emissivity and spectral radiative properties were carried out in the RBCC direct-connected test bench with laser-induced plasma combustion enhancement (LIPCE) and without LIPCE. The flame average temperatures at fuel global equivalence ratio (α) of 1.0b and 0.6 with LIPCE were 4.51% and 2.08% higher than those without LIPCE. The flame combustion oscillation of kerosene tended to be stable in the recirculation zone of cavity with the thermal and chemical effects of laser induced plasma. The differences of flame temperature at α = 1.0b and 0.6 were 503 K and 523 K with LIPCE, which were 20.07% and 42.64% lower than those without LIPCE. The flame emissivity with methane assisted ignition was 80.46% lower than that without methane assisted ignition, due to the carbon-hydrogen ratio of kerosene was higher than that of methane. The spectral emissivities at 600 nm with LIPCE were 1.25%, 22.2%, and 4.22% lower than those without LIPCE at α = 1.0a (with methane assisted ignition), 1.0b (without methane assisted ignition) and 0.6. The effect of concentration in the emissivity was removed by normalization to analyze the flame radiative properties in the RBCC combustor chamber. The maximum differences of flame normalized emissivity were 50.91% without LIPCE and 27.53% with LIPCE. The flame radiative properties were stabilized under the thermal and chemical effects of laser induced plasma at α = 0.6. [ABSTRACT FROM AUTHOR]

Green ammonia has become an increasingly popular fuel in recent years because of its combustion process without carbon oxide release. Adding ammonia to methane fuel for co-combustion has become one of the important research topics in the current combustion field. In the present study, the CH4/NH3/Air counterflow diffusion flame was taken as the research object, and Chemkin-2019 R3 software was used to explore and analyze the flame extinction limit and chemical kinetics characteristics under different ammonia mixing ratios, initial pressures, and air preheating temperatures. It was obtained that the flame extinction stretch rate was decreased by increasing the NH3 mole fraction in the CH4/NH3 mixed fuel. The increase in pressure or air preheating temperature would accelerate the chemical reaction rate of each component in the combustion process, increase the flame extinction limit, and counteract the "stretching effect" of the flame, thus restraining the flame extinguishing phenomenon. The results of a path analysis show that the formation and consumption of OH had an important influence on flame extinction in the chain reaction. The net reaction rate of OH increases with increasing the initial pressure or air preheating temperature, which leads to an increase in flame intensity, combustion stability, and the extinction limit. Furthermore, the function curve between the reaction influences the RIF factor and the stretch rate of the first-to-ten reactions, affected by the heat release of flame combustion, was drawn and quantitatively analyzed. Eventually, a sensitivity analysis of the flame under different working conditions was completed, which found that promoting the forward reaction R39 H + O2<=>O + OH also promotes the positive combustion as a whole when the flame was near extinction. The sensitivity coefficient of R39 in the CH4/NH3/Air flame increases with the growing initial pressure. The increasing air preheating temperature was capable of switching the reaction of R248 NH2 + OH<=>NH + H2O in the CH4/NH3/Air flame from an inhibiting reaction to a promoting reaction, while decreasing the sensitivity coefficient of inhibiting the forward reaction R10 O + CH3<=>H + CH2O, R88 OH + HO2<=>O2 + H2O, and R271 H + NO + M<=>HNO + M. Thus, the inhibition effect of flame extinction was weakened, and the positive progress of combustion was promoted. [ABSTRACT FROM AUTHOR]

In Japan, a country prone to earthquakes, numerous damaging earthquakes have been recorded throughout history, often accompanied by descriptions of mysterious earthquake lights (EQL), which may involve various mechanisms. In this article, the possible mechanisms for different types of EQL in 11 cases are reviewed among 21 selected earthquakes. These involve preseismic physicochemical variations in the geological structure of the fault in the lithosphere, which contains deep Earth gases such as radon, methane, and others, as a primary factor for EQL generation. Additionally, various seismic, atmospheric, hydrospheric, and ionospheric variations interact with each other, resulting in the visualization of characteristic anomalous phenomena, such as glowing or shining ground, mountains, offshore areas, and skies of various colors. These phenomena appear momentarily but can sometimes last for extended periods. Because EQL often appear just before an earthquake, their study might be significant for earthquake prediction. Additionally, EQL involving methane flames in the ground is an important research topic as it relates to public safety. Was what they witnessed paranormal? [ABSTRACT FROM AUTHOR]

Energy conversion from heat to acoustics remains one of the major challenges in high-performance propulsion systems. It has been found that structure vibration may lead to combustion instability, though the mechanism behind this is still unknown. In this paper, the impact of axial mechanical vibration on a premixed methane flame is studied experimentally, using an electromagnetic vibration exciter to drive the flame burner. The burner's movement is measured by the accelerometers. The flame morphology is captured by a high-speed camera with an image intensifier via long exposure times, and the local unsteady heat release rate is measured by a photomultiplier tube. The flame transfer functions are analyzed at various equivalence ratios and mean flow velocities. The results indicate that the mechanically induced vibration leads to a clear bandpass filtering effect on the premixed flame across different equivalence ratios and flow velocities without altering the peak frequency. In addition, as the vibration amplitude increases, the flame response saturates and the flame tips become wrinkled, especially when the driving frequency aligns with the response peak. [ABSTRACT FROM AUTHOR]

The introduction of the pilot flame can stabilize the lean premixed flame and promote its industrial application. However, the interaction mechanism between the pilot and main flames is complicated. To reveal the effect of the pilot flame on the main flame, a laminar lean premixed flame adjacent to a rich premixed pilot flame on one side and a similar lean premixed flame on the other side was considered. A two-dimensional numerical model was adopted with detailed chemistry and species transport, also with no artificial flame anchoring boundary conditions. The results show that the pilot flame could promote the main flame stabilized in different locations with various shapes, by adjusting the stretch, heat transfer, and preferential diffusion in a complicated manner. The pilot flame improves the local equivalence ratio and transfer more heat to the main flame. The growth of the pilot flame equivalence ratio and inlet velocity enhances the combustion on the rich side of the main flame and helps it anchor closer to the flame wall. Both the curvature and strain rate show a significant effect on the flame root, which contributes to the main flame bending towards the pilot flame. [ABSTRACT FROM AUTHOR]

Metal fiber surface combustion, with potential advantages of low NOx emission and flashback prevention, is a promising technology to burn hydrogen-enriched methane that is regarded as a low-carbon fuel when seeking the carbon-peaking and carbon neutrality target. In this piece of work, we studied two combustion modes, namely the infrared radiation mode (flame submerging in the metal fibers and the radiation of hot metal fiber dominating the heat transfer) and the blue flame mode (flame roots attached to the metal fibers forming visible blueish flame), of premixed hydrogen-enriched methane/air mixtures on two different metal fiber surface burners in terms of their blowout limit, high-temperature corrosion of the metal fibers, and pollutant emissions. The morphology of the premixed flames on the metal fiber surface burners was found to be essentially a group of tiny premixed jet flames. Higher excess air ratio under the fuel-lean condition, lower hydrogen volumetric fraction in the fuel, and lower porosity of the metal fiber favored the blue flame mode, accompanied by the decreased NOx and increased CO. The detailed characterization of the metal fibers confirmed that the infrared radiation mode may be accompanied by the high-temperature corrosion of the metal fibers. To obtain the optimized operation conditions for hydrogen-enriched methane flames on metal fiber surface burners, we proposed and experimentally verified a dimensionless semi-empirical criterion K * for high-temperature corrosion and flame blowout by considering the ratio of overall flow velocity to laminar flame speed, the ratio of characteristic jet diameter to laminar flame thickness, the porosity of the metal fiber, as well as the preferential diffusion effect induced by hydrogen. The results showed that there existed a high-temperature corrosion risk when K * <1.4, and a flame blowout risk when K * >4. Combining the criterion and NOx emission characteristics, combustion mode phase diagrams were also demonstrated as well as the optimized operation conditions. • The metal fiber burner with two different fiber porosities was adopted. • Two combustion modes on metal fibers were experimentally analyzed. • The morphological and elemental analyses were conducted to confirm the corrosion. • The stability and emissions of hydrogen-enriched methane/air flames were studied. • A criterion was proposed to quantitatively evaluate the risks of instability and corrosion. [ABSTRACT FROM AUTHOR]

AbstractThis work investigated the effect of high-pressure (10 MPa) methane gas jet impinging on the methane lean-burn premixed flame (equivalent ratio of 0.7) based on a three-dimensional numerical simulation by CONVERGE software. The results show that laminar premixed flame is accelerated to develop into a stable turbulent flame under the action of the methane jet, the whole process of flame front development is divided into three stages: laminar (Reynolds number maintains stable, 1–1.3 ms), transition (Reynolds number shows an increasing trend, 1.4–8 ms), and turbulent (Reynolds number tends to stabilize at a high value, 1.9–3 ms). The effect of high-pressure jet on flame development along jet direction (Z axis) is greater than that on vertical direction (Y axis). During turbulence stage, the momentum and kinetic energy of Z axis are 2.7 and 6.3 times greater than that of Y axis, respectively. The high-pressure methane jet causes a change in heat distribution, resulting in local flameout. The rate of change in the local flameout area is greater than that in the flame area, causing a temperature drop in Z axis. This temperature drop increases with the increase in equivalence ratio and with the decrease in distance between cross-section position and ignition center. [ABSTRACT FROM AUTHOR]

Gas cameras are primarily used to detect gas leaks, but their use has been increasingly extended to mass flow quantification (qOGI). We employ the previously published active illuminated gas camera [Bergau et al. "Real-time active-gas imaging of small gas leaks," J. Sens. Sens. Syst. 12, 61–68 (2023) and Bergau et al. "Flow rate quantification of small methane leaks using laser spectroscopy and deep learning," Process Saf. Environ. Prot. 182, 752–759 (2024)] in a real-world application for quantification, enhancing the camera with two new features: sensitivity adaptation and camera-gas distance detection. This technology was applied to a gas leak found in the pressure swing adsorption room of a biogas plant in Germany. We compare its performance with state-of-the-art quantification gas cameras (qOGI), such as Sensia Mileva 33. Such a comparison between active and passive gas cameras is possible for the first time due to the introduced sensitivity tuning. Additionally, we enclosed the gas leak and measure the methane concentration with a flame ionization detector, providing a gold standard for comparison. Our findings revealed relative offsets to our gold standard of −57% and +319% for the DAS-camera and the Sensia, respectively, suggesting that the accuracy of mass flow quantification could be improved through the use of active gas cameras. [ABSTRACT FROM AUTHOR]

Subgrid scale models for unresolved turbulent fluxes are investigated, with a focus on combustion for space propulsion applications. An extension to the gradient model is proposed, introducing a dependency on the local burning regimen. The dynamic behaviors of the model's coefficients are investigated, and scaling laws are studied. The discussed models are validated using a DNS database of a high-pressure, turbulent, fuel-rich methane–oxygen diffusion flame. The operating point and turbulence characteristics are selected to resemble those of modern combustors for space propulsion applications to support the future usage of the devised model in this context. [ABSTRACT FROM AUTHOR]

Nanosecond repetitively pulsed (NRP) discharges have been considered a promising technique for enhancing combustion efficiency and control. For successful implementation, it is necessary to understand the complex plasma–combustion interactions involving chemical, thermal, and hydrodynamic pathways. This paper aims to investigate the mechanisms enhancing a laminar methane–air flame assisted by NRP discharges by high fidelity simulations of the jet-wall burner employed in a previous experimental study. A phenomenological plasma model is used to represent the plasma energy deposition in two channels: (1) the ultrafast heating and dissociation of O 2 resulting from the relaxation of electronically excited N 2 , and (2) slow gas heating stemming from the relaxation of N 2 vibrational states. The flame displacement, key radical distribution and flame response under plasma actuation are compared with experimental results in good agreement. The computational model allows a systematic investigation of the dominant physical mechanism by isolating different pathways. It is found that the kinetic effect from atomic O production dominates the flame dynamics, while the thermal effect plays a minor role. Hydrodynamic perturbations arising from weak shock wave propagation appear to be sensitive to burner geometry and is found to be less significant in the case under study. [ABSTRACT FROM AUTHOR]

Four-wave mixing techniques, such as coherent anti-Stokes Raman spectroscopy (CARS), laser-induced grating spectroscopy (LIGS), and degenerate four-wave mixing (DFWM), have been widely used in combustion diagnostics due to their advantages of high signal-to-noise ratio (S/N), coherent signal, and spatial resolution. In this work, a nano-second pulsed laser is utilized to generate mid-infrared (near 3 µm) pump beams, exciting the rovibrational transitions of nascent water in flames. Combined LIGS and DFWM measurements are demonstrated in premixed laminar CH4/O2/N2 flames with equivalence ratios from 0.6 to 1.5, to achieve precise thermometry in a wide range of flame conditions. The flame temperatures were also measured by thermocouple as a reference, and the results from LIGS and DFWM align well with the trends shown in the thermocouple measurements. In fuel-lean flames, where the mass-to-specific-heat ratio variation is minimal, LIGS provides temperature data with a precision better than 16 K (0.8%). In fuel-rich flames, where the increased H2 concentration in the flame introduces uncertainty in gas constants thus affecting the accuracy of LIGS thermometry, DFWM is instead employed for temperature measurement since it is less sensitive to the gas composition within the measured volume. The high-precision LIGS temperatures in lean flames serve as temperature reference during the DFWM calibration of the degree of saturation, and a precision better than 90 K (4.5%) is achieved for DFWM thermometry. In addition to temperature, a theoretical model is employed to fit LIGS signal time waveforms, extracting the local speed of sound and thermal diffusivity with precisions better than 0.5% and 1.3%, respectively. These high-precision measurements contribute additional data for flame research and simulation calculations. This way, the combined use of DFWM and LIGS proves the potential for accurate thermometry and diagnostics of other thermodynamic parameters across a wide range of flame conditions. [ABSTRACT FROM AUTHOR]

Blending hydrogen into methane impacts flame stability, morphology, and pollutant emissions. This study, set against the backdrop of hydrogen-enriched natural gas combustion technology, employs a non-premixed coaxial burner with conical bluff body to create recirculation zones for flame stabilization. Combining EDC and GRI-Mech 3.0, the research fills in the gap of effects of hydrogen blending ratio (0–100%) and equivalence ratio vary (0.4–1.4) within a wide range. Keeping equivalence ratio and fuel volume flow rate as constants, as increasing hydrogen blending ratio, the mean velocity in the chamber decreases, the size of vortex structure located in outer recirculation zones reduces, and the flame length is shortened, illustrating the reaction zone shifts to chamber inlet. Furthermore, when Ф = 1.4, average temperature and temperature uniformity are averagely larger 59.82% and 4.21% than that under Ф = 0.4. The CO 2 emission at X H2 = 80% is 54.70% averagely lower than that at X H2 = 0%, but peak concentration of NO increases. [Display omitted] • Coupled Eddy Dissipation Concept and GRI-Mech 3.0 to CH 4 /H 2 diffusion flame. • Flame shifts toward combustion chamber inlet as hydrogen blending ratio increases. • Flame width of high-temperature zones increases a little when pure hydrogen burns. • Average temperature and temperature uniformity increase with hydrogen addition. • CO 2 and NO emissions firstly increase and then decrease from fuel-lean to fuel-rich. [ABSTRACT FROM AUTHOR]

In practical applications, rapid prediction and optimization of heat transfer performance are essential for premixed methane impinging flame jets (PMIFJs). This study uses computational fluid dynamics (CFD) combined with a methane detailed chemical reaction mechanism (GRI–Mech 3.0) to study the equivalence ratio ( ϕ ), Reynolds number ( R e ) of the mixture, and the normalized nozzle–to–plate distance ( H / d ) on the heat transfer performance of PMIFJs. Moreover, the Kriging model (KM) was used to construct a prediction model of PMIFJ heat transfer performance. A genetic algorithm (GA) was used to determine the maximum likelihood function (MLE) of the model parameters for constructing KM and identify the points with the maximum root mean square error (RMSE) as the new infilled points for surrogate–based optimization (SBO). Combining these methods to analyze the simulation results, the results show that the global heat transfer performance of PMIFJs is enhanced with the increase in ϕ , the increase in R e , and the decrease in H / d . Sensitivity analysis points out that R e and ϕ significantly affect enhanced heat transfer, while H / d has a relatively small effect. In addition, GA was also used to search for the optimal heat transfer performance, and the global heat transfer performance at specific conditions was significantly enhanced. This study deepens the understanding of the heat transfer mechanism of impinging flame jets and provides an efficient method framework for practical applications. [ABSTRACT FROM AUTHOR]

Low-velocity laminar flames gradually accelerates under the joint action of various mechanisms, and develops from deflagration to detonation, and the mechanism in this process is the focus of explosion dynamics research. This article presents experimental results of the propagation of premixed hydrogen/methane flames in square-section channels with flexible and rigid obstacles at a blockage ratio (BR) of 0.3. Theory and experiment show that the acceleration effect of flexible obstacles on flame is weaker than that of rigid obstacles. The results show that there are many differences in the physical mechanism of flame acceleration. It is mainly reflected in the difference between the two obstacles on turbulence guidance, and the difference in the peak pressure of flame propagation. The flexible obstacles create smaller turbulence and absorbs the transverse waves within the pipe that continuously accelerate the flame. At the same time, compared with rigid obstacles, flexible obstacles can effectively reduce the peak flame tip speed V max , and the peak flame tip speed reduction rate is the highest by 22.86%. In addition, unburned pockets formed by obstacles play a key role in the acceleration of the flames. The expansion of gas in the flexible obstacle pocket expands the obstacle spacing, accelerates the fuel combustion in the pocket, and avoids the concentrated release of fuel energy. The intensity of the turbulence and the velocity of the flame are lower after passing through the obstacle area, showing a lower explosion risk than rigid obstacles. • Rigid and flexible obstacles have different effects on turbulence guidance and peak flame propagation pressure. • Flexible obstacles generate smaller turbulence and dampen the transverse waves of the accelerating flame in the pipe. • The addition of hydrogen not only raises the upstream and downstream pressure peaks, but also makes the peaks appear earlier. • Flexible obstacles have a smoother change in peak flame velocity and a smaller change with hydrogen percentage. [ABSTRACT FROM AUTHOR]

Understanding the methane-hydrogen lifted flame behavior is significant for improving the stability and safety of practical combustion systems. In this study, the effects of the hydrogen addition ratios and oxygen-fuel ratio on the lifted flame stability, OH* chemiluminescence distribution, and lift-off height are investigated using spectral diagnostic techniques. The results show that the addition of hydrogen can enhance the stability of lifted flame. The flame exists in a single main reaction zone and the peak intensity decreases as the hydrogen addition ratio increases. The flame lift-off height decreases as the hydrogen addition ratio increases at low oxygen-fuel ratios, but there is an opposite trend at high oxygen-fuel ratios. In addition, a fitting line is obtained by considering the effect of co-flow to summarize the effect of jet velocity variation on flame lift-off height after hydrogen addition and the fitting correlation is well established. [Display omitted] • The OH* chemiluminescence characterize of lifted flame is investigated. • The stability of methane/oxygen lifted flame is greatly improved by adding hydrogen. • The lift-off height is related to the oxygen-fuel ratio and hydrogen addition ratio. • The dimensionless correlation between jet velocity and lift-off height is obtained. [ABSTRACT FROM AUTHOR]

This article introduces a novel, fully data-driven method for forming reduced order models (ROMs) in complex flow databases that consist of a large number of variables. The algorithm utilizes higher order dynamic mode decomposition (HODMD), a modal decomposition method, to identify the main frequencies and associated patterns that govern the flow dynamics. By incorporating various normalization techniques into an iterative process, clusters of variables with similar dynamics are identified, allowing the classification of different instabilities and patterns present in the flow. This method, known as hierarchical HODMD (h-HODMD), has been thoroughly tested in the development of ROMs using three different databases obtained from numerical simulations of a nonpremixed coflow methane flame. The effectiveness of h-HODMD has been demonstrated as it consistently outperforms HODMD in terms of modeling and reconstructing flow dynamics using a reduced number of modes. Additionally, the clusters of variables identified by h-HODMD reveal the algorithm's ability to group chemical species whose behavior is consistent from a kinetic perspective. h-HODMD allows for the construction of inexpensive reduced dynamical models that can predict flame liftoff, identify the occurrence of local extinction and blowout conditions, and facilitate control purposes. [ABSTRACT FROM AUTHOR]

To enhance ammonia's flammability, blending methane with ammonia is a viable strategy to improve the laminar burning velocity of ammonia combustion. We conducted three-dimensional numerical simulations to investigate the thermal performance, the 2nd thermodynamic law efficiency, and NO emissions of a micro-combustor fuelled by ammonia/methane. Three key parameters are identified and examined. They include: 1) the inlet volume flow rate, 2) the CH4 mole blended ratio, 3) the N 2 dilution rate. It is found that increasing the inlet volume rate give rise to an increase of the combustor outer wall temperature, and so the thermodynamic second-law efficiency, but a reduction of NO emissions. For example, when the inlet volume flow rate is set to 14.4 mL/s, the wall temperature and 2nd law efficiency are increased by 36 % and 21 % respectively, but the NO emission is reduced by 15 %, in comparison with those in the presence of 7.2 mL/s inlet flow. While an increase of the CH 4 blended ratio (molar fraction) is shown to have limited impact on the thermal performances. This variation of such blended ratio is found to be associated with a notable 22 % reduction of NO emissions. Additionally, injecting N 2 as a dilution gas is shown to be not beneficial to the thermal performance. However, NO emissions are reduced. When the N 2 dilution rate is set to 0.6, the wall temperature is found to be reduced by 173 K. However, 47 % more NO reduction is observed in comparison with those in the presence of the N_2 dilution rate being set to 0.3. • 3D numerical investigation of a micro-combustor fuelled by methane/ammonia is conducted. • Entropy generation analyses of the ammonia/methane-fuelled micro-combustor are performed. • Examining the effect of blending methane with ammonia on thermodynamic performances is conducted. • NO emissions from the ammonia/methane-fuelled micro-combustor are evaluated. • A reduced chemical reaction mechanism of ammonia-methane combustion is proposed. [ABSTRACT FROM AUTHOR]

Adding methane to methanol can solve the problem of difficult cold starts of methanol engines. Therefore, it is important to understand the combustion of methane and methanol fuel blends. Because of this, this study explores the effect of methane addition on the laminar burning velocity and Markstein length of methanol/methane premixed flames under stoichiometric conditions at the initial temperatures of 353 K, 373 K, and 393 K, initial pressures of 1 bar, 2 bar, and 4 bar, using methane addition ratios of 0%, 25%, 50%, 75%, and 100% in a constant volume combustion chamber. The results show that the laminar burning velocity decreases linearly with the increase of methane addition ratio due to the linear decrease of the Arrhenius factor. The sensitivity analysis revealed that the kinetic effect is the main reason for the inhibition of laminar burning velocity, which is insensitive to initial temperature but enhanced at a high initial pressure. The Markstein length decreases with the addition of methane and the increase of initial pressure, which is mainly caused by the high mass diffusivity of methane and the decrease of flame thickness due to the increase of initial pressure. [ABSTRACT FROM AUTHOR]

Experimental and numerical study of flame and emission characteristics in a tubular micro gas turbine combustor is reported. Micro gas turbines are used for distributed power (DP) generation using alternative fuels in rural areas. The combustion and emission characteristics from the combustor have to be studied for proper design using different fuel types. In this study methane, representing fossil natural gas, and biogas, a renewable fuel that is a mixture of methane and carbon-dioxide, are used. Primary air flow (with swirl component) and secondary aeration have been varied. Experiments have been conducted to measure the exit temperatures. Turbulent reactive flow model is used to simulate the methane and biogas flames. Numerical results are validated against the experimental data. Parametric studies to reveal the effects of primary flow, secondary flow and swirl have been conducted and results are systematically presented. An analysis of nitric-oxides emission for different fuels and operating conditions has been presented subsequently. [ABSTRACT FROM AUTHOR]

Measuring soot concentration in a burner flame is essential for an in-depth understanding of the formation mechanism and to abate its generation. This paper presents an improved emission spectroscopy (ES) method that uses an adaptive particle swarm optimization (APSO) algorithm for measuring the concentration of soot in methane burner flames. Experimental tests were conducted on a laboratory-scale facility under a methane flowrate ranging between 0.6 and 0.9 L/min. A comparison analysis of the soot concentration measured by the ES method, the improved emission spectroscopy (IES) method, and the thermocouple particle density (TPD) method (as a reference) was conducted. The ES method obtained a maximum absolute deviation of 0.84 ppm from the average soot concentration at the three measurement points compared to the TPD method, while that of the IES was only 0.09 ppm. The experimental results demonstrate that the proposed IES method can obtain a more accurate soot concentration of diffusion flames. [ABSTRACT FROM AUTHOR]

A numerical investigation is conducted using OpenFOAM to study the characteristics of ammonia flames enriched with methane and hydrogen, influenced by a diluted co-flow (exhaust gas). The study aims to explore various flame characteristics, including fuel consumption, ignition delay time, lift-off height and position, temperature, and CO and NO emissions while maintaining a constant energy input of 5 kW. Non-stabilized flames are identified by analyzing the OH peak in the computational domain among ammonia flames enriched with CH 4 /H 2 for different dilutions of O 2 (6 %, 9 %, and 12 %). The study is structured into three stages of numerical simulations corresponding to stabilized flames: (1) no reaction, (2) reaction in the absence of O 2 , and (3) reaction with O 2 to see the sole impact of the hot co-flow and local turbulence mixing on the primary cracking of ammonia. Furthermore, the rates of H 2 and N 2 formation resulting from the utilization of ammonia in the presence of hot co-flow are thoroughly examined. Additionally, the behavior of ammonia flames and CH 4 /H 2 -enriched ammonia flames under preheated air conditions (23 % O 2) is analysed. Stable flames are observed in the co-flow at 12 % O 2 , displaying characteristics of Moderate or Intense Low-oxygen Dilution (MILD) combustion. Conversely, ammonia flames under preheated air conditions exhibit conventional flame behavior. Pure ammonia flames exhibit stronger temperature gradients and higher levels of CO and NO emissions. As the enrichment of NH 3 with CH 4 /H 2 increases, peak temperatures decrease while lift-off heights increase, indicating a transition towards more distributed flame structures. Notably, the instance with 40 % H 2 enrichment exhibits lower NO emissions and a higher rate of ammonia consumption along with more distributive flame characteristics. • Thoroughly examined NH 3 flames in co-flow with varying levels of O 2 dilution. • The stability of ammonia flames is tested under various O 2 dilutions. • Primary cracking of NH 3 enhanced under the hot co-flow conditions. • Minimum NO emissions were observed under lower O 2 dilutions with H 2 enrichment. • MILD NH 3 flames are identified among the CH 4 /H 2 enriched NH 3 diluted flames. [ABSTRACT FROM AUTHOR]

Under an equivalency ratio of 1.1, this manuscript studies the fireballs of methane-combustible gas clouds with varying hydrogen doping ratios (5 %, 10 %, 20 %, 30 %, 50 %, and 75 %). It then modifies the semiempirical-theoretical fireball heat radiation model based on flame size. Under 5%–30 % mixing ratio, flame growth characteristics are more comparable, and peak flame propagation speed is 18 m/s, whereas the mixing ratios of 50 % and 75 % are higher, take 0.1s to complete the accelerated process, and reach 26.7 and 36.7 m/s, respectively. In flames including methane hydrogen-doped mixtures, thermal diffusion instability and hydrodynamic instability are common. With an increase in the hydrogen doping ratio, the flame self-acceleration index shows a steady rising trend. Additionally, heat flux meter peak heat radiation values increase with hydrogen blending ratio, while the time to peak decreases. These findings align with the developmental process of flame morphology. Based on the observed flame development characteristics, a semi-empirical-theoretical model of heat radiation from the fireball is simplified. The results show that as the hydrogen doping ratio increases, the safety zone must be moved further away. • The flame development characteristics of methane-hydrogen gas clouds are described. • The effects of different hydrogen mixing ratios on fireball thermal radiation are analyzed. • The semi-empirical prediction model of thermal radiation is modified and the safe distance is proposed. [ABSTRACT FROM AUTHOR]

This study investigates the impact of H 2 addition on soot formation in inverse diffusion flame (IDF) and explores the underlying mechanism of flame structure on soot formation. Experimental measurements and simulation kinetic calculations were performed to explore the influence of free radical (OH*/CH*/C 2 */CO 2 *) chemiluminescence on soot formation mechanism. The investigation ultimately determines the relationship between free radical chemiluminescence and soot formation. The findings reveal that similar to the simulation results, the peak soot volume fraction (SVF) is reduced by approximately 60 % when the hydrogenation increases to 40 %, and the initial soot production position moves downstream of the flame. The free-radical chemiluminescence distribution is mainly concentrated at the burner exit, where the increase in hydrogen concentration leads to a decrease in CH* and C 2 * and an increase in OH* content in the flame. The blue region on the fuel side is consistent with the peak CH* and C 2 * emission region. In the soot formation, the chemiluminescence signal of free radicals is suppressed, indicating that soot formation and free radical chemiluminescence are antithetical. The results of the component yields indicate that soot formation and chemiluminescence are fueled by gas phase components (CH 3 , C 2 H 3 , C 2 H 2 , etc.) and there are generated in opposite pathways, which supports the above statement. Furthermore, the increase of hydrogen content inhibits the production of C 2 H and CH 2 , resulting in the decrease of C 2 *, CH* and CO 2 * radical chemiluminescence by 77.0 %, 75.1 % and 81.2 %, respectively. The decrease of soot concentration is primarily due to the increase of H 2 , which hinders the formation of C 3 H 3 and C 4 H 5 -2. • The different effects of H 2 addition on soot generation and free radical chemiluminescence were determined. • The antiposition relationship between soot generation and free radical chemiluminescence was clarified. • Soot and chemiluminescence reaction pathways were analyzed to gain insight into how H 2 addition affects these pathways. [ABSTRACT FROM AUTHOR]

In this work, the laminar premixed flames for methane and propane at atmospheric pressure were numerically investigated with respect to different hydrogen concentrations, ranging from 5 % to 25 % (by vol.). Based on the two-dimensional (2D) flame predictions, hydrogen decreased the maximum temperatures and species concentrations such as formaldehyde (CH 2 O), acetylene (C 2 H 2), carbon monoxide (CO) and nitric oxide (NO) for methane and propane. However, the CH 2 O mole fractions were raised by a maximum deviation of 10 % when hydrogen was added into the fuel lean and stoichiometric propane mixtures. The decrease in C 2 H 2 eventually lowered the soot volume fractions and number densities. Besides, the axial locations of the maximum species concentrations, soot volume fractions and number densities were also affected by hydrogen. Meanwhile, the addition of hydrogen increased the flame temperatures and laminar burning velocities for the one-dimensional (1D) premixed flames of methane and propane. This trend was identically predicted for the formation of intermediate species such as CH 2 O, hydroperoxyl (HO 2) and hydroxyl (OH) whereas C 2 H 2 , CO and NO were lowered, as justified by the sensitivity analyses. For propane at an equivalence ratio (φ) = 1.3, the rates of production (ROPs) of C 2 H 2 were increased linearly with hydrogen, which resulted in higher C 2 H 2 concentrations. • Methane and propane coflow flames with hydrogen addition are modelled in 1D and 2D. • Hydrogen lowers flame temperatures, CH 2 O, C 2 H 2 , CO and NO for 2D flames. • For 1D flames, hydrogen raises flame temperatures and intermediate species. • Effects of hydrogen on emissions depend on fuel types and mixture conditions. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of counterflow methane/n-heptane dual fuel flames, investigating the influence of pressure on flame structure and NO generation characteristics. The results demonstrate that, as pressure increases, the peak temperature and heat release rate of the flame increase, while the NO emission index decreases. The prompt NO emission index experiences a monotonic reduction as pressure increases, while the thermal NO emission index initially increases and then decreases. NO emission index via reburn route displays non-monotonicity with increasing pressure. Elevated pressure induces a decline in the mole fraction of radicals (CH, CH 2 and HCCO) crucial for prompt NO formation, consequently suppressing the formation of prompt NO. The principal reaction responsible for the consumption of reburn NO at lower pressures is HCCO+NO CO+HCNO, whereas the primary reaction for generating reburn NO is H+HNO H 2 +NO. However, at higher pressures, the reaction H+HNO H 2 +NO takes precedence as the dominant reaction consuming NO. • NO emission index decreases with increasing pressure. • Sensitivity analysis of NO generation under different pressures is conducted. • The formation routes of NO under different pressures are identified and analyzed. • The primary NO formation route changes from prompt to thermal as pressure rises. [ABSTRACT FROM AUTHOR]

The combustion properties of hydrogen-containing fuel mixtures and the effect of the variation of the oxygen content of the oxidizer are at the center of recent research interest. Laminar burning velocity measurements with varied oxygen content can help in the validation of reaction mechanisms for better simulations of combustion systems using exhaust gas recirculation (EGR) or oxygen-enriched atmosphere. Such measurements were carried out in hydrogen-enriched methane−air flames using the heat flux method with higher accuracy and a wider range of initial oxygen and hydrogen concentrations compared to the similar studies in the literature. The mole fractions of the hydrogen and oxygen contents of the initial fuel and oxidizer mixtures were varied between x H2 = 0 and 0.20, and x O2 = 0.14 and 0.23, respectively. The initial gas temperature and pressure were 298 K and 1 bar, respectively. It is demonstrated that the increase of combustion rate by the hydrogen enrichment can be compensated with the decrease of the oxygen content. This compensating effect was investigated in detail in a wide range of equivalence ratio (φ). The experimental data were simulated with 11 widely used methane combustion reaction mechanisms. The prediction accuracies of the mechanisms at lean and rich equivalence ratios were significantly different and the important reaction steps were identified using sensitivity analysis for three mechanisms. Mechanisms POLIMI-2014 and Caltech-2015 gave the best overall predictions. • Influence of O 2 content on the LBV of premixed laminar CH 4 –H 2 flames was measured. • Higher H 2 content increases the LBV, linear trend in the range of 0–20 vol %. • Reduction of O 2 reduces the LBV, nonlinear trend in the range of 14–23 vol %. • Experimental data compared with simulation results of 11 reaction mechanisms. • Results of the 4 best mechanisms had an average deviation near 3σ error limits. [ABSTRACT FROM AUTHOR]

An interband cascade laser (ICL)‐based absorption sensor was developed for calibration‐free and high‐fidelity measurement of nitric oxide (NO) in laminar premixed methane/ammonia cofired flames using a combination of a micro‐probe and a low‐pressure, high‐temperature gas cell. The developed sensor, which has been shown to be particularly suitable for kinetic studies of ammonia combustion, used a tunable, continuous, narrow‐linewidth, free‐space ICL operating at ~5.2 μm to target the optimal NO transition centered at 1900.07 cm−1. A direct absorption spectroscopy technique with a reduced line model was implemented for calibration‐free measurements. Comprehensive experiments were first conducted to evaluate the sensor performance against the standard gas mixture at varied pressure from 20 to 101 kPa at 393 K, showing a relative difference of ≤2.5% and a measurement uncertainty of ≤2.0%. Such a sensor shows a minimum detection limit of ~2 ppm at the integration time of 1 s. The sensor was then applied for investigating the NO formation in ammonia‐methane cofiring flames with various ammonia blending ratios; the acquired experimental data was further used as benchmark data for evaluation of literature chemical mechanisms for ammonia combustion. The developed sensor system was demonstrated to be capable of providing quantitative and spatially resolved measurement of NO in ammonia‐methane cofired flames. [ABSTRACT FROM AUTHOR]

Adding hydrogen to methane has gained interest as a means of reducing CO 2 emissions from combustion systems. However, this process can lead to several issues, such as flashback and NOx emissions. Therefore, it is essential to determine the optimal conditions for flame stabilization and emission to design effective hydrogen burners. This study chose a turbulent fuel jet with coaxial air as the basic configuration, and its combustion characteristics were investigated under elevated pressures. Additionally, the depth of the fuel tube in the coaxial air tube was actively controlled to regulate flame stabilization, and its effects were examined for various pressures, velocities, and hydrogen concentrations. Three flame stabilization modes were observed clearly, i.e., inner-attached, outer-attached, and inner-lifted modes. Furthermore, the parameters that influenced the flame modes were identified, and the characteristics of soot and NOx emissions were studied. The inner-lifted mode demonstrated the best combustion performance, and the corresponding conditions were predicted to be extended through hydrogen enrichment and pressure elevation. Therefore, the inner-lifted flame is recommended as the conceptual design goal for hydrogen-enriched high-pressure burners. [Display omitted] • Non-premixed jet flames of hydrogen and methane mixtures were investigated. • Effects of coaxial air and pressure on flame stabilization were explained. • Emission characteristics of soot and NOx were examined. • An optimal condition for flame stabilization and emission was suggested. [ABSTRACT FROM AUTHOR]

This paper first presents particle image velocimetry measurements of the chemical time scales versus equivalence ratio and the concentration of the inhibitor trimethyl phosphate for premixed methane–air and dimethyl ether–air flames at atmospheric pressure. Comparison of the experimental results with theoretical estimates based on the Zel'dovich–Barenblatt hypothesis shows their qualitative agreement. Within the accuracy of the experiment, the chemical time scale depends only on the burning rate, rapidly decreasing as it increases. At fuel–air flame speeds close to and above 0.6 m/s, the results of the experiments show the high accuracy of theoretical estimates based on the Zel'dovich–Barenblatt hypothesis. [ABSTRACT FROM AUTHOR]

Flickering buoyant diffusion methane flames in weakly rotatory flows were computationally and theoretically investigated. The prominent computational finding is that the flicker frequency nonlinearly increases with the nondimensional rotational intensity R (up to 0.24), which is proportional to the nondimensional circumferential circulation. This finding is consistent with the previous experimental observations that rotatory flows enhance flame flicker to a certain extent. Based on the vortex-dynamical understanding of flickering flames that the flame flicker is caused by the periodic shedding of buoyancy-induced toroidal vortices, a scaling theory is formulated for flickering buoyant diffusion flames in weakly rotatory flows. The theory predicts that the increase of flicker frequency f obeys the scaling relation f - f 0 ∝ R 2 , which agrees very well with the present computational results. In physics, the external rotatory flow enhances the radial pressure gradient around the flame, and the significant baroclinic effect ∇ p × ∇ ρ contributes an additional source for the growth of toroidal vortices so that their periodic shedding is faster. [ABSTRACT FROM AUTHOR]

Pilot diesel ignition is an effective approach for achieving efficient and clean combustion of natural gas. In this study, a rapid compression and expansion machine (RCEM) was constructed for examining diesel-ignited premixed methane combustion. The effects of the methane equivalence ratio and pilot diesel mass on the combustion process of diesel-ignited premixed methane gas were investigated. The results show that the combustion process can be divided into two stages: diesel dominance and premixed methane combustion. An increase in the methane equivalence ratio inhibits diesel combustion, leading to delayed CA10 and OH radical generation. However, it enhances premixed methane flame propagation and improves the heat release rate, resulting in a shorter combustion duration. An increase in the pilot diesel mass contributes to a larger flame area and higher OH generation intensity in the ignition region; however, too large a diesel mass inhibits methane flame propagation towards the diesel nozzle due to an extended injection duration. In conclusion, a larger pilot diesel mass can achieve better overall combustion performance, but excessive amounts may be counterproductive. [ABSTRACT FROM AUTHOR]

Due to the severe damage of Halon to the stratospheric ozone layer, the urgent need for substitutions for Halon has driven the search for potential alternatives. As a perfluoroolefin substance, R1216 (1,1,2,3,3,3‐hexafluoro‐1‐propene) has a similar chemical structure to the widely used 2‐bromo‐3,3,3‐trifluoro‐1‐ene (CF3CBrCH2, 2‐BTP) extinguishants. This study revealed the thermal decomposition and fire‐extinguishing performance of R1216 using theoretical calculations and experimental measurements. It was found that R1216 has high thermal stability and does not decompose at 600°C, and not only achieves the purpose of chemical extinguishment by generating perfluoroalkanes, perfluoroolefins and CF3· radicals that can capture H· and OH· radicals in the flame to interrupt the chain reactions of combustion, but also achieve the goal of cooling by absorbing heat through bond breaking. A combination of physical and chemical inhibition makes R1216 ideal for fire suppression (6.78 and 7.40 vol% for methane and propane flames, respectively). R1216 does not contain Br· and has a global warming potential of 0, which is more environmentally friendly. These findings suggested that R1216 may be a potential Halon substitute with promising applications and deserved further evaluation. [ABSTRACT FROM AUTHOR]

Motivated by the pioneering activation-energy asymptotic analysis of strained laminar premixed flames in counterflow by Libby and his coworkers, a rate-ratio asymptotic analysis is carried out to elucidate the structure and predict the critical conditions of extinction of strained premixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams: a reactive mixture stream and a product stream toward a stagnation plane is considered. The temperature of the reactive mixture stream is T 1 and it is made up of methane (CH4), oxygen (O2) and nitrogen (N2), while the temperature of the product stream is T 2 , and it is made up of O2, carbon dioxide, water vapor and N2. The asymptotic flame structure is presumed to be made up of a thin reaction zone where all chemical reactions take place. On one side of the reaction-zone is an inert, preheat zone containing the reactants and on the other side a post-flame zone made up of products. Analysis of the preheat zone gives matching conditions that is required to analyze the structure of the reaction zone. A four-step, reduced mechanism is used to describe the chemical reactions. The reaction zone is presumed to be made up of an inner layer, where CH4 is consumed. The hydrogen (H2) and carbon monoxide that are formed in this layer are consumed in an oxidation layer that is made up of two layers: an H2-oxidation layer and a CO-oxidation layer. The results of the analysis are used to predict the flame location, y r , flame temperature, T r , and the speed of the convective flow, v b , in the reaction zone as a function of the strain-rate, a u . Classical C-shaped curves were obtained when y r , T r and v b are plotted as a function of a u and they were used to predict extinction. A key finding of this work is that v b is proportional to T r − T 0 , where T 0 is the crossover temperature predicted by the rate-ratio asymptotic analysis. Whether abrupt extinction will take place or not was found to depend on the value of T 2 relative to T 0 , which is different from the predictions of activation-energy asymptotic analysis where T 2 must be compared with the value of the adiabatic temperature. Similar to the analysis of Libby and his coworkers, the rate-ratio asymptotic analysis predicts the existence of "negative flame speeds," where the convective flow and the diffusive flow of reactants in the reaction zone, are in opposite directions. The predictions of the rate-ratio asymptotic analysis were found to agree with the results of computations with detailed chemistry and previous experimental data. [ABSTRACT FROM AUTHOR]

The article investigates liquid oxygen (LOx)–methane supercritical combustion dynamics in a multi-element rocket-scale combustor using large eddy simulation (LES). A complex framework of real gas thermodynamics and flamelet-generated manifold (FGM) combustion model is invoked to simulate transcritical oxygen injection and supercritical methane combustion. A benchmark Mascotte chamber, rocket combustion Modeling (RCM) test case, i.e., RCM-3 (V04)/G2 test case, is used to validate the real gas FGM model in the LES framework. The validation study accurately reproduces experimental flame structure and OH concentration, demonstrating the FGM model's importance in incorporating finite rate kinetics in LOx–methane combustion. Subsequently, the numerical framework investigates a specially designed multi-element combustion chamber featuring seven bidirectional swirl coaxial injectors. The analyses capture the complex hydrodynamics and combustion dynamics associated with multiple swirl injectors operating at supercritical pressure, effectively demonstrating the initiation of transverse acoustic waves and examining the effect of local sound speed on the evolution of acoustic modes in the combustor. The dominant frequency modes shed light on understanding the role of injectors in enhanced combustor dynamics. Spectral analysis reveals the interplay of the upstream injector and chamber acoustics due to possible frequency coupling. The results also highlight the effect of fuel injection temperature on the stability of the combustor, revealing a violent dynamic activity for lower fuel injection temperature associated with the longitudinal acoustic mode of the combustor. The investigation appropriately reproduces self-sustained limit-cycle oscillations at lower fuel injection temperatures and corroborates the conventional understanding of combustor instability. [ABSTRACT FROM AUTHOR]

A large-eddy simulation (LES) of an atmospheric confined jet flame test case is presented, which represents a model gas turbine combustor and for which detailed experimental data is available. A genetic algorithm approach is used to develop a new, cost-effective reduced mechanism for lifted lean premixed methane-air flames at atmospheric conditions. For the mechanism development, auto-ignition-delay time of mixtures of reactants and cooled down combustion products has been introduced as an optimization criterion. The new mechanism consists of 11 species and 12 reactions. The developed mechanism is validated by comparing the results of zero-dimensional (0D) reactor and one-dimensional (1D) flame simulations against results from the well-known reaction mechanism, GRI-3.0 and Lu19. The three-dimensional (3D) LES are carried out using a finite-rate chemistry (FRC) approach combined with the dynamic thickened flame (DTF) model. In the simulations, the developed mechanism is compared against the Lu19 mechanism and experimental data. Simulations using Lu19 show that the flame is predicted accurately. The new mechanism predicts the flame liftoff and position well, while slightly underpredicting the flame length and showing deviation in quenching behavior but achieves a very significant speedup factor of approximately 2.9 for the entire 3D simulation. For the DTF model, two flame sensor functions are compared, the first based on the progress variable and the second on the local heat release rate. The heat release based formulation is found to be preferable, as it detects the reaction region well, as opposed to the progress variable based formulation which additionally senses zones where reaction products are mixed with reactants, i.e. zones where no classical premixed flame propagation is observed. [ABSTRACT FROM AUTHOR]

[Display omitted] It was shown that at 298 K and 100–300 Torr, the flame front of a well-mixed dilute methane–oxygen mixture, propagating to the ends of hollow cylindrical and conical obstacles, does not form a vortex shedding behind them; however, under the same conditions, this instability occurs when hot products flow behind the obstacles. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study of flames propagating with an alternating-sign velocity in methane–air and coal dust–methane–air mixtures in a vertically located closed tube at coal dust concentrations of 0.10–0.42 kg/m3. The results of the study can be useful for developing combustion models and assessing dynamic and thermal effects during combustion of methane–air–coal dust mixtures in coal mines. [ABSTRACT FROM AUTHOR]

This work experimentally and numerically investigates the effect of the obstacle gradient on the characteristics of the methane/air explosion in obstructed ducts. The obstacle gradient is expressed through various blockage ratios, and three different obstacle gradients are investigated, i.e., C357, C555, and C753. Noted that C357 means that the blockage ratios of three obstacles arranged sequentially in the duct are 0.3, 0.5 and 0.7 respectively, and so do C555 and C753. A two-dimensional (2D) model is adopted and the Scale-Adaptive Simulation method with the thickened flame model is considered. Experimental results show that the obstacle gradient significantly affects the flame evolution structure, flame propagation speed and overpressure. The obstacle gradient is accountable for the "tulip flames" appearing downstream of the obstacle. For the case with a fixed methane volume fraction, the average flame front speed is fixed. However, with the increasing obstacle gradient, the maximum flame front speed increases until it achieves the maximum at C357, and so does the maximum overpressure. The numerical simulation can predict the flame evolution behaviour precisely. It is evident that the generation of different flame shapes appearing is derived from the flow field evolution of unburned gas downstream of the flame front. [ABSTRACT FROM AUTHOR]

Research and development of new fire-extinguishing agents to replace halon is very urgent, and the properties of some fire-extinguishing agents with potential application value have not been studied. The molecular structure of octafluorocyclobutane (C4F8, C-318) is a quaternary ring structure consisting of four CF2, resulting in suitable chemical activity and potential fire-extinguishing performance. However, the fire-extinguishing performance and mechanism of C-318 as a fire-extinguishing agent have not been studied in previous reports. In this paper, to explore environmentally friendly and high-performance chemical gas fire-extinguishing agents to replace halon agents, experimental measurements and theoretical calculations were performed to measure the extinguishing performance of C-318 against a methane flame and a propane flame, and to thoroughly analyze the reaction mechanism of C-318 with chain radicals. It was found that C-318 has relatively low minimum extinguishing concentrations (MECs) for suppressing methane–air (7.23 vol%) and propane–air (7.40 vol%) flames. Furthermore, combined with theoretical analysis of the decomposition mechanism and extinguishing mechanism, the feasibility of C-318 as a fire-extinguishing agent is verified, which lays a good foundation for large-scale experiments into its engineering application as a substitute for halon. [ABSTRACT FROM AUTHOR]

This article studies the generation of entropy disturbances by laminar premixed flames. The total entropy generation equals the integrated ratio of the local heat release rate and the local temperature, namely, ∫ (q ˙ / T) d V. Due to this path dependency, evaluating this integral requires an understanding of how the heat release is distributed in the temperature space. Several studies evaluate the local entropy generation as (∫ q ˙ d V) / T b , where T b refers to the burned gas temperature, implicitly assuming all the heat release occurs at T b. Such an approximation is motivated by the high activation energy nature of combustion chemistry. This work evaluates this assumption by comparing it to results from one-dimensional premixed flame calculations for hydrogen, methane, and propane-air flames over a range of pressures, equivalence ratios, and preheat temperatures, quantified via the ratio κ. We show that this assumption is quite reasonable for methane and propane-air flames (with errors ranging from 5% to 25%) but deviates significantly from the exact results for hydrogen flames (where errors can be as high as 50%). In general, the peak heat release moves to lower temperatures as preheat temperature is increased. Noting that the temperature sensitivity of heat release is directly related to the activation energy, we use Law's approach to extract global activation energies and show that the deviations of κ from unity can be approximately correlated with β e f f . Finally, we show that significant improvements in entropy generation calculations can be obtained by estimating ∫ (q ˙ / T) d V using (∫ q ˙ d V) / T p e a k , where T p e a k is the temperature at which the reaction rate peaks. This estimation leads to predictions of ∼5% within the exact value for the hydrocarbon cases but can still be in significant error for hydrogen at certain conditions. [ABSTRACT FROM AUTHOR]

Gas-fired boilers, gas stoves, and wall-mounted gas boilers are the main consumers of gas fuel, but they generally encounter problems when operating at high altitudes, such as reduced thermal efficiency and increased pollutant emissions. Previous studies on gas combustion characteristics under sub-atmospheric pressure were mostly carried out in a large space, which is quite different from chamber combustion equipment. Therefore, it is insufficient to guide the design and operation optimization of plateau gas equipment. In this paper, experimentations were carried out to explore the characteristics of a methane diffusion flame under sub-atmospheric pressures. The mass flow rates of methane and air remain consistent under different pressure conditions. The centerline temperature ( T c ) distribution, flame appearance, smoke point, CO emission, and NOx emission under different pressures (ranging from 61.66 to 97.75 kPa) were examined under both fuel rich and lean conditions. The results show that T c at the rear and front of furnace variation with pressure is opposite under fuel-lean and -rich combustion. The T c at the front of furnace decreases with decreasing pressure, whereas T c at the rear of furnace increases with decreasing pressure. With decreasing pressure, flame length decreases under lean combustion, but increases under rich combustion. The smoke point fuel flow rate, flame length, and residence time increases with decreasing pressure, following the law of negative exponent. The CO emission decreases with decreasing pressure, which indicates that the reduced pressure makes methane combustion more complete. For NO emission, the reduced pressure results in an opposite tendency under fuel-lean and -rich combustion. With decreasing pressure, the NO emission decreases under fuel-lean combustion but increases under fuel-rich combustion. [ABSTRACT FROM AUTHOR]

Flame Propagation in closed chambers is one of the most important process in the industry which may lead to a severe damage in the systems. The purpose of this study is to observe the flame propagation process of methane/air mixture and to investigate the flame velocity using perforated plates in a cubic enclosure. The experiments are performed at atmospheric pressure. The flame front of the methane/air mixture is visualized by using a high speed camera. In this study, perforated plates are used with hole sizes of 5 mm, 4 mm, 3 mm and 2 mm. by increasing the hole size of the perforated plates, the velocity of the flame increases after hitting the perforated plates. Therefore, in the case of a hole diameter of 5 mm in the perforated plate, the maximum flame tip speed (i.e. 12 m/s) is recorded. By increasing the hole diameter (between 2 mm to 5 mm), the absolute pressure generated by combustioninside the chamber is augmented up to 2.97 (bara). In addition to considering the effect of the hole diameter of perforated plates, the effects of the position of perforated barriers (quenching distance) on flame progress, flame velocity, combustion chamber pressure and flame quenchingare investigated. Also, flame quenching patterns (head on and sidewall) arevisualized and the effects of the position of obstacles from the spark plug on changing the flame quenching patterns are analyzed. By visualization of the quenching process of premixed flame in the presence of perforated plates, both head-on quenching, and sidewall quenching are analyzed by using nondimensional groups. According to the results, by increasing distance of the perforated obstacle from spark, the flame quenching pattern changes from head-on mode to side-wall mode. The quenching mode, in this condition, is related to the flame Reynolds number. [ABSTRACT FROM AUTHOR]

The Adiabatic Flame Temperature (AFT) in combustion represents the maximum attainable temperature at which the chemical energy in the reactant fuel is converted into sensible heat in combustion products without heat loss. AFT depends on the fuel, oxidizer, and chemical composition of the products. Computing AFT requires solving either a nonlinear equation or a larger minimization problem. This study obtained the AFTs for oxy-methane (methane and oxygen), oxy-hydrogen (hydrogen and oxygen), air-methane (methane and air), and air-hydrogen (hydrogen and air) for stoichiometric conditions. The reactant temperature was 298.15 K (25°C), and the pressure was kept constant at 1 atm. Two reaction mechanisms were attempted: a global single-step irreversible reaction for complete combustion and the GRI-Mech 3.0 elementary mechanism (53 species, 325 steps) for chemical equilibrium with its associated thermodynamic data. NASA CEARUN was the main modeling tool used. Two other tools were used for benchmarking: an Excel and a Cantera-Python implementation of GRI-Mech 3.0. The results showed that the AFTs for oxy-methane were 5,166.47 K (complete combustion) and 3,050.12 K (chemical equilibrium), and dropped to 2,326.35 K and 2,224.25 K for air-methane, respectively. The AFTs for oxy-hydrogen were 4,930.56 K (complete combustion) and 3,074.51 K (chemical equilibrium), and dropped to 2,520.33 K and 2,378.62 K for air-hydrogen, respectively. For eight combustion modeling cases, the relative deviation between the AFTs predicted by CEARUN and GRI-Mech 3.0 ranged from 0.064% to 3.503%. [ABSTRACT FROM AUTHOR]

Ammonia is a promising carbon-free fuel, while, understanding of ammonia jet flame is still in lack. In this work, a novel facility was applied and air coflowed ammonia jet flames were achieved in an elevated ambient temperature range, 723–923K. Stabilization regimes and limits were investigated. Stable lifted flame with a classical triple structure was observed, and critical aerodynamic parameters were measured at three specific regimes, liftoff, reattachment and blowoff. Attached flame can only be retained under laminar conditions with flow Reynolds number <150. A linear correlation between velocities of fuel jet and coflow under critical conditions was uncovered, which is different from the literature research on methane flames. Effects of partially premixing and N 2 dilution were considered. Partially premixing was found harmful to stabilization at 823K, while this influence becomes unclear at 923K. Differently, a linearly adverse effect was observed under both N 2 -diluted jet and coflow conditions at different temperatures. [Display omitted] • First insight into stabilization of air coflowed NH 3 jet flames. • Elevated-ambient-temperature flame facility. • Flame regimes and stabilization maps. • Effects of partially premixing and dilution. [ABSTRACT FROM AUTHOR]