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Methane (CH 4) serves as a significant hydrogen carrier and is the predominant component of natural gas. The comprehension of CH 4 combustion characteristics and chemical kinetics under unconventionally lean and rich conditions is essential for advanced industrial processes such as lean burn operations and syngas reforming. However, due to challenges in flame stabilization, corresponding experimental data on key CH 4 combustion characteristics such as laminar burning velocity are scarce, which in turn leaves existing kinetic models unvalidated. In this study, the laminar burning velocities of CH 4 +O 2 +N 2 , CH 4 +O 2 +Ar, and CH 4 +O 2 +CO 2 flames were measured using the heat flux method at 1 atm and 298 K. The equivalence ratio spanned from the very lean range of 0.3–0.6 to the very rich range of 1.5–1.8, through an oxy-fuel combustion approach, with most conditions being unexplored in existing literature. Simulations adopted six kinetic models that are widely-used for predicting CH 4 flames. These models exhibited mixed performance when predicting different diluent types and oxygen ratios, with none capable of accurately reproducing all experimental data within the uncertainty ranges. Analyses on A-factor reaction sensitivities were performed, along with rate constant uncertainty evaluations, based on which the model from the authors was updated. The updated model is compact, consisting of 91 species and 483 reactions, with shortened versions provided for different calculation purposes, and reproduce well the present experimental data. Furthermore, it maintained good predictability for previous validation targets of laminar burning velocities, ignition delay times, and NO speciation data in CH 4 , CH 3 OH, C 2 H 5 OH, and their blended flames, surpassing the capabilities of other literature models tested, thereby could help with the understanding and optimization of relevant applications. • New CH 4 S L data extend to extreme lean/rich conditions, expanding knowledge base. • Six kinetic models assessed against S L data, revealing model limitations. • Key CH 4 S L reactions identified through an innovative uncertainty analysis. • A new compact kinetic model offers improved predictability for extreme conditions. • The updated model also maintaining strong predictability against previous targets. [ABSTRACT FROM AUTHOR]

Numerical simulations were conducted to investigate the flame acceleration in a stoichiometric hydrogen-air mixture in obstructed channels with uniform and stepped distributions of obstacle height. The two-dimensional, fully-compressible reactive Navier-Stokes equations coupled to a calibrated chemical-diffusive model were solved using a fifth-order numerical algorithm. The numerical method was validated against the experiment. The results show that the effect of obstacle height on flame propagation speed varies with the arrival position of the flame front. In the channel with uniform height obstacles, flame-vortex interaction dominates flame acceleration in the early stage. Flame speed increases with the blockage ratio since higher obstacles facilitate the formation of large vortices. As the flame continues to propagate, flame-shock interaction becomes dominant in flame acceleration. At this stage, flame speed first increases and then decreases with the blockage ratio, since the obstruction to the flame-shock by obstacles increases with the blockage ratio. To further explore the influence of obstacle height distribution on flame acceleration, a stepped distribution of obstacle height was designed by using the obstacle heights that produce the highest speed in different parts of the channel in uniform cases. It was found that the flame in the channel with stepped distribution reaches the end faster than in the uniform cases since the stepped case is more conducive to the flame stretching and perturbation in the early stage and the flame-shock interaction in the later stage. Furthermore, an optimized analytical model was developed to predict the flame acceleration in the channel with a stepped distribution of obstacle height. • FA in hydrogen-air mixtures is simulated using high-order numerical method. • Effect of stepped distribution of obstacle height on FA is investigated. • A prediction model is developed for FA in channel with stepped height obstacles. [ABSTRACT FROM AUTHOR]

Experimental and theoretical investigations were performed on the propagation behavior of ammonia/hydrogen premixed flames, and an in-depth theoretical analysis of flame instability was carried out. The laminar burning velocity (LBV), burning flux, and net heat release rate (HRR net) were calculated. The instability was measured by flame thickness, thermal expansion ratio, effective Lewis number, and perturbation dimensionless growth rate. The effects of equivalence ratios (0.8, 1.0, 1.2, and 1.4), initial pressures (1, 2, and 3 bar), and hydrogen additions (15, 20, 25, and 30%) on ammonia/hydrogen premixed flames were determined. The findings indicated that as the hydrogen addition rises, both the LBV and burning flux increase, and the peak HRR net rises and shifts towards the low-temperature reaction zone, indicating that the diffusivity and reactivity of hydrogen can enhance the combustion intensity. The increment in the equivalence ratio improves the flame stability. Hydrodynamic instability effect always destabilizes the flame during the combustion process. In rich mixtures, the hydrogen addition improves thermal-diffusion stability, delaying the occurrence of flame instability whereas in lean mixtures, the hydrogen addition thermal-diffusionally destabilizes the flame. • Laminar burning velocity and burning flux of NH 3 /H 2 mixtures were measured. • The intrinsic instability of NH 3 /H 2 flame was investigated. • H 2 promotes the NH 3 /H 2 flame propagation and increases the heat release rate. • The hydrogen addition stabilizes the flame at high equivalence ratio. [ABSTRACT FROM AUTHOR]

The potential of chemiluminescence to develop non-intrusive sensors for the monitoring and control of turbulent ammonia–hydrogen flames is here investigated experimentally. This study looks into the impact of equivalence ratio (0.35 ≤ ϕ ≤ 1.70), NH 3 fuel fraction (0.55 ≤ X N H 3 ≤ 0.90) and Reynolds number (4000 ≤ Re ≤ 7000) on UV, visible and infrared chemiluminescence signatures and NO x emission of NH 3 /H 2 turbulent flames within an atmospheric tangential swirl burner. Chemiluminescence spectroscopy is employed to provide detailed information about the excited species (e.g., NO ∗ , OH ∗ , NH ∗ , NH 2 ∗ , and NO 2 ∗) in both in-flame and post-flame zones. Findings are compared to previous measurements in laminar flames and similar trends are observed. Many chemiluminescence intensity ratios are investigated but none are found to be potential surrogates of equivalence ratio and NH 3 fuel fraction across all the conditions considered. Therefore, a more advanced method based on machine learning is used to infer equivalence ratio and NH 3 fuel fraction from the chemiluminescence intensities of more than just two excited radicals at once. This method referred to as Gaussian Process Regression (GPR) is found to provide predictions of equivalence ratio and NH 3 fuel fraction with an accuracy better than 0.1 and 0.02, respectively, across the whole range of conditions. GPR is also able to predict the measured NO, N 2 O and NO 2 emissions using only measured chemiluminescence intensities, confirming the potential of chemiluminescence sensors coupled with a machine learning-based method for the monitoring and control of practical NH 3 /H 2 flames. • Chemiluminescence dataset of 252 test points for NH 3 -H 2 turbulent flames. • Chemiluminescence helps the detection and control of NOx emissions. • NO, NO 2 and N 2 O emissions are measured in 193 stable NH 3 -H 2 flame cases. • Machine learning used to build correlation between excited species and NO x. [ABSTRACT FROM AUTHOR]

To investigate the impact of ammonia volume fraction (X NH3) and obstacle blockage ratios on the explosion propagation of a hydrogen-ammonia mixture, this work conducts a numerical study of flame propagation in a closed duct under three different obstacle blockage ratio configurations, focusing on flame structure, propagation speed, overpressure development, key free radicals, and evolution process of vortex dynamics. The research reveals that increasing the X NH3 decreases the density gradient behind the flame front, weakens velocity shear across gas layers and flow instability, and inhibits the formation rate and quantity of key free radicals, which dampens flame propagation and explosion overpressure in closed ducts. Moreover, obstacle blockage ratios near the ignition source and at the end significantly influence flame propagation. As the X NH3 increased from 0 to 0.2, the steepest attenuation of the flame propagation speed is observed for the decreasing gradient of obstacle blockage ratio (BR-Down), with a 52% attenuation. The arrangement with an increased blockage ratio (BR-Up) exhibits the highest flame propagation speed and overpressure rise rate under all conditions, reaching 373.62 m/s and 2058.59 MPa/s respectively when the X NH3 = 0. Finally, the vortex distribution within the duct is visualized by the numerical simulation results, clearly illustrating the influence of the X NH3 and the obstacle blockage ratio on the generation and evolution of spatial vortices. The research offers a theoretical foundation for the safety of hydrogen-ammonia mixture explosive combustion, particularly in confined spaces containing obstacles. • Adding ammonia reduces flame speed and explosion pressure in ducts. • Flame propagation is significantly affected by obstacles near the ignition source and end. • Increased ammonia volume leads to more intense vortices and complex flame dynamics. • Faster flame propagation and higher pressure peaks are resulted from higher obstacle blockage ratios. [ABSTRACT FROM AUTHOR]

Understanding mechanisms controlling flame stabilization and pollutant emissions in swirled hydrogen flames at elevated pressures is crucial for advancing hydrogen-powered gas turbines. In this work, a single sector model gas turbine combustor operated with a coaxial dual swirl H 2 /air injector is installed in a high pressure test rig equipped with optical access. Flame stabilization and pollutant emissions of NO, NO 2 , and N 2 O are investigated at atmospheric injection temperature across a wide range of air and hydrogen injection velocities and operating pressures up to 8 bars. Two stabilization modes are identified: flames anchored to the hydrogen injector nozzle and flames lifted above the coaxial injector. It is shown that the air injection velocity required to lift the flame from the hydrogen injector rim increases with rising hydrogen velocity or pressure. However, with the current burner design, the lift-off air velocity reaches a plateau beyond 4 bars, regardless of the hydrogen inlet velocity. N 2 O emissions remain negligible for all operating conditions explored. Except at very lean operating conditions with global equivalence ratios below 0.3, NO 2 emissions are negligible too. It is finally shown that NO emissions scale with the adiabatic flame temperature, residence time in the flame volume, and pressure and that lifted flames typically yield lower NO emissions than anchored flames. The observations presented in this study help identifying critical flow parameters and lay solid foundations for the development of swirled hydrogen burners at elevated pressures. [Display omitted] • Air velocity at flame lift-off increases with pressure and hydrogen velocity. • NO emissions increase with pressure and equivalence ratio. • NO emissions decrease with hydrogen injection velocity and tend to a plateau. • NO 2 and N 2 O emissions are only significant at very low equivalence ratios. • NO x emissions scale with flame temperature, residence time and pressure. [ABSTRACT FROM AUTHOR]

This study aims to provide laminar burning velocity measurements of lean, stoichiometric, and rich H 2 -air and CH 4 -air flames at cryogenic temperatures, as well as to determine the accuracy of the existing high-temperature modeling approach (i.e., empirical and kinetic models) to simulate them. The lowest temperatures reached experimentally are 100–120 K and 150–160 K (depending on the equivalence ratio) for H 2 -air and CH 4 -air mixtures, respectively. Simulations are conducted with Cantera down to 100 K in all conditions with several kinetic models. This study summarizes both critical aspects of the experimental procedures by comparing the present results with previous data and important numerical considerations by evidencing sensitive simulation parameters and addressing the 200 K temperature limit of Cantera's solver. Quantitative analyses revealed that both empirical power laws and kinetic models extrapolations are accurately predicting, within the experimental uncertainties, the laminar burning velocities of H 2 -air and CH 4 -air flames in almost all conditions. • Laminar burning velocities of H 2 -air and CH 4 -air flames at 100 K and 150 K. • The presented experimental procedure correct biases from previous literature. • Important aspects to simulate cryogenic flames down to 100 K are presented. • Usual kinetic and empirical models can be extrapolated to cryogenic temperature. [ABSTRACT FROM AUTHOR]

Strong turbulence-chemistry interactions in turbulent spray combustion need to be closed by accurate combustion models in the framework of large eddy simulation (LES). The two-phase spray flamelet/ progress variable (TSFPV) model has exhibited good predictions in spray combustion simulation since it takes into account droplet evaporation effects during flamelet modelling. In our previous studies, the TSFPV model was validated on simple configurations of non-swirling jet spray flames. In this research, the model is further applied to the LES of the complicated configuration of the Cambridge model swirl burner. The M-shaped flame structure is well reproduced according to the comparisons of the simulation results with experimental images. The simulation results agree well with the experimental data in terms of quantitative statistics of droplet diameter and velocity. The complicated flame structures, especially for the inner flame brush in the two-phase spray combustion regime, are further analysed. Strong droplets/ flame interactions occur where some droplets penetrate the flame region, emphasising the influence of the evaporation source terms on the flame structure. The heat release rate in the inner flame region is greater than that in the outer flame region, which is consistent with the experiments. The droplet dynamics are analysed based on the scatter plots of droplet axial velocity and the results show that the behaviour of droplets of different sizes varies. The spray flame analyses promote the understanding of the complex spray flame structure in the complicated swirling flame configuration. [ABSTRACT FROM AUTHOR]

Ammonia/hydrogen blends are examined in a spark-ignition engine with variable hydrogen content. This is achieved by expanding an in-house CFD code focusing on its combustion model to account for ammonia-blended fuels. This model relies on a flame tracking approach and a characteristic conversion time-scale to calculate the species reaction rates, except from NO, whose reaction rate is calculated with the extended Zeldovich mechanism. The CFD code is validated against experimental data, revealing its good accuracy. Moreover, the calculated NO is close to the measured one, within a range of 25–30%, indicating that the thermal NO mechanism is an important one in the examined conditions. The next step was to examine in-cylinder processes and NO formation, including the flame propagation for variable ignition timing. Overall, the developed numerical tool adequately predicts the transport phenomena in ammonia/hydrogen fueled spark-ignition engines, enabling its use to examine other conditions and cyclic variability. • SI engine fueled with NH 3 /H 2 blends with variable hydrogen content. • RANS-based CFD code validated to simulate engine performance and emissions. • Combustion and thermal NO emissions for variable H 2 content and ignition timing. • Ignition timing highly affects performance but low effect on NO emissions. [ABSTRACT FROM AUTHOR]

To understand the effect of flame instability on the formation of the tulip flame, this paper modified the multi-phenomena combustion model and used the CFD code GASFLOW-MPI system to perform a numerical simulation of premixed stoichiometric hydrogen/air deflagration without considering the influence of flame stretch rate, Darrieus-Landau (DL) instability and thermal-diffusive (TD) instability based on the consideration of the heat transfer mechanism. The influence of different control factors on the formation of the tulip flame was compared and analyzed, and the accuracy of the numerical simulation was verified by combining it with experimental results. The research results showed that the coupling effect of DL instability and TD instability was the main reason for the formation of tulip flames, and DL instability played a dominant role. Furthermore, the DL instability determined the growth rate of the deflagration pressure amplitude and its fluctuation frequency. Finally, DL instability and TD instability had a more significant effect on flame propagation than heat loss. When either effect of DL instability and TD instability was ignored as a factor, the pressure gradient near the flame front decreased sharply and no vortex was generated. • The reason for the formation of tulip flames was obtained. • The Darius-Landau instability played a dominant role in the instability coupling. • Flame instability have a greater effect on flame propagation than heat loss. [ABSTRACT FROM AUTHOR]

Micro combustion offers a promising pathway for powering small-scale devices, yet achieving stable flame propagation at this scale remains challenging. Ammonia, a carbon-free fuel, has emerged as a potential candidate, but its intrinsic combustion characteristics pose challenges. Blending ammonia with hydrogen enhances its combustion properties. This study investigates the performance of a hydrogen/ammonia micro combustor, stabilized by both a bluff-body and swirling flows, under various flow parameters and bluff-body configurations. Key findings indicate that increasing the inlet mass flow rate and ammonia-to-hydrogen ratio enhances thermal efficiency and exhaust gas temperatures, albeit at the cost of decreased radiation efficiency. Furthermore, increasing the equivalence ratio diminishes thermal efficiency and reduces emissions, while oxygen enrichment significantly boosts combustion and radiation efficiencies, as well as mean outer wall temperatures, despite a decrease in thermal efficiency. Additionally, the size and half-angle of the bluff-body emerge as critical factors affecting combustion and thermal efficiencies. Larger bluff-bodies enhance combustion and radiation efficiencies, leading to more uniform wall temperatures. On the other hand, emissions decrease with increasing bluff-body size but increase with greater half-angles. These insights hold substantial implications for the design and optimization of micro combustion-based power generators, particularly in the pursuit of minimizing carbon emissions. • A novel bluff-body and swirl-stabilized micro combustor for micropower is proposed. • Effects of flow and structural parameters on thermal performance are examined. • Larger bluff-bodies enhance combustion and radiation efficiencies. • Stoichiometric conditions led to the highest combustion and radiation efficiency. • Oxygen enrichment raises exhaust gas temperature, radiation efficiency, and N O x. [ABSTRACT FROM AUTHOR]

Gas flow influences flame morphology and flame propagation. Resorting to large eddy simulation, this paper analyzes flow characteristics and dynamics of confined hydrogen-air explosion. The results reveal that the gas at flame surface accelerates, then decelerates, and finally reversely accelerates. The variation can be contributed to transition from pressure-induced acceleration to deceleration effects. With initial pressure increasing, the decelerating effect has stronger intensity and wider influencing area. With initial turbulent fluctuation velocity increasing, intensity and influencing scope of the decelerating effect rise under higher initial pressure. In unburnt zone, forward gas flow is dominated by accelerating effect of pressure for equivalence ratios of 1.0 and 2.0. In burnt zone, gas flow is alternately dominated by accelerating and decelerating effects of pressure gradient. Vortex appear after flame surface and is largest near flame wrinkles. Formation of the strongest vorticity is dominated by baroclinic torque which represent Rayleigh Taylor instability. [Display omitted] • Effects of initial pressures and turbulence on flow field are investigated. • Gas on flame surface accelerates, decelerates and finally reversely accelerates. • Increased initial pressure significantly promotes both acceleration stages. • Gas flow at over a half flame surface is dominated by pressure gradient. • Vortices distribute after flame surface and are controlled by baroclinic torque. [ABSTRACT FROM AUTHOR]

Three-dimensional Direct Numerical Simulations (DNS) of turbulent premixed lean H 2 -air flames are performed for different equivalence ratios and initial turbulence intensities to investigate the influence of differential diffusion on the invariants of the velocity gradient tensor and local flow topologies. It is found that the effects of differential diffusion strengthen with a decrease in equivalence ratio. This difference results in more frequent super-adiabatic temperatures and higher positive dilatation rates for smaller values of equivalence ratio, having a significant impact on the flow topology distributions and their contributions to the scalar-turbulence interaction and vortex-stretching terms. The topologies associated with positive dilatation rates tend to occur at the curvatures associated with super-adiabatic temperatures for highly lean H 2 -air premixed flames. However, this tendency weakens with increasing equivalence ratio. These findings suggest that the chemical timescale must be accounted for in the modelling of viscous and scalar dissipation rates in lean hydrogen-air premixed flames. • Statistically planar flame DNS of equivalence ratios of 0.4 and 0.7 considered. • Effects of equivalence ratio on flow topology distributions analysed. • The non-unity Lewis number effects strengthen with deceasing equivalence ratio. • Dilatation effects strengthen with deceasing equivalence ratio. • Topologies specific to positive dilatation strengthen for small equivalence ratio. [ABSTRACT FROM AUTHOR]

Large Eddy Simulations (LES) of turbulent lean-premixed flames of V- and M-shape are presented. A simple algebraic closure with the ability to capture finite-rate chemistry effects is used for subgrid reaction rate modeling. The V-shaped flame is stabilized in the inner shear layer between a swirling annular jet and a central recirculating bubble in a sudden expansion duct. The M-shaped flame is stabilized in the inner and outer shear layer, adjoining the corner recirculation zone induced by the vertical step. The focus of the study is on the flow fields and shapes of the flames, which distinguish themselves through different heat load and sensitivity to local extinction. Good agreement with measurements is observed for the cold and the reacting flow cases. The numerical results suggest that the entrainment of hot gases into the outer recirculation zone occurs close to the impingement point of the swirling annular jet on the wall and this process is strongly dependent on intense vortical structures in the outer shear layer. The results further suggest that local extinction influences the position of the flame in the inner shear layer and, thereby, also the intensity of the local entrainment process. [ABSTRACT FROM AUTHOR]

Silicone rubber‐based materials are important thermal protection materials for high‐temperature applications. In this work, a holistic analysis of silicone rubber‐based materials was conducted with an aim to comparatively study the ablative performance of different rubber matrices using the oxyacetylene flame. Six types of silicone rubber‐based composites were prepared in large quantities using a customized device, and the relationship between the ablative properties and influencing factors was elucidated, which was useful in guiding the design and preparation of flexible ablative materials for thermal protection purpose. The expansion and ceramicization of char layer, which was realized by introducing expandable graphite and alumina was found helpful in reducing line ablation rate. The increase in the thickness of char layer and the decrease in surface roughness indicated a successful implementation of the above strategy, which led to a significant decrease in line ablation rate and an increase in thermal insulation performance of the studied system. Moreover, a thermal ablative composite with excellent moldability after ablation process was prepared which showed a promising application as a reusable thermal protection material. This work provided guidelines for developing flexible thermal ablative composites, which can be targeted for practical applications in aerospace and fire protection sectors. [ABSTRACT FROM AUTHOR]

Considering the requirements for developing hydrogen combustion chambers and the application potential of micro-mix combustion, the flame macrostructure and evolution of hydrogen-air multi-microjet flames have been investigated to discover the relationship between flame macrostructure and thermoacoustic instability. A novel burner has been tested under different combustor liner lengths to simultaneously produce stable and unstable combustion under the same operation conditions. A compact conical flame shape without adjacent flame front interference is observed. In comparison with the combustor liner length, the flame temperature variation shows an insignificant effect on the thermoacoustic instability but triggers an oscillation mode transition from low-frequency (∼210–240Hz) to high-frequency (∼400–440Hz) under unstable combustion. Different flame evolutions are revealed for these oscillation modes. DMD analysis and LES simulation show that the high-frequency oscillation mode is mainly controlled by flame front oscillation and flame pinch-off process. However, the low-frequency oscillation mode is characterized by flame extinction on a large scale. Both equivalent ratio and velocity fluctuations contributed to flame and heat release oscillations under low and high flame temperatures. These findings help understand the mechanisms, driving hydrogen-air flame dynamics, and designing hydrogen combustors. • A novel hydrogen micro-mix combustion burner was investigated. • A change in the liner length significantly impacted the combustion instability. • An increase in the flame temperature induced a transition of the oscillation mode. • The relationship between flame evolution and combustion instability was revealed. [ABSTRACT FROM AUTHOR]

Integrating hydrogen into sustainable energy systems and existing natural gas infrastructures requires fast and reliable prediction of blast dynamics of hydrogen-methane-air mixtures to design effective protective structures. This study proposed an analytical model for predicting blast loading from hydrogen-methane-air (H 2 –CH 4 -air) explosions in unconfined spaces, with or without obstacles. The model includes a flame speed prediction formula that accounts for fuel-air characteristics, congestion levels, and fuel quantities, based on extensive experimental data. Modified open-source codes were utilized to calculate the laminar flame properties for the flame speed prediction formula under various initial temperatures, H 2 /CH 4 ratios, and equivalence ratios. Additionally, an empirical model based on theoretical calculations was devised for rapid computation of laminar flame properties. This flame speed prediction formula integrated into the blast loading prediction framework was validated by comparing predicted peak pressures and impulses against diverse experimental results, including scenarios with CH 4 -air, H 2 -air, and CH 4 –H 2 -air mixtures, both with and without congestion. The application of this model was demonstrated through a case study at Australia's first commercial hydrogen refueling station and another case study for the development of engineering chart for rapid blast loading estimation, illustrating its practical utility in blast loading determination. This research provides an efficient tool for predicting H 2 –CH 4 -air blast loading, offering a systematic approach to assess and mitigate the risks associated with vapor cloud explosions in unconfined industrial settings. • Developed a fast and reliable blast loading model for H 2 –CH 4 -air mixtures, validated with experiments. • Proposed an empirical formula to predict laminar flame properties for H 2 , CH 4 , and blends. • Demonstrated practical uses, including blast prediction at hydrogen refueling stations and engineering chart development. [ABSTRACT FROM AUTHOR]

The quantitative measurements of the soot volume fraction can deliver important validation data for detailed models of soot inception, growth, and oxidation in hydrocarbon-fueled flames – a process still not fully understood. Therefore, the sensitive and non-intrusive optical measurement of soot levels in fuel-rich steady flames is highly desirable. This work presents the first in situ Cavity Ring-Down Extinction (CRDE) setup for monitoring soot concentration levels in the low-ppb range in premixed ethylene/air stagnation flames stabilized on a flat flame burner at elevated pressure. The high-reflectivity CRD mirrors were mounted in nitrogen-flushed chambers attached to and separated from the flame area by small diameter apertures. CFD simulations supported the dimensioning of the purge flows and visualized possible perturbation of the cold- and hot-gas (from the flame) flows inside the burner housing. Within the uncertainties of the present experimental conditions and based on results from other labs for similar flame conditions, the pressure dependence of the evaluated soot volume fraction fV was fitted to a power-law ansatz with an exponent n between 2.1 and 2.6 depending on the equivalence ratio. A global thermal rate constant for surface growth, k SG ≈ 49 ± 20 s - 1 , was found for a measured flame temperature of 1750 ± 100 K in an atmospheric pressure flame, in gross agreement with results in the literature, which demonstrates the potential of the current setup for soot diagnostics in laminar premixed elevated pressure flames. A detection limit for fV of above 40 ppt has been determined from a comparative measurement of CO2 having a small absorption coefficient at the laser wavelength of 1064 nm. [ABSTRACT FROM AUTHOR]

Hysterosalpingography (HSG) remains a valuable diagnostic tool in current clinical practice, offering crucial insights into endometrial cavity, fallopian tubes and the adjoining part of the pelvic peritoneal cavity. Despite the emergence of alternative imaging and non-imaging options, HSG continues to be widely utilized due to its diagnostic accuracy, cost-effectiveness, and easy accessibility. Due attention to the correct technique and optimal image interpretation will further enhance its diagnostic accuracy and precision in the work-up of patients with fertility problems. [ABSTRACT FROM AUTHOR]

The low dissipation low dispersion (LD2) second-order accurate scheme is effective for scale-resolving simulations in finite volume computational fluid dynamics (CFD) solvers, thanks to its combination of a skew-symmetric split form for convective terms and matrix-valued artificial dissipation fluxes. However, reactive flow simulations face challenges due to steep gradients and varying gas properties. This study replaces the skew-symmetric scheme with the kinetic energy and entropy preserving (KEEP) scheme, utilizing quadratic and cubic split forms for convective terms, enhancing stability. The nonsmooth fluid interfaces in reactive flow simulations necessitate upwind fluxes for reactive scalars to limit total variation (TV), also requiring upwind fluxes for the mixture-dependent internal energy fluxes. Other convective terms use central discretizations from the KEEP scheme, leveraging LD2's spatial reconstruction to minimize dispersive errors. Numerical assessments show this approach reduces spurious pressure oscillations in single and multicomponent flows. The absolute flux Jacobian for dissipation flux calculation is efficiently computed using an expanded Turkel's approach for thermally perfect gas mixtures. Partial pressure derivatives are approximated when using the flamelet generated manifolds (FGM) combustion model. The proposed scheme is evaluated through scale-resolving simulations of the Cambridge burner flame SWB1 on an unstructured grid using the density-based solver TRACE, employing both finite rate chemistry (FRC) and FGM combustion models. Comparative analysis with the all-speed SLAU2 scheme shows the superior performance of the proposed scheme in handling turbulent reactive multicomponent flows. [ABSTRACT FROM AUTHOR]

In this article, the effect of different boundary conditions and different thermal and physical properties of walls and gas on flame characteristics and stability of hydrogen–air mixture are investigated using an analytical method. This method solves the gas–wall energy equation, and the hydrogen mass conservation equations. The jump conditions are obtained by integrating the energy and mass equation into a small control volume around the flame. For validation of this model, the temperature distribution on the outer surface of the wall is compared with experimental data that show the maximum relative error of 3.5% for Q = 400 mL/min and 4.9% for Q = 200 mL/min. The maximum variation of gas temperature is nearly 6.5 times of wall temperature variation. The wall can be considered one‐dimensional for conventional wall materials with K > 10. For the existence of combustion inside the chamber, when the value of K is greater than 10, the Péclet number should also be considered greater than 10. In a constant equivalence ratio, increasing the medium temperature increases flame stability. [ABSTRACT FROM AUTHOR]

This paper presents a detailed comparative analysis and discussion of two typical predictive methods for combustion instability in long flame combustion chambers: the coupled method and the decoupled method. Using large eddy simulation (LES), the coupled method directly predicts stability in typical long flame combustion chambers. In the decoupled method, stability in the combustion chamber is predicted by combining a low-order acoustic network for long flames with flame responses and mean parameters from numerical simulations. The research results indicate that the coupled method provides full-field information, while the decoupled method neglects certain factors, such as the coupling between combustion and acoustics. However, the decoupled method can directly determine combustion instability based on the growth rate of oscillation modes. The flow field undergoes periodic changes, with the region of fluctuation in the combustion heat release rate gradually increasing, resembling vortex development, which ruptures upon encountering the wall due to radial constraints. Furthermore, in the decoupled method, the periodic changes in the flow field are controlled by the frequency of incoming flow disturbances, whereas in the coupled method, they are controlled by the acoustic frequency of the combustion chamber. In the coupled method, the coupling among disturbances and the acoustic disturbances at the boundaries amplifies the disturbances, causing the radial scale of the fluctuation region in the combustion heat release rate to increase along the axial direction and approach a fixed value faster than in the decoupled method. • A detailed comparative analysis and discussion of two typical predictive methods for combustion instability in long flame combustion chambers: the coupled method and the decoupled method are investigated. • The coupled method directly predicts stability in typical long flame combustion chambers. • In the decoupled method, stability in the combustion chamber is predicted by combining a low-order acoustic network for long flames with flame responses and mean parameters from numerical simulations. [ABSTRACT FROM AUTHOR]

In the framework of comprehensive assessment of explosion risks on board of spacecrafts and on the facilities of launch places, the paper is focused on the detailed analysis of particular scenarios of deflagration-to-detonation transition taking place in smooth tubes filled with acetylene-oxygen mixtures of different compositions. By means of precise numerical simulation it is demonstrated that various scenarios of detonation onset can take place depending on the mixture composition and its initial thermodynamic state. It is demonstrated that independent on the particular scenario always the basic mechanism of detonation onset via the formation of strong enough shock wave takes place. In more reactive mixtures the strong shock originates from the self-sustained process of joint pressure build up and reaction intensification exactly at the flame front. In less reactive mixtures the transient flow behavior leads to the shock waves generation and interaction. As a result, a brand new reaction kernel could arise in the area of shock waves interaction. In number of cases, that leads to the coupling between the shock wave and the newborn reaction front and results in the strong shock formation and further detonation onset. • Deflagration-to-detonation transition in acetylene-oxygen mixtures is analyzed numerically. • Two basic scenarios of deflagration-to-detonation transition are distinguished and described. • In more reactive mixtures the detonation is formed exactly on the flame front. • In less reactive mixtures the compression waves interaction leads to the hot-spot initiation. [ABSTRACT FROM AUTHOR]

The stretch effects in large eddy simulation (LES) of a turbulent lean propane-air premixed Bunsen flame in the wrinkled flamelet regime are investigated. A simple approach to model the subgrid-scale flame stretch is proposed. It is based on the analysis which shows that, when the surface-filtered strain rate in a subgrid-scale stretch model is approximated using the volume-filtered velocity field solved for in LES, heat release and associated gas expansion in the filtered flame brush tend to artificially alter the wrinkling of resolved flame fronts. To minimize such artifacts, it is suggested that the volume-filtered strain rate on the unburned side of the filtered flame brush be used to approximate the surface-filtered strain rate and projected through the filtered flame brush. The results show the importance of mitigating the artificial heat release effects when considering the strain effects in LES. The relative importance of the curvature stretch is also investigated in terms of the mean and local effects. For a Bunsen flame with a positive Markstein number, the mean flame curvature effect tends to increase the total burning rate by enhancing the laminar flame speed near the flame tip, while the local flame curvature effect tends to decrease the total burning rate by suppressing the wrinkling of the resolved flames. It is found that the competition between the two makes the overall curvature effects not influence the total burning rate much for the flame investigated here, as compared with the strain effects. [ABSTRACT FROM AUTHOR]

A novel, modified coflow burner was developed to study the effect of charge injection from a non-thermal plasma into three helium-diluted laminar coflow diffusion ethylene flames. The frequency of the high voltage (HV) signal was varied to control the ion concentration (charge) injected into the flames. Optical emission spectroscopy was used to characterize the non-thermal plasma while a bias plate methodology was used to gauge the relative amount of charge generated. For different HV signal frequencies, the laser-induced fluorescence of OH, chemiluminescence of CH*, and laser-induced incandescence of soot in flames were measured. The OH and CH* measurements showed that the flames retained the classic flame shape with charge injection. Significant soot reduction was observed at low HV signal frequencies, corresponding to an increase in charge injection. Notably, at low HV signal frequency, soot reduction in highly concentrated (60%) ethylene flame is three times lower than the less concentrated (32%) ethylene flame. This can be attributed to the decrease in the injected charge to soot precursor concentration ratio when the concentration of ethylene in the flame is increased. These results demonstrate that the current system is a promising candidate for studying the charge effect from non-thermal plasma on soot formation in laminar coflow diffusion flames. [ABSTRACT FROM AUTHOR]

NOx emission reduction is one of the major challenges when using ammonia/hydrogen blends as an alternative fuel for carbon-free combustion. In this study, chemical reaction pathways of NO formation in planar premixed flames of stoichiometric ammonia/hydrogen/air at atmospheric pressure and reactants temperature of 298 K are investigated under laminar and decaying turbulent conditions using quasi-DNS with detailed chemistry and the mixture-averaged transport model. The sweep parameter, $$\alpha $$ α , is the volumetric ratio of hydrogen to the ammonia/hydrogen mixture. Here, for unstrained laminar conditions, $$\alpha $$ α = 0, 0.4, and 0.6 and for turbulent condition, $$\alpha $$ α = 0.4 within the thin reaction zones region ($$Ka$$ Ka $$ \approx $$ ≈ 34.2) are studied. Under 1D laminar conditions, increasing $$\alpha $$ α enhances NO formation drastically, around 3 times when comparing $$\alpha $$ α = 0 and 0.6. At different $$\alpha $$ α , despite the high activity of the reactions which belong to the Zeldovich pathway, the contribution of this pathway is insignificant in NO formation. However, HNO and N2O pathways have the most significant roles. For instance, at $$\alpha $$ α = 0.4, R85 (NH+NO $$ \Leftrightarrow $$ ⇔ N2O+H) in the N2O pathway and R144 (HNO+H $$ \Leftrightarrow $$ ⇔ NO+H2) in the HNO pathway have major roles in NO formation. Higher NO formation at larger $$\alpha $$ α is found to be due to the increased H and O radicals within the reaction zone as well as the increased reaction rates because of the higher flame temperature. Under the 3D turbulent condition ($$\alpha $$ α = 0.4), it is observed that turbulence does not switch the pathways trends across the flame brush compared to the laminar condition. In the reaction zone, however, it is observed that NO formation is lower (higher) in the regions convex (concave) toward the reactants. The underlying reason for this effect is the preferential diffusion of H2 to the regions of the flame front convex toward the reactants, which consumes NO and also depletes O and OH radicals, which are necessary for NO formation. [ABSTRACT FROM AUTHOR]

The ignition, combustion characteristics, and cenosphere formation of single droplets combustion of four vacuum residues (VRs) from different refineries with various asphaltene contents were studied experimentally. The single droplets of VRs were suspended at the tip of a silicon carbide fiber and heated in air at temperatures of 973 and 1023 K, respectively, in an electrically heated tube furnace. The ignition and combustion behavior of the VRs were recorded using a CCD camera, which enabled the determination of droplet size, ignition delay time, flame duration, and cenosphere size. The effect of initial droplet size, gas temperature, and asphaltene content on the ignition delay time, flame duration, cenosphere morphology, and particle size were investigated. The whole ignition and combustion process of single droplets of the VRs consisted of five stages in succession: (1) pre-ignition, mainly involving the evaporation of highly volatile components from the droplet surface; (2) steady combustion of fuel vapors evaporated from the droplet surface; (3) splashing combustion of fuel vapors evaporated from droplet interior; (4) disruptive combustion due to thermal decomposition of asphaltene; and (5) solid residue ignition and combustion. A visible and sooty flame was formed upon ignition and lasted during stages 2–4. The droplet size increased sharply in the stage 4 due to the thermal decomposition of asphaltene, which was more profound for VRs with higher asphaltene content and at higher gas temperatures. The ignition delay time increased with increasing initial droplet size and gas temperature but varied little as the asphaltene content in the VRs increased, suggesting that the ignition process of VRs was controlled by the vaporization of high volatile components on the droplet surface. The thermal decomposition of asphaltene produced solid residue, which was in the form of a cenosphere with the shell thickness being ca. 20 μm and a number of blowholes presented in the shell. The VRs with higher asphaltene content had more and bigger blowholes. The ratio of cenosphere particle size to initial droplet size is independent of the initial droplet size but almost increased linearly with the asphaltene content in the VRs. [ABSTRACT FROM AUTHOR]

This study examines the influence of the proximity of a ceiling on the burning rate of a fire in enclosures. The parameters included the pool diameter, distance to the ceiling and fuel type. The analysis focused on the average mass loss rate. The results confirmed the increase in mass loss rate with the elevation for all fuel types. This increase in the mass loss rate appears from a distance to the ceiling that is correlated with the flame height. It is caused by an increase in the heat flux emitted by the flame, which changes shape, and the temperature of the ceiling, which increases with the fire elevation. A correlation is proposed to predict the increase in mass loss rate as a function of the enthalpy ratio Δ H / Lv and flame height. A comparison of the results with previous experimental studies demonstrated consistency between the different tests. [ABSTRACT FROM AUTHOR]

In order to promote low-carbon sustainable development in the ecological environment and improve the efficiency of hydrogen and natural gas energy utilization, this project carried out research on the explosive effects of different thicknesses of ordered porous media on the hydrogen-methane gas mixture. A detailed discussion was conducted based on the critical quenching hydrogen blending ratio under the thicknesses of 50 mm and 60 mm of ordered porous media. The results indicate that the critical quenching hydrogen blending ratio is 9% for a thickness of 50 mm and 20% for a thickness of 60 mm, indicating that greater thickness enhances flame suppression capabilities. Between the critical quenching hydrogen blending ratio range for thicknesses of both 50 mm and 60 mm, the peak values of flame front velocity, reverse diffusion flame length, and explosion pressure initially decrease and then subsequently increase with an increasing hydrogen content. As the thickness of the flame retardant medium augments, there is an increase in both the flame velocity and the reverse diffusion length at the critical hydrogen concentration. However, the pressure peak observed at a thickness of 50 mm surpasses that at 60 mm. The pressure curve experiences sudden fluctuations due to the combined effects of explosion pressure and heat transfer, with the initial point of this abrupt change closely linked to the thickness of the ordered porous media. Therefore, it is imperative to maintain hydrogen content below the critical quenching hydrogen blending ratio to ensure the safe transport and utilization of hydrogen and natural gas energy. [ABSTRACT FROM AUTHOR]

Alcohol-based fuels have shown high compatibility with spark-ignition (SI) engines, which require improvements in fuel efficiency and emissions reduction to meet modern environmental standards. While extensive research has been conducted on ethanol and other lower-order alcohols, there has been comparatively limited investigation into higher-order alcohols like butanol and pentanol as fuel alternatives. Previous studies on pentanol-gasoline blends in SI engines have demonstrated improved engine performance and reduced emissions. Building on this, the present study focuses on analyzing the flame characteristics—specifically speed and distribution—of pentanol-gasoline blends within the engine. In this study, pentanol was blended with gasoline by the volume of 10%, 20%, and 30%, namely 1-PNL10, 1-PNL20, and 1-PNL30, and tested in a twin-cylinder gasoline engine with an MPFI system at various load conditions. The study has focused on investigating the flame propagation of gasoline-pentanol blends by examining the in-cylinder flame image. The in-cylinder combustion evolution was visualized and captured by using an AVL Visio scope camera. Flame characteristics such as spatial flame distribution and flame speed were evaluated from the captured flame images for pentanol–gasoline blends and compared with sole gasoline. The flame study indicates that the addition of pentanol favored to increase in the flame speed, which in turn improved the combustion rate. The flame intensity and distribution area increased with the addition of pentanol in gasoline, demonstrating improved in-cylinder combustion with increased peak in-cylinder pressure and heat release rate. The insights on the flame characteristics of pentanol–gasoline blends were used to rationalize the discussion on engine performance and emissions. The performance of the engine was enhanced while increasing the proportion of Pentanol in the gasoline. The 30% Pentanol gasoline blend showed 5.71% higher BTE than gasoline at full load condition. Emissions like CO and HC also decreased at the same time, and NO emission increased. From the test results, it can be concluded that Pentanol can be blended with gasoline up to 30% without any engine modifications. [ABSTRACT FROM AUTHOR]

To enhance the ablation resistance of porous Csf/MA–SiBCN composites, the polymer infiltration and pyrolysis (PIP) process is adopted to obtain layered Csf/SiBCN composites containing MA–SiBCN and PDCs–SiBCN. After ablation for 10 s, the ablative layer fails to remain on the Csf/MA–SiBCN surface with mass ablation rate of 0.062 g/s and linear ablation rate of 0.208 mm/s, while the dense Csf/SiBCN surface is covered by a dense and stable ablative layer mainly consisting of amorphous SiO2 glassy phase due to the synergistic ablation effect of MA–SiBCN and PDCs–SiBCN, which could resist the intense scour by ablation flame and prevent internal corrosion, and thus the ablation resistance improves with a 37% decrease in mass ablation rate (0.039 g/s) and an 18% decrease in linear ablation rate (0.171 mm/s), respectively. [ABSTRACT FROM AUTHOR]

To ensure the safe application of hydrogen energy in utility tunnels, it is crucial to study the propagation process of hydrogen explosion. In this paper, the explosion propagation process of non-premixed hydrogen-air mixture in gas compartments with different bending angles and with various hydrogen status was investigated. By analyzing overpressure, flame evolution state and flame velocity, the characteristics of the hydrogen explosion in utility tunnels were revealed. The results show that the explosion overpressure, flame velocity and flame brightness all first increase and then decrease with the increase of hydrogen concentration, due to the suppression effects in both fuel-lean and fuel-rich environments. Moreover, it was found that, in 90°, 120° and 150° bending compartments, the maximum overpressure values of hydrogen were respectively 28.52%, 3.52% and 8.53% higher than that in the straight compartment, while the average flame velocity values were respectively 34.19%, 11.96% and 34.13% lower than that in the straight compartment. This was closely related to the intense reflection of shock waves in the bending section, where the accumulation of reflected shock waves not only increased the maximum overpressure but also slowed down flame velocity. This research provides valuable guidance for the design and safety management of hydrogen transportation in utility tunnels and similar confined spaces. • Non-premixed hydrogen explosion shows enhanced overpressure and flame velocity. • Higher explosive power occurs in concentration slightly above stoichiometric ratio. • The effect of bending utility tunnel structure on hydrogen explosion is discovered. [ABSTRACT FROM AUTHOR]

In order to ascertain the true characteristics of the leakage explosion loads of hydrogen-blended natural gas (HBNG) in utility tunnels, the influence of leakage location on the concentration field distribution of HBNG leakage in utility tunnels was studied through the CFD technique. Furthermore, the characteristics of explosion flame and overpressure load distribution of HBNG of inhomogeneous distribution under effect of different ignition delay times and ignition positions were also investigated. The results show that the greater the distance between the leakage location and the air outlet, the more readily HBNG can propagate throughout the utility tunnel. This, in turn, leads to more prominent downstream gas cloud stratification. The development of explosion flame is primarily seen in the direction of low resistance and ample space within the utility tunnel. With the increase in ignition delay time (t id) and the backward shift of the leakage location (L lk), the average flame velocity generally exhibits a decreasing trend. Overall, the peak overpressure exhibits a concave trend. Under circumstances where the leakage source is in the front or middle section of the tunnel, the average peak overpressure demonstrates a general tendency of increase followed by decline with the increase in t id. In comparison, in the case of a leakage source located at the back end, it decreases gradually as t id increases. The most severe explosion overpressure load is readily induced when the leakage source is located at the front end. In the case of a leakage source located in the front section of the tunnel, the overpressure load at each measuring point decreases continuously and then rises as the ignition position moves backward. However, in the case of a leakage source located in the rear section of the tunnel, the average value of peak overpressure induced by ignition in the middle of the tunnel is the greatest, while the average value of peak overpressure induced by ignition at the back end is the smallest. The maximum explosion overpressure load is found at the front-most leakage source under front-end ignition conditions. The findings of this study offer scientific substantiation for the structure of the utility tunnel, which is designed for explosion withstanding, prevention, and control as well as risk assessment. • HBNG leakage concentration field distribution in utility tunnels were investigated. • Effects of leakage location on leakage and explosion were investigated. • Flame development process in inhomogeneous concentration field is revealed. • Dynamic evolution process of overpressure in utility tunnels is revealed. [ABSTRACT FROM AUTHOR]

Hydrogen is one of the most promising fuels for achieving zero carbon emissions in civil aviation. However, the rapid flame speed would easily lead to flashback issues, and its ultra-low density would weaken the jet penetration capabilities. Additionally, the high flame temperature during hydrogen combustion tends to produce significant amounts of NOx. To address these challenges, this paper proposes a honeycomb micromix diffusion combustion scheme that enhances mixing through the coupling of air streamwise vortex (ASV) and hydrogen jet vortex (HJV). The micromix element incorporates a disturbance vortex generator (DVG) embedded within a hexagonal microchannel to promote mixing uniformity. Numerical simulations were conducted to analyze the flow characteristics and combustion mechanisms of the DVG perturbation scheme, elucidating the low-emission principles of the micromix diffusion combustion design. The vortex-vortex coupling and vortex-flame coupling effects on the mixing quality were thoroughly analyzed. The DVG creates ASVs within the main airflow, forming an intensely turbulent stirring zone and accelerating the axial velocity decay. The HJV entrains mainstream air, forming a coupled vortex between the HJV and the ASV, which further enhances mainstream disturbance. Uniform mixing within a 30 mm axial length could be achieved and the HJV tube surface approaches an approximate premixed state and results in low NOx emissions. • A hydrogen micromix combustion scheme with vortex generator was proposed. • The coupling mechansim of air streamwise vortex and hydrogen jet vortex was obtained. • The hydrogen concentration probability reaches 100% within 30 mm downstream the DVG. • The flame propagation and combustion state were analyzed. [ABSTRACT FROM AUTHOR]

The article in the Journal of Fluid Mechanics explores flame acceleration and detonation transition in premixed and inhomogeneous supersonic flows using reactive Navier-Stokes equations and detailed chemical mechanisms. The study emphasizes the challenges of achieving detonation transition in premixed supersonic mixtures without obstacles, while transverse jet obstacles aid in promoting detonation transition by introducing perturbed vortices and shock waves. The research highlights the significance of fuel-lean mixtures and the role of transverse jet obstacles in facilitating detonation transition in supersonic mixtures with concentration gradients. The study provides insights into the impact of inhomogeneous mixtures on detonation propagation speed and instability, shedding light on the complex phenomena of flame acceleration and detonation transition. [Extracted from the article]

Combining experimental research and numerical simulation, this paper studies the effects of obstacle position and blockage ratio on hydrogen flame evolution and explosion characteristic. The mechanism of obstacle-induced turbulence on the hydrogen flame acceleration is revealed. The results show that in a double obstacle tube, When the obstacle spacing is larger, the turbulent combustion intensity and pressure rise rate is relatively higher, and the pressure oscillation is enhanced. The sufficient turbulent combustion promotes the formation of stronger vortex motion at the downstream obstacle. As the blockage ratio increases, the turbulent combustion intensity, flame front speed, and pressure rise rate increases significantly. While obstacles with a high blockage ratio have a blocking effect on the pressure, slightly weakening the pressure oscillation. As the blockage ratio increases, the jet speed at the obstacle increases, the scale and speed of vortex increase, and the vorticity magnitude of unburned and burned area increases. • Effects of obstacle position and blockage ratio on hydrogen explosion are investigated. • A numerical model is developed to analyze the flame evolution and pressure dynamic. • Mechanism of obstacle-induced turbulence on the hydrogen flame acceleration is revealed. [ABSTRACT FROM AUTHOR]

To evaluate the reliability of (Ti,W) 3 AlC 2 in aerospace ablation conditions, the ablation behavior of (Ti,W) 3 AlC 2 ceramic is investigated by using oxyacetylene flame above 2550 K. Results show a four-stage process exists with the formation of TiO 2 -Al 2 TiO 5 -Al 2 O 3. Once the central ablation temperature exceeds 2350 K, the innermost Al 2 O 3 layer melts and collapses, exposing the decomposed TiC x -Al matrix directly to the flame with rapid oxidation and a remarkable increase of the linear ablation rate. The melt finally flows out with the flame from the ablation center and solidifies into a stripe-like TiO 2 -Al 2 TiO 5 eutectic oxide above the protective oxide layer. The refractory W particles detected in the eutectic TiO 2 -Al 2 TiO 5 may retard the flowing rate of the fluid by increasing its viscosity, resulting in a better ablation resistance performance in the later period of the ablation process, making the linear ablation rate lower from 10.30 μm/s of Ti 3 AlC 2 to 9.01 μm/s of (Ti,W) 3 AlC 2. [ABSTRACT FROM AUTHOR]

The research in this paper focuses on the development of a recyclable, efficient and environmentally friendly fireproofing material for mining applications. Therefore, we formulated a novel composite gel named sodium alginate composite gel (PBSL-3) with sodium alginate, alkali lignin and polyvinyl alcohol as raw materials and borax as cross-linking agent. The success of the synthesis of PBSL gel material was verified by scanning electron microscopy and infrared spectroscopy experiments. The good self-healing and ability of PBSL-3 was verified by self-healing, thermal stability and rheological performance experiments; The material has a moderate viscosity and exhibits suitable rheological properties at different shear rates, making it suitable for pipeline transportation; In addition, the weight loss of the gel material was only 64.43% at the programmed warming to a temperature of 160°C, presenting a lesser rate of weight loss, and the material lost only 58% of its weight at temperatures up to 800°C. The flame retardancy of the PBSL-3 gel material was verified by programmed warming experiments and small-scale fire extinguishing experiments. The experimental results showed that the material critical temperature of the coal samples treated with the material was delayed from 100°C to 130 °C, and the CO and C2H4 gas inhibition was 41.98% and 58.12%, respectively, and the flame retardancy was 56.41% at a temperature of 200°C. Small-scale fire extinguishing experiments show that the overall fire extinguishing effect of PBSL gel material is better, and it can effectively extinguish spontaneous coal combustion fires. [ABSTRACT FROM AUTHOR]

OH* chemiluminescence and emissions of the convergent swirler module and the Venturi swirler module are studied in a single-element lean direct injection combustor. The OH* chemiluminescence images are captured by a CCD camera with an image intensifier, and the emissions are measured by a gas analysis system. The flame of the Venturi swirler module is closer to the inlet plane of the combustor compared to that of the convergent swirler module. At the equivalence ratio (ϕ) of 0.55, 0.65, and 0.75, the EINOx of the convergent swirler module is 1.02 g/kg, 1.44 g/kg, and 1.98 g/kg, and the EICO is 0.32 g/kg, 0.54 g/kg, and 3.65 g/kg separately. The EINOx of the Venturi swirler module is 0.95 g/kg, 1.37 g/kg, and 2.05 g/kg and the EICO is 0.32 g/kg, 0.93 g/kg, and 5.86 g/kg. The NOx emissions of the two swirler modules are similar. But the CO emission of the convergent swirler module is lower at the ϕ of 0.65 and 0.75. The convergent swirler module has an advantage in reducing emissions. In addition, the influence of the swirl number (S) on OH* Chemiluminescence and emissions of the convergent swirler module is studied. At the ϕ of 0.55, the EINOx is between 0.88 g/kg and 1.17 g/kg and the EICO is between 0.13 g/kg and 0.44 g/kg with different S. With the increase of the S, the NOx emission increases and the CO emission decreases. But at the ϕ of 0.65 and 0.75, with the increase of the S, the NOx emission first increases and then decreases and the CO emission first decreases and then increases. [ABSTRACT FROM AUTHOR]

In this paper, a methodology for experimental investigation of chemical and thermal structure of weakly stretched counterflow flames under terrestrial conditions is proposed. The non-premixed counterflow flames are stabilized in a narrow channel between quartz plates, and the flame gases are sampled by a microprobe inserted via a thin slit in one of the quartz plates. The gas samples are then analyzed by mass spectrometry. The results of the measurements of mole fraction profiles of the main components of methane-air non-premixed flames, namely, CH4, O2, H2O, CO2, and CO, are presented. The temperature profiles are also reconstructed using the measurement data for the gas composition. The experimental profiles are compared to the results of the 3D numerical simulations undertaken within the global reaction mechanisms (one- and four-step) and a detailed reaction mechanism (San-Diego). The results of this work clearly show that the thermal and chemical structures of the counterflow flame in the planar channel are essentially non-one-dimensional, and 3D-calculations are necessary to correctly predict the flame behavior in this configuration. The methodology demonstrated in the current work can be successfully employed for the investigation of the low stretched counterflow flames of various hydrocarbon fuels and fuel blends. This will allow verifying and developing the reaction kinetic models capable of quantitative prediction of weakly stretched flame behavior in confined conditions. [ABSTRACT FROM AUTHOR]

Obtaining quantitative experimental data on soot properties of aviation kerosene at elevated pressures is a significant concern for the surrogate fuel kinetic model of aviation kerosene. However, there are scarce studies on soot formation and properties of aviation kerosene in tractable flames at elevated pressures in literature. In this study, Chinese RP-3 aviation kerosene is used in relevant experiments and numerical simulations up to 3.5 atm to evaluate the effect of pressure on soot formation and properties. Soot morphology, nanostructure, and concentration are studied by transmission electron microscopy and laser extinction method. Then, an RP-3/PAH kinetic model containing the 5-aromatic rings (A5) growth mechanism is proposed to simulate the soot formation processes at elevated pressures. Quantitative experimental results show that pressure significantly affects the soot morphology, nanostructure parameters. The peak soot volume fractions in the RP-3 flame scale with the pressure as ${P^{1.62}}$ P 1.62 . The numerical results show that the RP-3/PAH model can depict the trends of soot particle size and soot volume fraction with pressure. At 1.0–3.5 atm, the rates of soot growth processes increase with pressure, especially the PAHs condensation. The significantly increased soot surface growth rate may be one of the main factors for the variation of soot properties with pressure. The quantitative experimental and numerical results contribute to the understanding of the kinetic mechanism of soot formation for RP-3 kerosene at elevated pressures. [ABSTRACT FROM AUTHOR]

Lifted multi-jet flames of hydrogen and steam-diluted oxygen are studied using direct numerical simulations (DNS). The DNSs have been carried out for laminar and turbulent flame configurations. The simulation data are analyzed to investigate the flame and flame-base structures and the flame-base propagation in a configuration akin to a gas turbine with multiple fuel jets. The laminar flames have a tribrachial structure, although lean premixed and diffusi on flame branches collide downstream owing to preferential diffusion. The turbulent multi-jet flames have a single connected base region, whereas the laminar flames have separate bases, each of which is associated with the corresponding fuel jet. Although turbulent multi-jet flames have substantially different flame-base characteristics from laminar multi-jet flames (and conventional single-jet lifted flames), the propagation speed is similar for both conditions. The results suggest that the well-known flame-base stabilization theory can be straightforwardly applied to the multi-jet configuration, despite the turbulent flame structure's unconventional features. [ABSTRACT FROM AUTHOR]

In order to study the flame retardant performance of corrugated flame arrester core on the explosion characteristics of 9.5% hydrogen/methane/air premixed gas, a self-built experimental platform was used to study the explosive characteristics and propagation rules and the impact of varying thicknesses and different volumes of hydrogen. The results indicated that the corrugated flame arrester core on the explosion flame of premixed gas was observed with two scenarios, quenching and penetration. The flame is dominated by reverse vortex flow after quenching, reversing the axial direction of flame propagation. However, the flame penetration leads to more intense burning reactions as φ increases. And the increase in thickness changes the flame structure forms a spherical flame to a mushroom-shaped flame and finally form a turbulent flame. The velocity of flame decreased significantly when L = 60 mm with φ = 10% and 20% respectively. The quenching effect is more obvious in P2 with hydrogen added, and its better performance with increasing thickness, with 9.76% and 3.70% decrease when L = 40 mm with φ = 10% and 20%, respectively, and with 57.05% and 48.55% decrease when L = 60 mm with φ = 10% and 20%, respectively. [ABSTRACT FROM AUTHOR]