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A computationally efficient model is proposed to analyze plasma-assisted combustion using nanosecond repetitive pulsed (NRP) plasmas. The NRP plasma discharge is placed in the oxidizer stream of a counter-flow diffusion flame. The effect of changing the flow rate and the pulse repetition frequency (PRF) of a continuous NRP plasma discharge on the temperature and species profiles of a counter-flow diffusion flame is investigated numerically. The results confirm that oxygen atom and nitrogen vibrational states are the most important species to enhance combustion. The results also show that kinetic effects are much more significant for higher PRF and lower pulse voltage. In addition, when steady plasma profiles are used instead of unsteady plasma profiles, the extinction strain rates increase by 25.8%, 21.1%, and 10.8% for PRF equal to 1, 2, and 4 kHz, respectively. [ABSTRACT FROM AUTHOR]

Considering the complex phenomenon of in-cylinder flame impingement occurring in internal combustion engines, flame-wall interaction in different impinging distance cases is presented for the non-premixed impinging flames. In this article, the quenching mechanism of impinging flame is focused on using experiments and numerical simulations. The quenching information on the flow and thermal field is obtained in the experiment. Large Eddy simulation is used to provide the details in the flow characteristics. In the wall jet region, multiple vortexes are formed accompanied by strong turbulence and large wall shear stress, which becomes more significant with the increase of the impinging distance. The low-temperature region corresponds to a large wall heat flux and Nusselt number, and this non-positive correspondence between temperature and heat transfer leads to excessive heat loss and weakens the flame stability. The quenching behavior in the impinging flame is analyzed in terms of the flow field and thermal characteristics. [ABSTRACT FROM AUTHOR]

This work is focused on the explosion performance of syngas/air premixed gas in a self-designed variable cross-section pipe under varying ignition positions (IP 1 , IP 2) and hydrogen volume fraction (α(H 2)). Results show that α(H 2) has a great influence on the propagation of the flame as well as the maximum flame front velocity (FFV max) and the maximum overpressure (P max). As α(H 2) in the syngas increases, the time the flame stays in the pipe is gradually reduced, the FFV max as well as the P max increase regardless of the ignition position. Differently, when α(H 2) < 50%, the t closed at IP 1 is shorter than that at IP 2. The opposite is true when α(H 2) ≥ 50%. This is attributed to the difference in flame velocity and the effect of film breakage. Both α(H 2) and variable cross-section chamber structure all affect the evolution of the flame morphology. When ignition at IP 2 , a "M" flame is observed for the first time at α(H 2) = 20%, and a flame resembling a "T" shape appears at α(H 2) = 70%. Ignition positions have a great influence on overpressure oscillation. In the case of α(H 2) = 15%, the Fourier transform is performed for the pressure curve. When ignited at IP 1 , secondary unstable oscillations occur during the later stage of flame propagation. However, when ignition at IP 2 , primary instability oscillations occur in both the early and late stages of flame propagation. The variable cross-section chamber structure plays a role in the flame front velocity (FFV) and overpressure. After the flame front enters the variable section chamber, the FFV reduces, and when the flame front leaves the variable section chamber, due to the sudden decrease of the cross-sectional area, the FFV increases significantly, and FFV max and P max are obtained at this moment. • A self-designed variable cross-section pipe is studied. • Open-end ignitions at varying positions are analyzed. • When ignition at IP 2 , a "M" flame is observed for the first time at α(H 2) = 20%. • Pressure oscillation frequency is different under different ignition positions. • When the flame front leaves the variable section, FFV max and P max will be obtained. [ABSTRACT FROM AUTHOR]

The proportion of flaming and smoldering activity occurring in landscape fires varies with fuel type and fuel characteristics, which themselves are influenced by ecology, meteorology, time since the last fire etc. The proportion of these combustion phases greatly influences the rate of fuel consumption and smoke emission, along with the chemical composition of the smoke, which influences the effects on the atmosphere. Earth Observation (EO) has long been suggested as a way to remotely map combustion phase, and here we provide the first known attempt at evaluating whether such approaches can lead to the desired improvements in smoke emissions estimation. We use intensively measured laboratory burns to evaluate two EO approaches hypothesized to enable remote determination of combustion phase and concurrent measurements of the smoke to determine how well each is able to improve estimation of smoke emission rates, smoke composition and the overall rate of fuel consumption. The first approach aims to estimate the sub-pixel 'effective fire temperature', which has been suggested to differ between flaming and smoldering combustion, and the second detects the potassium emission line (K-line) believed only to be present during flaming combustion. We find while the fire effective temperature approach can be suited to estimating Fire Radiative Power (FRP), it does not significantly improve on current approaches to estimate smoke chemical makeup and smoke emission. The K-line approach does however provide these improvements when combined with the FRP data, improving the accuracy of the estimated CO2 emission rate by an average of 17±4 % and 42±15 %, respectively, depending on whether the K-line detection is used to simply classify the presence of flaming combustion, or whether its magnitude is also used to estimate its relative proportion. Estimates of CO and CH4 emission rates were improved to a lesser extent than that of CO2, but the accuracy of the smoke modified combustion efficiency (MCE) estimates increased by 30±15 % and 46±10 %, respectively. MCE is correlated to the emissions factors (EFs) of many smoke constituents, so remotely deriving MCE provides a way to tailor these during smoke emissions calculations. Whilst we derived and tested our approaches on laboratory burns, we demonstrate their wider efficacy using airborne EO data of a boreal forest wildfire where we find that combined used of K-line and FRP data significantly change estimated smoke MCE and CO2 and CO emission rates compared to the standard approach. Our findings suggest that satellite EO methods that jointly provide K-Line and FRP data could enable marked improvements in the mapping of landscape fire combustion phase, fuel consumption and smoke emissions rate and composition. [ABSTRACT FROM AUTHOR]

Thermal protection materials (TPMs) are crucial in advancing aerospace technology. In order to explore the applicability limits and reusability of MoAlB ceramics used as TPMs, the cyclic ablation performance and mechanisms were studied in a nitrogen plasma flame at about 2000 °C. After 1 to 5 ablation cycles (5 min per cycle), MoAlB ceramics retained their structural integrity. The linear and mass ablation rates were measured as 4.73 µm/s and 2.98 mg/s, respectively, after five ablation cycles (total of 25 min). The ablation resistance of MoAlB ceramics was primarily ascribed to the oxidation layer formed through their decomposition and oxidation. As the cycles extended, the plasma flame gradually damaged the oxide layer and led to the erosion and failure of the MoAlB matrix. Nevertheless, MoAlB ceramics exhibit exceptional resistance against cyclic and long-duration ablation, rendering it a highly promising application for TPMs. [ABSTRACT FROM AUTHOR]

In this study, short carbon fibre (C sf) reinforced ZrB 2 -SiC composites were fabricated via direct ink writing followed by precursor infiltration and pyrolysis. The ablation behaviour of the composites was investigated at heat fluxes of 4–6 MW/m2 under an air plasma flame, which revealed excellent ablation resistance with negative linear ablation rates of − 0.1 to − 0.5 µm/s. The results showed that ZrO 2 -SiO 2 oxide layer was formed on the ablation surface after ablation, which provided effective protection to the internal materials. With an increase in heat flux, ZrO 2 precipitated from the SiO 2 melt first in the form of nano-tetragonal ZrO 2 (t -ZrO 2), owing to the vaporisation of SiO 2. During cooling, the nano t -ZrO 2 particles were retained, and some of them reacted with SiO 2 to form ZrSiO 4 , thus improving the thermal stability of the SiO 2 -ZrO 2 oxide layer. In contrast, t -ZrO 2 agglomerated and grew owing to capillary force and finally transformed into monoclinic ZrO 2 (m -ZrO 2). Moreover, a higher heat flux led to the size of precipitation ZrO 2 increases, meanwhile increasing the content of t -ZrO 2 and ZrSiO 4 in the oxide layer. • Short carbon fibers reinforced C sf /ZrB 2 -SiC composites were fabricated by direct ink writing, followed by precursor impregnation and pyrolysis. • During cooling, the nano t -ZrO 2 particles were retained, and some of them reacted with SiO 2 to form ZrSiO 4 , thus improving the thermal stability of the SiO 2 -ZrO 2 oxide layer. • The capillary force leads to the nano crystals gradually agglomerate and form dendritic structure. • From 4 to 6 MW/m2, the size of the dendritic structure ZrO 2 increased. [ABSTRACT FROM AUTHOR]

The objective of this paper is to assess the capabilities of the Conditional Source-term Estimation (CSE) approach applied to a laboratory scaled turbulent buoyant flame without extinction. CSE is coupled with the Large Eddy Simulation (LES) solver, FireFOAM. Tabulated detailed chemistry is included. Radiation is treated using the optically thin model using four different implementations and the effect of subgrid scale (sgs) Turbulence-Radiation Interactions (TRI) is considered. Predictions of time-averaged temperatures and corresponding root mean square (rms) are compared with the experimental measurements at several locations. Further, flame shape, unconditional and conditional species mass fractions, axial velocity and mixture fraction are also examined for qualitative analysis. The predicted temperatures are in good agreement with the experimental data, except very close to the fuel inlet. The predicted temperatures increase more slowly compared to the experimental data in the first 0.09 m above the burner. The temperature rms is in reasonable agreement with experimental data with the peak being overpredicted by approximately 17%. The predicted flame height closely matches the experimental value. The temperature predictions are consistent with previously published results. Best predictions are obtained when the effect of sgs TRI is included. For this flame, the optically thin assumption is found to be valid, in agreement with previous investigations. The calculated radiative fraction is found to be close to the experimental value, but the postprocessed heat release rate is larger than the experimental finding. This first LES-CSE study provides a good foundation for further developments, for example more detailed radiation models, for more complex fire cases. [ABSTRACT FROM AUTHOR]

This study is devoted to the characterisation of lean premixed hydrogen–air laminar flames stabilised on an annular burner. The flames studied are chosen to lie close to the lean flammability limit, spanning a range of Damköhler and Reynolds numbers, and correspond to situations where instabilities arise. The experimental characterisation is achieved by examining the statistical moments of the flame hydroxyl radical (OH*) chemiluminescence and schlieren. The standard deviation and the skewness, that are needed to fully characterise the measured probability density functions, are shown to exhibit a non-monotonic behaviour with the distance from the burner surface, first increasing, then decreasing downstream the flame tip. An exponentially modified Gaussian distribution model is proposed to describe the skewed schlieren fluctuations probability density function. This model is closed by developing relations for the probability density function parameters based on the Damköhler and Reynolds numbers, and then used to describe the OH* stochastic behaviour. The relation between the distribution parameters and the transport equation of a reactive scalar gradient is addressed also. [ABSTRACT FROM AUTHOR]

This study provides a chemical mechanism reduction strategy based on entropy production analyses in both auto-ignition and laminar flames, which enhances the predictive accuracy for laminar burning velocities. In addition to chemical reactions, other irreversible sources causing entropy generation, such as mass diffusion and heat conduction, are considered in the modified approach. Specifically, initial skeletal mechanisms are first generated based on important reactions that contribute to entropy production in auto-ignition processes. Mechanism patches are then constructed to include important species and reactions, which contribute to entropy production from mass diffusion and heat conduction in laminar premixed flames beyond the pre-defined thresholds, respectively. Finally, the initial skeletal mechanisms and mechanism patches are combined to establish the final skeletal mechanisms. In this way, two final skeletal mechanisms for n-dodecane, consisting of 162 species and 2276 reactions, and 160 species and 1916 reactions, respectively, are developed from the detailed POLIMI mechanism with 451 species and 17,848 reactions. The two final skeletal mechanisms are proven to accurately predict laminar burning velocities and entropy production in n-dodecane flames with insignificant variations in the simulation results compared to the detailed mechanism, while their accuracy in predicting ignition delay times relies on the initial skeletal mechanisms. Specifically, the reduced mechanism with 160 species and 1916 reactions exhibits less satisfactory performance in predicting ignition delay compared to that with 162 species and 2276 reactions, indicating that a lower threshold is required to generate the initial skeletal mechanism through entropy production analysis of homogeneous auto-ignition processes. Additionally, compared with the reduced mechanisms with similar sizes obtained with other mechanism reduction strategies, the two final skeletal mechanisms accurately capture the characteristics of laminar burning velocities and ignition delay times, with similar calculation time required. [ABSTRACT FROM AUTHOR]

The objective of the present study is to investigate two new formulations of the Conditional Source-term Estimation (CSE) model using Reynolds Averaged Navier Stokes (RANS) calculations applied to Sandia flames D and F. The first method relies on a first-order Tikhonov regularisation and the second approach denoted by CSEBP, includes Bernstein polynomials to approximate the conditional averages. Current predictions for temperature, main product and minor species are consistent with previously published CSE results with a different implementation. However, smoother conditional profiles are obtained with less a priori information. Both formulations have good predictions for flame D with minor discrepancies near the inlet and one position downstream, with occasional small advantages for CSEBP. In contrast to previous RANS-CSE attempts, stable solutions are obtained for flame F in good agreement with the experiments. Considering the RANS and single conditioning limitations to capture transient effects, both formulations predict the changes of conditional averages and Favre averaged quantities from flame D to F well, except at one location where the predicted re-ignition occurs earlier than what is seen in the experiments. Additionally, the computational cost of the CSE routine is decreased significantly from $ 85\% $ 85 % of the total computational cost to only $ 10\% $ 10 % for the first formulation and under $ 3\% $ 3 % for CSEBP by means of using hash tables for storing the results of interpolations from the chemistry tables and avoiding on-the-fly interpolations. [ABSTRACT FROM AUTHOR]

Microcombustors are microscale combustion devices that can be used to power microelectromechanical systems. Many combustor configurations are reported in the literature and, among them, combustion in a microscale recirculating heat exchanger is a feasible option. In this work, a simple, double‐channel, recirculating heat exchanger is considered. The novelty of the present work lies in the heat transfer analysis approach to design a microcombustor. A combustor is designed using thermal resistance networks for a premixed fuel containing a methane–air mixture in stoichiometric ratio. The length of the combustor is designed based on the position of the combustion flame. Computational fluid dynamics is utilized to validate the theoretical results. The analysis is carried out for adiabatic and nonadiabatic conditions. The combustor lengths for adiabatic and nonadiabatic (ceramic) combustors vary from 39 to 242 mm and 49 to 276 mm, respectively, for variations in the mass flow rate of the premixed gases from 6 to 10 mg/s. A minimum limiting flow rate of 6 mg/s was identified. The average error in the maximum combustion gas temperatures between the theoretical and CFD results obtained in this work is 4.2%. The theoretical approach presented can be suitably applied to more complex geometries involving multichannels and variations in geometrical properties. [ABSTRACT FROM AUTHOR]

This paper presents improved single- and multi-objective algorithms based on the original moth flame optimizer (MFO) to tackle the dynamic economic emission dispatch (DEED) problem that affects power systems operations. The DEED problem is a multi-objective optimization problem that is strongly constrained, multi-dimensional, nonlinear, and non-convex. It comprises several optimization criteria, many of which are in direct opposition to one another; therefore, no one solution is optimal with regard to all of those criteria. Firstly, an enhanced flame generation strategy is incorporated into the MFO algorithm to improve performance. Then, the improved MFO is combined with the crowding distance mechanism and non-dominated sorting framework to enhance the convergence rate and the quality of the results. This helps improve the convergence pace. Firstly, the proposed multi-objective moth flame optimizer (MOMFO) algorithm is validated using 15 ZDT and UF benchmark multi-objective test functions. Then, the nonlinear DEED problem is also solved by determining the feasible optimal solution using the MOMFO algorithm. The implementation of the MOMFO on 10-unit systems and the IEEE 30-bus test system is being done to display the ability to solve a nonlinear, non-convex, and constrained DEED optimization problem. The DEED problem is solved using the MOMFO algorithm and other state-of-the-art algorithms, such as the non-dominated sorting genetic algorithm-II (NSGA-II), the multi-objective teaching–learning-based optimization (MOTLBO) algorithm, and multi-objective reptile search algorithm (MORSA). The selection of the control parameters of the MOMFO can be decided from the algorithm's findings on different IEEE bus systems. This study also introduces a new technique for incorporating loss predictions using artificial neural networks into the DEED model. During each phase of the dispatch time, the trained neural network can make only a single forecast of the transmission loss. The performance of MOMFO is compared with NSGA-II, MOTLBO, and MORSA, and the results obtained for both benchmarks and DEED proved the superiority of the proposed algorithm in solving the DEED of the power systems. [ABSTRACT FROM AUTHOR]

Cigarette butts (CBs) are one of the most common types of litter in the world. Due to the toxic substances they contain, the waste generated poses a harmful risk to the environment, and therefore there is an urgent need for alternative solutions to landfill storage. Thus, this work presents a possible revalorization of this waste material, which implies interesting environmental benefits. CBs were used as sacrificial templates for the preparation of CaO-based materials by impregnation with calcium and magnesium nitrates followed by flaming combustion. These materials presented enhanced porosity for their use in the Calcium Looping process applied either to thermochemical energy storage or CO 2 capture applications. The influence of the concentration of Ca and Mg in the impregnating solutions on the multicycle reactivity of the samples was studied. An improved multicycle performance was obtained in terms of conversion for both applications. [ABSTRACT FROM AUTHOR]

The present study investigates the characteristics and periodic behavior of H2/air tubular flames in a 1 mm diameter microtube under exciting and nonexciting inlet conditions. Under unstimulated inlet conditions, increasing inlet velocity positively impacted flow stratification and self-sustaining of the tubular flames, leading to higher maximum temperatures within the flame kernel due to reduced flow temperature gradients near the wall. Conversely, under stimulated conditions, varying excitation amplitudes resulted in two flame propagation modes: flame with semi-repetitive extinction/ignition (FSREI) and pulsating flame, observed across different exciting amplitude ranges. It was found that the formation of recirculation fields generated by negative propagation speed temporarily stored the released heat of combustion and prevented it from extinguishing in the flowing phase. From the kinetics point of view, the maximum reaction rate during the pulsating mode belongs to H + O2 = HO2, while competition between H + O2 = HO2 and H2 + OH = H2O + H occurred in the FSREI mode. Results revealed that in the pulsating mode, fluctuations in mass fractions of the heavier species are more considerable near the outlets. However, radical mass fraction fluctuations were significant near the inlet slot in pulsating mode. [ABSTRACT FROM AUTHOR]

The purpose of this work is to investigate the effects of adding hydrogen to the fuel on the combustion induced vortex breakdown flashback phenomenon in a combustion chamber. An atmospheric research burner is selected for studies and the analysis is performed using numerical simulations. In order to model the turbulence-chemistry interactions, the EDC combustion model is utilized and the GRI 2.11 reaction mechanism is used to investigate the combustion process as carefully as possible. The present studies are performed keeping the exit temperature constant. To do this, by adding any amount of hydrogen to the premixed inlet flow, the methane is decreased by using the trial-and-error until the mass averaged exit temperature remains almost constant. The purpose of applying this limitation is to keep the cycle operational condition unchanged that is important for adding hydrogen to the existing combustion chambers especially from technological viewpoint. In addition, the thermal NO x production is controlled. In the following, the effect of hydrogen addition on the flame thickness and flame shape is investigated and finally, the effect of operating pressure on the flashback behavior is analyzed. • Effect of hydrogen addition to the fuel on the combustion induced vortex breakdown flashback is investigated. • The studies are performed keeping the exit temperature constant. • By adding hydrogen and reducing methane, the equivalence ratio decreases but the flame moves upstream. • Adding hydrogen reduces the flame thickness, but at higher hydrogen percentages, flame thickness will no longer depend on hydrogen addition. • It is illustrated that increasing the pressure moves the upstream in both pure methane and hydrogen-methane fuels. [ABSTRACT FROM AUTHOR]

The effects of the vent burst pressure (P v) on the vented explosion of hydrogen-methane-air mixtures are investigated in a cylindrical vessel connected with a duct. The results demonstrate that P v significantly affects the flame behaviors and overpressure inside and outside the venting configuration. The flame front velocity at the vent increases with P v. Two pressure peaks, P a c , caused by the acoustically enhanced combustion, and P s e , caused by the secondary explosion, dominate within the vessel at P v ＜ 132 kPa and P v ≥ 132 kPa, respectively. The maximum overpressure (P max) inside the duct increases with P v. The maximum pressure rise (d P / d t) of the duct is always higher than that of the vessel. As P v increases, increased turbulence of unburned gases within the duct results in higher d P / d t. Exit flame speed increases with P v. When P v ≥ 153 kPa, "Mach disk" appears outside the relief duct. Three pressure peaks, P a , P b , and P e x t , due to vent failure, the secondary explosion, and the external explosion, respectively, occur in the external pressure curve. P b dominates outside the configuration except for P v = 153 kPa. • Initial flame velocity in the duct increased with P v. • P v was positively correlated with P max inside the duct. • "Mach disk" appeared only when P v ≥ 153 kPa. • Both P a and P e x t increased with the increase of P v. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3), as a zero carbon, easy to synthesize, and easily obtainable fuel, is one of the choices for future internal combustion engine fuels. However, the slow laminar flames speed and high ignition temperature of NH 3 seriously hinder the development of NH 3 engines. Efficiently igniting NH3 and ensuring its stable combustion is currently a challenging research topic. Utilizing diesel to optimize the NH3 combustion process is one effective solution. However, the increase of unburned ammonia (UNH 3) and unstable combustion, under high load conditions have always been technical challenges that constrain the development of ammonia-diesel dual-fuel (ADDF) engines. Previous studies have found that pre-chambers (PC) have a significant effect on improving ammonia combustion characteristics. Therefore, based on the PC system, this study proposes two combustion modes: Reactivity Assisted Jet Ignition (RAJI) and Reactivity Turbulent Jet Disturbance (RTJD), which greatly optimize the combustion characteristics of ADDF engines under high load conditions. The research results indicate that the RAJI mode effectively improves the thermal environment inside the cylinder. Setting the start of diesel injection of pre-chamber (SODI-PC) at −14 °CA after top dead center (ATDC) can increase the cylinder temperature by 25.05% before the start of diesel injection of main chamber (SODI-MC). The improvement of thermal environment reduces the average generation of N 2 O to 13.56% of the traditional mode. However, if SODI-PC is postponed to −8 °CA ATDC, the effect of creating a thermal environment is no longer significant. Research on the RTJD mode has found that adopting the RTJD mode can significantly optimize emissions. Compared with traditional modes, N 2 O, UNH 3 , and Soot decreased by 28.81%, 27.79%, and 35.08%, respectively. Additionally, SODI-PC is delayed until after 4 °CA ATDC, CO and UHC will be further reduced to 50.00% and 12.24% of the traditional mode, respectively. In addition, the RTJD mode overcomes the deficiency of the slow laminar flame speed of NH 3 , shortening the combustion duration by 23.46%, and effectively improving the gross indicated thermal efficiency (ITEg) by an average increase of 1.08%, with the highest increase reaching 47.57%. This paper provides ideas for optimizing the structure and strategy of ADDF engines, as well as theoretical support and empirical reference for achieving clean and efficient combustion of engines. [Display omitted] • Two combustion modes: RAJI and RTJD are proposed. • The RAJI mode can reduces the average generation of N 2 O to 13.56% of the traditional mode. • In RAJI mode the N 2 O and UNH 3 , was decreased by 28.81% and 27.79%, respectively. • The RTJD mode shortens the combustion duration by 23.46%. [ABSTRACT FROM AUTHOR]

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

To investigate the effect of potassium compound on the combustion characteristics of hydroxyl-terminated polybutadiene (HTPB) composite solid propellants. Potassium compound were added to the HTPB propellants as flame inhibitors at different concentration gradients, and the effects of the potassium compound on the thermal decomposition and combustion characteristics of these propellants were investigated by thermogravimetric differential calorimetric scanning calorimetry (TG-DSC) and laser ignition combustion experiments. The results showed that the effect of potassium compound on the thermal decomposition of the HTPB composite solid propellant predominantly occurred in the AP/HMX decomposition stage, and the potassium compound affected the synergistic decomposition of AP/HMX. The entire process of the HTPB composite solid propellant combustion slowed down with increasing potassium compound content; this was mainly shown by the increase in ignition delay time and longer combustion time. The intensity of the secondary combustion flame decreased, the maximum flame area and image grayness decreased, and the intensity of the maximum emission spectrum of the secondary combustion flame decreased. With the increase in potassium compound content, the spectral intensity of free radicals produced by combustion shows a decreasing intensity with a delayed trend with the change rule of time. Overall, the potassium compound weakened the combustion of the HTPB composite solid propellant. [ABSTRACT FROM AUTHOR]

Pressure has important effects on soot formation processes in kerosene flames. The impact of pressure on soot processes, particularly surface growth, in kerosene flames remains incompletely understood, primarily due to insufficient experimental support. Although several soot surface growth models in previous studies exhibit excellent predictive capability at atmospheric pressure, deviations of the prediction by these models and experimental result still need to reduce at high pressure. This paper presents a novel soot surface growth model which utilizes the Chebyshev polynomials method to capture the impact of pressure on the reaction rate of surface growth. The impact of pressure on soot production of RP-3 kerosene flames from 1 to 20 atm is numerically studied, with experimental validation from the pressure range from 1 to 3.5 atm. A comparison is made between the experimental and numerical results, highlighting the favorable predictive performance of the current surface growth model on the soot volume fraction and soot yield. Furthermore, a subsequent numerical study shows that the pressure dependency of the soot yield decreases as the pressure further increases from 5 to 20 atm. This study presents a pressure-dependent model for soot surface growth, which serves as a basis for further researches of soot formation in kerosene flames at high pressure. [ABSTRACT FROM AUTHOR]

Ammonium phosphate fire extinguishing agents, due to their excellent cost-effectiveness and suitability for forest fires, have become one of the most commonly used fire suppressants for forest fire control. Among them, NH 4 H 2 PO 4 fire extinguishing agent is the most widely applied. In this study, NH 4 H 2 PO 4 (>90%) as the main component of dry powder fire extinguishers was selected as the suppressant to investigate the reaction mechanism of NH 4 H 2 PO 4 interacting with red pine wood pyrolysis gas flames. Initially, we constructed a Bunsen burner experimental system suitable for NH 4 H 2 PO 4 /red pine wood pyrolysis gas/air. Although experimental methods yield the most intuitive empirical conclusions, measuring only the laminar flame speed of red pine wood pyrolysis gas/NH 4 H 2 PO 4 /air mixture does not adequately analyze the inhibitory mechanism of NH 4 H 2 PO 4 on red pine wood pyrolysis gas flames. Additionally, due to limitations in experimental equipment and methods, it is not feasible to extend equivalence ratios and NNH 4 H 2 PO 4 doses to large or small working conditions. Therefore, in this study, we utilized Chemkin software to construct theoretical models of flames under different conditions and performed calculations. We comprehensively analyzed the kinetic regulatory mechanisms of NH 4 H 2 PO 4 interacting with red pine wood pyrolysis gas flames from aspects such as chemical inhibition, sensitivity of elementary reactions, elementary reaction yields, and chemical reaction pathways. • The laminar flame velocity of NH 4 H 2 PO 4 interacting with the pyrolysis gas flame of red pine was measured. • A comprehensive analysis of the dynamic regulation mechanism of NH4H2PO4 interaction with pyrolysis gas flame of red pine. • The inhibition mechanism of phosphorus-containing active substances on pyrolysis gas flame of red pine wood was elucidated. • Provide theoretical and experimental reference for development of more efficient fire extinguishing agents for forest fires. [ABSTRACT FROM AUTHOR]

This study proposes a novel design concept that leverages the geometric effects of channels with varying diameters and bends to boost flame acceleration and detonation in microchannels. A quasi-direct numerical simulation with detailed chemical kinetics is used to evaluate the processes of flame acceleration and detonation. A comparative study on the impact of equidistant spiral and converging spiral shapes of detonation channels on flame acceleration is conducted and discussed. The results indicate that in the low-speed and high-speed stages of flame propagation, Fermat's spiral channel exhibits a more significant promoting effect on flame acceleration compared to the Archimedes spiral channel. Fermat's spiral channel has significant advantages in terms of combustion efficiency and can save about 30% of fuel during the detonation process. This study helps to further reduce the scale of the detonation system. [ABSTRACT FROM AUTHOR]

A new unsteady flamelet model is developed to be used for sub-grid modeling and coupling with a resolved flow description for turbulent combustion. Difficulties with prior unsteady flamelet models are identified. The model extends the quasi-steady rotational flamelet model, which differs from prior models in several critical ways: (i) the effects of shear strain and vorticity are determined, in addition to normal-strain-rate impacts; (ii) the strain rates and vorticity are determined from the conditions of the environment surrounding the flamelet without a contrived progress variable; (iii) the flamelet model is physically three-dimensional but reduced to a one-dimensional, unsteady formulation using similarity; (iv) variable density is fully addressed in the flamelet model; and (v) non-premixed flames, premixed flames, or multi-branched flame structures are determined rather than prescribed. For both quasi-steady and unsteady cases, vorticity creates a centrifugal force on the flamelet counterflow that modifies the transport rates and burning rate. In the unsteady scenario, new unsteady boundary conditions must be formulated to be consistent with the unsteady equations for the rotating counterflow. Eight boundary values on inflowing scalar and velocity properties and vorticity will satisfy four specific relations and, therefore, cannot all be arbitrarily specified. The temporal variation of vorticity is connected to the variation of applied normal strain rate through the conservation principle for angular momentum. Limitations on the model concerning fluctuation of the interfacial plane are identified and conditions under which interfacial plane fluctuation is negligible are explained. An example of a rotating flamelet counterflow with oscillatory behavior is examined with linearization of the fluctuating variables. [ABSTRACT FROM AUTHOR]

Through comprehensive experimental and modeling efforts, this work unravels the underlying mechanisms governing flame development and misfire at advanced engine conditions that are representative of low-load and lean blow-out operations. Toward this, preliminary heat release, autoignition, and flame developing patterns are characterized, via a case study of n-heptane, at ultra-lean conditions in a well-controlled optical engine under various combustion modes including homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), and reactivity-controlled compression ignition (RCCI). Changes in preliminary heat release and flame developing patterns at three overall equivalence ratios (0.12, 0.18, and 0.24) are first characterized under the PPC mode. Flame development characteristics including flame areas and number of initial flame kernels at close-to-misfire conditions are further extracted and compared across the HCCI, RCCI, and three PPC modes, with two distinctive and one transition regimes identified. Further analyses indicate that sustainable flame development and misfire are largely controlled by the spatial distribution of local equivalence ratio (phi) and local temperature in the mixture, which dictate the initial flame kernel generation and the subsequent flame propagation through localized preliminary heat release and autoignition. Chemical kinetic modeling is also undertaken, using a recently updated gasoline chemistry model, in conjunction with a backpropagation neural network, where the predicted ignition delay map well captures the different regions of flame development. Further kinetic analysis and heat rate of production per reaction analysis corroborate the CH2O planar laser-induced fluorescence experiments and highlight the important chemical kinetics that govern the initial flame development patterns. [ABSTRACT FROM AUTHOR]

The necessity of minimizing NOx emissions drives the pursuit of ultra-lean premixed combustion in aeroengines and gas turbines, characterized by susceptibility to combustion instabilities. To tackle this issue, swirling flow design is widely incorporated into lean premixed combustor design, enhancing flame stability, and shortening flame length. This study utilizes the tomographic background-oriented Schlieren (TBOS) to reconstruct the spatial distribution of the refractive index gradient of lean premixed turbulent swirl flames with an aeroengine combustor configuration. A parametric study of the TBOS reconstruction quality is conducted, and the results reveal that view sparseness primarily degrades the reconstruction quality compared to the specific iterative algorithm used. The classic visual hull approach is explored to address this challenge, highlighting the significance of visual hull size. Furthermore, to improve the reconstruction quality, a posterior support constraint method is proposed, involving the removal of voxels of nearly constant refractive index within the central volume surrounded by flames. Results demonstrate that implementing this posterior support constraint further improves the reconstruction quality of lean premixed turbulent swirl flames. Finally, the robustness of this posterior support constraint method is validated by introducing high-level noise to the light deflection data, showcasing the potential of combining the dedicated designed visual hull and proposed posterior support constraint in addressing the view sparseness challenge for TBOS measurements. [ABSTRACT FROM AUTHOR]

Within the scope of reacting flow simulations, the real-time direct integration (DI) of stiff ordinary differential equations for the computation of chemical kinetics stands as the primary demand on computational resources. Meanwhile, as the number of transport equations that need to be solved increases, the computational cost grows more substantially, particularly for those combustion models involving direct coupling of chemistry and flow such as the transported probability density function model. In the current study, an integrated graphics processing unit-artificial neural network (GPU-ANN) framework is introduced to comply with heavy computational costs while maintaining high fidelity. Within this framework, a GPU-based solver is employed to solve partial differential equations and compute thermal and transport properties, and an ANN is utilized to replace the calculation of reaction rates. Large eddy simulations of two swirling flames provide a robust validation, affirming and extending the GPU-ANN approach's applicability to challenging scenarios. The simulation results demonstrate a strong correlation in the macro flame structure and statistical characteristics between the GPU-ANN approach and the traditional central processing unit (CPU)-based solver with DI. This comparison indicates that the GPU-ANN approach is capable of attaining the same degree of precision as the conventional CPU-DI solver, even in more complex scenarios. In addition, the overall speed-up factor for the GPU-ANN approach is over two orders of magnitude. This study establishes the potential groundwork for widespread application of the proposed GPU-ANN approach in combustion simulations, addressing various and complex scenarios based on detailed chemistry, while significantly reducing computational costs. [ABSTRACT FROM AUTHOR]

During the top blowing process of the basic oxygen furnace, the oxygen jet may react with CO before it reaches the surface of the molten bath. This combustion reaction affects the dynamic characteristics of the jet, which, in turn, affects its interaction with the bath. In this study, a numerical model is developed to investigate the transient and steady combustion characteristics of the supersonic jet emitted from the top lance. The transient combustion simulation shows that the CO ratio gradually decreases while the CO2 mole fraction increases at the interaction boundary region between jet and ambient due to the chemical reactions. In addition, the jet profile is changed when the postcombustion is introduced. Due to the flame formation and turbulence change at the jet combustion interface, the axial velocity at height of 2 m (x = 73 re) is 89% higher than that of the noncombustion reaction jet flow. In the results, it is demonstrated that the combustion interface is favorable to reduce the jet decay, resulting a higher dynamic pressure before the jet reaches the bath surface. Furthermore, a higher core temperature and lower density are obtained for the combustion jet compared to that of without combustion reaction. [ABSTRACT FROM AUTHOR]

For more than a century, smoke explosions have been documented in the fire research literature. Yet even with this long history, smoke explosions have received very little attention in the fire research community. The few review articles on unexplained explosions (overpressure events) have adopted the conventional names of smoke explosions, backdraft, smoke gas explosions or rapid fire progression. Anecdotal evidence from firefighters have shown a number of overpressure events that cannot be explained as a gas leak or flammable liquids ignition. In this study, experiments were conducted in a plywood lined compartment with a timber crib as a sustained fire source and the ignition source. The compartment had a single vent that was open for the entire time. After an extended period of burning, the flames detached from the crib, traveled around the compartment, and finally self-extinguished. Sometime after the flames self-extinguished, the compartment erupted in flames culminating in a large horizontal flame projecting more than 2 m from the vent opening. In this study, a consistent cycle that leads to an overpressure event has been identified. A total of 29 overpressure events were created in 13 experiments with many experiments experiencing multiple overpressure events. Gas species, compartment temperatures, vent velocities, and compartment pressure were all recorded during the experiments. Conditions in the compartment immediately prior to the overpressure event are reported. Having demonstrated that an overpressure event within a combustible compartment can be reproduced, future research is recommended to better quantify the conditions leading to an overpressure event. [ABSTRACT FROM AUTHOR]

Large and downed woody fuels remaining behind a wildfire's flame front tend to burn in a smoldering regime, producing large quantities of toxic gases and particulate emissions, which deteriorates air quality and compromises human health. Smoldering burning rates are affected by fuel type and size, the amount of oxygen reaching the surface, and heat losses to the surroundings. An external wind has the dual effects of bringing fresh oxidizer to the fuel surface and porous interior, while at the same time enhancing convective cooling. In this work, a series of experiments were conducted on single and adjacent poplar dowels to investigate the effect of fuel geometry and wind speed on smoldering of woody fuels, including its burning rate and combustion products. Dowels had variable thickness (19.1 and 25.4 mm), aspect ratios, and arrangement (number of dowels and spacing between them). Using measurement of mass loss, CO, and HC production as indicators of the smoldering intensity, the results indicate that the arrangement of smoldering objects significantly affects burning rates and emissions. Specifically, spacings of 1/8 and 1/4 of the dowel thickness enhanced the smoldering process. The smoldering intensity was also enhanced by increased external wind (ranging between 0.3 m/s and 1.5 m/s), but its effect was dependent upon the spacing between the dowels. The convective losses associated with the spacing were further investigated with a simplified computational model. The simulations show that the wind significantly increases convective losses from the smoldering surfaces, which in turn may offset the increase in smoldering intensity related to the higher oxygen flux at higher wind speeds. [ABSTRACT FROM AUTHOR]

The effects of the slope and the trench inclination on the spread of wildfires with a homogeneous fuel bed in inclined trenches, were studied numerically by Fire Dynamics Simulator. This simulation is based on a solid–gas two-phase numerical model that incorporates the physicochemical combustion characteristics of Platycladus orientales leaves. The results show that the rate of fire spread accelerates with increasing slope and trench inclination. The flame front inclines until it attaches to the fuel bed for slope angles ranging from 20.9° to 35.2°, and it was found a critical angle for full attachment is about 35.2° for trench inclination of 45°. Increasing the trench inclination causes a decrease in the critical angle because the trench wall restricts air entrainment at the bottom flame, promoting the flame to adhere to obtain sufficient air. Flame radiation is the dominant heat transfer mechanism at low slopes, and as the slope increases, convective heat transfer starts to be relevant and significantly changes with the trench inclination. This study provides scientific insights and guidance for early prevention and fire fighting in inclined trenches of wildfires. [ABSTRACT FROM AUTHOR]

Two kinds of novel modified C/C‐SiC‐ZrC composites were prepared via precursor infiltration and pyrolysis, as pyrocarbon (PyC)/silicon carbide (SiC) and PyC/boron nitride (BN) dual‐layer interphases were separately structured on the fibers by means of chemical vapor infiltration. Data analysis and conclusions are served for investigating the effects of these two interface layers on mechanical and anti‐ablative properties. On the mechanical property hand, both PyC/BN and PyC/SiC interphase layers play positive roles, resting with the reduction of fiber damage during the fabrication process. Compared with BN, SiC shows better enhancement as the flexural strength of PyC/BN and PyC/SiC interphase‐modified composites are 214.9 and 229.2 MPa, respectively. On the ablative property hand, after oxyacetylene flame ablation for 60 s, the mass and linear ablation rates of the composites modified by PyC/SiC interface were 2.2 mg/s and 9.7 μm/s, which is much lower than that modified by PyC/BN. The inferior ablation properties of PyC/BN‐CSZ were attributed to the vaporization of the B2O3 gas that destroys the integrity of the oxide film and oxygen erosion on the substrate through the damaged BN interface. [ABSTRACT FROM AUTHOR]

The steam-diluted oxyfuel combustion of the hydrogen-enriched fuels is promising to attain zero-emission in the energy system. One critical challenge for this technology is the flame stabilization, which was examined in this work mainly through the flame structure analysis. The burner adopted three swirlers to enhance the mixing, which leads to V-shaped diffusion-like flames. Starting from the purely hydrogen fuel, the CH 4 addition effects with fractions of 20% and 40% were studied. Meanwhile, combustion with different equivalence ratios were also compared, when the volume flowrate ratio of the O 2 and steam was kept constantly at 1:4. For these diffusion-like flames, the reaction zone was obtained using the OH-PLIF. Then, the flame structure statistics, such as the number and area of the broken reaction zone and its thickness, were all examined. Results show that the fuel properties and steam flowrate intensively affect the flame stabilization. With increased CH 4 addition in the hydrogen fuel, the reaction zone tends to be broken seriously. This is characterized by frequently appeared small reaction zone fragments, along with largely decreased reaction zone area. With a larger stream flow rate, the local flame extinction is also increased obviously and the flame branches can be completely distinguished due to the intensive straining effects. The extinction upstream is observed to promote an enveloped reaction zone downstream, due to the unburned fuel leakage. It is found that the mean reaction zone thickness is overall close, which varies from 1 to 2 mm, but can be modified by different factors. An analogical comparison of the reaction thickness to the laminar counter-flame thickness reveals that the CH 4 addition and flow straining can decrease the reaction zone thickness, but the accompanied turbulence effects can broaden it to at least about 1.2 times. These results are beneficial to attain a stable ultra-clean combustion utilizing hydrogen-based flexible fuels. [Display omitted] • Steam-diluted swirling CH 4 /H 2 /O 2 flames are studied. • The flame is a diffusional type and can be unstable. • Extinction is increased with either CH 4 addition or high steam flowrate. • Extinction and fuel leakage lead to enveloped reaction zone downstream. • The reaction zone thickness can be broadened by turbulence. [ABSTRACT FROM AUTHOR]

This work is aimed at exploring the effects of obstacle opening structure on premixed hydrogen-air flame propagation characteristics using experimental and numerical method. The flame shape evolution, flame front speed, and overpressure are obtained. The velocity distribution is further analyzed. The experimental results show that under the given equivalence ratio, the peak flame front speed generated by circular opening obstacle is 16% and 27% higher than that of triangular opening obstacle and square opening obstacle at most. When the equivalence ratio is 1.5, the peak overpressure of circular opening is 5% and 10% higher than that of triangular opening obstacle and square opening obstacle. The established numerical model is able to reproduce the premixed hydrogen-air flame propagation characteristics under different obstacle opening structures. The decreasing order of vortex intensity is circle, triangle and square opening. The flame-vortex interaction directly results in the difference of flame shape, flame front speed and overpressure under different obstacle opening structures. The results of this study are helpful for the prevention of hydrogen related risks. • Effects of obstacle opening structure on hydrogen-air flame front dynamics are obtained. • LES method of predicting hydrogen-air flame front dynamics is proposed. • Hydrogen-air flame front dynamics are analyzed using velocity vector and vorticity distribution. [ABSTRACT FROM AUTHOR]

While ammonia (NH 3) and hydrogen (H 2) are promising carbon-free fuels, there are significant challenges presented by their combustion properties. Blending them can help mitigate some of these issues; however, the fundamental behaviors of these blends need to be well understood. A constant volume combustion chamber (CVCC) is used to study the emissions of nitrogen oxides (NO, NO 2 , N 2 O), laminar burning velocity (LBV), and intrinsic instabilities for H 2 -NH 3 blends across a wide range of equivalence ratios (Φ = 0.5 to 1.5) and volume fractions of H 2 in the blends (0% to 50%). Measurements of LBV are consistent with existing data where available. The largest NO x and N 2 O emissions are found at lean conditions and decrease significantly for rich mixtures. NO x emissions tend to increase with H 2 fractions in fuel for lean mixtures. Intrinsic instabilities are more prominent at lean conditions and heavily influenced by the amount of H 2 in the blend. [Display omitted] • NO x production peaks at Φ =0.7, increases with H 2 content for lean mixtures. • N 2 O production peaks at lowest Φ , no significant variation with H 2 content. • Higher H 2 content results in earlier and more pronounced flame wrinkling. • LBV and emissions data extends available range for mechanism validation. [ABSTRACT FROM AUTHOR]

This study investigates experimentally and numerically the stability, combustion, and emissions characteristics of premixed swirl-stabilized methane/air flames with enhanced lean blowout (ELBO) injection of hydrogen in the non-premixed mode for higher turndown ratio gas turbines. The study analyzed the effects of hydrogen fraction (HF : volumetric percentage of H 2 in the total fuel) ranging from 0 to 25% and equivalence ratio (φ) ranging from 0.5 to 0.893 on flow/flame interactions and flame stability, structure, and emissions. The introduction of non-premixed H 2 yields two conflicting outcomes, with enhancements in reaction kinetics and flame speed yet counteracted by increased hydrogen jet momentum dispersing the flame. The combustor lean blowout (LBO) limit remains relatively unaffected (φ overall ≈ 0.5) by H 2 injection. Flame temperature is primarily influenced by φ and minorly by HF. Increasing HF resulted in localized temperature rises around H 2 jets without an overall increase in flame temperature, offering a promising strategy for reduction of NOx emissions. Nevertheless, using H 2 as a pilot fuel leads to an increase in intermediate species like OH, H, and O, contributing to flame stabilization. However, this practice also results in higher CO and unburned hydrocarbons emissions due to the inherent characteristics and heightened reactivity of H 2. • Flow/flame/emissions of premixed CH 4 -air flames with enhanced lean blowout H 2 injection. • H 2 injection resulted in counteracting effects of enhanced kinetics and increased momentum. • Lean blowout (LBO) limit remains relatively unaffected (φ overall ≈ 0.5) by H 2 injection. • Non-premixed H 2 injection proves to be an effective strategy for reducing NOx emissions. • Non-premixed H 2 injection resulted in higher CO and unburned hydrocarbons emissions. [ABSTRACT FROM AUTHOR]

Background: In recent years, simultaneous or sequential occurrence of MOG antibody disease and anti-NMDAR encephalitis in the same patient has been reported with increasing frequency. Scholars refer to the overlapping occurrence of these two disorders as MOG antibody disease and anti-NMDAR encephalitis overlap syndrome (MNOS). Cortical T2-weighted fluid-attenuated inversion recovery (FLAIR) -hyperintense lesions in anti-MOG-associated encephalitis with seizures (FLAMES) is a rare clinical phenotype of MOGAD in which cortical FLAIR high-signal lesions are unilateral, with little spread to the cortex and meninges bilaterally. Although cases of FLAMES have been consistently reported. However, to our knowledge, such cases of FLAMES combined with NMDARE are rare. Case presentation: Here, we describe a case of FLAMES combined with anti-NMDARE. The patient was a young male, 29 years old, admitted to our hospital with isolated seizures, whose MRI showed unilateral thalamic and bilateral frontal and parietal leptomeningeal involvement. Since we were unaware of the possibility of bilateral meningo-cortical MOGAD manifestations, the case was initially diagnosed as viral encephalitis and was given antiviral therapy. The diagnosis was not clarified until anti-NMDAR-IgG and MOG-IgG positivity was detected in the cerebrospinal fluid and serum. The patient was then treated with high-dose corticosteroids and his symptoms responded well to the steroids. Therefore, this case expands the clinical spectrum of MNOS overlap syndrome. In addition, we describe the clinical features of MNOS by summarizing the existing literature and exploring the possible mechanisms of its immune response. Conclusions: Our case serves as a reminder to clinicians that when patients present with atypical clinical manifestations such as seizures, consideration should be given to MNOS and conduct testing for various relevant autoantibodies (including MOG abs) and viruses in both serum and cerebrospinal fluid, as it is easy to misdiagnose the disease as other CNS diseases, such as viral meningoencephalitis. This syndrome exhibits a high responsiveness to steroids, highlighting the critical importance of recognizing the clinical and neuroimaging features of this overlap syndrome for prompt diagnosis and treatment. Furthermore, it enriches the disease spectrum of MNOS. [ABSTRACT FROM AUTHOR]

To investigate the flame propagation characteristics and chemical kinetics of premixed propane/hydrogen/air mixtures at elevated pressure, a series of experiments on propane/hydrogen/air mixtures with varying initial pressures (P 0 = 100–300 kPa) and equivalence ratios (φ = 0.6–1.4) at a hydrogen fraction of 0.6 and an ambient temperature of 298 K were carried out in a closed duct with an aspect ratio of 15. The chemical kinetics of the elementary reaction were analyzed numerically. The results revealed that the combustion-induced rapid phase transition (cRPT) occurred in the premixed C 3 H 8 /H 2 /air flames. Both the combustion pressure and flame propagation velocity increase with initial pressure except at P 0 = 300 kPa, where the flame propagation velocity decreases. Moreover, the linear dependence between the laminar burning velocity and peak mole fraction [H + O + OH] max was found by the effect of initial pressure on the peak mole fraction (H, O, OH). The explosion pressure P rea and the maximum rate of production of free radical H/O show excellent correlations. The maximum rate of pressure rise can be predicted by the maximum heat release rate (HHR). These results provide essential data on elevated-pressure duct accidents and have important implications for explosion prevention. • C 3 H 8 /H 2 /air flames at elevated pressure in a closed duct were investigated. • Combustion-induced rapid phase transition was observed in C 3 H 8 /H 2 /air flames. • An equation is established for combustion pressure with peak rate of H production. • A relationship between explosion parameters and fundamental parameters are found. [ABSTRACT FROM AUTHOR]

The vented deflagration of hydrogen-air mixture is studied with the influence of hydrogen concentration. The pressure oscillation and flame propagation mechanism are investigated by analyzing the internal and external pressure histories and flame propagation images. The experimental results present that the pressure oscillation phenomenon has a great effect on the internal pressure peak. All of the internal pressure curves present the remarkable pressure oscillation. The pressure oscillation curve is processed by the fast Fourier transform(FFT) method. The internal pressure peaks of origin curves are higher than the results of the FFT filter curves, especially the fuel-rich conditions. The internal pressure peak of origin curve increases with the increase of the hydrogen concentration(c (H 2)) approximatively, but the filtered internal pressure peak increases first and then decreases as c (H 2). The duration of the external pressure rise is significantly shorter than the internal results. The external pressure curves have one or two pressure peaks. The external pressure histories just have one peak as c (H 2) ranging from 15 vol% to 25 vol%. But the external pressure histories turn out to two peaks as c (H 2) is more than 30 vol%. It can be found that the duration of the internal pressure history increases monotonically with the decrease of c (H 2). The duration of external flame propagation is contrary to the duration of internal pressure history in the change rule. The duration of external flame propagation increases with the increase of c (H 2). The external explosion phenomenon can be found as c (H 2) is higher than 30 vol%. And the backdraft phenomenon in external flame propagation process can be found as c (H 2) is higher than 40 vol%. • The pressure oscillation phenomenon has a significant impact on the internal overpressure peak. • The duration of the external pressure rise is significantly shorter than the internal results. • The external explosion phenomenon can be found as c (H 2) is higher than 30 vol%. • The backdraft phenomenon can be found as c (H 2) is higher than 40 vol%. [ABSTRACT FROM AUTHOR]

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

Improvement of the fuel mixing inside the cavity flame-holder is important for the development of this injection system in scramjet engines. This article presents significant flow results on the mixing role of multi-extruded nozzles inside dual cavity flame holders with a single shock generator. A three-dimensional model of the dual cavity with injectors located on the inclined rear wall is produced to simulate real flow physics associated with proposed jet configurations. The impression of air injection from the inclined rear wall is also examined by computational modeling of the compressible flow inside the dual cavity. Based on the achieved results, the injection of multiple extruded nozzles enhances fuel diffusion in the cavity when the shock generator is applied. Our findings disclose that the injection of the fuel from the inclined rear wall results in more efficient fuel mixing inside the dual cavity flame holder. Mixing of the annular jet is increased about 14% via backward air jet inside the cavity. • Fuel mixing inside the cavity flame-holder is improved via dual cavity flame holders. • A three-dimensional model of the dual cavity with extruded injectors is simulated. • Fuel mixing and flame holding inside the dual cavity are investigated in the present work. • Injection of the fuel from the inclined rear wall results in more efficient fuel mixing inside the dual cavity flame holder. [ABSTRACT FROM AUTHOR]

Based on computational fluid dynamics technology, the effect of hydrogen blending ratio (HBR) on the concentration distribution of leaked gas diffusion in utility tunnels was investigated, and based on this, the explosion characteristics of hydrogen-blended natural gas (HBNG) were studied in an inhomogeneous concentration field. The results showed that the concentration field of HBNG gradually stabilized in the downstream of the leak site, and the peak concentration increased linearly with HBR. When HBR≤5%, a high concentration region above the upper explosive limit is formed inside the gas cloud. This characteristic of the distribution induces secondary combustion in utility tunnels. The maximum flame velocity and steady acceleration distance increase with the increase of HBR. The flame shape is significantly affected by the concentration field distribution. The explosion overpressure induced by the real concentration field created multiple peak structures that exacerbated the explosion disaster complexity. The first overpressure peak at the ends was formed by the superposition of the precursor shock wave and its reflected wave, and those at other locations are caused by the shock waves. However, all maximum overpressure peaks are formed by the superposition of multiple reflections of the explosion wave. The maximum peak overpressure at both ends of the tunnel is higher than that in the middle. The explosion overpressure induced by the inhomogeneous gas cloud is lower than that caused by homogeneous gas cloud under the same volume. The research results can provide an effective reference for the anti-explosion design and risk assessment of utility tunnel. • Characteristics of HBNG leakage and diffusion in utility tunnels were investigated. • Effects of hydrogen blending ratios on leakage and explosion were investigated. • Flame evolution process under real concentration field of HBNG leakage is revealed. • Distribution law of explosion overpressure loads in utility tunnels is revealed. [ABSTRACT FROM AUTHOR]

C/C-HfC-ZrC (noted as CHZ) composites were prepared by a combined technique of polymer infiltration and pyrolysis (PIP) and reactive melt infiltration (RMI) to resist long-term ablation. The phase composition, microstructure and ablation properties of the composites were studied. Moderate HfC introduced by PIP cooperated with ZrC generated during RMI improved the ablation resistance of CHZ significantly, whose mass and linear ablation rates were only 0.544 mg/s and 0.107 μm/s, respectively, after ablation for 600 s (15 × 40 s) using oxyacetylene flame with a heat flux of 4.18 MW/m2. The excellent long-term ablation performance was attributed to the formation of a HfO 2 -ZrO 2 solid solution layer, which resisted the flame scouring and prevented the oxygen permeation. This work will provide some insight into long-term anti-ablation design and the combination of multiple techniques. [ABSTRACT FROM AUTHOR]

Traditional optical flame detectors (OFDs) in flame detection are susceptible to environmental interference, which will inevitably cause detection errors and miscalculations when confronted with a complex environment. The conventional deep learning-based models can mitigate the interference of complex environments by flame image feature extraction, which significantly improves the precision of flame recognition. However, these models focus on identifying the general profile of the static flame, but neglect to effectively locate the source of the dynamic flame. Therefore, this paper proposes a novel dynamic flame detection method named Dynamic Deformable Adaptive Framework (DDAF) for locating the flame root region dynamically. Specifically, to address limitations in flame feature extraction of existing detection models, the Deformable Convolution Network v2 (DCNv2) is introduced for more flexible adaptation to the deformations and scale variations of target objects. The Context Augmentation Module (CAM) is used to convey flame features into Dynamic Head (DH) to feature extraction from different aspects. Subsequently, the Layer-Adaptive Magnitude-based Pruning (LAMP) where the connection with the smallest LAMP score is pruned sequentially is employed to further enhance the speed of model detection. More importantly, both the coarse- and fine-grained location techniques are designed in the Inductive Modeling (IM) to accurately delineate the flame root region for effective fire control. Additionally, the Temporal Consistency-based Detection (TCD) contributes to improving the robustness of model detection by leveraging the temporal information presented in consecutive frames of a video sequence. Compared with the classical deep learning method, the experimental results on the custom flame dataset demonstrate that the AP0.5 value is improved by 4.4%, while parameters and FLOPs are reduced by 25.3% and 25.9%, respectively. The framework of this research extends applicability to a variety of flame detection scenarios, including industrial safety and combustion process control. [ABSTRACT FROM AUTHOR]

Laminar flame speeds of ammonia/hydrogen blends diluted in a mixture of 79% Ar and 21% O 2 (referred to as "airgon") were measured for the first time at high temperatures exceeding 500 K and atmospheric pressure using the shock tube flame speed method. Lean (ϕ = 0.8), stoichiometric (ϕ = 1) and rich (ϕ = 1.2) mixtures, with hydrogen fuel fractions x H 2 = 0.2, 0.4, and 0.6 were studied in the temperature range of 500–820 K behind reflected shock waves. A significant enhancement in flame speeds and improved flame morphology was observed with increasing concentrations of hydrogen. Equivalent fuel/air data were computed from fuel/airgon measurements using a mixture-scaling approach and compared against available literature data. Although current data do not overlap with the temperatures of data in the literature, the full 300–800 temperature range of experimental data remain consistent with a non-Arrhenius correlation, while a power-law fit does not capture the full data trend. 1-D flame speed simulations were conducted using five recent chemical kinetic models of ammonia combustion available in the literature. Large disagreement was found among the models, with the model by Zhou et al. agreeing well with experiments at all conditions studied in this work. Sensitivity analyses were performed, and the most relevant reactions involved in the computation of flame speed, according to the models, were identified. Overall, the experimental data and modeling demonstrate the value of using blends of ammonia and hydrogen to improve the combustion characteristics of ammonia as a carbon-free fuel. • First measurements of ammonia/hydrogen flame speeds at temperatures above 500 K. • Hydrogen fuel fraction varying from 0.2 to 0.6, with ϕ = 0.8, 1, and 1.2 were studied. • Inclusion of hydrogen significantly improves ammonia flame morphology. • Empirical correlations were constructed using measurements and literature data. • Significant differences in the predictions of chemical kinetic models were observed. [ABSTRACT FROM AUTHOR]

Hydrogen's wide flammability range causes its high-pressure jet flames to exhibit large-scale, high-temperature characteristics, posing risks of casualties and property damage. This study investigates the impact of different nozzle shapes and release pressures on flame behavior and temperature distribution through high-pressure hydrogen jet flame experiments. Circular outlets (0.5–4 mm diameter) and rectangular outlets (aspect ratios 1 to 8) are used, covering release pressures from 20 to 220 bar. High-pressure hydrogen is released through a nozzle into ambient air, igniting to form a flame. Flame length, width, and axial temperature are measured. Findings indicate that visible flame size increases with release pressure. The models for dimensionless visible flame length and width, considering the dimensionless heat release rate, are proposed for different nozzle shapes. Additionally, the dimensionless temperature rise along the flame axis is observed to be a function of the dimensionless distance and dimensionless heat release rate. • The hydrogen jet flame length and width varied regularly with the outlet shape. • The model of the flame length and width at rectangular outlets was proposed. • The flame temperature varied regularly with the outlet pressure, area and shape. • The model of the flame temperature distribution at circular outlets was proposed. • The model of the flame temperature distribution at rectangular outlets was proposed. [ABSTRACT FROM AUTHOR]

This study investigated the effect of the H 2 fraction on reaction dynamics and NO x formation in a premixed NH 3 /H 2 /air jet flame within a hot coflow (JHC) through numerical simulations. We found that even a small mole fraction of H 2 (e.g., X H2, F = 5%) significantly enhances the ignition of the premixed NH 3 /air flame. However, the presence of H 2 markedly intensifies the main combustion reactions only when X H2, F ≥ 30%, resulting in a sharp increase in temperature and heat release rate. Moreover, as X H2, F increases, the NO x emission first increases until X H2, F = 75% and then decreases. Without any H 2 addition, a high emission of unburned NH 3 occurs in the pure NH 3 JHC flame. On the other hand, when X H2, F ≥ 50%, the emissions of unburned H 2 and OH increase rapidly due to extremely high temperatures. Notably, maintaining X H2, F between 5% and 20% minimizes the emissions of NO x , NH 3 , H 2 , and OH. Furthermore, the preferential diffusion of H 2 plays a key role in enhancing the production of active radicals (OH, O, and H), thereby significantly boosting the initiation and combustion reactions of NH 3 /H 2 blended fuel. [Display omitted] • Effect of H 2 addition on the premixed NH 3 /H 2 /air jet flames is investigated. • A small amount of H 2 (5%) in the fuel notably enhances the ignition of NH 3 flame. • H 2 significantly boosts major combustion reactions only when its proportion ≥30%. • Optimal H 2 proportion is 5%–20% to reduce emissions of NO x , unburned NH 3 and H 2. • Preferential diffusion of H 2 greatly enhances the ignition and combustion reactions. [ABSTRACT FROM AUTHOR]

The hydrogen carrier ammonia is considered a prominent carbon-free alternative fuel. This paper examines the impact of flue gas recirculation (FR) on combustion instability and emission behavior of premixed CH 4 /NH 3 /air swirl flame at different equivalence ratios (Φ) and ammonia mole fractions (χ NH 3 ). Experimental results show that FR is beneficial to suppressing the self-excited pulsating pressure and pulsating heat release rate of CH 4 /NH 3 /air flame, and the pulsating frequency decreases monotonically with the increase of FR. Under fuel-lean and stoichiometric conditions, FR is beneficial to suppressing NOx formation but will increase CO and NH 3 emissions. Under fuel-rich conditions, FR reduces CO emissions but increases NOx emissions. The NO emissions obtained from the chemical reactor network simulation are in qualitative agreement with the experimental results. The results show that the elementary reactions related to HNO and NHi radicals play a crucial role in the influence of FR on NO formation. • Flue gas recirculation (FR) suppresses combustion instability of CH 4 /NH 3 /air flame. • FR decreases NOx emission but increases CO emission under non-rich conditions. • FR increases NOx emission but decreases CO emission under fuel-rich conditions. • Chemical reactor network reveals the main reactions of FR affecting NO formation. [ABSTRACT FROM AUTHOR]

Combustion of hydrogen-enriched natural gas is a valuable short-term strategy for reducing CO 2 emissions from high temperature industrial heating. This paper presents several experiments on combustion characteristics and the formation of nitrogen oxides. The experiments included hydrogen contents up to 100% and fuel heat inputs up to 75 kW. Water-cooled lances were used to influence the furnace temperature. The analysis includes the distribution of furnace temperatures, the composition of flue gas, the cooling capacity of the lances under steady-state operating conditions, and OH*-chemiluminescence imaging of the near burner region. The presented results demonstrate the dependence of furnace conditions and NO X formation on various factors, such as different air inlet fluxes, furnace temperature, and fuel composition, for constant heat inputs. Efficiency increased by up to 5.5% and significant changes in flame shaped along with a maximum increase in NO X emissions when comparing natural gas to hydrogen was measured at 167%. [Display omitted] • Combustion of hydrogen-enriched natural gas with 0 to 100% H 2 • Experimental analysis of an industrial burner in a steady state, false air-free furnace. • Converged measurements for temperature, flue gas composition and cooling heat fluxes. • Dependence of NO X in mg/kWh on flame shape, temperature, fuel blend and air ratio. • Imaging of self-emitted OH*-chemiluminescence from turbulent jet flames. [ABSTRACT FROM AUTHOR]

Hydrogen may become a substitute for liquid fossil fuels, contributing to greenhouse gas emissions reductions in internal combustion engines. Numerical simulations play a critical role in the advancement of these engines, with laminar flame speed being the main input. Experimental data of hydrogen flame speed at elevated pressures are scarce, due to the instability of the flames. Nonetheless, stable hydrogen flames can be predicted using chemical kinetics models. Moreover, the injection of water into the hydrogen fuelled engine could offer several benefits to engine combustion and emission performance, as it modulates the laminar flame speed within the combustion chamber and this effect has not been completely understood. Currently, no correlation exists to predict the laminar flame speed of hydrogen-air combustion with water addition under lean mixture engine operating conditions. In this study, we have extended the newly developed laminar flame speed correlation of hydrogen-air combustion to account for the effects of water addition under engine relevant conditions by using chemical kinetic laminar flame speed values. The laminar flame speed correlation was derived for pressures from 10 to 70 bar, temperatures from 400 to 800 K, equivalence ratios from 0.35 to 1 and water addition by mole from 0 to 20%. The hydrogen laminar flame speed correlation was expressed using polynomial forms with reduced order and number of terms with optimized values of coefficients. Additionally, a new exponential term was proposed to the power term α of the laminar flame speed correlation to capture the coupled effects of pressure and temperature on laminar flame speeds under engine-relevant lean burn water-diluted operating conditions. • Water injection with hydrogen offers benefits for combustion and emission performance. • New correlation for predicting flame speed of hydrogen combustion with water addition. • Hydrogen laminar flame speed under engine relevant conditions using chemical kinetics. • New Hydrogen laminar flame speed correlations using reduced order polynomial form. • New term proposed to capture the coupled effects of pressure and temperature. [ABSTRACT FROM AUTHOR]