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The laminar burning velocity (LBV) and cellular instability of ammonia/syngas/air were investigated using the spherically expanding flame method at an initial pressure of 3 atm and temperatures of 298–448 K. The impacts of equivalence ratio, temperature, and hydrogen ratio on LBV were scrutinized. The results demonstrate that the LBV varies non-monotonically with the equivalence ratio and increases with increasing temperature and hydrogen ratio. Flame instability was analyzed using flame uplift rate, Lewis number, thermal expansion ratio, flame thickness, critical radius, critical Peclet number and logarithmic growth rate of perturbation in combination with the schlieren images. Of these, the flame uplift rate is used to quantify buoyancy instability, a flame instability that has received little attention. At high pressures, lower LBV makes the flame susceptible to buoyancy instability. The flame uplift rate increases with increasing flame radius, suggesting that the buoyancy instability becomes increasingly significant as the spherical flame expands. The diffusional-thermal instability is weaker at the fuel-rich side and enhanced at the leaner side. As the hydrogen ratio increases, the flame instability increases. Hydrodynamic instability is not susceptible to temperature, which is only slightly weakened with increasing temperature. Additionally, the phenomenon of superadiabatic flame temperature (SAFT) in ammonia/syngas/air flames was first found and analyzed its temperature dependence. Increasing the initial temperature suppresses SAFT occurrence. [Display omitted] • LBVs of ammonia/syngas/air mixtures were measured at 298–448 K and 3 atm. • The buoyancy instability was studied in detail using flame uplift rates. • The effect of initial temperature on the flame instability was analyzed in detail. • The phenomenon of SAFT in ammonia/syngas/air flames was first found. [ABSTRACT FROM AUTHOR]

This study evaluates the laminar burning velocity of oxyfuel syngas mixtures. Experimental measurements were performed in standard conditions (298 K and 1 atm), varying the oxyfuel syngas composition (H2/CO/CO2/O2) and equivalence ratio for lean mixtures. The experimental setup uses the heat flux method, validated with CH4/air and H2/CO/air measurements. The results were compared with six chemical kinetic mechanisms developed for syngas, aiming at evaluating their predictive capabilities. The mixtures presented a tendency to the appearance of cellular instability in the flame front that was prevented to a certain extent. The CO2 effects on the flames were explored through numerical analysis, isolating the thermodynamic properties, transport properties, thermal radiation, and chemical effects. The CO2 or N2 effect as bath gas in the oxidizer is also evaluated in terms of the laminar burning velocity and mixture's effective Lewis number. Limitations due to cellular instability were studied, analyzing the influence of fuel dilution and composition on the onset of cellular instability at the flames. The results showed a good prediction of the experimental data by the evaluated kinetic mechanisms. The expected reduction in the laminar burning velocity with CO2 dilution is primarily due to the effect of carbon dioxide's thermodynamic properties, followed by its transport properties (when compared to N2 dilution), chemical effects, and heat losses by radiation. Cellular instability is attenuated with high dilution levels due to the reduction of hydrodynamic instability. At the same time, the increase of the H2 concentration in the mixture leads to more unstable flames due to both hydrodynamic and thermal-diffusive instability. A laminar burning velocity threshold above which the measurements were not possible due to the effects of cellular instabilities was identified and analyzed in detail. [ABSTRACT FROM AUTHOR]

Due to the carbon dioxide (CO2) content in biogas, using biogas is relatively limited in many industrial applications. Hydrogen (H2) is a highly reactive gas that is used extensively to enhance the burning rate of biogas, on the other hand, nitrous oxide (N2O) is a powerful oxidizer that can improve the burning rate of biogas. The laminar burning velocities of biogas-hydrogen-air and biogas-N2O mixtures at various equivalence ratios were investigated experimentally using the spherical flame methodology. It was found that the laminar burning velocities were enhanced in biogas-hydrogen-air and biogas-N2O mixtures. However, the enhanced laminar burning velocities of biogas-N2O showed a more significant increase than biogas-hydrogen-air due to the significant energy released by the N2O decomposition reaction. The flame thickness of the biogas-N2O mixture indicated lower values than biogas-hydrogen-air mixtures, suggesting a higher flame instability of the biogas-N2O mixture than biogas-hydrogen-air mixtures. The Lewis number of biogas-N2O mixture showed lower values than all biogas-hydrogen mixtures, indicating higher diffusive-thermal instability influence on biogas' flame. The reactions of H + O2⇔ OH + O and H + CH3(+M) ⇔ CH4(+M) represented the most significant reactions influencing the laminar burning velocities of biogas-hydrogen mixtures, while the N2O + H⇔N2 + OH, and N2O(+M) ⇔ N2 + O(+M) reactions represented the most important reactions in biogas-N2O combustion. [ABSTRACT FROM AUTHOR]

This article investigates the impact of H 2 and O 2 enrichment on the laminar burning velocity (S L) of syngas in a constant-volume combustion chamber. By analyzing changes in both chemical and thermal effects, it examines the impact of variations in H 2 and O 2 content on the S L independently. Furthermore, this article investigates the pressure variations of spherical expanding flames, flame instability, and the formation of mixed fuel NO. Results show increased H 2 and O 2 content enhance S L , with O 2 addition leading to a linear increase. H 2 affects thermal-diffusive instability differently in lean and rich fuel regions, while increasing O 2 content mitigates it. The synergistic influence of flame thickness and thermal expansion ratio amplifies fluid dynamic instability with higher H 2 and O 2 content. O 2 addition is more effective in promoting the rate of production for thermal NO and H 2 addition is more effective in promoting the rate of production for non-thermal NO. • The influence of H 2 and O 2 enrichment on combustion is analyzed. • The change of laminar combustion pressure is analyzed. • Decoupling analysis of the influence of the virtual gas method on H 2 and O 2. • The influence of H 2 and O 2 on the instability of spherical flame is analyzed. • The effects of H 2 and O 2 on different NO formation mechanisms are analyzed. [ABSTRACT FROM AUTHOR]

A theoretical method was optimized based on a wrinkled flame model for predicting the explosion pressure in confined chambers for typical low-carbon fuels, e.g., hydrogen (H 2), methane (CH 4), and ammonia/dimethyl ether/hydrogen (NH 3 /DME/H 2) blends, etc. It was found that the previous model with a wrinkling factor of Ξ Δ = e t (t is time) is only applicable to the explosions of H 2 fuel, while considerably underestimating the explosion pressures of other fuels such as CH 4 and NH 3 /DME/H 2 blends. For H 2 fuel where the pressure power exponent β > 0, the effect of flame wrinkling on the laminar burning velocity S L is less significant compared to that of adiabatic compression, so that an underestimated wrinkling factor does not affect the overall prediction of pressure by the model. However, for most fuels with β < 0, a more accurate representation of the wrinkling effect is necessary for pressure prediction. The results show that the model can be optimized by increasing the wrinkling factor to Ξ Δ = e 20 t that satisfactorily represents the effect of flame instabilities. The wrinkling factor remains constant (Ξ Δ = 2.46) after it reaches its maximum value of 2.46. Comparison of the model predictions to experiments shows that the optimized model can well predict the explosion pressures of various low-carbon fuels that experienced flame instability. • The wrinkled flame model for predicting the explosion pressure in confined chambers was optimized. • For H 2 , the effect of flame wrinkling on flame speed is less significant compared to that of adiabatic compression. • For fuels with β < 0, the wrinkling factor was increased to Ξ Δ = e 20 t with the maximum value of 2.46. • The optimized model can well predict the explosion pressures of different fuels at various initial conditions. [ABSTRACT FROM AUTHOR]

In the present paper, it was aimed to consume pure NH 3 by using an existing non-premixed burner with H 2 blending strategy, and to examine the temperature and NO emission profiles emerged in a model combustor for NH 3 /H 2 and pure NH 3 fuel compositions. The Mixture Fraction/PDF combustion model with a reduced reaction mechanism was used for the numerical modelling. During the experimental studies, a stable NH 3 /air flame was able to achieve by introducing H 2 into the burner since NH 3 /air mixture has a low reactivity and needs a high ignition energy. However, any flashback or lift-off tendency was not observed for NH 3 /H 2 /air or NH 3 /air flames. The numerical and experimental results showed that H 2 addition into NH 3 /air mixture provided higher temperature values both in the flame region and in the entire combustor. As the concentration of H 2 in the mixture was raised, the flame approached towards to the burner outlet with increment effect in burning velocity. However, H 2 addition into NH 3 /air mixture led to more NO emissions formation in the combustor. This can be attributed with that H 2 addition promoted both thermal and fuel-NO mechanisms because H 2 addition induced to increase the decomposition rate of NH 3 and flame temperature of NH 3 /air mixture. This finding was validated with the prediction of the concentrations of O and HNO radicals for all fuel compositions. Consequently, H 2 introduction contributed to the flame stability of NH 3 /air mixture even if more NO formation was observed. • H 2 addition on the performance of NH 3 /air combustion have been investigated. • A stable flame for NH 3 /air combustion has been achieved. • H 2 addition provided higher flame temperature distributions in the combustor. • H 2 addition promoted NO emissions in the combustor. [ABSTRACT FROM AUTHOR]

Combustion is the cornerstone for generating energy in various sectors and applications. The predominance of hydrocarbons combustion causes several environmental and human health issues. The ever-increasing demand for decarbonization of combustion technologies in order to reduce or eliminate undesirable emissions has created a major challenge, putting tremendous pressure on the international research community to develop alternative fuels. Ammonia has recently been explored as a promising carbon-free fuel and a great alternative to hydrocarbon fuels to achieve zero-carbon emissions. However, despite its advantages over hydrogen, ammonia combustion has some drawbacks including low burning velocity, flame stability, and high NOx emissions. This review begins with energy and environmental issues before moving on to hydrogen and ammonia as promising potential solutions, their advantages and drawbacks, and ammonia combustion kinetics. It summarises the principles of chemical kinetics of both ammonia combustion and NOx reduction coupled with the chemistry, kinetic mechanisms and impact of plasma on ammonia combustion. The effects of plasma on ammonia combustion and NOx reduction are also discussed, with a comparison of NOx reduction mechanisms and plasma approach. Finally, the review discusses the perspectives and challenges of using plasma in ammonia combustion. [Display omitted] • Plasma assisted ammonia combustion. • Advantages and drawbacks of ammonia combustion with kinetics. • NOx reduction coupled kinetics and impacts of plasma on ammonia combustion. • Perspectives and challenges of plasma ammonia combustion technologies. [ABSTRACT FROM AUTHOR]

Ammonia allows for long-distance and large-scale storage transport of renewable energy sources. The reactivity of NH 3 -Air flame can be enhanced by doping with hydrogen and by increasing the initial temperature. The flame combustion characteristics of NH 3 –H 2 -Air are studied in a rectangular closed pipe with a mole fraction of 45% H 2 , an equivalence ratio (φ) of 0.8–1.2 and 0.1 MPa. The initial temperatures (T u) of the mixture are 300 K, 375 K and 450 K. Experimental studies of flame front speed, flame tip structure evolution, and overpressure dynamics are performed. Laminar burning velocity (S L), peak mole fraction of key radicals, and flame instability of NH 3 –H 2 -Air flames are simulated in Chemkin Pro-software using the Mathieu and Petersen mechanism. The results show that the relatively low equivalence ratio and initial temperature promote the structural changes of the flame tip, forming a "tulip" flame and a distorted "tulip". As the equivalence ratio increases, the flame front speed and overpressure increase and the flame oscillates slightly. The distortion in the lip of the "tulip" is more pronounced. The increase in temperature promotes flame propagation in the early stages and the pressure decreases. Reactive activity increases with increasing temperature, the peak mole fraction of NH 2 and H radicals increases, and S L increases. At normal temperature, the flame is affected by thermo-diffusive instability only when lean mixture. The Darrieus-Landau instability increases as the equivalence ratio increases. Conversely, the Darrieus-Landau instability decreases with increasing temperature. Therefore, the current experimental and simulation results help to understand the laminar combustion characteristics of NH 3 –H 2 -Air at high temperatures. • The distortion of "tulip" is most obvious at T u = 300 K and φ = 1.1. • Increased temperature promotes flame propagation in the finger-shaped phase. • As The temperature increases (300–450 K), the H and NH 2 mole fractions increase. • DL instability decreases with increasing initial gas temperature. [ABSTRACT FROM AUTHOR]

Due to zero-carbon nature of ammonia and hydrogen, their blend fuel is proposed in many fields of combustion. In this study, the feasibility of using Bunsen flame burning ammonia-hydrogen-air for impingement heating is evaluated. It is found that within the ranges of Re and Ф , there exists a threshold value of X H2 beyond which cone-shaped flames stably burn on Bunsen burner. For X H2 = 5–50%, addition of hydrogen promotes flames stability by increasing laminar burning velocity to match the high flow speed at 800 ≤ Re ≤ 1000. Besides, richer mixture also improves flame stability due to more abundant radicals. In the range of 0.8 ≤ Ф ≤ 1.4, shorter flame height while higher flame temperature occurs near stoichiometric conditions of 1.0 ≤ Ф ≤ 1.2, which is favorable for compact and rapid heating applications. Examination of NO formation mechanism from free flame reveals that increasing X H2 or decreasing Ф leads to more NO emission, finding that it is much sensitive to the operational parameters of X H2 , Ф and Re. But NO emission becomes less sensitive when the flame is under impingement. Further, NO emission becomes lower, which a favorable feature of using ammonia-hydrogen-air flame for impingement impinging. Lastly, comparison of thermal power and efficiency between natural gas and ammonia-hydrogen reveals that the latter fuel is sufficient to replace the former in industrial heating applications. • Stable Bunsen flame of ammonia-hydrogen-air relies on hydrogen fraction over a threshold value. • Increased laminar burning velocity by hydrogen addition boosts ammonia flame stability. • Hydrogen addition promotes ammonia flame temperature. • NO emission declines during the impingement course. • Similar heating efficiencies between the flames of ammonia-hydrogen-air and natural gas. [ABSTRACT FROM AUTHOR]

A detailed experimental and numerical investigation of the combustion characteristics of carbon-free NH 3 /H 2 blends in N 2 O was performed in the present work. The experiments with various hydrogen fractions (α = 0–1.0) and equivalence ratios (φ = 0.6–1.4) were performed at ambient pressure (1 atm) and temperature (283 K) in a standard 3.375-L cubic combustion vessel. The explosion behaviors (maximum explosion pressure P max and maximum pressure rise rate (d p / d t) max) were obtained from the pressure-time curves. Further, the combustion characteristics (e.g. laminar burning velocity (S L), thermal expansion ratio (σ), flame thickness (δ), effective Lewis number (L e e f f ) and Markstein length (L b)) were calculated numerically by means of chemical kinetic analysis using Zhang's mechanism (Zhang et al. , Combust. Flame, 2017, 182: 122–141). Besides, sensitivity analyses of the explosion pressure and laminar burning velocity were performed to identify the contribution of dominant elementary reactions. The results show that, for combustion supported by N 2 O, the flame instability and the potential of deflagration to detonation transition (DDT) should be considered for the evaluation of P max and (d p / d t) max. Moreover, the hydrodynamic instability of premixed H 2 /NH 3 /N 2 O flame reaches its peak near the stoichiometric ratio and increases monotonously with the hydrogen fraction. The diffusion-thermal instability is more significant in the leaner mixture, and it is not invariably enhanced with the increase of hydrogen fraction but is relevant to the composition of the mixture. In addition, the sensitivity analysis of explosion pressure demonstrates that R80 (N 2 O + H 2 N 2 +H 2 O) and R70 (N 2 O+(M) = N 2 +O (+M)) are the most dominant reactions contributing to the pressure rise. The sensitivity analysis of laminar burning velocity shows that R70 (N 2 O+(M) = N 2 +O (+M)), R71 (N 2 O + H N 2 +OH) and R80 (N 2 O + H 2 N 2 +H 2 O) are the most dominant reactions enhancing laminar burning velocity. • The combustion characteristics of NH 3 /H 2 blends in N 2 O were firstly investigated. • The flame instability and DDT were considered for the combustion supported by N 2 O. • The parameters of hydrodynamic and diffusion-thermal instability were analyzed. • The sensitivity analysis based on different mechanisms was performed. [ABSTRACT FROM AUTHOR]

The effects of hydrogen addition on the premixed laminar burning characteristics of n-dodecane reacting in air were experimentally investigated using the freely expanding spherical flame method. The initial operating conditions were temperature = 425 K, pressure = 1 ˗ 4 bar, equivalence ratio = 0.8–1.4, and mole fraction of H 2 in n-dodecane-H 2 mixture = 0–40%. Nonlinear stretch extrapolation schemes were implemented to minimize the uncertainty in the estimation of unstretched flame speed as the binary fuel mixtures of n-dodecane and H 2 had non-unity Lewis numbers. All experimental operating conditions were simulated in CHEMKIN using a planar flame model with three different reaction mechanisms. The statistical uncertainty estimated for the LBV was below ±6.93%. The LBV increased by three/two times at off–stoichiometric/stoichiometric mixtures as the mole fraction of hydrogen (X H2) increased from 0 to 40% in n-dodecane. The rich mixtures of n-dodecane-air (ϕ = 1.4) were unstable to thermo-diffusive effects due to the lower mass diffusivity of n-dodecane and Le < 1, in contrast, H 2 blending to the n-dodecane by keeping the other parameters being same transformed it from an unstable to a stable one. The effective Lewis number and Markstein length decreased with an increasing X H2 in n-dodecane, indicating that the mixtures were stable to thermo-diffusive effects, but with further increase in X H2 , there may be a transition in flame stability. H 2 addition resulted in an earlier onset of hydrodynamic instability due to a reduction in the flame thickness. In addition, sensitivity analysis was performed to identify the key reactions responsible for the enhanced reactivity associated with H 2 addition. The reaction pathway and emission analysis showed a significant reduction in CO 2 and CO emissions. Finally, an increase of initial pressure decreased the LBV for all investigated mixtures. • First experimental study on LBV and burned gas Markstein-lengths of n-dodecane + H 2 -air mixtures at 1–4 bar. • Hydrogen blending improved the thermo-diffusive stability of rich mixtures. • LBV enhancement due to H 2 blending was dominated by the kinetic effect than the thermal effect. • CO 2 and CO emissions decreased significantly with the addition of hydrogen in n-dodecane. [ABSTRACT FROM AUTHOR]

27 m3 unconfined hydrogen explosion experiments are conducted to investigate the effect of barrier wall on explosion characteristics. The results show that the flame propagation velocity is highly related with the development of the flame instability, not only dependent on the laminar burning velocity. In front of the wall, due to the wave reflection, the overpressure peak P max and positive overpressure impulse I P increase while the positive overpressure duration time t dur decreases. In the rear of the wall, due to the wave diffraction, P max and I P decrease while t dur increases. The time reaching overpressure peak t peak increases on both sides of the wall. P max is about twice the incident overpressure on the wall surface because of the pressure reflection. The enhancement or mitigation effect of the barrier wall only works in a certain range. The mitigation effect is highly related with the relative height between gas cloud and barrier wall. [Display omitted] • The effect of a barrier wall is analyzed in 27 m3 hydrogen cloud explosion. • The explosion intensity is enhanced before the wall and mitigated behind the wall. • Positive overpressure duration time rises before the wall and drops behind the wall. • The times reaching overpressure peak are delayed on both sides of the wall. • The mitigation effect of the barrier wall with different height is discussed. [ABSTRACT FROM AUTHOR]

The purpose of this work is to evaluate the characteristics of hydrogen-blend methane explosion. The explosion characteristics without initial turbulence or with initial turbulence are firstly explored. Then the factors affecting laminar burning velocity and turbulent flame regime are revealed. Finally, the different prediction models are developed to evaluate peak explosion pressure of hydrogen-blended methane fuel. The results indicated that the MFFV, PEP, PRPR and laminar burning velocity of MR = 10% and MR = 30% continue to increase with the increase of ER, the peak value of MFFV, PEP, PRPR and laminar burning velocity of other MRs is arrived at ER = 1.0. The MFFV, PEP and PRPR increase gradually with increasing root-mean-square velocity. Regardless of initial turbulence, the MFFV, PEP and PRPR continue to decrease with increasing MR, the PEP and PRPR decrease obviously with increasing distance from ignition location. The turbulent flame regime is highly sensitive to MR, not root-mean-square velocity. The predicted value of laminar model, wrinkled model and turbulent model is relatively conservative, maximum wrinkling coefficient induced by flame instabilities and flame front itself needs to accurately evaluated. • Characteristics of hydrogen-blended methane explosion are explored. • Laminar burning velocity and turbulent flame regime are revealed. • Different theoretical models of predicting PEP are established. [ABSTRACT FROM AUTHOR]

Experimental and dynamic analyses of the deflagration characteristics of laminar premixed syngas-air at different preheating temperatures and with different CO 2 volume fractions were carried out in a rectangular half-open pipe. The effects of CO 2 concentration and different initial temperatures on the flame structure evolution, flame structure profile and reaction rate of critical radicals, flame propagation speed, overpressure dynamics and hydrodynamic instability of syngas-air mixture were studied. The FFCM-1 mechanism was used to predict the laminar burning velocity of syngas-air under relevant conditions. The results revealed that the addition of CO 2 inhibited the flame propagation and reduced the concentration of H, OH and O, thus reduced the laminar burning velocity. The increase in temperature promotes the chemical effect of CO 2 , and the interaction between the flame front and the pressure wave is more pronounced, prolonging the duration of the " tulip " flame. Adding CO 2 reduces the flame front speed and overpressure, decreases the oscillation amplitude in late flame propagation, and inhibits the explosion intensity. Meanwhile, the temperature increase accelerates the flame propagation in the spherical and finger stages, and the maximum flame propagation speed and peak pressure appear earlier. In addition, as CO 2 content and temperature rise, flame hydrodynamic instability is difficult to ignore. However, there is a lack of data from studies of syngas deflagration dynamics at higher temperatures and with higher CO 2 additions. This suggests a focus on studies at higher temperatures as well as with higher CO 2 additions to enable the development of accurate kinetic models for wide range of syngas combustion. Also, the higher the initial temperature, the longer the time required for heating. • The appearance time of the tulip flame increased with the initial temperature. • CO 2 inhibits flame propagation and reduces the concentration of H, OH and O. • The maximum flame front speed appear earlier as the initial temperature increases. • The peak pressure decreases with increasing initial temperature. [ABSTRACT FROM AUTHOR]

The report deals with the investigation of explosion safety parameters of hydrogen-air mixtures in a 17.17 L cylindrical closed-vessel with different concentrations, obstacles, and ignition locations. The experimental data including the maximum explosion pressure, laminar burning velocity, and corresponding flame radius were confirmed by using GASEQ code and theoretical calculation, respectively. The report shows the orifice plate reduced the maximum explosion pressure of the low-concentration hydrogen (φ ＜20% v/v), while the maximum explosion pressure of high-concentration hydrogen (φ ＞20% v/v) was increased, and the oscillation of the explosion pressure in the closed-vessel was obvious. The effect of the ignition location on the maximum explosion pressure was related to the interaction between the flame instability and the orifice plate for the φ = 30% v/v hydrogen-air mixture. · Multiple effects on safety parameters are captured in a self-made 17.17 L vessel. · P max obtained in the experiment is lower than P ad , calculated using GASEQ. · r f corresponding to S l become large as the characteristics of ignition energies. · Orifice plate contribute positively and negatively to P max ; oscillation is obvious. · Ignition location on P max of φ = 30% hydrogen-air is negligible without obstacles. [ABSTRACT FROM AUTHOR]

Currently, hydrogen-enriched n-butane blends present a real interest due to their potential to reduce emissions and increase the efficiency of combustion processes, as an alternative fuel for internal combustion engines. This paper summarises the recent research on laminar burning velocities of hydrogen-enriched n-C4H10–air mixtures. The laminar burning velocity is a significative parameter that characterises the combustion process of any fuel–air mixture. Accurately measured or computed laminar burning velocities have an important role in the design, testing, and performance of n-C4H10–H2 fuelled devices. With this perspective, a brief review on the influence of hydrogen amount, initial pressure and temperature, and equivalence ratio on the laminar burning velocity of hydrogen-enriched n-C4H10–air mixtures is presented. Hydrogen has a strong influence on the combustion of butane–air mixtures. It was observed that a parabola with a maximum at a value slightly higher than the stoichiometric ratio describes the variation in the laminar burning velocity of hydrogen-enriched n-butane–air mixtures with the equivalence ratio. An increase in initial pressure or hydrogen amount led to an increase in this important combustion parameter, while an increase in initial pressure led to a decrease in laminar burning velocity. Overall, these studies demonstrate that hydrogen addition to n-C4H10–air mixtures can increase the laminar burning velocity and flame temperature and improve flame stability. These findings could be useful for the optimisation of combustion processes, particularly in internal combustion engines and gas turbines. However, the literature shows a paucity of investigations on the laminar burning velocities of hydrogen-enriched n-C4H10–air mixtures at initial temperatures and pressures differing from those in ambient conditions. This suggests that experimental and theoretical investigations of these flames at sub-atmospheric and elevated pressures and temperatures are necessary. [ABSTRACT FROM AUTHOR]

The laminar burning velocity and cellular instabilities of premixed flame in H 2 -N 2 O mixtures were studied experimentally at various equivalence ratios (φ) and initial pressures (p 0). High-speed schlieren was used to record the morphology of the spherical flame. Based on the spherical flame method, the laminar burning velocity and Markstein length were derived. In addition, the Lewis number, flame thickness, thermal expansion and Zel'dovich number were evaluated numerically. The results show that both the laminar burning velocity and Markstein length increase with the equivalence ratio. The Lewis number (less than unity) increases with the equivalence ratio while the flame thickness exhibits an inverse evolution. Hence, the most unstable flames were found to be at φ = 0.4–0.6 due to the combined effects of the thermal-diffusion and hydrodynamic instabilities. At lower initial pressure, the flame front is smooth in spite of the diffusion-thermal instability. With the increase of initial pressure, both the diffusion-thermal instability and hydrodynamic instability dominate the cellular structure of the flame front. The flames undergo self-acceleration at φ = 0.4–0.6 and p 0 = 40 kPa, with the acceleration exponents close to 1.5 (the value of self-turbulization). Due to the local oscillation of velocity, the flame propagation speed decreases after the self-similar regime. [ABSTRACT FROM AUTHOR]

Adding the product of water electrolysis (i.e. 2:1 volume of H 2 and O 2) is an effective strategy to enhance the combustion intensity of NH 3 /air mixtures. In this work, the laminar burning velocity (LBV) of the obtained NH 3 /H 2 /O 2 /air mixtures was measured at 303 K, 0.1 MPa and compared with the values predicted by seven mechanisms. To improve the prediction performance, a new mechanism is developed based on the existing mechanism and adopted for numerical simulation. The results of this study show that the LBV of NH 3 is significantly increased by additional H 2 and O 2. By comparison, it is found that H 2 shows a more significant promoting effect on LBV when the volume ratio of additional H 2 and O 2 is 2. The concentration of key radicals and the flame temperature increase remarkably due to the addition of H 2 and O 2 , which promote the flame propagation. Furthermore, the experimental results also indicated that the additional H 2 and O 2 make the burned gas Markstein length decrease on the lean side and increase on the rich side. • The combustion of NH 3 was enhanced by 2:1 volume of H 2 and O 2. • The laminar burning velocity of NH 3 /H 2 /O 2 /air was measured. • A new kinetic model for NH 3 /H 2 /O 2 /air was obtained based on the existing model. • The promotion effect of H 2 and O 2 was analyzed. • The effect of H 2 /O 2 on flame instability was analyzed. [ABSTRACT FROM AUTHOR]

Mixtures of CH4 and CO2, known as biogas, are considered as renewable gas with low heat energy, which makes it extremely difficult to use as fuel in conventional natural gas equipment. To better understand the behavior of biogas mixtures and their properties, numerical results of the impact of carbon dioxide on laminar burning velocity and flame temperature of methane (CH4)–carbon dioxide (CO2)–air combustion were done. One of the most promising methods to enhance combustion is the use of a plasma source to assist unstable flames. For this purpose, experimental results to investigate the effect of microsecond pulsed plasma on non-premixed biogas flames were conducted. In the first section, laminar burning velocities and adiabatic flame temperature have been numerically calculated by one-dimensional steady code using several chemical kinetic mechanisms. Numerical results were compared with several experimental results from the literature. The laminar burning velocity and the adiabatic temperature of biogas/air combustion were calculated with CO2 concentration ranging from 0% to 50% in the mixture. The equivalence ratio was varied from 0.4 to 1.4, the initial pressure was set at 1 bar, and the temperature was set to 300 K. The impact of CO2 addition on the CO and NOx emissions at equivalence ratios from lean to rich (0.4–1.4) was investigated. It was found that increasing CO2 in the biogas lowers NOx and increases CO. In the second section, the effects of microsecond pulsed plasma on non-premixed swirling biogas/air flames were experimentally investigated. The plasma-assisted combustion was carried out in a coaxial burner. It consists of two concentric tubes; the central tube delivers the biogas flow to the injector and the annular tube contains a swirler and supplies the airflow. To create plasma in the stabilization zone of the flame, a rod-ring microsecond plasma configuration was used. OH* chemiluminescence measurements are used to describe the structure and stability of the flame. Results showed that plasma generated by microsecond high-voltage (HV) pulses can improve flame stability. An analysis of the optical emission spectra (OES) showed that plasma generates chemically active species such as excited N2*, CH*, OH* molecules, and H* atoms, leading to improved flame stability. The dependence of their intensities on both the pulse repetition frequency (PRF) and the input electrical voltage was investigated. The gas analysis of the exhaust gas was carried out with and without plasma. The exhaust gas concentration measurements revealed that the rod-ring pulsed plasma reduces CO with a low impact on the NOx level. [ABSTRACT FROM AUTHOR]

Biogas is a potential alternative energy source with low environmental impact. However, the practical applications of biogas are relatively limited due to the existence of CO2 which acts as a dilutant that decreases the calorific value and the burning rate of biogas. Nitrous oxide (N2O) is known to be a powerful oxidizing agent for propulsion applications which can enhance the combustion rate of biogas. In the present paper, the laminar burning velocity (LBV), hydrodynamic instability, and diffusive–thermal instability of biogas/N2O oxide were experimentally studied at different equivalence ratios. The spherical propagating premixed flames for various mixtures of biogas-N2O were determined using the constant volume combustion vessel at 303 K and atmospheric pressure. Two mechanisms were used in CHEMKIN-PRO software in order to estimate the predicted combustion characteristics of biogas/N2O mixtures. The results indicate that the decline in LBVs was prominent in the fuel-rich mixtures than in the fuel-lean mixtures with CO2 dilution. It is found that the influence of curvature on the flame front is weakened at the fuel lean-to-stoichiometric mixture due to the decrease in the flame thickness; therefore, flame instability tends to increase at the lean-to-stoichiometric region. Thermal diffusivity values decline with increasing the CO2% except in the equivalence ratios of φ = 1.0 and φ = 1.4, which showed no impact on the thermal diffusivity. The thermal reaction of N2O decomposition is the most significant reaction in biogas/N2O combustion at lean mixtures of φ = 0.6 and 0.8. [ABSTRACT FROM AUTHOR]

The effects of density ratio and differential diffusion on premixed flame propagation of H 2 /O 2 /N 2 mixtures are investigated by constant volume combustion chamber. The density ratio and differential diffusion are controlled independently by adjusting the O 2 /N 2 ratio and equivalence ratio. Results show that the density ratio has no effect on turbulent burning velocity while the differential diffusion has a promotion effect on turbulent burning velocity. The onsets of laminar flame acceleration are promoted by both density ratio and differential ratio. The turbulent flames perform a continuous acceleration propagation and the dependence between flame propagation speed and flame radius can be characterized as (d R /d t)/(σ · S L) ∼ R 0.33∼0.37, which is lower than the 1/2 power law. The acceleration parameters of laminar flames and turbulent flames (u ' / S L = 1) are around 0.17 and 0.36 respectively, and both of them are not affected by density ratio and differential diffusion. The empirical formula m = 0.19·(u ' / S L)0.4+0.17 is concluded to quantitatively describe the accelerative characteristics of laminar and turbulent flames. The current study indicates that the acceleration of laminar flames is mainly induced by flame intrinsic instability, and the latter can affect the acceleration onset but not affect the fractal excess. The acceleration of turbulent flames is dominated by turbulent stretch, while the effects of density ratio and differential diffusion can be ignored. • Flame intrinsic instability promotes the onset of laminar flame acceleration but does not affect fractal excess. • The turbulent flame follows the law of continuous acceleration propagation as (d R /d t)/(σ · S L)∼ R 0.33∼0.37 for u ' / S L = 1. • Τhe turbulent flame acceleration is dominated by turbulent stretch while the effects of σ and Le can be ignored. • The acceleration parameter m = 0.19·(u ' / S L)0.4+0.17 is concluded to quantitatively describe turbulent flame acceleration. [ABSTRACT FROM AUTHOR]

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Copyright of Advances in New & Renewable Energy is the property of Editorial Office of Advances in New & Renewable Energy and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• Compound inhibition mechanism of CF 3 I and CO 2 on CH 4. • Effects of CF 3 I and CO 2 on instability of CH 4 flame. • Physical and chemical effects of CO 2 and CF 3 I on CH 4. Natural gas is widely used, but explosions are highly destructive. This work uses a 20L spherical device to study the suppression effect of CF 3 I and CO 2 on 9.5 % CH 4 deflagration flame. The results show that the inhibitory effect of CF 3 I on the laminar burning velocity of methane is significantly better than that of CO 2. Mixing a small amount of CF 3 I into CO 2 can improve the explosion suppression effect. The addition of CO 2 and CF 3 I can inhibit the hydrodynamic instability and enhance the diffusional-thermal instability and the buoyancy instability of the flame. In general, the cellular structure of the flame gradually disappears with the addition of CO 2 and CF 3 I, while the flames float upward. Through numerical simulation analysis, it was found that CO 2 basically does not change the combustion reaction path of methane, but mainly reduces the concentration of active free radicals, rate of production and heat release rate through the inerting effect. CF 3 I can consume H, O and OH free radicals through the HI cycle reaction and CF 3 I decomposition reaction, changing the methane combustion reaction path, reducing the heat release rate and rate of production, especially the inhibition of H free radicals. Through quantitative analysis, the physical inhibitory effect of CO 2 accounts for more than 80 %, and the chemical inhibitory effect of CF 3 I accounts for more than 60 %. When the total amount of explosion suppressant is 8 %, the amount of CF 3 I mixed in CO 2 increases from 0 to 1.5 %, and the proportion of chemical suppression effect increases from 10 % to 60 %. It shows that the inhibition mechanism of CO 2 is mainly physical inerting, and the main inhibition mechanism of CF 3 I is chemical inhibition. Mixing a certain amount of CF 3 I into CO 2 can simultaneously exert the inhibitory effect of CO 2 on reducing oxygen and the effect of CF 3 I on reducing active free radicals. [ABSTRACT FROM AUTHOR]

[Display omitted] • First experimental and theoretical study of parametric instability of ammonia flames. • Reliable method to investigate premixed flame parametric instability is demonstrated. • Thermoacoustically unstable and cellular ammonia flames are visualized. • NH3 flames are more unstable than CH4 flames at the same laminar burning velocity. • The Zeldovich number strongly influences the acoustic instability of ammonia flames. This work investigates the self-excited thermoacoustic parametric instability of downward propagating laminar premixed ammonia flames in an open-closed combustion tube. NH 3 /O 2 /N 2 flames were propagated at different laminar burning velocities at equivalence ratios of ɸ = 0.8 and ɸ = 1.2. Different unstable flame regimes were observed through pressure fluctuation measurements and synchronized high-speed imaging capturing the lateral and longitudinal perspectives of flame propagation. The observed propagation speed of quasi-planar flames showed a strong correlation with the calculated laminar burning velocities of NH 3 /O 2 /N 2 mixtures. For the first time, the cellular structures of laminar premixed ammonia flames at the onset of parametric instability were clearly observed from the present experimental approach. The experimental cellular flame wavenumbers and acoustic velocity fluctuation amplitudes at the onset of parametric instability are in qualitative agreement with the theory of stability of laminar flames under acoustic excitation. Experiments have shown that NH 3 /O 2 /N 2 flames are more unstable than CH 4 /O 2 /N 2 flames at the same laminar burning velocity independent of Lewis number within the range of tested conditions. The influence of activation energy on parametric instability is investigated through a comparative thermoacoustic analysis between two different fuels at varied equivalence ratios. Ammonia flames are characterized by large overall activation energy and Zeldovich numbers compared to hydrocarbon flames. This work elucidates how this unique property characterizes the acoustic instability of downward propagating premixed ammonia flames in a tube based on classical theories on acoustic instability of laminar premixed flames. [ABSTRACT FROM AUTHOR]

This study presents, for the first time, a comprehensive investigation into the laminar flame propagation of ammonia/biogas/hydrogen/air mixtures. The laminar burning velocities (LBVs) were determined using the outwardly propagating spherical flame method under atmospheric temperature and elevated pressures across various equivalence ratios. The Okafor and CEU–NH 3 –Mech-1.1 mechanisms showed the best agreement with flame speed measurements. The influence of equivalence ratio, hydrogen ratio, and initial pressure on LBV was examined. Results indicate that LBV varies non-monotonically with the equivalence ratio, decreases with increasing pressure, and increases with higher hydrogen ratios. The enhancement of LBV due to hydrogen addition is mainly attributed to chemical effects, with thermal effects also playing a role. Given the significant CO 2 content in biogas, which is a key radiative species, the study also analyzed the radiative heat loss of ammonia/biogas/hydrogen mixtures. It was found that as the equivalence ratio nears the flammability limit, LBV is more affected by radiative heat loss, with a lower hydrogen ratio increasing this effect. Additionally, higher CO 2 mole fractions in biogas increase radiative heat loss. Detailed analysis of cellular instability, considering factors such as thermal expansion ratio, flame thickness, effective Lewis number, the logarithmic growth rate of perturbations and in conjunction with schlieren images, revealed that flame instability is enhanced with higher initial pressure and hydrogen ratios, while higher equivalence ratios inhibit instability. • Laminar burning velocities of ammonia/biogas/hydrogen/air mixtures were measured at room temperature and high pressure. • The effect of radiative heat loss on laminar burning velocity was analyzed. • The effects of hydrogen ratio and carbon dioxide concentration in biogas on radiative heat loss were analyzed. • The effects of hydrogen ratio, initial pressure and equivalence ratio on flame instability were analyzed. [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]

Unstretched laminar burning velocity (LBV) and intrinsic instabilities of Refined producer gas (H 2 :CO:CH 4)-Air mixtures were systematically investigated at 300 K, 1–4 bar and ϕ = 0.8–1.2 using freely expanding spherical flame method. In H 2 /CO/CH 4 rich mixtures, LBV increased with increase in CO (and reduction in CH 4)/H 2 (and reduction in CH 4)/H 2 (and reduction in CO) at any given equivalence ratio, while peak LBV occurred at ϕ = (1.2 and remained at 1.2)/(1.2 and shifted to 1.1)/(1.1 and remained at 1.1). Computed unstretched LBV using GRI Mech 3.0 and FFCM mechanisms deviated from measurements with initial pressure. From the comprehensive susceptibility analysis (to instabilities), the composition H 2 :CO:CH 4 = 0:1:1 had the highest resilience towards thermo-diffusive and hydrodynamic instabilities. Refined producer gas with higher mole fractions of H 2 were vulnerable to intrinsic instabilities, while increment in CH 4 suppressed the susceptibility to hydrodynamic instability and increment in CO suppressed the thermo-diffusive instability. • H 2 :CO:CH 4 -Air mixtures were systematically studied at 300 K, 1–4 bar & ϕ = 0.8–1.2. • Unstretched LBV, thermo-diffusive & hydrodynamic instabilities were measured. • CO & CH 4 added resilience to mixtures against the thermo-diffusive instabilities. • New ranking system introduced to analyze susceptibility of mixtures to instabilities. • Composition H 2 :CO:CH 4 = 0:1:1 has the highest resilience to intrinsic instabilities. [ABSTRACT FROM AUTHOR]

The explosion characteristics of H 2 –N 2 O over a wide range of equivalence ratio (φ) at a reduced pressure were studied experimentally in a cylindrical explosion vessel. Two orifice plates with the blockage ratios of 0.4 and 0.6 were employed to characterize the influence of an obstacle on the explosion behaviors. The experimental results showed that maximum explosion pressure could be higher than the adiabatic value due to the potential of flame acceleration and deflagration to detonation transition (DDT). The maximum explosion pressure was increased by the orifice plate at the fuel-lean side due to the flame instability while decreased at the fuel-rich side. This means that the roles played by the orifice plate are different for affecting the maximum pressure. The maximum pressure rise rate ( (d p / d t) max) was increased by introduction of an orifice plate due to the turbulence-generator effect. The maximum (d p / d t) max was obtained at φ = 0.6, indicating that the flame instability rather than the adiabatic explosion pressure or laminar burning velocity plays a leading role. • Effects of an orifice plate on the explosion behaviors of H 2 –N 2 O were studied. • The orifice plate plays a dual role on the maximum explosion pressure. • The orifice plate acts as a turbulence-generator on the maximum pressure rise rate. • The flame instability dominates the evolution of the maximum pressure rise rate. [ABSTRACT FROM AUTHOR]

Hydrogen is a potential aviation alternative fuel. To explore the influence of hydrogen addition on the combustion of aviation kerosene, the spherical expanding flame method was used to measure the laminar burning velocity and Markstein length of n-decane/hydrogen/air mixtures at 1bar, 2bar and 470 K using experimental and numerical analysis. The experimental LBVs agreed well with the simulation at an initial pressure of 1 bar, but lower than the simulation at 2bar. The laminar burning velocity showed a linear relationship with the hydrogen addition ratio (R H) at the different effective fuel-air equivalence ratios (ϕ F). The Markstein length decreased with increased R H and initial pressure, hence, increasing the susceptibility of flame front instability. Based on a one-step overall reaction assumption, sensitivity analysis of the premixture mechanism showed that the kinetic effect greatly influenced n-decane/hydrogen/air mixture combustion. Further kinetic analysis indicated that the maximum (H + OH) mole fraction of the chemical reaction is highly responsible for the linear relationship between laminar burning velocity and R H. • LBV and Markstein length of n-decane/hydrogen/air mixtures were measured. • [ X H + X O H ] m a x and LBV had similar linear relationships with the hydrogen addition ratio. • Markstein length decreased with increased hydrogen addition ratio and pressure. • Sensitivity analysis of the premixture mechanism was investigated. [ABSTRACT FROM AUTHOR]

• The concavity of spherical flame formed by the synergy of buoyancy vs. pure hydrodynamics. • A linearity of maximum pressure vs. LBV was established by incorporating the flame thickness. • Inhibition of methane combustion by the thermal effect of diluents is stronger than chemistry. • CO 2 /H 2 O chemical effect can weaken the thermal-diffuion and Darrieus-Landau instabilities. The evolutions of spherical flame of methane/air/diluent mixtures in a cylindrical chamber were observed by setting different equivalence ratios ϕ and diluent volume fraction V d i l u t i o n s , and the relationship of combustion pressure vs. chemical parameters was improved. Moreover, the influences of ϕ on the methane combustion characteristics and flame instability were explored by chemical pathways. Based on the results obtained, the spherical flame exhibits asymmetric propagation and generates a concavity in its lower flame front that closely resembles the "Tulip flame" at higher V d i l u t i o n s. The dominant mechanism responsible for this phenomenon is the pure hydrodynamics influenced by stronger buoyancy effects (i.e., F r o u d e n u m b e r ≤ 0.016). Moreover, a linear correlation of laminar burning velocity (LBV) with the maximum combustion pressure p m a x modified by flame thickness δ , which explains that p m a x is primarily determined by the heating rate and combustion velocity. Results from the sensitivity analysis of LBV reveal that simplifying the mechanisms for CH 4 -rich and CH 4 -lean mixtures must be differentiated to reduce the calculated quantity and enhance calculated accuracy. Furthermore, CO 2 /H 2 O chemical effects only contribute to the inhibition of LBV under the extremely CH 4 -lean conditions due to the influences of ϕ on CO 2 /H 2 O chemical pathways. In addition, the thermal diffusion and flame thickness are sensitive to the chemical pathways associated with OH and H radicals, respectively. Therefore, CO 2 /H 2 O chemical effects both weaken the influences of Thermal-diffusion and Darrieus-Landau instabilities on methane flames by affecting the productions of H and OH radicals, and H 2 O chemical effect can relieve flame concavity due to the improvement of the sensitivity of flame front on stretch effect. [ABSTRACT FROM AUTHOR]

• Experimental and calculated laminar burning velocities of EMC flames were obtained. • The explosion characteristics of EMC flames were measured. • Flame instabilities were calculated through crack length and average cell area. • The minimum critical flame radius occurs at an equivalence ratio of 1.6. • The acceleration exponent and fractal dimension were investigated. The intrinsic instabilities of ethyl methyl carbonate flames are investigated at equivalence ratios (0.6–1.8) and initial pressures (70–160 kPa) by using a 36-liter constant volume combustion chamber. The unstable laminar premixed flame is evaluated by the laminar burning velocity, explosion characteristics, flame cellularity, and flame self-acceleration. The experimental laminar burning velocity is well reproduced by the chemical kinetic mechanism. The extent of cellularization characterized by the crack length and average cell area non-monotonically varies with increasing equivalence ratio but monotonically increases with increasing initial pressure. Meanwhile, the critical flame radius for the onset of cellularity exhibits non-monotonic variation with the equivalence ratio, while it decreases monotonically with increasing initial pressure. The experimental critical Peclet number and critical flame radius are well captured by flame stability theory. The linear correlation between the critical Peclet number and the Markstein number is Pe cl = 26.46 Ma + 122.22. In addition, the dependence of the critical Karlovitz number on the Markstein number is predicted as Ka cl = 0.0226e-0.0880Ma. The flame self-acceleration increases as the flame instabilities increase. The acceleration exponent and fractal dimension are smaller than 1.5 and 2.33, respectively, which are the suggested values for flame self-turbulization. [ABSTRACT FROM AUTHOR]

• Laminar burning characteristics at high temperatures and pressures were studied. • The laminar burning velocities and Markstein lengths were measured and analyzed. • Trends in hydrodynamic instability with temperature and other factors were studied. • The simulation of ammonia/ethanol mechanism is higher than experimental value. • The sensitivity of laminar flame speed and N-containing reaction paths was analyzed. Ammonia, as a carbon free fuel, is expected to play a key role in carbon neutrality and social energy shortages. However, the flame propagation speed of ammonia is low. Blending ammonia with ethanol is one of the methods to increase laminar burning velocity of ammonia, but the existing research on the combustion characteristics of ammonia/ethanol focuses on low temperature and pressure. This paper investigated the effect of ethanol addition on ammonia combustion characteristics under high temperature and pressure. The initial conditions for this work include initial temperature (400, 450, and 500 K), initial pressure (0.5, 1.0, 1.5 MPa), equivalence ratio (0.8–1.3), and ethanol energy fraction (0 %, 20 %, 40 %, 60 %, 80 %). The results illustrate that the laminar burning velocity has a positive correlation with the ethanol energy fraction and the initial temperature, but is negative correlation with initial pressure. And the laminar burning velocity and the adiabatic flame temperature appear to be maximum when the equivalence ratio is between1.0 and 1.1. The strongest hydrodynamic instability occurs within the equivalence ratio range from 1.0 to 1.1. In addition, initial pressure increased flame hydrodynamic instability and initial temperature has little effect on it. The changes in initial pressure and temperature have little effect on thermal diffusion instability. When the ethanol energy fraction is less than or equal to 20 %, ammonia has a significant impact on the Markstein length, but ethanol dominates the Markstein length with a proportion of ethanol energy over 40 %. Under current experimental conditions, the existing ammonia/ethanol mixing mechanisms cannot comprehensively and accurately predict laminar combustion velocity. The CEU mechanism is relatively accurate, especially under lean combustion and high pressure. The three most sensitive reactions to laminar burning velocity are H + O2<=>O + OH, H + O2(+M)<=>HO2(+M), and NH2 + NO<=>NNH + OH. When adding 40 % ethanol compared to 20 % ethanol, the molar fraction of OH and O free radicals is higher, and the peak value of NO total ROP is higher. [ABSTRACT FROM AUTHOR]

• The characteristics of laminar combustion of NH 3 -CH 4 -air at high temperatures and pressures are investigated in CVCC. • The mechanism of OTOMO has the greatest agreement with the NH 3 -air laminar burning velocity, and the mechanism of OKAFOR has the greatest agreement with the NH 3 -CH 4 -air laminar burning velocity. • The reaction order of ammonia is calculated in the range from 1.24 to 1.48 and the temperature exponent is calculated in the range from 1.88 to 2.68. • The laminar burning velocities and Markstein lengths of NH 3 -CH 4 -air are measured, and their trends with temperature, pressure and equivalence ratio are analyzed. • Trends in hydrodynamic instability, thermodynamic instability, and buoyancy instability of flames with temperature, pressure, equivalence ratio, and methane energy ratio are investigated. Nowadays, ammonia is considered a potential carbon-free fuel. However, the research on the combustion characteristics at high temperatures and pressures which has significance to the research of real engine in-cylinder combustion is little. In this study, the Laminar Burning Velocity (LBV) and Markstein Length (L b) of NH 3 -air at high temperature and high pressure were investigated within a constant volume combustion chamber, and methane with various energy ratios was introduced for comparative analysis, as well as the reaction order and temperature exponent were calculated. Furthermore, flame instability in spherical expansion flames has also been studied in depth. The results show that when initial temperature (T u) = 450 K, the reaction order of ammonia ranges from 1.32 to 1.48, when T u = 500 K, the reaction order of ammonia ranges from 1.24 to 1.41. The temperature exponent is calculated between 1.88 and 2.68, when T u from 400 K to 550 K. In the range of T u from 400 K to 500 K, initial pressure (p) from 0.5 MPa to 1.5 MPa, Φ from 0.8 to 1.3 and methane energy ratio (E CH4) from 0 to 0.8, increasing T u and E CH4 , decreasing p all have the potential to increase the LBV. Notably, the optimal equivalence ratio shifts from 1.1 to 1.0 with the increase of E CH4. The L b is unaffected by variations in temperature, decreasing with increasing pressure but increasing with increasing equivalence ratio. As the E CH4 increases, L b increases within 0.8 < Φ < 1.0, but decreases within 1.1 < Φ < 1.3. As the p and E CH4 increase, the hexagonal structure of the flame surface by an increasing trend, indicating an increase in hydrodynamic instability, while the temperature has little effect on this instability. In addition, as the equivalence ratio increases, the hydrodynamic instability first increases and then decreases. With an increase in T u , Φ , and E CH4 , the thermal diffusive instability is slightly weakened, and the response to pressure is not significant. Lower flame velocity of ammonia, and decreasing T u , increasing the p , and decreasing E CH4 enhances the effect of buoyancy on the flame, which becomes more pronounced as the flame develops. [ABSTRACT FROM AUTHOR]

In this work, the effects of oxygen enrichment coefficient (Ω = 0.21 − 0.42) and initial temperature (T u = 298 − 448 K) on the laminar burning properties of NH 3 /H 2 /O 2 /N 2 flames are systematically investigated by using the spherical flame propagation method at equivalence ratios (Φ = 0.8 − 1.4) and initial pressure (P u = 3 atm). LBV is analyzed by obtaining experimental data. In order to analyze flame instability, thermal diffusion and hydrodynamic instability are separated by linear stability theory. The thermal and chemical effects of oxygen enrichment on LBV and its effect on heat release rate are analyzed by numerical simulation. The results show that the laminar burning velocity (LBV) exhibits a positive correlation with both the oxygen enrichment coefficient and the initial temperature. The temperature index (α) exhibits a non-monotonic variation in response to the initial temperature of the NH 3 /H 2 /O 2 /N 2 flame. Le eff is insensitive to temperature and positively correlated with the oxygen enrichment coefficient. The NH 3 /H 2 /O 2 /N 2 flame is mainly affected by hydrodynamic instability when the oxygen enrichment coefficient is large. The peak NHRR scales positively with the oxygen enrichment coefficient. Oxygen enrichment helps to contribute to the overall net reaction rate. • The LBV of mixture flame with oxygen enrichment coefficient 0.21–0.42 was measured. • The effects of initial temperature on the LBV of the mixture flame were studied. • Thermal and chemical effects of oxygen enrichment were analyzed by adding virtual gas. • Diffusion thermal and hydrodynamic instability were separated by linear stability theory. [ABSTRACT FROM AUTHOR]

The composition of biomass gas is complex, in order to promote the more extensive and efficient utilization of low-calorific syngas, it is necessary to master the basic combustion characteristics of low-calorific syngas under different compositions. In this paper, the effects of addition of CH 4 (0% – 20%) on the combustion characteristics of laminar flow flame of high-H 2 syngas (H 2 :CO = 3:1) were studied by experiments and numerical simulations under the dilution of N 2 and CO 2 (40%). The laminar burning velocity (LBV) and Markstein length are obtained by experiments. CHEMKIN-PRO software package was used to compare the prediction results of the four mechanisms with the measured data. Finally, GRI3.0 mechanism was selected as the simulation mechanism in this paper. The experimental results indicate that the LBV of N 2 dilution is higher than that of CO 2 dilution, and the flame is more stable, with adding CH 4 , the LBV of syngas mixtures is decreased, and the LBV of rich-fuel mixtures is decreased faster, but the stability of premixed flames of syngas is improved. Numerical simulation shows that the dominant branch chain reaction changes from R99 to R38 after CH 4 is added, and the mole fraction of H, O decreases while the mole fraction of OH, CH 3 increases. The dilution of CO 2 results in a lower concentration of active radicals than N 2 because CO 2 inhibits the important reaction R99. The GRI 3.0 mechanism was used to simulate the formation of NO: the addition of CH 4 promoted the formation of hot NO. The NO production of NNH and N 2 O intermediate mechanism is decreased, and a new way of N 2 and CH reaction to HCN is increased. The dilution of CO 2 can reduce NO emission more than that of N 2. The insights gained from this study have practical implications in a variety of production applications. For example, optimizing mixture equivalence ratios can improve combustion efficiency, thereby increasing energy efficiency and reducing environmental emissions. • The addition of CH 4 in N 2 /CO 2 dilution for high-H 2 syngas combustion characteristics. • Adding CH 4 reduces the laminar burning velocity but increases the flame stability. • The addition of CH 4 increases the NO production, and the NO production path is change. • When CO 2 dilution, the flame is stable and NO production is smaller than N 2 dilution. [ABSTRACT FROM AUTHOR]

• Measured the laminar burning velocity of ethane-hydrogen-air mixtures. • Validated the combustion kinetics Aramco Mech 1.3 and USC Mech II. • Developed correlation of Pecl as a function of Mab for stable/unstable flame regime. Laminar flame characteristics of ethane/hydrogen/air (with X H 2 = 25 %, 50 %, 75 % and 100 % by volume) within an equivalence ratio from 0.7 to 1.3 were determined in a spherical constant volume combustion vessel at elevated pressure up to 0.5 MPa, temperature up to 360 K. Key combustion characteristics such laminar burning velocity, Markstein length/number, flame thickness, effective Lewis number, thermal expansion coefficient and critical conditions at the onset of cellular instability including critical radius and Peclet number, were either measured or calculated. These measurements were compared with the available literature data and with the predictions from chemical kinetics mechanisms, showing perfect fitting with the literature data, although the kinetics were overpredicted at a temperature of 360 K. The dependence of the ethane/hydrogen/air laminar burning velocities on temperature, pressure, and hydrogen ratio was analyzed using a datum empirical expression and blending law, yielding excellent agreement. A general correlation, based on the Peclet number against the Markstein number, has been proposed for various mixtures and conditions, aimed at defining the stable and unstable regimes of flame propagation. This correlation exhibits an R 2 value of 0.82, signifying a strong predictive capacity. The findings highlight that, compared with methane, ethane plays a promising role in enhancing the resistance of hydrogen mixtures to cellular flame instability, thereby fostering more stable flame propagation. [ABSTRACT FROM AUTHOR]

The extreme explosiveness and high flame velocity of hydrogen challenge its application. Overcoming these challenges requires improving the fundamental flame characteristics of H 2 /O 2 mixtures. In this study, the propagation characteristics of H 2 /O 2 flames are investigated. The laminar burning velocity (LBV) is evaluated using nonlinear extrapolation. The empirical relations of LBV are given with the equivalence ratio (ER) and initial mixture pressure (IMP). The LBV increases first and then decreases as the ER increases and reaches its maximum value at the ER slightly higher than 1.0 (φ = 1.1–1.2). The LBV increases monotonically with increasing IMP. The critical instability radius and Markstein length increase as the ER increases, while decreasing with the IMP increase. The flame thickness decreases significantly with increasing IMP. The flame remains stable and smooth throughout the propagation process for all examined ERs only at the lower IMPs of 0.1 atm and 0.3 atm. • The LBV is evaluated by using the nonlinear extrapolation method owning to the high stretch effect of H 2 /O 2 flame. • The LBV increases monotonically with the increase in initial pressure at the experimental pressure range. • The flame instability is induced only by the hydrodynamic mechanism for mixtures in rich-hydrogen conditions. • Only at the lower pressures of 0.1 atm and 0.3 atm does the flame remain stable at all examined equivalence ratios. [ABSTRACT FROM AUTHOR]

The effect of H2/CO addition on flame dynamics of propane-air mixtures in a stepped microcombustor with heat recirculation was investigated experimentally. Unstable flame propagation regimes and their transition modes were observed for a range of mixture velocities (5–8 m/s) and equivalence ratios (0.6–0.95). Up to 60% H2 addition to lean propane-air mixtures helps improve the flame stability range in the combustor. The suppression of these flame instabilities with H2 addition is attributed to the increase in the overall mixture reactivity and burning velocity. The enhancement in OH intensity with H2 addition indicated a change in the elementary reactions, thereby affecting the flame dynamics. Contrary to H2 addition, CO addition does not show any noticeable effect on the formation of stable flame modes. The change in mixture properties and burning velocity leads to a change in the flame behavior and alters the heat release rate due to H2 addition to propane-air mixtures. A higher percentage of H2 addition led to the reappearance of the unstable (rotating) flames in the microcombustor. Intrinsic flame instabilities appeared at higher H2 percentages due to differential diffusion and increased thermal-wall coupling effects. [ABSTRACT FROM AUTHOR]

Biogas is a renewable fuel predominantly composed of carbon dioxide (CO2) and methane (CH4) in varying proportions. The effects of the varying CO2 proportion need to be clarified for the development of engines. The laminar burning velocity and the burned gas Markstein length of premixed CH4/CO2/Air were measured with CO2 concentration ranging from 0.3 to 0.7 dilution ratios. The equivalence ratio was varied from 0.8 to 1.2, the initial pressure was set at 0.5 MPa, and the temperature was set to 298 K. The experiment was performed using a high-pressure constant volume combustion chamber. One-dimensional simulation of the flames was conducted using GRI-Mech 3.0. The results showed a reduction in the laminar burning velocity of CH4/CO2/Air mixtures with an increase in CO2 dilution ratio. A non-monotonic relationship was discovered between measured Markstein length and CO2 dilution ratio with different equivalence ratios. It was found that an increase in the CO2 dilution increased the response of the flames to stretch. For the lean and stoichiometric flames, the Markstein length was nearly constant with CO2 dilution of 0–0.5 and decreased with CO2 dilution of 0.7, suggesting an increase in susceptibility of the flame to the intrinsic flame instability. This was found to be mainly due to an increase in the Zel'dovich number and a decrease in the effective Lewis number with CO2 dilution. The Markstein length of the rich flame increased with CO2 dilution as it was more sensitive to CO2 dilution. Thermo-diffusive effects and pure stretch effects had similar influences on the burning velocity of the rich flames with an increase in stretch rate. [ABSTRACT FROM AUTHOR]

The paper studies experimentally the stability of an inverted conical plane-symmetrical premixed methane-air flame under normal and reversed gravity. The conical flame is stabilized by a thin transverse rod. Flow velocity is varied within the range of 1–8 m/s, fuel equivalence ratio— within the range of 0.8–1.4. It is shown that such a flame could be both V-shaped (attached only to the stabilization rod) and M-shaped (attached both to nozzle edge and stabilization rod) depending on the set of conditions. The transition between two modes is studied experimentally under normal and reversed gravity. The hysteresis properties for the M–V and V–M transitions under the normal gravity conditions and their absence under the reverse gravity ones are reported. The most unstable flames are observed under reversed gravity at the maximum burning velocity (φ ≈ 1.1 ±). For such conditions, periodical oscillations between M-shaped and V-shaped flames occur over a wide range of velocities. In the experiments under reverse gravity, the V shape prevails over the M shape. It is found that a reverse flow exists above the stabilizer at any velocity under normal gravity and at high velocities (>5 m/s) under reverse gravity. In both cases, a linear increase in the longitudinal size of the vortex zone with increasing velocity is observed. It is concluded that gravity noticeably contributes to rich flames stability. [ABSTRACT FROM AUTHOR]

In response to the global warming crisis, NH 3 /H 2 blended fuel has aroused great interest as a promising energy carrier. Consequently, this study aims to construct a laminar burning velocity (LBV) correlation and perform a comprehensive analysis of combustion characteristics for NH 3 /H 2 /air flames with elevated initial pressure and temperature. For a = 0–0.4 (H 2 mole fraction), the model (S L = e x p [ ∑ i = 1 2 ( a i ⋅ Δ H i ∑ i = 1 2 (a i ⋅ Δ H i) ⋅ l n S L , i ) ]) based on the energy fraction mixing rule has sufficient accuracy in predicting the LBVs of NH 3 /H 2 /air flames at various work conditions. For a = 0.4–1, the model S L = e x p [ ∑ i = 1 2 (a i ⋅ l n S L , i ) ] has a better performance. For NH 3 /H 2 /air flames, branching reactions (R1: H + O 2 <=> O + OH and R68: NH 2 + NH <=> N 2 H 2 + H) are the main reactions that promote LBVs, while the recombination reactions (R9: H + O 2 (+M) <=> HO 2 (+M) and R67: NH 2 + NH = N 2 H 3) are the main reactions that suppress LBVs. Raising the initial pressure and temperature can accelerate the reaction rates of NH 3 /H 2 /air flames. Furthermore, it is discovered that the decrease in LBVs of NH 3 /H 2 /air flames with elevated initial pressure is mainly due to the thermal diffusivity greatly decrease. The increase in LBVs of NH 3 /H 2 /air flames with elevated initial temperature is the result of the increase both in reaction rate and thermal diffusivity. The overall reaction orders of NH 3 /H 2 /air flames decrease with rising initial pressure and H 2 mole fraction. Finally, it is discovered that the hydrodynamic instabilities of NH 3 /H 2 /air flames obviously increase with elevated initial pressure, and decrease with elevated initial temperature. The diffusional-thermal instability slightly increases with elevated initial temperature, and the initial pressure has little influence on it. [Display omitted] • Constructed a laminar burning velocity correlation of NH 3 /H 2 /air flames. • Investigated the effect of initial environment on NH 3 /H 2 burning characteristics. • Analyzed the reasons that initial environment affect reaction rates and LBVs. • Studied the overall reaction order and flame instabilities of NH 3 /H 2 /air flames. [ABSTRACT FROM AUTHOR]

For flames with very low burning speed, the flame propagation is affected by buoyancy. Flame front evolution and laminar flame parameter evaluation methods of buoyancy-affected flame have been proposed. The evolution and propagation process of a center ignited expanding ammonia/air flame has been analyzed by using the methods. The laminar flame parameters of ammonia/air mixture under different equivalence ratio (ER) and initial pressure have been studied. At barometric pressure, with the increase of ER, the laminar burning velocity (LBV) of ammonia/air mixture undergoes a first increase and then decrease process and reaches its maximum value of 7.17 cm/s at the ER of 1.1, while the Markstein length increases monotonously. For ammonia/air flames with ER less than unity, the flame velocity shows a decreasing trend with stretch rate, resulting in the propensity to flame instability, but no cellular structure was observed in the process of flame propagation. As the initial pressure increases, the LBV decreases monotonously as well as the Markstein length. The flame thicknesses of ammonia/air mixtures decrease with initial pressure and are much thicker than those of hydrogen flames, which makes a stronger stabilizing effect of curvature on the flame front. The most enhancement of LBV is contributed by the dehydrogenation reaction of NH 3 with OH. The NO concentration decreases significantly with the increase of ER. • Flame front evolution and laminar flame parameter evaluation methods of buoyance affected flame have been proposed. • Evolution and propagation process of a center ignited expanding ammonia/air flame has been analyzed. • Laminar burning velocity of ammonia/air mixture under different equivalence ratio and initial pressure has been obtained. • Markstein length and flame thickness decrease with the increase in initial pressure. • NO concentration decreases significantly with the increase of equivalence ratio. [ABSTRACT FROM AUTHOR]

The present work reports thermo-acoustic instability in a mesoscale tube of diameter 8 mm (> quenching diameter) and length 702 mm. Mixtures with Lewis number, Le~ 0.8 (rich C 2 H 4 /O 2 /CO 2), 1.05 (lean C 2 H 4 /O 2 /CO 2) and 1.34 (lean C 2 H 4 /O 2 /N 2) are used. Several new flame responses are observed. For lower burning velocity mixtures, flame extinction is observed due to heat loss when primary instability transforms to secondary instability for all Le mixtures. However, parametric cellular structures which are characteristic of parametric instability are observed only for Le of 0.8. It is proposed based on calculations and experiments that parametric structures will be observed only when diameter of tube is two times larger than the characteristic wavelength of parametric instability. If the tube diameter doesn't allow formation of parametric structure whirling and counter-rotating flames are observed instead of parametric structures. Suppression of acoustic parametric instability is observed for higher burning velocity mixtures with Le >1 and its mechanism is discussed. For a range of S L, a beating instability is observed for CO 2 diluted mixtures of Le >1, where pressure oscillation and flame motion show beating oscillations of frequency around 15 Hz. This beating instability is believed to be caused by non-linear interaction of acoustic instability with pulsating instability of flame front which is caused due to combined radiative and convective heat loss. Due to CO 2 dilution, the radiative heat losses could play a significant role in inducing pulsating instability. [ABSTRACT FROM AUTHOR]

Turbulent combustion is usually operated at elevated pressures for practical applications to achieve high power density. There will be more wrinkles on the turbulent flame surface with pressure rising, resulting in a higher turbulent burning velocity. Wrinkling process of the turbulent flame at elevated pressure is still not well understood. One reason is that the large and small-scale wrinkles can not be distinguished because of their multi-scale nature. In this paper, the Proper Orthogonal Decomposition (POD) scale analysis is used to describe the multi-scale nature of turbulent premixed flames at elevated pressure. Turbulent premixed CH4/air flames are conducted in a high-pressure combustion chamber under 0.1 to 0.3 MPa. Turbulence is generated and controlled by multi-grid perforated plates. Turbulent flame front is detected by OH-PLIF technique. Results show that the POD scale analysis can decompose the multi-scale of turbulent flame surface to different modes. Reconstruction of turbulent flame front shows that there is an inverse relationship between POD modes and physical length of scales. Small mode number represents large scale of flame wrinkles, while large mode number stands for small-scale wrinkles. The large and small scale of the flame wrinkles both increase with pressure rising, leading to higher flame surface density and turbulent burning velocity. This indicates that the increased turbulent flame wrinkling at elevated pressure is not only caused by flame instability, which mainly acts on the small wrinkles. Spectral analysis of the turbulent premixed flame shows that the decreased flame thickness, which enlarges the turbulence–flame interaction range, and the decreased laminar burning velocity, which extends the vortex–flame interaction time, are two dominant factors promoting the flame wrinkling at elevated pressure. Turbulent burning velocity correlation shows a linear relationship with the analytical formulation when it takes laminar burning velocity and flame thickness into account. [ABSTRACT FROM AUTHOR]

• Laminar flame velocities of n-dodecane and ammonia mixtures were experimentally determined. • The presence of ammonia in the gas mixture diminishes the laminar flame velocity. • The proposed coupling mechanism accurately predicts the laminar flame velocity development. • The presence of ammonia in the gas mixture results in an increase in the thickness of the laminar flame. • The application of coupling mechanism in CFD serves as a valuable reference for the advancement of novel compound combustion engines. Laminar burning velocity is a crucial characterization parameter in fuel combustion progress. Investigating the combustion properties is significant to improve combustion efficiency. This paper employs a constant volume combustion bomb and high-speed ripple shadow camera technology to explore the influence of equivalence ratios (0.8–1.4) and ammonia energy shares (0%–40%) on the laminar burning velocity of n-dodecane/ammonia mixture at an ambient temperature of 400 K and a pressure of 1 bar. The results illuminated that the unstretched laminar burning velocity of n-dodecane increased from 46 cm/s at ϕ = 0.8 to 61 cm/s at ϕ = 1 and then decreased to 32 cm/s at ϕ = 1.4. When the ammonia energy ratio reaches 40%, the maximum laminar burning velocity experiences a reduction of 44% because the blended ammonia inhibited the formation of H and O radicals based on kinetic analysis in the fast reaction zone. With the increase of ammonia ratio, the Markstein length of mixed burned gas gradually increases. Particularly under a 1.4 equivalent ratio condition, the Markstein length increases from 0.2 mm to 0.8 mm, indicating that ammonia components significantly enhance flame stability. To gain a deeper understanding of the influence mechanisms of laminar burning velocity, a simplified Yao-Otomo (Y-O) model with 92 species and 355 reactions is proposed, coupling the mechanisms of n-dodecane and ammonia. The Y-O model has demonstrated a predictive trend that is consistent with more detailed WUT-NH 3 model (176 species and 2839 reactions), and the Y-O model reduces the amount of computation to save calculation time, which has a higher engineering applicability. The turbulent kinetic energy (TKE) induced by premixed fuel in engine cylinders is estimated through comprehensively analyzing the laminar burning velocity and flame thickness characteristics. The findings indicate a 45% reduction in the peak value of TKE as the ammonia energy ratio increases. [ABSTRACT FROM AUTHOR]

• Combustion characteristics of NH 3 /H 2 is investigated experimentally and numerically. • Effects of temperature, pressure, equivalence ratio, and composition are evaluated. • Adding diluents enhances flame stability by decreasing hydrodynamic instabilities. • Exhaust gases negatively affect LBV, prominently at non-stoichiometric conditions. • EGR diminishes the NO emissions efficiently, up to 84% at fuel-rich condition. This paper investigates the potential of exhaust gas recirculation in hydrogen/ammonia mixtures. The effects of artificial exhaust gas recirculation were evaluated on flame morphology and unstretched laminar burning velocities experimentally, while the flame stability and NO emissions were simulated using previous kinetic models. The experiments and numerical simulations were performed for various equivalence ratios, initial temperatures, pressures, exhaust gas recirculation rates, and compositions. Results show that recirculation of exhaust gases can enhance flame stability by diminishing the hydrodynamic instabilities while negatively affecting the flame laminar burning velocities. This negative impact, which is more sensible at non-stoichiometric conditions, reduces the flame speed between 19% and 58% based on the flue gas content of the combustible mixture. Despite this reduction, recirculating the exhaust gases can considerably reduce the high NO emissions of hydrogen/ammonia/air mixtures. Indeed, adding flue gases can decrease the maximum NO concentrations up to 45% at near stoichiometric conditions. The effects are even more significant in fuel-rich mode, diminishing the NO emissions up to 84% at an equivalence ratio of 1.3. Finally, these evaluations support the idea of exhaust gas recirculation application for using hydrogen/ammonia blends as a clean fuel while considering the expenses of burning rate reduction. [ABSTRACT FROM AUTHOR]

This paper presents results of experimental investigations on spherical and cylindrical flame propagation in pre-mixed H 2 /air-mixtures in unconfined and semi-confined geometries. The experiments were performed in a facility consisting of two transparent solid walls with 1 m2 area and four weak side walls made from thin plastic film. The gap size between the solid walls was varied stepwise from thin layer geometry (6 mm) to cube geometry (1 m). A wide range of H 2 /air-mixtures with volumetric hydrogen concentrations from 10% to 45% H 2 was ignited between the transparent solid walls. The propagating flame front and its structure was observed with a large scale high speed shadow system. Results of spherical and cylindrical flame propagation up to a radius of 0.5 m were analyzed. The presented spherical burning velocity model is used to discuss the self-acceleration phenomena in unconfined and unobstructed pre-mixed H 2 /air flames. • We study spherical and cylindrical H 2 /air flames in un- and semi-confined geometries. • A large-scale shadowgraphy for high-speed applications used to measure visible flame. • Mixture specific flame acceleration observed for spherical flame propagation. • A spherical burning velocity S (sph) for undisturbed flame propagation was derived. • The model is in agreement with experimental data for spherical flame propagation. [ABSTRACT FROM AUTHOR]

Co-firing NH 3 with H 2 /CO/syngas (SYN) is a promising method to overcome the low reactivity of NH 3 /air flame. Hence, this study aims to systematically investigate the laminar premixed combustion characteristics of NH 3 /air flame with various H 2 /CO/SYN addition loadings (0–40%) using chemical kinetics simulation. The numerical results were obtained based on the Han mechanism which can provide accurate predictions of laminar burning velocities. Results showed that H 2 has the greatest effects on increasing laminar burning velocities and net heat release rates of NH 3 /air flame, followed by SYN and CO. CO has the most significant effects on improving NH 3 /air adiabatic flame temperatures. The H 2 /CO/SYN additions can accelerate NH 3 decomposition rates and promote the generation of H and NH 2 radicals. Furthermore, there is an evident positive linear correlation between the laminar burning velocities and the peak mole fraction of H + NH 2 radicals. The reaction NH 2 + NH <=> N 2 H 2 + H and NH 2 + NO <=> NNH + OH have remarkable positive effects on NH 3 combustion. The mole fraction of OH × NH 2 radicals positively affects the net heat release rates. Finally, it was discovered that H radicals play an important role in the generation of NO. The H 2 /CO/SYN additions can reduce the hydrodynamic and diffusional-thermal instabilities of NH 3 /air flame. The NH 3 reaction pathways for NH 3 –H 2 /CO/SYN-air flames can be categorized mainly into NH 3 –NH 2 –NH–N–N 2 , NH 3 –NH 2 –HNO–NO(−N 2 O)–N 2 and NH 3 –NH 2 (−N 2 H 2)–NNH–N 2. CO has the greatest influence on the proportions of three NH 3 reaction routes. [Display omitted] • Studied the effects of H 2 /CO/SYN additions on NH 3 combustion characteristics. • H 2 has the largest effects on improving NH 3 combustion intensity. • The mole fraction of OH × NH 2 positively affects the net heat release rates. • H radicals play an important role in the generation of NO. • The H 2 /CO/SYN additions can reduce the flame instabilities of NH 3 /air flame. [ABSTRACT FROM AUTHOR]