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Ammonia's distinctive chemical makeup, coupled with its ability to combust fully without carbon emissions, position it as a promising carbon-free fuel but requires careful management to mitigate NO x emissions. In practice, supplementing ammonia with hydrogen addresses its slow combustion rate and high ignition energy requirements. Meanwhile, the introduction of a bluff body (BB) within the pipe can alter the way of flame propagation. This study examines the combustion dynamics of the premixed ammonia/hydrogen/air flame in a pipe with a variable cross-section. The experimental findings demonstrate that increasing both equivalence ratio (Φ) and hydrogen volume fraction (α H 2 ) significantly alters the flame front velocity (FFV) and maximum overpressure (P max), while also enhancing the symmetry of the flame across variable cross-section. The addition of BB creates a vortex in the flame, the formation of which intensifies the combustion of the flame behind the BB, which can promote the generation of Helmholtz oscillations, elevate the amplitude of the pressure, improve FFV, and shorten the flame propagation time in the pipe. With the increase of Φ and α H 2 , the time is shortened less. When Φ = 0.8, α H 2 = 40%, the flame propagation time was shortened by 709 ms. When Φ = 1.2, α H 2 = 60%, the flame propagation time was shortened by 5.67 ms only. In addition, P max and maximum flame front velocity (FFV max) are more sensitive to α H 2 than Φ. After the addition of BB, the increase amplitude of P max and FFV max is increased. However, when Φ = 1.0 and Φ = 1.2, the amplitude of FFV max increase decreases with the increase of α H 2 . This study can provide some suggestions for the application of NH 3 /H 2 fuels in variable cross-sections as well as under obstacles. • Ammonia-hydrogen premixed flames were studied. • The BB makes the propagation time is shortened by 709 ms at Φ = 0.8, α H 2 = 40%. • The BB promotes the generation of Helmholtz oscillations. • The addition of BB creates a vortex in the flame, the formation of which intensifies the combustion of the flame behind the BB. • The BB makes the propagation time is shortened by only 5.67 ms at Φ = 1.2, α H 2 = 60%. [ABSTRACT FROM AUTHOR]

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

This work presents a Lagrangian model, referred to as the integral model in this study, for aluminum-dust combustion, with the intent to reduce the computational burden and numerical stiffness. A comparison with available experimental data and a detailed model explicitly resolving chemical kinetics is presented for the canonical case of a planar flame. The influence of particle diameter, dust concentration and fresh-gases temperature on the flame speed and flame temperature is presented. Most results are within experimental uncertainties, unfortunately significant for aluminum-dust flames. Paths for improvements are proposed when needed. Regarding computational burden, it is shown that the integral model is faster and more robust than its counterpart using chemical kinetics. Insights on polydisperse flows and validation with constant volume combustion cases are provided. This study paves the way for a use in more complex cases such as turbulent flames or afterburning in energetic materials. [ABSTRACT FROM AUTHOR]

The research has examined the effects of CO 2 dilution on ignition characteristics in H 2 /air mixtures. An ignition kernel was initiated by a laser-induced plasma in H 2 /air/CO 2 mixtures with a bulk velocity of 6.5 m/s. An infrared camera was used to measure radiation intensity emitted from H 2 O and ignition probabilities. A high-speed schlieren image system was utilized to measure ignition kernel areas and ignition time. High-speed chemiluminescence was performed to examine OH* intensities. Numerical simulations were conducted using GRI 3.0 mechanisms to calculate reaction rates and laminar flame speeds. The ignition probability is highest for the undiluted mixtures and decreases with increasing the CO 2 dilution fraction at constant adiabatic temperatures. A prolonged ignition time is observed at elevated CO 2 dilution concentrations. Increasing the CO 2 mole fraction decreases the ignition kernel growth, the integrated OH* intensity, and the integrated radiation intensity for each adiabatic temperature. CO 2 reacts with H radicals and decreases the production of OH*, OH, and H 2 O. The CO 2 dilution in the H 2 /air flames significantly contributes to the reduced reaction rate and flame speed, resulting in the low ignition probability. The high ignition probability is found at the large integrated OH* intensity in all cases. The ignition probability is high at the large integrated radiation intensity under the constant adiabatic flame temperatures. The OH* intensity is a critical parameter to estimate the ignition probability in the diluted lean hydrogen flames. • The effects of CO 2 dilution on the ignition process have been studied in lean premixed H 2 /Air flames. • The ignition probability decreases with increasing the CO 2 dilution fraction. • An increase in the CO 2 dilution fraction in the mixtures reduces OH* intensity and radiation intensity emitted from H 2 O. • The presence of CO 2 in the mixtures decreases reaction rates and flame speed. [ABSTRACT FROM AUTHOR]

The active injection system was designed to study the explosion flame propagation behavior of premixed CH 4 /H 2 /air mixtures in the closed tube with inert gas injection. The injection of inert gases (N 2 , CO 2 , and air) was controlled by the injection time, t inject , which is set from 0 ms to 210 ms. The effects of injection disturbance and inert gas dilution on flame morphology, flame tip dynamics, and overpressure build-up have been investigated. Results show that the injection of inert gas into premixed CH 4 /H 2 /air flame wrinkles the early laminar flame, changing the flame structure. The increase in t inject shortens the flame propagation time and increases the maximum explosion pressure, but the maximum value is attained at t inject = 120 ms. The linear relationship between maximum explosion pressure and maximum flame tip velocity is presented. The addition of inert gas dilutes the fuel-air mixture, but the inert gas shows some inhibition effect on the turbulent explosion flame propagation at t inject > 120 ms. The injection disturbance plays a dominant role in the flame propagation process compared to inert gas dilutions. Finally, the effect of Darrieus-Landau and Rayleigh-Taylor instabilities on explosion flame propagation behavior has been examined. The injection of inert gas into premixed CH 4 /H 2 /air flames creates local turbulence, enhances flame instabilities, and provides positive feedback on the flame's acceleration and oscillation. • Effect of inert gas injection on premixed CH 4 /H 2 /air explosions is investigated. • The injection disturbance plays a dominant role in explosion flame propagation behavior. • The injection of inert gas shortens flame propagation time and increases the explosion pressure. • Effect of inert gas type and injection time on explosion parameters are confirmed. • Injecting inert gas into the explosion flame changes the DL and RT instabilities. [ABSTRACT FROM AUTHOR]

The combustion process of both pure NH3 and a NH3/H2 fuel blends is here analyzed using two kinetics processors, i.e., Chemkin-Pro-and CANTERA: detailed kinetic mechanisms have been tested and compared in terms of laminar flame speed and ignition delay time (IDT) with the aim to identifying the most suitable ones for the evaluation of NOx emissions. The generic swirl burner being used in Cardiff University's Gas Turbine Research Center has been considered as validation test case. In addition, this paper presents an experimental campaign followed by a computational fluid dynamics (CFD) approach for the assessment of NOx emission using axisymmetric Reynolds-Averaged Navier-Stokes (RANS) simulations, leading to a significant reduction of the computational time. Different pressures and mass flow rates are evaluated to understand correlations of NOx formation for pollutants reduction purpose. A direct comparison between experimental and numerical results is carried out in terms of flow field, flame shape, and NOx emissions. Results show that the increase in pressure from 1.1 bar to 2 bar results in reduction of NOx emissions from 2515 ppmv to 885 ppmv, also indicating guidelines for using a simplified RANS analysis, which leads to improved computational efficiency, allowing wide sensitivity and optimization analysis to support the design development of an industrial combustion system. [ABSTRACT FROM AUTHOR]

Chemical ionization in combustion systems forms concentrated ions of both polarities. Applying electric fields and plasmas to combustion systems has been shown to reduce emissions (such as particulate matter) potentially by controlling the ionic properties. Detailed flame-generated ion properties need to be characterized to better understand and predict the dynamics and roles of these ions in combustion and particle formation processes. In this work, we used a high-resolution differential mobility analyzer (HR-DMA) to map the mobility and size distributions of positive and negative ions generated from a premixed methane-air flat flame under atmospheric pressure. Measurements were conducted over a wide range of stoichiometric ratios (0.8 to 1.2) and heights above the burner (HAB, 7–42 mm). Positively charged ions are relatively stable over the entire range of experimental conditions, showing two major modes, one at 1.16 nm, and another at 1.42 nm. The mode corresponding to 1.42 nm showed a gradual increase in size with HAB, indicating the charging of hydrocarbon precursors. With the mobility values of the ions, we calculated their approximate mass values based on the empirical mobility–mass relationship and estimated the charging characteristics of particles in the flame. The ion profiles and particle charging characteristics obtained in this study will improve our understanding of electrostatic interactions in flame systems. [ABSTRACT FROM AUTHOR]

This study developed a one-step chemical reaction model for hydrogen combustion and investigated the effect of heat loss on premixed hydrogen/air deflagration in closed pipelines using numerical simulations with the CFD code GASFLOW-MPI. The simulation results were compared and verified with experimental data. The numerical simulations accurately predicted the flame front structure, pressure, and flame tip position during the deflagration of premixed hydrogen/air. Furthermore, a comparison of adiabatic simulation and heat transfer simulation results revealed that large-scale vortices within the turbulent flow were closely linked to heat loss during the tulip flame stage. Simultaneously, heat loss reduced pressure and flame tip position by diminishing the flame wrinkling factor caused by thermodiffusive instability. Finally, convective heat transfer identified as the most critical factor contributing to heat loss during the deflagration of premixed hydrogen/air in closed pipelines. Specifically, during the tulip flame stage, heat loss was mainly due to convective heat transfer (73.8%∼76.3%), with radiation playing a minor role (23.7%∼26.2%). Therefore, accurate prediction of characteristic parameters during the deflagration of premixed hydrogen/air requires numerical simulations that account for both convective and radiative heat transfer. • Heat transfer simulation can better predict the evolution of hydrogen deflagration. • Heat loss can cause a reduction in pressure and flame tip position. • Large-scale vortices within the turbulent flow are closely linked to heat loss. • Thermodiffusive instability is closely related to heat loss. • Convection heat transfer plays a crucial role in the heat loss. [ABSTRACT FROM AUTHOR]

Practical combustors in furnaces, industrial heaters, and gas turbine engines face a sudden loss of flame due to rich blowout (RBO) at an extremely rich fuel-air mixture or lean blowout (LBO) at an extremely lean fuel-air mixture. Thus, the stability limits of the combustion regime are governed by RBO and LBO. In the present study, we focus on the dynamics of swirl-stabilized premixed combustion near lower and higher flammability limits, where the possibilities of LBO and RBO, respectively, are observed. Near RBO and LBO, we employ metrics from statistics and dynamical systems theory to characterize the transition in flame dynamics. We observe that the range of frequencies obtained using Fourier Transform near RBO and LBO is alike. The emergence of dominantly low-frequency oscillation near those blowout limits is investigated using Hilbert Transform. The mean frequency of the combustion system gradually reduces near both blowout limits due to behavioral oscillations prominently observed. The scaling property of flame oscillation is examined using the Hurst exponent, and we observe a decreasing trend of the parameter as combustion approaches both LBO and RBO. Therefore, for the gradual reduction of the Hurst exponent near both flammability limits, we can use the parameter as a precursor for both RBO and LBO. [ABSTRACT FROM AUTHOR]

The dynamics of the burner-stabilised flame is investigated theoretically in the frame of diffusive-thermal model. The nonlinear system of equations describing dynamics of curved flame front and the flame front temperature is derived in the quasi-stationary approximation from the basic diffusive-thermal model with one-step chemical reaction of Arrhenius type. The reduced model successfully describes nonlinear flame oscillation that is confirmed by comparison of oscillation characteristics obtained in the reduced and full models with the volumetric chemical reaction. The comparison of the instability increments depending on wave numbers agrees with the increments following from the linear stability analysis. The proposed model makes it possible to significantly reduce calculations by reducing the dimension of the system, as well as to obtain estimates of the critical parameters at which oscillations occur. Specifying the critical parameters by simple model will facilitate the performance of complex calculations in the models with detailed kinetics of chemical combustion reactions. A reduced model that describes the flame front spatial instability is proposed too. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the unsteady heat release rate response of industrially relevant axisymmetric premixed flames to harmonic velocity perturbations. The heat release rate response, quantified using the flame transfer function (FTF) definition, is measured from an acoustically forced swirl burner under perfectly premixed conditions. To understand the features of the measured FTF, a physics-based analytical model is developed in this study. To describe the heat release rate dynamics, a model for the flame spatiotemporal response is derived in the linear limit using the G-equation formulation. Inputs to the flame response model are selected to be consistent with values observed in the corresponding industrial burner, based on experimental and numerical studies. The relative contributions of acoustic and convecting vortical disturbances on specific features of the FTF are explored in this study. The results highlight the importance of capturing the appropriate disturbance velocity field as an input to the flame response model for accurate FTF predictions. [ABSTRACT FROM AUTHOR]

In this paper, computational fluid dynamics (CFD) numerical simulation is employed to analyze and discuss the effect of obstacle gradient on the flame propagation characteristics of premixed hydrogen/air in a closed chamber. With a constant overall volume of obstacles, the obstacle blocking rate gradient is set at +0.125, 0, and −0.125, respectively. The study focuses on the evolution of the flame structure, propagation speed, the dynamic process of overpressure, and the coupled flame–flow field. The results demonstrate that the flame front consistently maintains a jet flame as the obstacle gradient increases, with the wrinkles on the flame front becoming increasingly pronounced. When the blocking rate gradients are +0.125, 0, and −0.125, the corresponding maximum flame propagation speeds are measured at 412 m/s, 344 m/s, and 372 m/s, respectively, indicating that the obstacle gradient indeed increases the flame propagation speed. Moreover, the distribution of pressure is closely related to changes in the flame structure, with the overpressure decreasing in the obstacle channel as the obstacle gradient increases. Furthermore, the velocity vector and vortex distribution in the flow field are revealed and compared. It is found that the obstacle tail vortex is the main factor inducing flame evolution and flow field changes in a closed chamber. The effect of the blocking rate gradient on flow velocity is also quantified, with instances of deceleration occurring when the blocking rate gradient is −0.125. [ABSTRACT FROM AUTHOR]

Using nearly 20L explosive ball, the effects of various N 2 and CO 2 ratios (5%, 10%, 15%, and 20%) on the explosion characteristics of hydrogen-rich mixtures were studied. The transient emission spectral intensities of H*, OH*, and O* were simultaneously collected in order to compare the influence laws of inert gas on hydrogen explosion from both macroscopic and microscopic aspects. Using the Fundamental Fuel Chemistry Model (FFCM-2), the thermodynamic parameters and exothermic pattern of the reaction were also computed and analyzed. The results are as follows: When the proportion of N 2 is greater than 10%, the explosion pressure rise rate (dp/dt) max is inhibited more clearly; CO 2 has a greater effect than N 2 on the maximum explosion pressure P max and (dp/dt) max of the high equivalency ratio hydrogen explosion; under N 2 , the accumulation of free radicals OH* is greater than that of H* and O*; the reduction of the peak radiation signal intensity I max is significantly greater than the effect of P max ; and CO 2 has a greater effect than N 2 on the adiabatic flame temperature AFT and thermal diffusivity α. CO 2 had a considerably stronger inhibitory effect than N 2 on both α and AFT. [Display omitted] • The suppression of the explosive characteristics under hydrogen-rich conditions is stronger for N 2 ratios higher than 10%. • In N 2 circumstances, the I max of OH* tends to be bigger than that of H* and O*. • The effect of inert gas on I max is significantly larger than the influence on the P max. • CO 2 had a stronger impact on t Imax and t Pmax. [ABSTRACT FROM AUTHOR]

The influence of gas expansion on the velocity of freely propagating curved stationary premixed flames in channels with free-slip adiabatic walls is studied utilizing 2D computational simulations, with the focus on the strongly nonlinear effects of the Darrieus-Landau instability on the stationary flame shape and velocity. The complete system of unsteady compressible Navier–Stokes equations is solved, including the transport properties (heat conduction, diffusion, and viscosity), and the chemical kinetics in the form of a one-step irreversible Arrhenius reaction. The parametric study focuses on the gas expansion, with a wide yet realistic range of expansion coefficients employed, and on the channel width. It is shown that the stationary curved flame velocity increases with gas expansion coefficient, in line with previous studies. The second critical channel width, at which the stationary curved flame velocity exceeds the one predicted by weakly nonlinear analyses and starts to grow rapidly with channel width, is identified with a sufficient accuracy for a wide range of gas expansion coefficients. The critical channel width for the Darrieus-Landau instability development depends on the gas expansion coefficient non-monotonically. The second critical channel width is found to decrease with the gas expansion coefficient, in a qualitative agreement with previous theoretical predictions. [ABSTRACT FROM AUTHOR]

A transport equation for the flame displacement speed evolution in premixed flames is derived from first principles, and the mean behaviours of the terms of this equation are analysed based on a Direct Numerical Simulation database of statistically planar turbulent premixed flames with a range of different Karlovitz numbers. It is found that the regime of combustion (or Karlovitz number) affects the statistical behaviour of the mean contributions of the terms of the displacement speed transport equation which are associated with the normal strain rate and curvature dependence of displacement speed. The contributions arising from molecular diffusion and flame curvature play leading order roles in all combustion regimes, whereas the terms arising from the flame normal straining and reactive scalar gradient become leading order contributors only for the flames with high Karlovitz number values representing the thin reaction zones regime. The mean behaviours of the terms of the displacement speed transport equation indicate that the effects arising from fluid-dynamic normal straining, reactive scalar gradient and flame curvature play key roles in the evolution of displacement speed. The mean characteristics of the various terms of the displacement speed transport equation are explained in detail and their qualitative behaviours can be expounded based on the behaviours of the corresponding terms in the case of 1D steady laminar premixed flames. This implies that the flamelet assumption has the potential to be utilised for the purpose of any future modelling of the unclosed terms of the displacement speed transport equation even in the thin reaction zones regime for moderate values of Karlovitz number. [ABSTRACT FROM AUTHOR]

Experimental and numerical simulation methods were employed to investigate the effect of gravity orientation on the dynamics of premixed conical flames. The study focused on a typical propane-air flame established on a Bunsen burner, under normal gravity (+ g), reverse gravity (-g), and transverse gravity (⊥g). In the initial phase of the research, flame shapes were examined using flame chemiluminescence imaging. Result shows that gravity orientation has a slight impact on the flame height, and buoyancy caused flame asymmetry in ⊥g case is first discovered. In addition, flame flickering frequencies were collected through heat release signal experiments, and a wide range of data is acquired. Though being affected by the same pattern by equivalence ratio and Reynolds number, the frequencies in ⊥g case are generally lower than those in + g case. Based on this, the research also obtained the new empirical correlation for ⊥g case. For clearer explanations of the flame behavior under different gravity orientations, velocity fields were visualization using Particle Image Velocimetry (PIV) experiments and Direct Numerical Simulation (DNS). Results indicated that the gravity orientation mainly influences the flame through effects on shear layer between ambient air and burnt gas, which cause different forms of K-H instability and vortex shedding motions. [ABSTRACT FROM AUTHOR]

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

This study theoretically explored the effects of parameter settings on thermoacoustic oscillations with a low-order model. Three factors were explored—combustor length, inlet gas temperature and thermal power. The research findings indicate that optimizing the parameter settings can yield better thermoacoustic oscillation suppression results. The sound pressure amplitude decreased from 3.2 × 105 Pa to 2.1 × 105 Pa as the combustor length increased from 1.2 m to 6.0 m. The triggering time increased from 0.32 s to 0.91 s when the combustion chamber length increased. The climb rate declined from 23.38 × 105 Pa/s to 3.75 × 105 Pa/s when the combustor length was elongated. The sound pressure amplitude decreased from 3.44 × 105 Pa to 2.4 × 105 Pa as the gas temperature rose from 0 to 100 °C. The triggering time and climb rate variation tendency were similar when the gas temperature increased—both declined as the gas temperature rose. The sound pressure amplitude experienced a slight fluctuation when the thermal power rose. However, the triggering time decreased from 0.26 s to 0.043 s when the thermal power improved. The climb rate increased from 18.72 × 105 Pa/s to 27.65 × 105 Pa/s when the thermal power rose. The oscillation frequency presented was completely different in three cases that had different wavelengths and oscillation intensities. The triggering time and climb rate fluctuated extensively in varying conditions, and the above two factors were interrelated and contradictory to each other when thermoacoustic oscillation was excited. This study explored parameters' effects on triggering time and climb rate, thereby providing references for constructing a model-based control system for thermoacoustic oscillation feedback control. [ABSTRACT FROM AUTHOR]

The addition of renewable fuel to hydrocarbon fuel mitigated the carbon monoxide, carbon dioxide and soot particle emissions of internal combustion engines. A comparative study of the ammonia and hydrogen effect on combustion and emission characteristics of n-hexadecane flames was executed at various flame conditions. The results showed that alternative fuels had a stronger impact on peak temperature at diffusion flame conditions rather than at premixed flame conditions. The premixed flame released carbon monoxide emission lower than the diffusion flames with/without additional fuels. The carbon monoxide emissions decreased highly with ammonia enrichment instead of hydrogen. At premixed and coflow diffusion flame conditions, the ammonia highly reduced carbon dioxide emission compared to hydrogen. Hydrogen addition reduced the nitrogen oxide emission, whereas it was increased by ammonia. The soot particle number density was reduced highly by hydrogen addition instead of ammonia at the premixed flow flame and diffusion coflow flame conditions. The impact of hydrogen on the reduction of soot mass concentration emissions was greater than that of ammonia at all combustion conditions. It is suggested that H 2 ≥ 70% should be mixed in n-hexadecane fuel because a higher burning velocity can be achieved with a higher emission reduction. A reduction in the emission of carbon-related species can be realized with a small reduction in burning velocity at NH 3 < 70%. • Comparative study of H 2 and NH 3 impact on n-Hexadecane flame. • Premixed flow flame and diffusion counterflow and coflow flames studied. • Combustion species along with PAH and NOx species were discussed. • Soot surface equations along with Moss-Brookes model were used. • Results suggest H 2 ≥ 70% and NH 3 ≤ 70% should be used in n-Hexadecane fuel. [ABSTRACT FROM AUTHOR]

Development of efficient and safe energy systems for space objects includes ensuring fire safety. This problem requires a deep understanding of influence of gravitational forces on combustion processes, which explains the importance of studying this issue. The paper analyzes the dynamics of inverted conical methane-air flame under conditions of normal and reverse gravitational force with external acoustic excitation. Applied frequencies of acoustic excitation could be parasitic or be produced during proper work of different technical systems of aircraft. High speed recording of flame chemiluminescence and PIV analysis were conducted. Decomposition of flow vector pictures into the modes was calculated using POD method. Energy contribution into the flow was analyzed for its separate elements at the different frequencies of excitation. At the high excitation frequencies vortex generation intensity in the shear layer with reverse gravity is about the same as it was observed in the experiments with normal gravity direction, that could indicate dominance of acoustic mechanism of the vortexes stimulation over the mechanism of shear instability. At external excitation frequency equals 240 Hz significant growth of shear vortex diameters and increasing of perpendicular oscillations amplitude of flame branches were obtained. Thereby in such experiment with 240 Hz external acoustic excitation the most intensive large scale instability of flow was observed. • The smallest vortices sizes are observed at 0 and 60 Hz excitation frequencies. • At excitation of 160, 240 and 420 Hz the vortices are non-affected by the gravity. • Vortice sizes and transverse oscillation range approach maximum at 240 Hz excitation. [ABSTRACT FROM AUTHOR]

This paper presents a new micro combustor design based on four trapezoidal ribs equidistant distributed on its inner wall. The aim is to highlight its promising thermal performance improvements, using numerical simulations as a tool of investigation, compared to other micro combustors. The achieved improvement reflected the favorable effect of the trapezoidal ribs through quantitative computation of mean temperature, temperature difference, and standard deviation. The hydrodynamic study shows that the ribs shape and height elongate the equidistant distributed recirculation zones, resulting in a significant increase in the flow residence time. Furthermore, the outer wall temperature level increases significantly from 1371.25 K to 1417.71 K and the non-uniformity zone is reduced compared to the simple backward facing step micro combustor. A better understanding of the mechanisms involved in heat transfer and their respective contributions to the overall thermal performance of the different micro combustors are explicitly analyzed. Moreover, the results highlight the favorable effect of the inlet velocity of 10 m/s to obtain the optimal outer wall temperature level and uniformity. The rich mixture composition provides the closest thermal performance to the stochiometric case, which widens the operating range for MTPV systems and improves their efficiency and reliability. • A novel micro combustor with trapezoidal shaped ribs for MTPV system is proposed. • Effects of implementing the trapezoidal ribs and its height are investigated. • The thermal performance is improved by the increased height of trapezoidal ribs. • Hydrodynamic and H 2 /air mixture inlet key parameters are examined. • The outer wall temperature level and uniformity are quantified. [ABSTRACT FROM AUTHOR]

Hydrogen-rich syngas, produced from biomass-based feedstocks, is a promising alternative energy source. However, its combustion characteristics are completely different from those of typical hydrocarbon fuels due to the presence of hydrogen. As a result, the kinetic mechanism of hydrogen-rich syngas premixed flame (HRSPF) propagation is very complex. In this work, the propagation of HRSPF was investigated employing different fan speeds (0–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0 bar–3.0 bar), and equivalence ratios (0.6–1.0). It was found that the propagation speed of turbulent HRSPF increased when the mixing process of turbulent flow was enhanced or the stretching effect of flame front was weakened. When the turbulence intensity increased, the effect of flame inherent instability on the propagation speed of HRSPF was weakened. When the relative changes of thermal diffusion and mass diffusion of flame front were considerable, the effect of diffusional-thermal instability on flame acceleration process was more significant. On the contrary, the effect of hydrodynamic instability on flame acceleration process was more significant. [Display omitted] • The stretching effect of flame front on flame propagation was studied. • Effect of both turbulence and flame inherent instabilities on the u t were evaluated. • Self-similar propagation coupled with diffusional-thermal instability was analyzed. • Effect of accelerated propagation process of the HRSPF was analyzed. [ABSTRACT FROM AUTHOR]

Since hydrogen does not emit carbon dioxide when it is burned, burning hydrogen directly is one option. However, as the hydrogen ratio in the total fuel increases, the visible flame emission becomes weaker. Therefore, when using fuels with hydrogen mixed with city gas, it is necessary to have a system that monitors incomplete combustion to ensure safe use of combustion equipment. In this study, we measured the C2 and CH chemiluminescence intensities of a laminar premixed flame of hydrogen and methane. These chemiluminescence intensities were evaluated using the ICCD camera, compared with flame images taken by the commercially available digital camera. Especially, we proposed a method to determine the C2 and CH chemiluminescence intensities simultaneously in terms of the signals composed of three primary colors of red (R), green (G), and blue (B) obtained by the digital camera images. The G signal of the flame obtained by the digital camera is almost proportional to the C2 chemiluminescence intensity. The B signal also shows a good linear relationship when data are divided into two groups according to the presence or the absence of the outer flame. We concluded that if the C2 and CH chemiluminescence intensities are predicted from the G signal and the B signal.

Effects of species diffusion models on the laminar burning velocity SL of lean hydrogen-air premixed flame are investigated by performing one-dimensional numerical simulations of lean hydrogen-air premixed flames at an equivalence ratio of 0.5. Maxwell-Stefan (MS) diffusion, mixture-averaged (MA) diffusion, and unity Lewis number diffusion are compared as the concentration diffusion models at a pressure of 0.1 MPa. Moreover, the contribution of the species thermal diffusion is also investigated under three different pressure (P) and unburnt gas temperature (Tu) conditions. Results show that the MA diffusion well reproduces the results of MS diffusion including SL while saving computational cost to three-fourths. The unity Lewis number diffusion overestimated SL by 30 % because of the underestimation of the mass flux of H2. By considering the species thermal diffusion, SL decreases by 6.4 % and 3.0 % under the reference (P = 0.1 MPa and Tu = 300 K) and HPT (P = 2 MPa and Tu = 673 K) conditions, respectively, and increases by 1.3 % under the HP condition (P = 2 MPa and Tu = 300 K). Under the reference and HPT conditions, the species thermal diffusion removes the H radical from the reactive region. This mitigates the chain-branching reaction of H + O2 → OH + O and decreases SL. On the other hand, under the HP condition, the species thermal diffusion supplies the H radical toward the reactive region. This enhances the aforementioned chain-branching reaction and increases SL. Under the HP condition, the recombination reaction of H + O2 + M → HO2 + M is enhanced by a high molar concentration of third body M because of high gas density. The large contribution of this reaction to the consumption of H radicals in the high-temperature region makes peak positions of mass fraction of H and mass flux of H by the species thermal diffusion lower temperature side, which leads to the supply of H radicals to the reactive region and the increase in SL.

The thermal expansion induced by the exothermicity of chemical reactions taking place in a turbulent flame affects the flow dynamics so deeply that the velocity field can be imposed by chemistry rather than turbulence. Moreover, thermal expansion is known to be responsible for flame-generated turbulence (FGT) as well as non-gradient or counter-gradient diffusion (CGD) phenomena. In the present study, a specific description of the joint probability-density function (PDF) of the progress variable and velocity is introduced. The corresponding PDF accounts for the finite thickness of the local flame. On the basis of this theoretical framework, the evolution of the scalar fluxes is analyzed across a planar premixed turbulent flame brush described as a boundary layer. The corresponding analysis recovers a CGD region in the planar flame brush as well as a region controlled by gradient diffusion (GD) transport at its leading edge. This region, which corresponds to small values of the mean progress variable, is dominated by finite Damköhler number effects. Finally, the dependency of the normalized turbulent scalar flux to classical nondimensional numbers – i.e., the Bray, Karlovitz and Reynolds numbers – is put into evidence. The obtained results provide a relatively simple basis for the development of closure models for the turbulent flux of the progress variable. [ABSTRACT FROM AUTHOR]

The Head-on Quenching (HoQ) of laminar premixed ammonia–hydrogen-air flames under lean to stoichiometric condition is numerical investigated. Detailed chemistry including 34 reactive species and detailed multi-component transport model including thermal diffusion (Soret effect) are applied. The quenching distance is considered as a representative quantity for the HoQ process, and the influence of different system parameters on it has been investigated. These parameters involve fuel/air equivalence ratios, hydrogen content in gas mixture and pressure. It was found that an increase of quenching distance can be caused by a lower hydrogen addition and a leaner mixture condition. Furthermore, it was found that, regardless of the gas mixture, the quenching distance decreases monotonically with increasing pressure, obeying a power function with the exponent - 0.7. Moreover, numerical results show a relation between the quenching Peclet number and the dimensionless wall heat flux normalized by the flame power. Additionally, sensitivities of quenching distances with respect to the transport model, considering the heat loss in the wall and the chemical kinetics are studied. [ABSTRACT FROM AUTHOR]

This study presents new CH4/O2 laminar flame speed (SL) data for a wide range of equivalence ratios (ϕ=0.2-2.4) at low pressure (0.5 atm) and multiple initial pressures (0.5-3 atm) at an equivalence ratio far from stoichiometry (ϕ=2). All experiments were conducted at room temperature using spherically expanding flames. At 0.5 atm, SL peaked Slightly lean, reaching a maximum of 411 cm/s near ϕ=0.9. Far from stoichiometry, SL was approximately 72 and 34 cm/s at ϕ=0.2 and 2.4, respectively. SL predictions were computed using several commonly cited chemical kinetics mechanisms for methane combustion, with performance approximately the same for all mechanisms. Every mechanism underpredicted SL at 0.5 atm for 0.3<ϕ<1.9, with performance especially poor near stoichiometry (∼100 cm/s underprediction near the peak). For ϕ=2.0, experiments were conducted for pressures from 0.5 to 3.0 atm, showing that SL varies approximately according to the power law SL∼P-0.1. The mechanisms captured this pressure dependency, with GRI-Mech 3.0 performing especially well. Sensitivity analysis shows that many reaction adjustments are necessary to solve the underprediction near stoichiometry, with CH3+OH⇌CH2O+H potentially a reaction of interest. [ABSTRACT FROM AUTHOR]

Blending hydrogen has proven an efficient method to enhance the combustion stability of gaseous ammonia flames. Heat release rate (HRR), as an important parameter to indicate combustion process, is hard to be directly measured and highly dependent on the fuel components, equivalence ratios, and operation conditions. This paper presents a comprehensive study on developing a general HRR surrogate (HRRS) for ammonia–hydrogen premixed flames under various conditions. Firstly, reaction mechanisms for ammonia/hydrogen premixed combustion are evaluated under various conditions, by comparing the predicted laminar flame speeds with the experimental data collected from published literature. The reaction mechanism developed by Shrestha et al., (2021) performs the best under various conditions. Then, series of one-dimensional freely propagating premixed flames under various conditions are calculated using this reaction mechanism and analyzed to explore the effects of blending ratios, pre-heat temperatures, equivalence ratios, and pressures on the HRR reconstruction for ammonia–hydrogen premixed flames from the viewpoint of chemical kinetics, and also serve as a database for the identification of HRRS. It is found that equivalence ratio and blending ratio have significant effects on the HRR reconstruction, while the effects of pressure and initial temperature are relatively limited. Subsequently, the general HRRS, [NH2]1.53[OH]0.28, is identified for ammonia–hydrogen premixed flames under various conditions using a grid-research optimization method. Finally, the general HRRS is further comprehensively validated on several 2D and 3D turbulent premixed flames of ammonia–hydrogen under various conditions. The comparisons with the two previous HRRSs also demonstrate that the present developed HRRS is superior and more stable temporally.

The analysis of the exergy loss is an effective tool for evaluating second-law irreversibility in laminar flames. However, despite numerous studies underlining the importance of taking into account thermal diffusion in laminar flame studies, especially in hydrogen/air flames, this phenomenon is usually neglected in the exergy analysis of these flames. Therefore, this work investigates the effect of Soret diffusion on the exergy loss in laminar premixed flames for hydrogen/air mixtures using a detailed reaction mechanism and the multicomponent transport model. The study starts from conditions in which the importance of the Soret effect is well established in the scientific literature. It is found that, while the exergy losses directly due to the Soret effect are negligible, the Soret effect can appreciably affect the other exergy loss contributions and hence the total exergy loss. Hence, the Soret effect, unlike what has usually been assumed, is not negligible in flame calculations at least when this effect is known to affect laminar flame speed. • The Soret diffusion role on the exergy loss in laminar premixed flames is studied. • Hydrogen/air detailed reaction and the multicomponent transport models are adopted. • Soret diffusion can appreciably affect the exergy loss contributions. • Soret effect is not negligible especially when this affects laminar flame speed. • The derivation of the Soret diffusion contribution to entropy generation is presented. [ABSTRACT FROM AUTHOR]

Recently, there has been an increasing emphasis on the efficient utilization of energy resources. Precise control of gas injection can aid in achieving this objective. However, measuring the temperature and concentration of gases simultaneously presents a challenge. Therefore, it is essential to monitor and track irregular behavior, such as combustion processes, in real-time without interfering with the combustion flow. This study proposes a non-contact optical measurement method that does not disrupt the combustion flow. Direct absorption spectroscopy (DAS) is used to extract the absorption line during optical measurements. Since the choice of absorption line extraction method can affect temperature measurement results, the performance of linear and non-linear interpolation methods is compared. Temperature measurements of stable flames were compared using a thermocouple and the computed tomography-tunable diode laser absorption spectroscopy (CT-TDLAS) method. The relative temperature error at the center of the measurement area between the two measurement methods was found to be approximately 2%. To overcome the real-time temperature measurement limitation of thermocouples, the combustion process of premixed flames is measured using computed tomography (CT) technology. Additionally, to evaluate the real-time measurement superiority of CT-TDLAS, the combustion state is assessed by comparing the intensity of a single-camera image with simultaneous measurement using CT-TDLAS. The results demonstrate that the image intensity of the flame captured by a single camera and the temperature field measured by CT-TDLAS exhibit the same trends during the extinguishing process. Consequently, the superiority of CT-TDLAS measurement for temperature measurement is demonstrated, and the real-time measurement superiority is confirmed. [ABSTRACT FROM AUTHOR]

As a carbon-free fuel, hydrogen has received significant attention recently since it can help enable low-carbon-economy. Hydrogen has very broad flammability range and very low minimum ignition energy, and thereby there are severe safety concerns for hydrogen transportation and utilization. Cryo-compressed hydrogen is popularly used in practice. Therefore, it is necessary to investigate the combustion properties of hydrogen at extremely low or cryogenic temperatures. This study aims to assess and interpret the effects of cryogenic temperature on premixed hydrogen/air flame propagation and acceleration in a thin closed channel. Different initial temperatures ranging from normal temperature (T 0 = 300 K) to cryogenic temperature (T 0 = 100 K) are considered. Both one- and two-dimensional hydrogen/air flames are investigated through transient simulations considering detailed chemistry and transport. It is found that when the initial temperature decreases from T 0 = 300 K to T 0 = 100 K, the expansion ratio and equilibrium pressure both increase substantially while the laminar flame speeds relative to unburned and burned gasses decrease moderately. The one-dimensional flame propagation is determined by laminar flame speed and thereby the combustion duration increases as the initial temperature decreases. However, the opposite trend is found to happen to two-dimensional flame propagation, which is mainly controlled by the flame surface area increase due to the no-slip side wall constraint and flame instability. Based on the change in flame surface area, three stages including the initial acceleration, steady burning and rapid acceleration are identified and investigated. It is demonstrated that the large expansion ratio and high pressure rise at cryogenic temperatures can significantly increase the flame surface area in early stage and promote both Darrieus-Landau instability (hydrodynamic instability) and Rayleigh-Taylor instability in later stage. These two instabilities can substantially increase the flame surface area and thereby accelerate flame propagation in hydrogen/air mixtures at cryogenic temperatures. The present study provides useful insights into the fundamental physics of hydrogen flames at extremely low temperatures, and is closely related to hydrogen safety. [ABSTRACT FROM AUTHOR]

This study investigated experimentally the effects of H2-enrichment on premixed oxy-methane flames stabilized on a micromixer-like burner for clean energy production in gas turbines. The stable combustion zone was established over ranges of fuel hydrogen fraction (HF: 25–70% by vol.) and equivalence ratio (ϕ: 0.1–1) at fixed oxidizer oxygen fraction (OF = 30% by vol.) and fixed jet velocity (5.2 m/s). The blowout limit was plotted within the HF-ϕ space, superimposed on the contours of some operating parameters, namely adiabatic flame temperature (AFT), combustor power density (PD), jet Reynolds number (Re), and unburnt-mixture density (ρmix). Furthermore, images of visible flame shape were acquired at distinct points within the stable zone to study the effects of ϕ, HF, AFT, PD, Re, and ρmix on flame macrostructure. Results show that H2-enrichment extends the blowout limit from ϕ = 0.35 down to ϕ = 0.1, thereby widening the stable combustion zone. The blowout limit has little correlation with AFT and PD but correlates strongly with ρmix, i.e., occurs consistently at the same ρmix. The analyses of blowout limit and flame images were used to infer that the flames blow out possibly at constant ignition delay. [ABSTRACT FROM AUTHOR]

The second-order factor effect of burner optical ports and edge inter-matrices (EIM) and the first-order factor of pressure on the soot formation process and behavior of premixed sooting flames in a high-pressure burner are numerically investigated here. Three-dimensional computational fluid dynamics (CFD) simulations of a premixed flame C2H4/air at p = 1.01 and 10 bar using a one-step chemistry approach are first performed to justify the satisfied predictability of the prospective axisymmetric two-dimensional (2D) and one-dimensional (1D) simulations. The justified 2D simulation approach shows the generation of an axial vorticity around the EIM and axial multi-vorticities due to the high expansion rate of burnt gases at the high pressure of 10 bar. This leads to the development of axial multi-sooting zones, which are manifested experimentally by visible luminous soot streaks, and to the boosting of soot formation conditions of a relatively low-temperature field, fV) profile, fV < 3%, is manifested only at high heights above the burner of the atmospheric sooting flame C2H4/air ϕ = 2.1, and early at the high pressure of 10 bar of this flame, fV < 10%. Enhancing the combustion process reactivity by decreasing the rich equivalence ratio of the fuel/air mixture and/or rising the pressure results in the prior formation of soot precursors, which shifts the sooting zone upstream.

Understanding boundary-layer flashback is critical to the design of safe and efficient gas turbines, especially as the addition of highly reactive hydrogen to these devices becomes a prevailing trend for reduction of carbon dioxide emissions. In this work, the boundary-layer flashback of lean hydrogen-air mixtures is studied using large-eddy simulations based on a generic turbulent swirl burner previously investigated experimentally. Simulations at increasing equivalence ratios are used to identify the flashback limits, showing reasonable qualitative agreement with experimental measurements. Flame dynamics during flashback are compared to previous experimental studies, indicating that important physics are captured in the simulations even if the exact flashback limits are not. In particular, a change in the swirl vane angle is shown to dramatically change the flame dynamics, with flame propagation occurring in the direction of swirl at high angles and against the direction of swirl at low angles, consistent with experimental observations. The near-wall nonreacting mean velocity field is shown to be controlled by a temporally stationary flow instability, which is directly responsible for the counterswirl flame flashback exhibited at low swirl angles. Finally, the interaction of local axial flow velocity and flame propagation speed of the leading flame point is investigated with varying swirl angles both before and after the onset of flashback, elucidating the differences in flame dynamics in all four of these cases. In particular, the importance of local flame extinction on flashback limits is emphasized. [ABSTRACT FROM AUTHOR]

The propagation of an isobaric premixed flame into a quiescent gas mixture of fuel and oxidizer contained between two parallel plates is investigated numerically. The plates are separated by a small distance h and considered as adiabatic. The mixture is assumed to be lean in fuel and the combustion model includes a single-step Arrhenius-type reaction, constant heat capacity and unity fuel Lewis number. Transport properties are considered to be temperature dependent or constant, which allows us to decouple two different instability mechanisms of hydrodynamic nature: (i) Darrieus-Landau (associated with the density change due to thermal expansion) and (ii) Saffman-Taylor (associated with the viscosity contrast). By performing three-dimensional (3D) simulations, the propagation rate and the flame front shape is analyzed as a function of the dimensionless parameter a = h / δ T , where δ T is the thermal thickness of the planar flame. The parameter a ranges from very small values to large enough ones so that flame curvature between the plates manifests itself. Results show that, as the distance between the plates decreases, loss of momentum enhances the hydrodynamic instability in comparison with that of a freely (unconfined) propagating flame. Likewise, viscosity contrast across the flame brings about an additional destabilizing mechanism. When distance between the plates increases, flame curvature can become important and contribute significantly to the overall propagation rate. Finally, by comparison with the 3D simulations, we show that confinement effects can be effectively described by a two-dimensional formulation written in the limit a → 0 , in which momentum conservation is reduced to a linear equation for the velocity similar to Darcy's law. [ABSTRACT FROM AUTHOR]

Ensuring fire safety is one of the most important problems in the creation of habitable spacecraft. It is necessary to thoroughly study the issues of the reliability of the functioning of space systems, including those using chemical fuel. The paper studies experimentally the dynamics of an M-shaped premixed methane-air flame under normal and reversed gravity at various external acoustic excitation. The stable M-shaped flame is studied experimentally by means of high-speed video recording of flame chemoluminescence and PIV-methods. The paper shows that at low frequencies of acoustic excitation, the flame front remains practically undisturbed. At a 160 Hz frequency, the oscillatory motion is energetically more saturated than the main gas flow. With a further frequency increase, the main flow again becomes dominant in terms of energy, and the effect on the flame structure decreases. • Intense oscillations of the M-shaped flame occur when excited by acoustic oscillations of 160 Hz. • The intensity of forced oscillations rapidly decreases with an excitation frequency increase. • An external acoustics leads to a less regular flame front structure under reverse gravity. • The flame responds less intensely to external acoustic excitation under reverse gravity. [ABSTRACT FROM AUTHOR]

The second-order factor effect of burner optical ports and edge inter-matrices (EIM) and the first-order factor of pressure on the soot formation process and behavior of premixed sooting flames in a high-pressure burner are numerically investigated here. Three-dimensional computational fluid dynamics (CFD) simulations of a premixed flame C2H4/air at p = 1.01 and 10 bar using a one-step chemistry approach are first performed to justify the satisfied predictability of the prospective axisymmetric two-dimensional (2D) and one-dimensional (1D) simulations. The justified 2D simulation approach shows the generation of an axial vorticity around the EIM and axial multi-vorticities due to the high expansion rate of burnt gases at the high pressure of 10 bar. This leads to the development of axial multi-sooting zones, which are manifested experimentally by visible luminous soot streaks, and to the boosting of soot formation conditions of a relatively low-temperature field, <1800 K, and a high mixing rate of gases in combustion around and above the EIM location. Nevertheless, a tolerable effect on the centerline soot volume fraction (fV) profile, fV < 3%, is manifested only at high heights above the burner of the atmospheric sooting flame C2H4/air ϕ = 2.1, and early at the high pressure of 10 bar of this flame, fV < 10%. Enhancing the combustion process reactivity by decreasing the rich equivalence ratio of the fuel/air mixture and/or rising the pressure results in the prior formation of soot precursors, which shifts the sooting zone upstream. [ABSTRACT FROM AUTHOR]

The combustion and thermal characteristics of impinging premixed flames for low heating value gases (LHVGs) composed of methane, propane, and nitrogen are experimentally investigated. For the experiment, a combustor with an impingement plate is used to study the flame stability regions based on flame shapes and visualize OH radical distribution. The OH radical distribution is observed via OH* chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF). Thermal characteristics are investigated by measuring the temperature distribution of the plate and the heat transfer rate into the cooling water. The results show that the flame stability region narrows as the heating value of the fuels decreases. Also, as the equivalence ratio increases (i.e., »1), the flame stability region is expanded. The OH radical intensity of LHVGs is observed to be lower than that of methane. The OH-PLIF images show where OH radicals are more distributed in the flame. The temperature distribution of the plate is determined by the flame shape according to the heating value, and the heat transfer rate into the water has a linear tendency to the fuel's thermal power. Based on these findings, LHVGs can be utilized without system improvement within the flame stability range in the impinging flame system.

This study theoretically explored the effects of parameter settings on thermoacoustic oscillations with a low-order model. Three factors were explored—combustor length, inlet gas temperature and thermal power. The research findings indicate that optimizing the parameter settings can yield better thermoacoustic oscillation suppression results. The sound pressure amplitude decreased from 3.2 × 105 Pa to 2.1 × 105 Pa as the combustor length increased from 1.2 m to 6.0 m. The triggering time increased from 0.32 s to 0.91 s when the combustion chamber length increased. The climb rate declined from 23.38 × 105 Pa/s to 3.75 × 105 Pa/s when the combustor length was elongated. The sound pressure amplitude decreased from 3.44 × 105 Pa to 2.4 × 105 Pa as the gas temperature rose from 0 to 100 °C. The triggering time and climb rate variation tendency were similar when the gas temperature increased—both declined as the gas temperature rose. The sound pressure amplitude experienced a slight fluctuation when the thermal power rose. However, the triggering time decreased from 0.26 s to 0.043 s when the thermal power improved. The climb rate increased from 18.72 × 105 Pa/s to 27.65 × 105 Pa/s when the thermal power rose. The oscillation frequency presented was completely different in three cases that had different wavelengths and oscillation intensities. The triggering time and climb rate fluctuated extensively in varying conditions, and the above two factors were interrelated and contradictory to each other when thermoacoustic oscillation was excited. This study explored parameters’ effects on triggering time and climb rate, thereby providing references for constructing a model-based control system for thermoacoustic oscillation feedback control.

Summary: The utility tunnel plays a vital role in improving city comprehensive bearing capacity, while whether the gas pipeline is set into the utility tunnel is a focus as chain and coupling accidents will happen in case of fire and explosion. In this study, a scaled‐down gas chamber experiment was carried out, and full‐scale two‐dimensional (2d) and three‐dimensional (3d) numerical models were compared. The combustion process of premixed gases could be roughly divided into four development stages, that is, (I) rapid development stage, (II) stable development stage, (III) jump stage, and (IV) oscillation stage. The over‐pressure on different walls had the same development trend and the peak value appeared in the jump stage. When the methane‐air premixed concentration was 9% and the ignition source was located at the center of utility tunnel (x = 0 m), the most unfavorable working condition occurred as its peak over‐pressure was about 1.57 MPa with an increase rate of 70%. Similarly, when the ignition position was at the left end (x = −99 m), the peak over‐pressure value was 1.51 MPa, which was 65% more than the initial value. While when the ignition position was not located at the center or ends of utility tunnel, the maximum wall pressure had a "double peaks" characteristic. When x = −80 m, the pressure jumped at 27 and 91 ms with the peak values up to 1.33 and 1.49 MPa, respectively. Arising findings provide guidance for assessing risks of setting the gas pipeline into utility tunnel and help to improve the safety design concept of utility tunnel. [ABSTRACT FROM AUTHOR]

In this work, the behavior of a lean premixed stretched flame stabilized in a flat channel near a heated wall was studied. Dependences of the flame front position on the stretch rate parameter at temperatures of the heated wall of 1000 and 1200 K and the combustible mixture composition (ϕ = 0.7 and 0.6) were obtained experimentally. The reduced thermal diffusive model was used in numerical simulation for an explanation of obtained experimental results. Theoretical estimates are in qualitative agreement with the experiment. The performed qualitative analysis may be useful in estimation of the combustion product temperature and the residence time of the nanoparticles forming in combustion products before their impact with the hot wall. [ABSTRACT FROM AUTHOR]

Raman spectroscopy is a powerful technique that allows for simultaneous measurements of multiple species; however, it suffers from low signal intensity and, in diagnostics of ammonia (NH 3) combustion, strong flame fluorescence. The current work re-introduces a multipass setup to perform enhanced quantitative Raman spectroscopy measurements of major species and nitric oxide (NO) in a premixed laminar lean (φ = 0.9) NH 3 flame. A way to deal with the strong flame reaction-zone fluorescence is proposed and validated. The measurements were performed using both the second (532 nm, NO detection) and third (355 nm, major species and temperature) harmonics of an Nd:YAG laser. The acquired data sets were compared to the simulation data obtained with two chemical kinetic mechanisms. The reported average post-flame concentration of NO reaches a plateau at 3900 ± 120 ppm, and the detection limit is 700 ppm. The 355 nm excitation wavelength proved to provide close to no flame fluorescence, which allowed for measurements across the entire flame from reactants to products. Thus, major species profiles were acquired, and the concentration and temperature results are in good agreement with the simulation data. Lastly, possible error sources are estimated and ways to minimize them are proposed. [ABSTRACT FROM AUTHOR]

• The influence law on combustion and explosion parameters of real multicomponent natural gas/H 2 /air was disclosed. • The laminar burning velocity of real multi-component natural gas/H 2 /air was obtained. • The mechanism of flame stability in real multi-component natural gas/ H 2 /air had been revealed. • The effect of hydrogen content on the overpressure and temperature of real multi-component natural gas was elucidated. Hydrogen is widely taken for the most potential clean energy for the moment. Currently, pipeline transport of natural gas mixed with hydrogen is one of the primary approaches of hydrogen transportation. However, compared to pure natural gas (CH 4), the explosion and combustion of natural gas mixed with hydrogen may lead to more severe consequences, and its reliability needs to be considered. Existing research of combustion and explosion using methane instead of natural gas have yet to be considered for application to real multi-component natural gas. To address this issue, real multi-component natural gas is configured, and the premixed flame and combustion characterization parameters of natural gas/H 2 /air are obtained through experiment. The chemical reaction kinetics of natural gas/H 2 /air mixtures are analyzed using Chemkin Pro software. These results prove that the laminar burning (LB) velocity varies between 0.33 m/s and 2.34 m/s by adding H 2. The primitive reactions R1 (H 2 + OH → H + H 2 O) and R3 (H + O 2 → O + OH) predominantly contribute to enhancing the LB velocity. The Markstein lengths rang from −0.19 mm to −0.049 mm, most lewis numbers are less than 1, the flame thicknesses and critical destabilization radii are reduced. Additionally, hydrodynamic instability and thermal-diffusion instability are strengthened by adding the hydrogen. As for smaller hydrogen content, the average size of flame cell from about 8 mm program first rises and then falls cyclic fluctuations, fluctuation amplitude gradually reduces, fluctuation frequency accelerates. In larger hydrogen content, this fluctuation amplitude is reduced and the average size of flame cell decreases. In addition, H 2 enhances the explosive strength of natural gas. Finally, methane instead of natural gas may exaggerate the velocity of laminar burning and exaggerate or weaken the ignition time delay of natural gas with different hydrogen contents. The current research might provide crucial information for the safe transport of hydrogen mixed with natural gas. [ABSTRACT FROM AUTHOR]

Premixed flame ignition is a fundamental issue in combustion. A basic understanding of this phenomenon is crucial for fire safety control and for the development of advanced combustion engines. Significant efforts have been devoted to understanding the mechanisms of ignition and determining critical ignition conditions, such as critical flame radius, minimum ignition energy, and minimum ignition power, which have remained challenging research topics for centuries. This review provides an in-depth investigation of the forced-ignition of laminar premixed flames in a quiescent flammable mixture, with emphasis on theoretical developments, particularly those based on activation energy analysis. First, the fundamental concepts are overviewed, including spark ignition, characteristic time scales, and critical ignition conditions. Then, the chronological development of premixed flame ignition theories is discussed, including homogeneous explosion, thermal ignition theory, flame ball theory, quasi-steady ignition theory, and, more importantly, transient ignition theory. Premixed flame ignition consists of three stages: flame kernel formation, flame kernel expansion, and transition to a self-sustaining flame. These stages are profoundly affected by the coupling of positive stretch with preferential diffusion, characterized by the Lewis number. Specifically, positive stretch makes the expanding ignition kernel weaker at larger Lewis numbers, consequently increasing the critical ignition radius and MIE. The premixed flame ignition process is dominated by flame propagation dynamics. Both quasi-steady and transient ignition theories demonstrate that the critical flame radius for premixed ignition differs from either flame thickness (by thermal ignition theory) or flame ball radius (by flame ball theory). Particularly, the transient ignition theory appropriately acknowledges the "memory effect" of external heating, offering the most accurate description of the evolution of the ignition kernel and the most sensible evaluation of minimum ignition energy. In addition, the effects of transport and chain-branching reactions of radicals, finite droplet vaporization, and repetitive heating pulses on premixed flame ignition are discussed. Finally, a summary of major advances is provided, along with comments on the applications of premixed flame ignition theory in ignition enhancement. Suggested directions for future research are presented. [ABSTRACT FROM AUTHOR]

• Confinement induces significant differences in flow field and flame macrostructure. • Unconfined and confined flames are stabilized in the shear layer far from LBO. • Unconfined and confined flames exhibit different flame dynamics approaching LBO. • Confinement effects on extinction mechanisms during the LBO process are discussed. This work investigates lean blowout (LBO) dynamics of dimethyl ether (DME)/air premixed swirl flames in a gas turbine model combustor with and without confinement. CH* chemiluminescence imaging and 2 kHz high-speed OH* chemiluminescence imaging are performed to capture the swirl flame macrostructures. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) are used to visualize the flow fields and reaction zones, respectively. Compared to the unconfined flame, the addition of confinement results in the appearance of an outer recirculation zone (ORZ), variation in flame topology, enhanced overall heat release (HR) rate, increased turbulent kinetic energy (TKE), and extended LBO limits. Far from LBO, all flames are stabilized in the shear layer. Approaching LBO, the confinement enhances the flow vortex structures along the shear layers, which is associated with extinction at the flame base and shear layer. However, the unconfined flame is still stabilized in the shear layer due to the weaker flow vortex structures. Different extinction mechanisms during the LBO process are also discussed between the unconfined and confined flames. The unconfined flame suddenly leaves the inner shear layer (ISL) and recedes to the inner recirculation zone (IRZ) within about 100 ms before LBO. Abundant CH 2 O is transported into IRZ mainly by recirculation, which reduces the HR region area and give rise to LBO. For the confined flame, more and more cold reactants gather near the base plate and then are gradually brought into IRZ by ISL vortices, which makes the flame lift from the base plate and decrease the HR region area until LBO happens. [ABSTRACT FROM AUTHOR]