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The combined effects of heater position, mean flow parameters and flame models on thermoacoustic instability in a one-dimensional Rijke tube are studied systematically by classic linear stability analysis (LSA) and lattice Boltzmann method (LBM) simulation. In the former, the stability range of the linear flame model under low Mach number assumption is solved analytically, while in the more general case, it is obtained by numerically solving the dispersion relation. Both the linear and nonlinear flame model cases are studied using the LBM with a spectral multiple-relaxation-time collision model and a newly developed heat source term. With the linear flame model, the LBM is in good agreement with LSA in predicting the transition point and growth rates, while with the nonlinear flame model, LBM simulations are consistent with solutions of limit cycle theory in the fully developed state. These results demonstrate the applicability of the LBM in solving complex thermoacoustic problems. [ABSTRACT FROM AUTHOR]

Intelligent impact‐protection wearable devices often require intricate circuitry to operate, which hinders the timely display of impact‐related injuries. Consequently, it is imperative to develop intelligent protective materials that are self‐sufficient and capable of visualization. In this work, the impact protection material shear‐stiffening gel (SSG) is combined with the mechanoluminescent (ML) material ZnS:Cu/PDMS@SiO2 to create ML‐SSG. This material embodies various protective features, including impact protection, force visualization, flame resistance, and long‐distance passive control, making it ideal for intelligent wearable devices. In light of the significant shear stiffening effect of SSG, ML‐SSG effectively dissipates up to 80% of the impact energy and exhibits excellent impact resistance. Concurrently, ML‐SSG is also capable of visualizing impact injuries, displaying and warning in real‐time via mechanoluminescence, and assessing the impact force based on the intensity of mechanoluminescence. The incorporation of SiO2 and ZnS:Cu has resulted in ZnS:Cu/PDMS@SiO2 with remarkable flame‐retardant property. This innovative material significantly improves the performance of ML‐SSG in complex environments. In addition, ML‐SSG realizes human–computer interaction through neural network based on mechanoluminescence display characteristics. This research significantly expands the potential applications of multifunctional protective materials in various complicated environments, thereby promoting the development of wearable protective devices. [ABSTRACT FROM AUTHOR]

With the gradual improvement of the hydrogen energy industry chain and the continuous expansion of application scenarios, the hazardous consequences brought about by hydrogen combustion and explosion need to be emphasized. In this paper, compounded superfine dry powder fire extinguishing media (CSFDP) and flame retardant compounded ultrafine dry powder fire extinguishing media (CSFDP-MgAl-LDHs) were prepared, respectively. On this basis, a hydrogen deflagration experimental platform was constructed to investigate the inhibition efficiency of different extinguishing media. The results showed that the inhibition rates of CSFDP-MgAl-LDHs on the peak overpressure of hydrogen explosion were 68.57%, 68.43% and 81.72%, which were significantly higher than the corresponding inhibition rates of CSFDP (28.28%, 53.23% and 54.85%) when the hydrogen release pressure was 2.0, 4.0 and 6.0 MPa, respectively. Furthermore, under a release pressure condition of 2.0 MPa, the suppression rates of jet flame front propagation speeds under the action of CSFDP and CSFDP-MgAl-LDHs reached 15.99% and 23.65%, respectively. In addition, the pyrolysis properties of CSFDP-MgAl-LDHs were explored and the chemical reaction mechanism between them and the reactive radicals within the hydrogen jet flame was predicted by thermogravimetric analysis, differential scanning calorimeter and fourier transform infrared spectroscopy, respectively. Ultimately, the physicochemical synergistic inhibition mechanism exerted by CSFDP-MgAl-LDHs during pure hydrogen deflagration was revealed. This study demonstrates broad prospects and value in the areas of hydrogen energy safety and sustainable development, as well as providing a basis for the establishment of safety standards and regulations in the hydrogen energy industry. • A novel fire extinguishing agent containing flame retardants is prepared. • The suppression of pure hydrogen deflagration is taken as the research object. • The physicochemical inhibition mechanism of compounded media is predicted. [ABSTRACT FROM AUTHOR]

Pre-chamber spark ignition system enhances the combustion stability and efficiency of lean burn engines. Pre-chamber spark ignition system performance directly relies on the characteristics of the turbulent flame jet, which are governed by the pre-chamber geometrical parameters and nozzle hole configurations. Based on the existing literatures, limited research on nozzle hole numbers and patterns is noted with passive pre-chamber, and thus, the impact of these nozzle hole configurations on the combustion and flame development characteristics for hydrogen-methane-air mixtures is investigated in a constant volume combustion chamber. In the current study, single-layer nozzle hole configurations with three, four, six, and eight nozzle holes and double-layer nozzle hole configurations with six and eight nozzle holes are designed and tested in an optically accessible combustion chamber under various equivalence ratio conditions. The results indicated that the combustion characteristics deteriorated with an increase in nozzle holes, except with the single-layer four nozzle hole configuration. Compared to the single-layer eight nozzle hole configuration, the ignition delay and combustion duration for hydrogen-methane-air mixtures with single-layer four nozzle hole configuration was reduced by 31.9% and 30.7% under lean conditions. A reduction in flame jet penetration length, area and velocity and increased adjacent flame jet interaction was noticed for the single-layer configuration with more nozzle holes, and thus, a different nozzle hole pattern was studied in the current study. The double-layer configurations with more nozzle holes showed better combustion and flame development characteristics than single-layer configurations due to their unique flame structure, uniform and faster flame jet distribution, and reduced adjacent flame jet interaction. The ignition delay and combustion duration for hydrogen-methane-air mixtures with the double layer eight nozzle hole configuration were reduced by 30.5% and 40.7% compared to the single-layer eight nozzle hole configuration. [Display omitted] • Combustion characteristics deteriorated with an increase in pre-chamber nozzle holes. • SL-H4 configuration exhibited better combustion and flame development characteristics. • Increased jet interaction and reduced jet penetration was noted for more nozzle holes. • DL-H8 configuration improved the combustion process with a unique flame structure. [ABSTRACT FROM AUTHOR]

In this study, the dispersion and explosion of liquid hydrogen (LH 2) in leakage accidents are investigated using the Flame Accelerator Simulator (FLACS) code. The study focus was a liquid hydrogen research facility in Gimhae, South Korea. Simulations were conducted under various conditions by varying the mass flow rates (0.61 and 0.3 kg/s), directions (lateral in areas with minimal structures, lateral towards a tube trailer truck, and downward), and durations (90 and 180 s) of leakage. The results indicated that higher leakage mass flow rates produce a larger flammable cloud volume (FCV), resulting in higher maximum overpressures than that caused by lower flow rates. Downward leakage generally resulted in a larger FCV and higher overpressure than lateral leakage. The explosion characteristics were also influenced by geometry of LH 2 facility, including the presence of structures in the leakage path. Contrastingly, leakage duration had a minimal effect on the maximum overpressure because an increased duration causes a greater dilution of hydrogen, only slightly expanding the FCV. According to the overpressure–impulse diagram, higher leakage flow rates cause severe health risks, approaching thresholds for lung rupture and 100% fatality. Contrary to the minor damages caused by lower leakage flow rates. High flow rates, particularly with downward leaks, can cause significant structural damage in brick buildings, potentially leading to partial demolition. • Higher leakage mass flow rates result in larger flammable cloud volumes and increased maximum overpressures. • Downward hydrogen leaks lead to larger flammable clouds and higher overpressures than lateral leaks. • Facility geometry and structures significantly affect explosion overpressure magnitude and distribution. • Leakage duration has a minimal effect on maximum overpressure due to hydrogen dilution during leakage. [ABSTRACT FROM AUTHOR]

To guide the nozzle design for a novel oxy-fuel combustion facilitated pyrolysis technology, oxy-fuel rich combustion of pyrolysis gas is investigated experimentally and numerically. Two numerical models are compared and validated by experimental observations, among which the Eddy Dissipation Concept model performs better in prediction of flame length, maximum temperature, species conversion and flame stability with the modified fine scale constant. Flame length is found to be inversely proportional to the mixing rate of fuel and oxidizer. For non-premixed nozzles, the mixing rate increases with the increase in difference between the inner and annular velocity. For partially premixed nozzles, higher annular velocity promotes the mixing. The selective combustion molar ratio of three fuel components in practical nozzles are between those in premixed flame and at thermodynamic equilibrium assumption. CH 4 is consumed the most, while H 2 consumption molar ratio to CH 4 is 56%–69% and CO is net produced after the combustion. • The modified EDC model reasonably predicts flame appearance and blow-off limits. • Flame length is inversely proportional to the mixing rate of fuel and oxidizer. • For partially premixed nozzles, higher annular velocity promotes the mixing. • H 2 consumption molar ratio to CH 4 is 56%–69%, while CO is net produced. • For oxy-fuel rich combustion, yellow flame occurs at better premixed conditions. [ABSTRACT FROM AUTHOR]

Hydrogen and ammonia, as currently popular zero carbon fuels, have received widespread attention. These two fuels have complementary advantages in combustion characteristics, cost, safety, etc. Based on the vase shaped micro combustor, a new structure proposed for application in micro thermophotovoltaic systems in our previous work. This article numerically investigated the combustion characteristics and NOx emission of non-premixed H 2 /NH 3 /Air in vase shaped micro combustor which proposed in our previous work. The variation patterns of flame temperature, wall temperature, flammability limit, and NOx emission with the blended ratio of NH 3 and equivalence ratio were obtained. It was found that the flame cannot move and stabilize when the mixing ratio is 0.6. The rate of production (ROP) and sensitivity of NO and N 2 O were analyzed. Four precursors of NO, HNO, NH, N, N 2 have been discovered. Under the conditions of this article, HNO is the key component affecting NO emissions. When the blending ratio is greater than 0.3, the lower reaction rate of HNO leaded to a decrease in NO. The elementary reaction R63: NH + NO N 2 O + H is the key elementary reaction that affects NO consumption and N 2 O generation. The blending ratio of 0.4 and the equivalence ratio of 1.2 are the optimized operating conditions with the highest comprehensive performance. • Combustion and emission characteristics of H 2 /NH 3 in vase shaped micro combustor. • H 2 /NH 3 non-premixed combustion limit in novel micro combustor was studied. • ROP and sensitivity of NO and N 2 O were analyzed. • HNO and R63 are key species and elementary reaction effect on the NO emission. [ABSTRACT FROM AUTHOR]

In the context of the growing use of hydrogen for decarbonization in combustion, understanding NO x emissions from hydrogen burners is crucial. This study focuses on the impact of partial premixing on NO x production in hydrogen flames using a coaxial injector with swirl. The goal is to expand conventional jet scaling laws to encompass additional physical phenomena. The flexible injection system employed allows the stabilization of various flame types depending on the operating conditions, and double-front flames are obtained in several configurations. Observing the limitations of known scaling laws, particularly in coaxial flows with low-velocity differences or swirl, we conduct experiments measuring NO x and flame volume. Two Large Eddy Simulations explore distinct flame configurations to deepen our understanding about coaxial jet and recirculating flames, especially in the presence of a secondary premixed front due to rich premixing. Our investigation delves into the relationship between NO production and strain rate, considering turbulent and strain-related timescales. Notably, the presence of a premixed flame front alters the velocity profile on the shear layer, and the observed relationship between strain and turbulence in jet flames does not hold for recirculating flames. Thanks to this new insight, we propose a novel formulation for NO x scaling in coaxial jets, leveraging existing literature on coaxial jet modeling. Additionally, we propose a simple model to represent the piloting strain rate for NO emission in recirculating flames. Intricate effects of both swirl and partial-premixing are then included in the derived NO x scaling laws. [Display omitted] • Nitrogen oxide (NO x) emissions are measured in many swirled hydrogen flames. • Large-eddy simulations are used to enlighten the formation mechanisms. • Scaling laws for NO x emissions are derived to work for different flame topologies. [ABSTRACT FROM AUTHOR]

The paper reports simulation results on the influence of a direct-current driven radial electric field on the emission characteristics; especially NOx and CO of a premixed methane/air laminar jet flame. A multi-physics computational model is developed in the OpenFOAM framework to simulate electric-field-coupled premixed combustion process. The computational framework consists of coupled species, momentum and energy conservation together with a Poisson's equation solver to resolve the electric field distribution. Electron and ion conservation equations are resolved to consider the ionic wind body force in the momentum conservation equation and the associated possible electric field distortion due to the space charge distribution. The simulations are conducted for a stochiometric and fuel rich condition and over a range of jet flow rates for a configuration representative of a test-scale experimental setup. The model predictions show that for an applied voltage of 50 kV, the flame structure changes significantly for both the stoichiometric and fuel rich conditions. The flame is stretched significantly by the electric field due to ionic wind. For the fuel rich condition, the ionic wind allows additional mixing of the fuel rich stream with the surrounding air and drastically altering the flame structure. The electric field was found to reduce the NOx emission significantly for both stoichiometric and rich conditions. Over the entire range of flowrate conditions, the stochiometric fuel-oxidizer mixture showed a decrease in maximum NOx by a factor of 1.6 in presence of electric field. For the fuel rich case, however as the flow rate is increased, the NOx reduction factor decreased from 12.0 to 1.6. For CO emissions, the presence of electric field reduces the concentration under fuel rich conditions and vice versa for the stoichiometric flame. The role of kinetics is analyzed and discussed. [ABSTRACT FROM AUTHOR]

As a commonly applied ignition composition, Zr/KClO 4 has attracted various interests and plenty of efforts have been made to improve the combustion performance. In this study, Si nanoparticles were incorporated into Zr/KClO 4 by sonication. Around 2.5× peak pressure and 9.2× reactivity were achieved by Si/50%Zr/KClO 4 when compared to Zr/KClO 4. In addition, the peak pressure and pressurization rate of Si/Zr/KClO 4 ternary systems are all superior to ones of both Zr/KClO 4 and Si/KClO 4 binary systems. The synergetic effect of Si/Zr/KClO 4 is a combination of shorter ignition delay of Zr/KClO 4 and more gas production of Si/KClO 4 , which is also reflected on the higher flame propagation rate of Si/Zr-based thermites than composites with pure Si or Zr as the fuel. Meanwhile, the heat release during the redox reaction has been measured. The results show ∼700–1000 J/g higher energy release than Zr/KClO 4 , and higher combustion efficiency can be attained by ternary systems than binary ones. Thus, adding Si powders is an effective way to improve the energetic behavior of Zr/KClO 4 ignition composition, and Si/Zr/KClO 4 ternary composites can be a promising candidate for application in pyrotechnics. [ABSTRACT FROM AUTHOR]

The article presents the results of the computational and theoretical analysis of the mechanisms of formation of a vortex flow during flame propagation in a channel filled with a combustible gas mixture. It is shown that at the initial stage of combustion development the flow formed in the near-wall region can be described in accordance with the Blasius boundary layer theory. It is established that the nonstationary nature of flame evolution leads to disturbance of the boundary layer and that under certain conditions the development of the boundary layer instability determines the formation of vortex structures ahead of the flame front. In such a case, depending on the composition of the combustible gas mixture, stable flow regimes without the formation of vortex structures and regimes with flow stabilization are possible. [ABSTRACT FROM AUTHOR]

Large Eddy Simulation (LES) coupled with the Linear Eddy Model (LEM) provides a robust method for studying turbulent combustion, but it is computationally expensive due to the need for highly resolved sub-grid LEM domains. These domains simulate sub-grid stirring through stochastic rearrangements of scalar fields, while large-scale transport is modelled using a Lagrangian ‘splicing’ scheme. To address the computational cost of LES-LEM, a super-grid (SG) framework for LEM closure was developed by the authors (Comb. Theor. Model. 28, 2024), which uses coarse-graining, on-the-fly chemistry tabulation and a presumed PDF approach to reconstruct thermochemical fields at LES resolution. This study applies SG-LEM to a challenging setup, Case 1 of the Volvo Validation Rig, which involves a bluff-body-stabilised turbulent premixed propane-air flame, as a stress test to identify limitations that were not revealed by the previous application, in particular that of the coarse-graining parameters used to generate the super-grid. The intent is to yield a more realistically constrained assessment of the current capabilities of the method, and insight into possible ways for improving it. Four simulations were conducted using three SG cluster sizes. The finest resolution was tested with a global 2-step mechanism, showing good agreement with experimental data for temperature and velocity, particularly near the bluff body. The two larger cluster sizes used a 66-step skeletal mechanism for more detailed chemical closure but led to unphysical quenching due to splicing inaccuracies. To mitigate these issues, two novel additions were introduced: an intra-cluster-stirring routine and a method to control SG cluster shapes to reduce numerical dissipation. These methods improved flame stability with coarser SG clusters and more detailed mechanisms. Comparison with experiments showed good agreement for temperature and velocity, though elevated CO levels were observed in the recirculation region. Potential methods for further improving SG-LEM's capabilities are discussed. [ABSTRACT FROM AUTHOR]

Early detection of a fire during its development stage is highly desired to minimize human and material losses. The improvement of the efficiency of these monitoring systems through the analysis of fire video data drawn from unmanned aerial vehicles is considered. Classification of video images when monitoring a fire event on the territory is used based on their segmentation into identical rectangular segments of a given size and then assigning them to one of three classes: Smoke, Flame, and Indifferent classes. The Walsh-Hadamard transform was used to form descriptors for the Weak classifiers. Descriptors were calculated for three Weak classifiers; first, the Walsh-Hadamard transform was calculated for the window of the entire segment, and its spectral coefficients were used for the first Weak classifier. Next, the descriptors were calculated in windows whose sizes were half and a quarter of the original window size. The classifier consisted of three independently-trained neural networks. A simple ensemble averaging block was used to combine the outputs of those networks. A special software code has been developed to classify video images. The code allows us to create a database of segments of the 'Smoke' and 'Flame' classes, determine the two-dimensional Walsh-Hadamard spectrum of video image segments, train fully connected neural networks, conduct exploratory analysis and evaluate the relevance of the calculated two-dimensional spectral coefficients. For validation we used real data from Сlosed Circuit Television cameras presented in open databases. The experimental studies on the classification of video data containing flames and smoke showed that the proposed method has an average smoke detection accuracy of 86% and an average flame detection accuracy of 89.5%. Errors II in smoke detection and flame detection had the averages of 13% and 4.5%, respectively. [ABSTRACT FROM AUTHOR]

Ammonia, regarded as a zero-carbon fuel, presents significant challenges due to its notable combustion and chemical inertness. To mitigate these challenges, blending liquid ammonia with carbon-neutral methanol offers a promising avenue for creating an innovative, stable, and large-scale producible carbon-neutral fuel. This study investigates the impacts of xCH 3 OH (methanol mole ratio in methanol-ammonia mixture) and v (mean jet velocity at nozzle outlet) on forming stable flames of a premixed methanol-ammonia argon-oxygen (MA-AO) gaseous jet in the atmospheric surrounding. Results indicate that the pure ammonia argon-oxygen (A-AO) mixture can be ignited by an electrical spark with the energy of about 52.1 mJ, but it cannot grow to be a stable jet flame although the fire nucleus can form. Blending methanol is very helpful in forming a stable jet flame of the MA-AO mixture. When xCH 3 OH is lower than 49% at λ = 1.0, only the fire nucleus could be formed without a stable jet flame. When xCH 3 OH is more than 49%, there is a certain mean jet velocity range for different xCH 3 OH to form a stable flame. At the low-velocity limit, the flame would be quenching. While the flame would be blown off at the high-velocity limit. The velocity limits of quenching and blow-off become higher with higher xCH 3 OH , and the blow-off velocity limit increases more evident. • The ammonia combustion could be triggered with spark energy of 52.1 mJ. • There is a threshold of methanol-ammonia ratio to form a stable methanol-ammonia flame. • The methanol-ammonia flame stability has a close relationship with mean jet velocity. [ABSTRACT FROM AUTHOR]

The grate-kiln process for iron-ore pellet induration utilizes pulverized coal fired burners. In a developed infrastructure for H 2 , it might be desirable to heat the existing rotary kilns with renewably produced H 2. Technical challenges of H 2 heating of grate-kilns include high emissions of NO X and maintaining sufficient heat transfer to the pellet bed. This article examined cofiring (70% coal/30% H 2) in 130 kW experiments using two different integrated burner concepts. Compared to pure coal combustion, cofiring creates a more intense, smaller flame with earlier ignition and less fluctuations. The process temperature and heat transfer are enhanced in the beginning of the kiln. The co-fired flames emit 32% and 78% less NO X emissions compared to pure coal and H 2 combustion, respectively. We can affect the combustion behavior and NO X emissions by the burner design. H 2 /coal cofiring using integrated burners is probably an attractive solution for emission minimization in rotary kilns. [Display omitted] • Pure H 2 combustion faces obstacles replacing coal in current induration kilns. • Coal and H 2 cofiring is investigated as an alternative. • Cofiring enhances the ignition of coal particles and stabilizes the flame. • Cofiring emits less NO X emissions compared to traditional pure coal and pure H 2. • Cofiring and pure H 2 flames offer elevated heat transfer close to the burner. [ABSTRACT FROM AUTHOR]

Blending a low ratio of hydrogen into traditional hydrocarbon fuels can reduce the modification in structure of existing combustors. However, even a low hydrogen ratio can increase the propensity of flashback due to the extremely high reactivity of hydrogen. Swirl number is a key parameter of swirl combustors which determines the flow structure. To design reliable gas turbine combustors for hydrogen-blended fuels, the study in the effect of swirl strength on flashback characteristic and the underlying mechanism is necessary. In this study, the flashback characteristics of premixed 30% H 2 /70% CH 4 /air flame in a bluff-body swirl burner is investigated via both experiment and large-eddy simulations with the flamelet-generated-manifold method (FGM-LES). The flashback mode and flame-flow interaction are analyzed based on the LES results. It is found that the increase in swirl strength will increase the propensity of flashback and promote the flashback speed of premixed 30% H 2 /70% CH 4 /air flame in the bluff-body swirl burner. The turns from Mode II (small-scale flame bulges leading flashback) to Mode I (large-scale swirling flame tongue leading flashback) during flashback of hydrogen-blended flames due to the production of stronger reversed flow by swirling flow. It is also found that stronger swirl strength will result in a smoother flame front and narrower strain rate distribution range which suppresses the local hydrogen enrichment due to preferential diffusion of hydrogen, thus suppressing the flame propagating speed. However, this effect is not dominant in flashback process compared to the promotion of reversed flow by increased swirl strength. • Efficient FGM tabulation including heat loss and differential diffusion for premixed hydrogen/methane flame is applied. • The flashback processes of premixed hydrogen-blended flame in experiment were reproduced well through FGM-LES. • The interaction between swirl strength and preferential diffusion of hydrogen was investigated. [ABSTRACT FROM AUTHOR]

With the rise in load demand, improving the voltage profile and reducing line loss is vital to ensure reliable power delivery to the customer. However, increasing power plant generation capacity is limited by environmental and economic factors, requiring a careful assessment of locally optimal options. Distributed Generation (DG) installation into the electricity grid is one of the reliable remedial actions to ensure a smooth power delivery. Installation of DG requires an optimization process to identify the appropriate placement and sizing. Inaccurate sizing and placement of the DG installation may result to over-compensation or under-compensation phenomena. This paper proposes a novel approach termed the Integrated Immune Moth Flame Evolutionary Programming technique (IIMFEP) for optimizing the installation of DG sources in distribution systems. It handles various scenarios, including multi-DG single-type and multi-DG multi-type installations, with the main objective of minimizing power loss in the system. This technique employs a hybrid approach that combines elements of immune algorithms, moth flame optimization, and evolutionary programming to achieve more accurate and efficient results. Two cases were considered in this study termed Case 1 and Case 2. Results in Case 1 discovered that the optimal sizing and placement of four Type III DGs exhibit the lowest power loss worth 2.74 kW (98.78 % reduction) for the 69-Bus RDS, while for the 118-Bus RDS the power loss is 319.89 kW (75.36 % reduction). In Case Study 2, the combination of DG Types I and III provided the highest power loss reduction. With one DG Type I and two DG Type III units installed, power loss was reduced by 97.15 % to 6.41 kW for the 69-Bus RDS and by 62.01 % to 493.21 kW for the 118-Bus RDS. The proposed IIMFEP managed to alleviate the setback experienced in the traditional EP, AIS and MFO which found to be stuck at local optimum. The IIMFEP method is compared to Moth Flame Optimization, Artificial Immune System and Evolutionary Programming and validated using the IEEE 69-Bus and 118-Bus Radial Distribution Systems, resulting in outstanding performance. [ABSTRACT FROM AUTHOR]

In this work, a series of porous medium with Gyroid Triply Periodic Minimal Surfaces (G-TPMS) structures were established for lean concentration methane combustion. Based on pore-scale numerical simulations, the influence of TPMS parameters on porous media combustions, as well as the shape of flame surface in different TPMS structures, heat transfer characteristics between solids and fluids, the radiation characteristics and heat flux on the solid surface were investigated. Results show that, under the same inlet flow rate and equivalence ratio, the value and axial location of the maximum average fluid temperature will be changed due to the difference in porosity and flow resistance of each structure. For the G-TPMS structures' porous medium, the change of structure period had a greater impact on the combustion compared with the wall thickness. And, in the G-TPMS structures' porous medium, the flame surface always shows a finger structure. The flow rate that shows the same increase trend with the fluid temperature in the preheating region, which has a periodic change like the porosity in the axial direction. Based on the results, it can be seen that the region of solid-to-solid surface radiative heat transfer and fluid-solid heat flux on the interface was also changed due to the influence of flame front position. [ABSTRACT FROM AUTHOR]

Large hydrocarbon fuels are used for ground and air transportation and will be for the foreseeable future. A key parameter when burning these fuels is the turbulent consumption speed, which is the velocity at which fuel and air are consumed through a turbulent flame front. Such information can be useful as a model input parameter and for validation of modeled results. In this study, turbulent consumption speeds were measured for three jet-like fuels using a premixed turbulent Bunsen burner. The burner was used to independently control turbulence intensity, unburned temperature, and equivalence ratio. Each fuel had similar heat releases (within 2%), laminar flame speeds (within 5–15%), and adiabatic flame temperatures. Despite this similarity, for constant ReD and turbulence intensity, A2 fuel (i.e. jet-A) has the highest turbulent flame speeds and remains stable (i.e. without tip quenching) at lower ϕ than the other fuels. In contrast the C1 fuel, which consists of heavily-branched alkanes, contains no aromatics and has a relatively high average molecular weight, has the slowest turbulent flame speeds and is the most prone to tip quenching. The C1 fuel has the highest stretch sensitivity, in general, as indicated by calculated Markstein numbers. This work shows that turbulent flame speeds and tip stability of multi-component large hydrocarbon fuels can be sensitive to the chemical class of its components. [ABSTRACT FROM AUTHOR]

Values of laminar burning velocity obtained by the constant volume method suffer from the inaccuracies introduced by the semi-analytical burned gas mass fraction model and flame stretch. A cylindrical combustion chamber with full optical access was developed in the present study, enabling the photography of the flame until the flame touches the inner wall of the combustion chamber. This enables direct visualization of the flame, thus eliminating the dependency on burned gas mass fraction models. The values of laminar burning velocity obtained by simultaneous measurement of flame radius and pressure-time data using the constant volume method are not stretch-free. This paper also develops a methodology for stretch correction based on the determined laminar burning velocity. Experiments were performed on mixtures of methane and air at initial pressures (0.8–1.05 bar), initial temperatures (293–320 K) and equivalence ratios (0.8–1.2). Stretch-corrected laminar burning velocity for lean, stoichiometric, and rich methane-air flames has been obtained in the present study, and the results obtained are validated against available experimental data and 1-D flame computations. It is expected that the proposed method significantly reduces the inaccuracies in laminar burning velocity obtained by the constant volume method. [ABSTRACT FROM AUTHOR]

To explore the effect of pressure on the combustion and luminescence properties of magnesium/nitrate pyrotechnics, combustion tests and thermal analysis of magnesium/sodium nitrate (N) and magnesium/strontium nitrate (S) pyrotechnic samples with zero oxygen balance were conducted under the pressure range of 101kPa-1kPa in this study. The results show that the flame height of N increases continuously as the pressure decreases, and the flames gradually change from a steady-state convergent flame to a turbulent evanescent flame. However, due to the different diffusion rates of the gas-phase combustion products, the diffusion of S at low pressure is lower and the flame still maintains a stable laminar flow. Although the light intensity and burning speed of both N and S satisfy Vier's law, the luminous intensity of S is less sensitive to pressure and still has good luminous performance at 1 kPa. Thermal analysis test results show that N and S can react almost completely at atmospheric pressure, and their enthalpies of reaction are 6.75 kJ/g and 6.07 kJ/g, respectively. The difference in the thermal decomposition histories of strontium nitrate and sodium nitrate at 1 kPa results in a reaction enthalpy of 4.24 kJ/g for S and only 2.45 kJ/g for N. This indicates that the reaction degree of S at low pressure is higher than that of N. The SEM-EDS test results further confirm this opinion. These scientific findings provide useful insights into the low-pressure combustion behavior of pyrotechnics and support the growing need for military and civilian applications. [ABSTRACT FROM AUTHOR]

Corner tangential and bias combustion technology have significant advantages in optimizing combustion and pollutant emissions. This paper combined the corner tangential, bias combustion and jet technology for the first time to control thermoacoustic oscillations and pollutant NOx emissions synchronously. The control feasibility was examined by three variables: widths of corner tangential slots, bias jets ratios, and jet flow rates. Results indicate that the widths of corner tangential slots and bias jets ratios significantly affect the suppression effectiveness of thermoacoustic oscillations and NOx emissions. The synchronous control effectiveness with larger widths of corner tangential slots and smaller bias jets ratios is better than that with smaller widths of corner tangential slots and larger bias jets ratios. The maximum damping ratio of dynamic pressure and CH* can achieve 90.1% and 82%, respectively. The dynamic pressure drops from 39.2 Pa to 3.9 Pa and CH* reduces from 0.006 arb.units to 0.001 arb.units. A lower oscillations frequency will form by the bias jets, which avoids the occurrence of high-frequency and high-amplitude thermoacoustic self-excited oscillations. Besides, the NOx emissions drop from 26.2 ppm to 13.9 ppm reaching a maximum reduction of 12.3 ppm. Meanwhile, a slimmer flame root and flatter flame front are triggered. Two flame modes are summarized as Model A and Model B. This research realized an efficient simultaneous control of thermoacoustic oscillations and pollutant NOx emissions, which could be a helpful guideline for designing lean premixed flames. [ABSTRACT FROM AUTHOR]

This study comprehensively investigates the response of a combusting droplet during its interaction with a high-speed transient flow imposed by a coaxially propagating blast wave. The blast wave is generated using a specially designed miniature shock generator that produces blast waves using the wire-explosion technique, facilitating a wide range of Mach numbers (1.03 < Ms < 1.8). The experiments are performed in two configurations: open field and focused blast wave. The charging voltage and the configuration determine the Mach number (Ms) and flow characteristics. The flame is found to exhibit two major response patterns: partial extinction followed by reignition and full extinction. Increasing the Mach number (Ms > 1.1) makes the droplet flame more vulnerable to extinction. Additionally, the flame exhibits stretching and shedding, followed by reignition at lower Mach numbers (Ms < 1.06). In all cases, the flame base lifts off in response to the imposed flow, and the advection of the flame base interacting with the flame tip results in flame extinction. The entire interaction occurs in two stages: (i) interaction with the blast wave and the decaying velocity profile associated with it, and (ii) interaction with the induced flow behind the blast wave as a result of the entrainment (delayed response). Alongside the flame's response, the droplet also interacts with the flow imposed by the blast wave, exhibiting different response modes including pure deformation, Rayleigh–Taylor piercing bag breakup and shear-induced stripping. [ABSTRACT FROM AUTHOR]

In the present work, the laminar burning velocities of NH3 + hydrous C2H5OH + air flames were measured using the heat flux method at 1 atm with varied equivalence ratios and mixing ratios (with hydrous ethanol molar fractions of 45%–75%). The measurements were carried out at unburned temperatures of 358 and 378 K. The results show that the laminar burning velocities increase with the increase of hydrous ethanol mixing ratio and temperature, indicating that hydrous ethanol contributes to the combustion of NH3 flames. Based on Wang et al.'s CEU‐NH3 mechanism, sensitivity, reaction pathways, and product formation rate analyses were conducted. The results show that the addition of water reduces the laminar burning velocities of mixed fuel. Intermediate radicals such as NH and HNO are crucial for the formation of NO. After adding water, in the preheating zone, the total production rates of key species such as NH3, H, O, and OH radicals and intermediate species like NH2, NH, HNO, and N decrease, leading to a reduction in the total NO generation rate and the peak mole fraction of NO in the reaction zone. [ABSTRACT FROM AUTHOR]

The article "Onset of global instability in a premixed annular V-flame" delves into the self-excited oscillations of a lean premixed V-flame through computational simulations. It uncovers distinct families of eigenmodes and identifies subcritical behavior and hysteresis in the system at different Reynolds numbers. The study also reveals the presence of nonlinear attractors and the onset of chaos in the annular V-flame, shedding light on the challenges of linear analysis in predicting the nonlinear dynamics of such systems. This compilation of research studies and PhD theses explores various aspects of fluid mechanics and combustion dynamics, offering valuable insights into the complexities of these phenomena. [Extracted from the article]

The article delves into the hydrodynamic theory of premixed flames in closed vessels, emphasizing flame speed and Markstein lengths. It explores the impact of pressure-dependent Markstein numbers on flame speed, influenced by heat release and mixture properties. The study aims to provide insights into combustion processes in confined spaces, with practical implications for engineering design and safety considerations. The document also includes research articles on combustion and flame propagation in various settings, covering topics such as turbulent flame development, burning velocity measurement, and flame instability effects. The research seeks to deepen comprehension of combustion processes in closed vessels and high-pressure conditions. [Extracted from the article]

Internal combustion engines (ICEs) fuelled with hydrogen can play a major role in the short-term future transportation sector since they abate all criteria pollutants at engine-out reducing tailpipe CO2 emissions to near-zero levels. However, optimizing hydrogen ICEs is a challenging task that can be addressed through the development of a robust simulation tool capable to predict the H2 combustion process. In this study, a previously developed two-zone combustion model has been updated considering different laminar flame speed computations, both based on a detailed chemistry scheme: a polynomial correlation function and a tabulated approach. The predictive capabilities of the combustion model have been validated against experimental data coming from a 0.5L PFI single-cylinder engine under several operating conditions. The tabulated approach for laminar flame speed definition proved to be the best solution, leading to a combustion duration average error lower than 3 deg over a dataset containing more than 45 different operating conditions. • A predictive combustion model for hydrogen combustion was developed. • The air-hydrogen laminar flame speed calculated using detailed chemical kinetics. • Two approaches tested for implementing the laminar flame speed calculations. • Tabulated laminar flame speed provides high predictive capabilities. [ABSTRACT FROM AUTHOR]

Compared with pure NH 3 fuel, the lower NO emission conditions of NH 3 –H 2 mixtures under the oxygen-enriched combustion mode have been identified quantitatively. Furthermore, the effects of the H 2 addition on the NO productions of premixed oxygen-enriched NH 3 –H 2 flames are studied numerically, while the major reactions responsible for the variation of total NO are identified. Results show that the H 2 addition is eligible to reduce the NO emissions of the oxygen-enriched NH 3 combustion if the laminar burning velocity is fixed to a larger value (>23 cm/s). The quantitative equations are obtained to determine the optimum oxygen-enriched conditions of the NH 3 –H 2 mixture, aiming to achieve lower NO emission and similar or improved combustion stability compared to pure NH 3 fuel by restricting the minimum and maximum O 2 percentages for the NH 3 –H 2 mixtures. Based on the rate of production (ROP) analysis, for the oxygen-enriched NH 3 –H 2 combustion, R144 (HNO + H<=>H 2 +NO) is consistently the most significant NO production reaction, while R85 (NH + NO<=>N 2 O + H) and R91 (N + NO<=>O + N 2) play significant roles in the NO consumption. When the laminar burning velocity (S L) is kept to a lower value, the increased total NO of the oxygen-enriched NH 3 flames with the H 2 addition is ascribed to more efficient suppressions on the NO consumption reactions of R85, R91 and R77 (NH 2 +NO<=>NNH + OH). In contrast, thanks to the extra impact of the higher O 2 percentage on the H/O/OH radical pool at larger S L , the NO production reactions begin to suffer stronger suppressions of the H 2 addition, resulting in the reduced total NO of NH 3 –H 2 mixtures. Furthermore, R144 is identified as the key reaction influencing the variation trend of total NO at both lower and higher S L values. • NO emissions of oxygen-enriched NH 3 –H 2 combustion are investigated. • Lower NO emission conditions of NH 3 –H 2 mixture than NH 3 are identified. • H 2 addition leads to opposite change of NO emission at small and large S L. • Effects of H 2 addition on NO formations are analyzed at different S L. • R144 plays a decisive role in determining the opposite change of NO emission. [ABSTRACT FROM AUTHOR]

In this study, a three-dimensional, high fidelity LES of a fully premixed, lean hydrogen-air deflagration, in a confined and obstructed channel is performed. The experimental configuration studied is the GraVent explosion channel (L. Boeck et al., Shock Waves, 2016). A complete methodology to perform LES of lean hydrogen, strongly compressible deflagrations is presented. The capability of LES to quantitatively reproduce the main Flame Acceleration (FA) mechanisms of the fast deflagration is illustrated. The physics of FA are analysed and the contribution of the unburnt mixture flow aerodynamics to the absolute flame propagation speed, is evaluated. This is made possible by the access to the complete reactive flow fields, which are not available in the experiments. It is shown that the flow contraction, at fence-type obstacles, and the flame/vortex interaction, between the flame front and the turbulent structures in the wake of the obstacles, interact constructively, driving FA. [Display omitted] • 3-D Large Eddy Simulation of a fast deflagration in the GraVent BR30hS300 channel. • Methodology for LES of lean hydrogen explosions with Adaptive Mesh Refinement. • Excellent agreement with experimental measurements of flame velocity and overpressure. • Flow contraction at obstacles and flame/vortex interaction drive flame acceleration. [ABSTRACT FROM AUTHOR]

The reaction kinetics mechanisms of KAUST PAH mechanism 2(KM2) and AramcoMech are commonly used to simulate and predict the generation of polycyclic aromatic hydrocarbon (PAH) and soot. The two kinetics mechanisms were evaluated based on the flame multi-parameter measurement data using methane mixed with ethylene opposed diffusion flame as research carrier. The results show that the two mechanisms can accurately predict the thermochemical structure of methane mixed with ethylene diffusion flame. However, the prediction performance of the influence of methane mixing on PAH and soot generation is poor. Neither mechanism fails to predict the inhibition of PAH and soot generation in methane mixed with ethylene flame. Comparing the similarities and differences of PAH and soot generation paths between the two mechanisms, it is found that both mechanisms overestimate the sensitivity of propargyl radical (C3H3·)generation to methane blending. The flame multi-parameter measurement data are helpful to further optimize and verify the reaction mechanism of methane fuel. [ABSTRACT FROM AUTHOR]

Ammonia has emerged as a promising carbon-free fuel for mitigating greenhouse gas emissions. However, its application in practical combustion systems is limited by several issues including its low heating value and slow flame propagation speed, which have posed challenges in maintaining stable combustion. As an attempt to resolve these issues, we numerically investigated premixed ammonia–air combustion using a detailed chemical mechanism in a heat-recirculating, Swiss-roll burner that was proposed based on the concept of "excess-enthalpy." The main focus was put on the flame stabilizations as well as characteristics of NO/N2O emissions across a wide range of operational conditions. The results showed that the use of the Swiss-roll burner led to a significantly broadened stability regime for pure ammonia combustion, which could be attributed to the effective preheating from combustion products to unburnt mixture. The relationship between the dimensionless heat transfer parameter and excess-enthalpy was quantified and a linear correlation was revealed. In addition, flow expansion and recirculation within the combustion chamber led to the generation of vortices, which was also beneficial for flame stabilizations. NO emissions at the burner outlet were witnessed to have a linear growth in the laminar flow region, a gentler increase in the flow-transition stage, and a final leveling off at the turbulent flow condition with the increase in Reynolds number. For a given Reynolds number, the NO emission showed a non-monotonic variation with equivalence ratios, with relatively low emission levels at either the fuel-lean or fuel-rich conditions. As another major concern, N2O emission was found more significant in the laminar flow region and at fuel-lean conditions, both of which should be avoided in practical operations. [ABSTRACT FROM AUTHOR]

The characteristic behavior of the wall exerts a strong influence on the flame acceleration (FA) and deflagration-to-detonation transition (DDT) processes in microchannels [Ramachandran et al., "A numerical investigation of deflagration propagation and transition to detonation in a microchannel with detailed chemistry: Effects of thermal boundary conditions and vitiation," Phys. Fluids 35, 076104 (2023)]. In this work, motivated by the catalytic microcombustors in realistic industrial settings, we study the influence of catalytic nickel walls on the FA & DDT processes. Highly resolved numerical simulations (spanning 10–20 grid points across the flame thickness) are performed, employing a 9-species 21-reaction combustion mechanism for H2-combustion by Li et al. ["An updated comprehensive kinetic model of hydrogen combustion," Int. J. Chem. Kinet. 36, 566–575 (2004)] for the gas-phase chemistry and a 5-species 12-reaction submechanism derived from a methanation microkinetic mechanism by Schmider et al. ["Reaction kinetics of CO and CO2 methanation over nickel," Ind. Eng. Chem. Res. 60, 5792–5805 (2021)] for the catalytic surface chemistry. Stoichiometric H2/air with and without 25% (by mole) of H2O dilution/vitiation are investigated. The simulations demonstrate that catalytic walls enhance flame propagation in the vitiated mixture (which exhibits lower flame speeds) by providing additional radical production and heat release at the surface. As a result, the traditionally observed parabolic-like flame front profile in microchannels inverts due to preferential propagation of the flame along the wall. In contrast, the unvitiated mixture exhibits rapid flame acceleration, and the influence of catalytic walls is found to be minimal. These observations are due to the fact that the residence time available for coupling the heterogeneous wall chemistry with the gas-phase combustion is smaller at higher flame speeds (in unvitiated mixtures). [ABSTRACT FROM AUTHOR]

Based on the OpenFOAM platform, a numerical study was conducted to investigate the propagation characteristics of a detonation wave in uniformly premixed gases within a semi-confined channel, specifically examining the changes in wave's structure and analyzing the detonation reinitiation-extinguishment process. The results indicate that, due to the weak confinement, the detonation wave experiences lateral expansion, with transverse waves on the detonation wave front penetrating into the inert gas and forming segmented protrusions on the oblique shock wave. During the propagation process, the number of transverse waves decreases gradually, and the reflected waves formed by the interaction between transverse waves and the inert gas are weak, collectively leading to a gradual decay in the strength of the detonation wave and a reduction in the frequency of pressure oscillation on the detonation wave front. Furthermore, when transverse waves interact with the inert gas and undergo Mach reflection, the reflected waves would detach the inert gas and form new transverse waves promoted by upward-moving old transverse waves and disturbances on the flame front, thereby extending the duration distance of the detonation wave. Subsequently, after the detonation wave decouples, a transverse wave reflected from the inert gas interface interacts with the lower wall, triggering local detonation reinitiation, which generates a higher-intensity longitudinal wave that couples with the leading shock wave, temporarily restoring the detonation wave and forming lateral detonation in the decoupled region. However, the detonation wave will ultimately extinguish due to the attenuation of transverse waves' intensity and decreasing number of these waves. [ABSTRACT FROM AUTHOR]

The low-emission technology of gas turbine combustors is currently an active area of research. In light-duty lean-premixed combustors, achieving rapid and uniform fuel mixing presents significant challenges. Additionally, combustion instability issues are also likely to occur. To address these challenges, large eddy simulation and the flamelet generation manifold combustion model are used to predict the velocity field, fuel distribution, vortex structure, flame structure, and flame liftoff phenomenon in a low-emission tower-type coaxial-staged combustor. The results indicate that variations in the position of the fuel holes in the second main stage result in two types of fuel injection modes: coupling and decoupling. These variations do not significantly influence the velocity and vortex structure in a non-reacting flow. The dominant frequency of the non-reacting flow field in the combustor is 810 Hz. The position of the precessing vortex core affects the distribution of fuel. Furthermore, the uniformity of fuel distribution at the outlet of the second main stage is notably affected by different fuel injection modes. The spatial distribution of fuel is more uniform. In the reacting flow, compared to the decoupling mode, the fuel expansion angle decreases by 4.5° under the coupling mode, and the heat release at the flame front is more intense. Additionally, it is found that fuel injection modes significantly influence the dynamic characteristics at the flame root. Better flame stability is observed under the decoupling mode, while flame liftoff phenomena occur under the coupling mode. The lifted flame root shifts downstream by 12.3 mm. [ABSTRACT FROM AUTHOR]

This paper aims at assessing a hypothesis that resolution required to evaluate fuel consumption and heat release rates by directly (i.e., without a subgrid model of unresolved influence of small-scale turbulent eddies on the local flame) processing filtered fields of density, temperature, and species mass fractions should be significantly finer than resolution required to directly compute flame surface density by processing the same filtered fields. For this purpose, box filters of various widths Δ are applied to three-dimensional direct numerical simulation data obtained earlier from a statistically one-dimensional and planar, moderately lean H2/air complex-chemistry flame propagating in a box under conditions of sufficiently intense small-scale turbulence (Karlovitz number is larger than unity, and a ratio of laminar flame thickness δ L to Kolmogorov length scale is about 20). Results confirm this hypothesis and show that the mean flame surface density and area can be predicted with acceptable accuracy by processing filtered combustion progress variable fields computed using a sufficiently wide filter, e.g., Δ / δ L = 4 / 3 . Such an approach does not require a model of the influence of subgrid turbulent eddies on flame surface density provided that Δ and δ L are of the same order of magnitude. Good performance of this approach is attributed to inability of small-scale (when compared to δ L ) turbulent eddies to substantially change the local flame structure, which, nevertheless, is significantly perturbed by larger turbulent eddies that strain the local flame. [ABSTRACT FROM AUTHOR]

This work presents an experimental set of Bunsen flames characterized by a moderate Reynolds number and a variable turbulence intensity. Ten lean hydrogen-enriched methane–air mixtures at three turbulence levels are investigated, ranging from methane–air to hydrogen–air mixtures. Such mixtures are selected to have an almost constant laminar flame speed while inducing the onset of thermal-diffusive (TD) instability by gradually increasing the hydrogen content of the blend. The flames' global consumption speed, stretch factor, and flame surface area are investigated and discussed as functions of the effective Lewis number of the mixture. As the interplay between TD instability and turbulence enhances the overall flame propagation, below a transitional Lewis number, flames are observed to be particularly sensitive to external turbulent forcing. This synergistic interaction is discussed in terms of Karlovitz and Lewis numbers. A parameterization of the turbulent flame speed is thus proposed, based on a functional form depending, concurrently, on both Karlovitz and Lewis numbers. The proposed form is shown to fit the experimental results at different turbulence levels and to capture the flame speed enhancement across the transitional Lewis number. [ABSTRACT FROM AUTHOR]

This study investigates the flame dynamics of an annular combustor under various azimuthal acoustic forces. The experimental platform was applied to external azimuthal acoustic force through four loudspeakers installed on the wall outside the plenum. Experiments were conducted with an equivalence ratio of Ф = 0.71 and a combustion power of P = 9.5 kW. Therein, four loudspeakers were used to precisely control the acoustic pressure amplitude, spinning ratio, and spinning direction, which stimulated the first-order standing mode, spinning modes, and second-order standing mode acoustic mode of the plenum, respectively. Our analysis of flame dynamics and acoustic response characteristics was based on different azimuthal modes, combined with dynamic mode decomposition (DMD) of the flame image sequence and acoustic pressure signal. The results corroborate the consistency between the acoustic field quaternion analysis and the flame image sequence DMD results despite varied external forcing conditions. Specifically, the characteristics of the flame vary by the applied forces. Moreover, DMD results indicate that the flame structure of the eigenfrequency mode is asymmetrical in spinning modes and tilts in the direction of the pressure gradient. [ABSTRACT FROM AUTHOR]

Hydrogen is regarded as a promising energy source for the future due to its clean combustion products, remarkable efficiency and renewability. However, its characteristics of low-ignition energy, a wide flammable range from 4% to 75%, and a rapid flame speed may bring significant explosion risks. Typically, accidental release of hydrogen into confined enclosures can result in a flammable hydrogen–air mixture with concentration gradients, possibly leading to flame acceleration (FA) and deflagration-to-detonation transition (DDT). The current study focused on the evolutions of the FA and DDT of homogenous/inhomogeneous hydrogen–air mixtures, based on the open-source computational fluid dynamics (CFD) platform OpenFOAM and the modified Weller et al.'s combustion model, taking into account the Darrieus–Landau (DL) and Rayleigh–Taylor (RT) instabilities, the turbulence and the non-unity Lewis number. Numerical simulations were carried out for both homogeneous and inhomogeneous mixtures in an enclosed channel 5.4 m in length and 0.06 m in height. The predictions demonstrate good quantitative agreement with the experimental measurements in flame-tip position, speed and pressure profiles by Boeck et al. The characteristics of flame structure, wave evolution and vortex were also discussed. [ABSTRACT FROM AUTHOR]

Cryogenic compressed hydrogen (CcH2) technology combines the advantages of high pressure and low temperature to achieve high hydrogen storage density without liquefying the hydrogen, which has broad application prospects. However, the safety concerns related to cryogenic hydrogen need to be carefully addressed beforehand. In the present work, cryogenic hydrogen jet flames are experimentally investigated for various release pressures and initial temperatures. The flame length and thermal radiation flux were measured for horizontally releasing with nozzle diameters of 0.5–2 mm, temperatures ranging from 93 to 298 K, and initial pressures of 2–10 MPa. The results show that the flame length is dependent on the nozzle diameter, stagnation pressure and temperature. At a given pressure, the flame length, size and total radiant power increase with decreasing temperature, which is attributed to the lower jet flow velocity and higher density of low-temperature hydrogen. The normalized flame length Lf/D is correlated with the pressure ratio and temperature ratio. The correlation can be used to predict the flame length at various hydrogen pressures and temperatures. The normalized flame length of the cryogenic hydrogen jet flame is greater than that of the room-temperature hydrogen jet flame. The radiative heat flux of the flame can be predicted by the mass flow rate of the jet flow. [ABSTRACT FROM AUTHOR]

Large quantities of water are often used to extinguish fires when accidental fuel leakage occurs during storage and transportation. This may lead to spill fires on water. Boilover and splash of heavy oil spill fires in particular can pose a serious thermal threat to surrounding facilities and personnel. In this work, a series of diesel spill fire experiments were conducted on the surface of water. The results showed that, for the non-ignition cases, the fuel spread velocity was fast at first, then maintained a long period of steady spread, which can be successfully predicted by a developed spread model. During the ignition process, the burning of diesel fuel is divided into four phases. Following a brief quasi-steady burning phase, we observed an expansion of the burning area during the intermittent boilover phase, which was primarily driven by boilover. During the quasi-steady burning phase, the burning rate was lower than that of pool fires, which is attributed to the heat loss between the diesel and water layers. This heat loss also results in a lower flame height than the pool fire, and a dimensionless equation was proposed to eliminate discrepancies. During the intermittent boilover phase, the increase in the burning area was used to characterize the boilover intensity, which was found to be negatively correlated with the number of boilovers. Furthermore, the emergence of the boilover also caused flame radiation to rise rapidly; it was about 19% to 30% higher than that in the quasi-steady burning phase. [ABSTRACT FROM AUTHOR]

Aiming at the complex conditions of the coexistence of the explosive gases in the coal mines and the risk of spontaneous coal combustion, the effect of encapsulation, oxygen barrier and different microcapsules on methane and long-chain alkanes has been studied. A non-toxic microcapsule comprising the anti-explosion fire-extinguishing polymeric material with neutral pH value, biodegradability and full solubility in water has been developed. The fire-extinguishing platform system has been used to test and analyze the fire-extinguishing effect, explosion suppression efficiency and package efficiency of the oil-pan fires and solid stacks. It is revealed that the microcapsule fire-extinguishing technology has a strong fire-extinguishing effect and can better inhibit the methane explosion, owing to its effective enveloping effect on methane, thus making it difficult to reignite. The developed technology is of theoretical significance and has a practical application value for studying the flame retardation and fire-extinguishing behavior of combustible substances. [ABSTRACT FROM AUTHOR]

The smoldering of pine needle fuel beds (PNBs) has been a common subject of research because of its importance in initiating the rekindling of forest floor fires. Experimental studies of the coupling effects of the bulk density and external heat supply on smoldering in PNBs have been scarce up to now. In this study, laboratory smoldering experiments were conducted to study the coupling effects of bulk density (30–55 kg m−3) and heat supply (ignition-off temperature Toff = 190 °C and 230 °C). Different ignition modes were observed under the same conditions, including non- ignition (NI), flaming ignition (FI), and the smoldering-to-flaming (StF) transition. The results in this study showed that the bulk density had distinct effects on different ignition modes: the increase in the bulk density facilitated the StF transition but impeded the FI. The coupling effects between the bulk density and heat supply became more intricate, especially at lower bulk densities and at a reduced heat supply. Additionally, a simple energy balance equation was established to explain the coupling effects of bulk density and heat supply on ignition behavior. The critical mass loss rate (MLR) for the StF transition ranged from 0.01 g s−1 to 0.03 g s−1, while the critical MLR for FI was 0.035 g s−1. The modified combustion efficiency (MCE) index for the StF transition decreased from approximately 79.6% to 70.1% as the density increased from 30 kg m−3 to 55 kg m−3. In contrast, the MCE for FI was approximately 90% across all the bulk densities. The StF transition delay time increased from 50 s at 30 kg m−3 to 1296 s at 55 kg m−3 when Toff = 230 °C. Further reduction in heat supply led to an increase in the delay time for the StF transition by diminishing the intensity of smoldering combustion. This work advances the fundamental understanding of how heat supply and bulk density impact smoldering ignition modes, ultimately aiding in the development of wildfire prevention strategies. [ABSTRACT FROM AUTHOR]

Carbon nanomaterials have been widely applied for cutting edge therapeutic applications as they offer tunable physio-chemical properties with economic scale-up options. Nuclear delivery of cancer drugs has been of prime focus since it controls important cellular signaling functions leading to greater anti-cancer drug efficacies. Better cellular drug uptake per unit drug injection drastically reduces severe side-effects of cancer therapies. Similarly, carbon dots (CDs) uptaken by the nucleus can also be used to set-up cutting edge nano delivery systems. In an earlier paper, we showed the cellular uptake and plasma membrane impact of combustion generated yellow luminescing CDs produced by our group from fuel rich combustion reactors in a one-step tunable production. In this paper, we aim to specifically study the nucleus by establishing the uptake kinetics of these combustion-generated yellow luminescing CDs. At sub-lethal doses, after crossing the plasma membrane, they impact the actin and microtubule mesh, affecting cell adhesion and migration; enter nucleus by diffusion processes; modify the overall appearance of the nucleus in terms of morphology; and alter chromatin condensation. We thus establish how this one-step produced, cost and bulk production friendly carbon dots from fuel rich combustion flames can be innovatively repurposed as potential nano delivery agents in cancer cells. [ABSTRACT FROM AUTHOR]

In this work, iron-oxide nanoparticle formation in the spray-flame synthesis (SFS) process of the standardized SpraySyn 2.0 burner was investigated in situ using laser-induced incandescence (LII). For the evaluation of these measurements, prior LII-experiments within iron-oxide aerosols (Fe3O4 and α-Fe2O3) with known primary particle size distribution and morphological properties were performed to determine the thermal accommodation coefficient (TAC) α, which led to approx. α = 0.08. The applicability of the TAC results within the flame was validated using spectrally and temporally resolved measurements in the flame at 65 mm HAB employing a spectrograph. Data for a bimodal particle size distribution, obtained from Transmission Electron Microscopy (TEM), were used in the LII-evaluation. The validated TAC was then used to evaluate the primary particle size evolution from in situ Time-Resolved (TiRe) LII-measurements using PMTs along the centre axis of the burner, ranging from 10 mm to 50 mm HAB. These measurements reveal a relatively constant effective particle diameter along HAB with dp,eff ≈ 300 nm. To further investigate particle formation in SFS, 2-dimensional time-resolved LII-measurements in the SFS flame were performed, showing a clear particle formation region up to approx. 30 mm HAB, from where on a constant particle mass is observed. [ABSTRACT FROM AUTHOR]

To address the challenges of low detection accuracy, slow detection speed, coarse feature extraction, and the difficulty of detection deployment in complex forest fire backgrounds, this paper presents a forest fire object detection algorithm based on an improved YOLOv5s (YOLOv5s-ACE). The algorithm not only realizes the accurate identification of small objects, but also guarantees the accuracy and speed of detection. Firstly, YOLOv5s-ACE uses Copy-Pasting data enhancement to expand the small object sample set to reduce the overfitting risk in the process of model training. Secondly, it choose Atrous Spatial Pyramid Pooling (ASPP) to replace Spatial Pyramid Pooling (SPP) module in backbone part of YOLOv5 network. Therefore, the proposed algorithm can enlarge the receptive field while ensuring the resolution, which is conducive to the accurate positioning of small object forest flame. Third, after adding the Convolutional Block Attention Module (CBAM) module to the C3 module of the Neck layer, the key features of the forest flame object can be further screened, while irrelevant information that interferes with the flame detection, such as background information, can be eliminated. The network performance of forest fire detection is improved without increasing the depth, width and resolution of the input image. Finally, we replace CIOU losses (Complete-IoU) with EIOU losses (Efficient-IoU) to optimize the performance of the model and improve accuracy. The experimental results show that compared with the original algorithm, the proposed object detection algorithm improves mean Average Precision (mAP) by 5.6%, Precision by 2.7%, Recall by 6.5% and GFlops by 6.7%. Even compared with the YOLOv7 algorithm, the proposed algorithm YOLOv5s-ACE increases mAP by 0.9%, Precision by 2.2%, and Recall by 0.3%. [ABSTRACT FROM AUTHOR]

This article proposes an empirical expression to describe the pyrolysis and charring of spruce wood in bench-scale experiments for a wide range of incident heat fluxes. Spruce wood samples were exposed to a cone radiant heater oriented vertically with varying intensities, ranging from q ˙ cone ′ ′ = 22 kW m - 2 to 93.5 kW m - 2 over 53 test samples. The mass loss rate (MLR), the position of the char front and a preliminary additional heat source from smoldering or flaming combustion were experimentally determined. The experimental data were processed to express the burning rate as a function of heat flux and char front position. A grouping of the experimental curves was obtained, allowing to predict the MLR outcome over time regardless of the incident heat flux. A linear regression at the quasi-steady state regime allowed the determination of the fitting coefficients of the correlation, which ultimately correspond to the mass of volatiles produced per unit of energy input into the material. A comparison was made with theoretical analysis of the pyrolysis of charring materials from the literature, and the discrepancies with the proposed approach and its limitations were finally discussed. The main advantage of this approach is that it provides a generalized expression, requiring minimal input of material properties, which predicts the MLR change over time for any heat flux within engineering accuracy. [ABSTRACT FROM AUTHOR]

The interrelation between Reynolds stresses and their dissipation rate tensors for different Karlovitz number values was analysed using a direct numerical simulation (DNS) database of turbulent statistically planar premixed H2-air flames with an equivalence ratio of 0.7. It was found that a significant enhancement of Reynolds stresses and dissipation rates takes place as a result of turbulence generation due to thermal expansion for small and moderate Karlovitz number values. However, both Reynolds stresses and dissipation rates decrease monotonically within the flame brush for large Karlovitz number values, as the flame-generated turbulence becomes overridden by the strong isotropic turbulence. Although there are similarities between the anisotropies of Reynolds stress and its dissipation rate tensors within the flame brush, the anisotropy tensors of these quantities are found to be non-linearly related. The predictions of three different models for the dissipation rate tensor were compared to the results computed from DNS data. It was found that the model relying upon isotropy and a linear dependence between the Reynolds stress and its dissipation rates does not correctly capture the turbulence characteristics within the flame brush for small and moderate Karlovitz number values. In contrast, the models that incorporate the dependence of the invariants of the anisotropy tensor of Reynolds stresses were found to capture the components of dissipation rate tensor for all Karlovitz number conditions. [ABSTRACT FROM AUTHOR]

The combustor is the core component of the scramjet engine, and stable combustion is the prerequisite for the efficient operation of the scramjet engine. This paper investigates combustion stability in a combustor with a thin strut for oxygen supplementation at the trailing edge. LES (Large eddy simulation) was carried out under the combustor inlet conditions of Ma = 2.8, T t = 1680 K, and P t = 1.87 MPa. The results show that as the amount of oxygen supplementation inside the combustor increases, the oscillation forward propagation characteristics of the flame front inside the combustor are formed. First, the combustion instability phenomenon of the flame inside the combustor is analyzed, and then the oscillation frequency of the flame is analyzed by FFT (Fast Fourier Transform) of the flame front and pressure and DMD (Dynamic Mode Decomposition) of the velocity field within one cycle. Finally, the flame forward propagation mechanism inside the combustor is analyzed by pressure-velocity-heat release distribution. The results show that the injection of excess oxygen inside the combustor forms a strong coupling phenomenon of flow and combustion, and the premixed gas downstream of the trailing edge of the strut forms a state of slow combustion to explosion, forming a flame propagation forward and periodic oscillation phenomenon. • LES method is used to capture the flame oscillation behavior with different oxygen supplementation. • The multi-frequency characteristics of combustion oscillations are obtained by using spectrum analysis. • Different combustion modes in flame oscillation and stable regions are discussed. • The mechanism of flame oscillation and propagation upstream is discussed. [ABSTRACT FROM AUTHOR]

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In order to investigate the premixed combustion characteristics of CH4/H2/air in a micro-mixing combustor, the effects of different micro-mixing head-ends (HE1, HE2, HE3) and hydrogen mixing ratios on the temperature distribution, heat transfer process, emission characteristic, flames shape are analyzed. The results show that compared with swirl head-end combustion, the micro-mixing combustion performance is better. Among the three head-ends, HE3 has the best combustion characteristics and stable flames. The temperature distribution in the high-temperature zone is uniform, and low-temperature zone is concentrated near the jet, which can suppress the flashback. The velocity and temperature gradient near the central axis of jet streams show a strong synergistic effect. The flames are plume shaped and flames stability is mainly influenced by the H2 combustion process. Increasing the jet diameter, decreasing the jet spacing and increasing the hydrogen mixing ratio all contribute to the flames stability, but these three methods can stabilize the flames by affecting fluid Reynolds number, interaction between small flames and combustion rate, respectively. Moreover, small jet diameter and high hydrogen mixing ratio can reduce OTDF, which contributes to improve outlet temperature uniformity. [ABSTRACT FROM AUTHOR]