The explosion thermal behavior of H2/CH4/air mixtures in a closed 20 L vessel

In this work, the explosion thermal behavior of H₂/CH₄/air mixtures, at different equivalence ratios (0.6–1.6) and hydrogen volume fractions (0%–100), was investigated in a confined 20-L chamber. The parameters of explosion time and pressure, as well as the explosion heat loss were quantitatively studied and analyzed. Moreover, the dominant chain reactions of the explosion process and heat release were identified via the detailed mechanism of the Foundational Fuel Chemistry Model (FFCM1). The results indicated that an increased H₂ volume fraction in the mixtures increased the peak explosion pressure, maximum pressure rise rate and deflagration index. In addition, the explosion duration and fast-burning period were greatly shortened. Both the adiabatic flame temperature and thermal diffusivity monotonically increased with increasing H₂ volume ratio. Moreover, the enhancement effect of the H₂ ratio on the thermal diffusivity of H₂/CH₄ mixtures was more prominent for fuel-rich mixtures than for fuel-lean mixtures. The obtained quantitative results are helpful for developing measures to prevent the potential explosion accidents.     

Performance characteristics of methanol and kerosene fuelled meso-scale heat-recirculating combustors

A meso-scale heat recirculating combustor has been developed for the combustion of methanol and kerosene fuels with oxygen enriched superheated steam as an oxidizer. The steam oxygen mixture is a surrogate for the decomposition products of hydrogen peroxide, and as such the combustor development is toward meso-scale bi-propellant propulsion. Both the extinction behavior and thermal performances have been examined under partially-premixed and non-premixed configurations of a unique design incorporating heat recirculation. Stable combustion with thermal efficiencies of ∼90% has been demonstrated with both methanol and kerosene. Global flame behavior is investigated through direct image photography of the flame that revealed different flame modes at various equivalence ratios (Φ), including “flameless” combustion of kerosene. Density impulse values calculated based on exhaust temperatures and simulated equilibrium gas properties and assuming 1 atm chamber pressure and expansion to vacuum show that the maximum density impulse of kerosene/steam/oxygen combustion to be within 6% of the adiabatic density impulse of hydrazine/nitrogen tetroxide.     

Inerting effect of carbon dioxide on confined hydrogen explosion

Taking maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse as index, this paper is aimed at evaluating the inerting effects of carbon dioxide on confined hydrogen explosion by varying initial pressure, carbon dioxide addition and equivalence ratio. The results indicated that under enhancing hydrodynamic instability, the stronger flame destabilization occurs with the increase of initial pressure. At Φ = 0.8 and Φ = 1.0, the destabilization effect of thermodiffusive instability continues to increase with the increase of carbon dioxide addition. At all equivalence ratios, the destabilization effect of hydrodynamic instability decreases monotonously with the increase of carbon dioxide addition. All of maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse reach the peak value at Φ = 1.5, and decrease significantly with increasing carbon dioxide addition. The inerting effect of carbon dioxide could be attributed to the reduction of thermal diffusivity, flame temperature and active radicals. The chemical effect of carbon dioxide reaches the peak value at Φ = 1.0. With the increase of carbon dioxide addition, the chemical effect continues to decrease at Φ = 0.8 and Φ = 1.0, and increase monotonously at Φ = 2.5.     

氮气稀释气对天然气燃烧特性的影响

为获得氮气稀释气对天然气燃烧特性的影响规律,在定容燃烧反应器中对不同当量比与初始压力下天然气的火焰传播特性、燃烧稳定性及燃烧特性进行了试验测试,并分析了氮气稀释度对天然气火焰传播特性、燃烧稳定性及燃烧特性的影响规律。研究结果表明:随着初始压力与氮气稀释度的升高,火焰前锋面将出现细小裂纹,火核逐渐向定容燃烧反应器上部漂移,火焰稳定性变差;随着初始压力的提高,马克斯坦长度明显变短,火焰稳定性变差,无拉伸火焰传播速度与层流燃烧速度明显降低,但最大燃烧压力显著升高。随着当量比的提高,层流燃烧速度与最大燃烧压力出现先增加后降低的趋势,两者的最大值出现在当量比为1.0时。马克斯坦长度随氮气稀释度的增加逐渐变短,表明火焰逐渐趋于不稳定;同时,无拉伸火焰传播速度、层流燃烧速度与最大燃烧压力随氮气稀释度的增加显著降低。     

Effect of hydrogen on explosion characteristics of liquefied petroleum gas-air mixtures

The effects of hydrogen addition on the explosion characteristics of liquefied petroleum gas (LPG)-air mixtures were experimentally investigated under initial conditions of 1 atm and 298 K. Furthermore, with reference to the detailed USC-Mech II mechanism, sensitivity and the rate of production (ROP) analyses were conducted. When the hydrogen proportion is constant, the maximum explosion pressure and the maximum rate of pressure rise first increase and then decrease, reaching peaks at an equivalence ratio of 1.3. The experimentally measured maximum explosion pressure is lower than the numerically calculated adiabatic pressure. The calculated adiabatic pressure decreases slightly with an increase in the hydrogen proportion. However, under the experimental conditions, owing to the high reactivity of hydrogen, the LPG-H₂-air mixtures experience a small heat loss in the early stage of the explosion, and the maximum explosion pressure and maximum rate of pressure increase considerably, with the arrival time of the pressure peak is advanced. The addition of hydrogen promotes the sensitivity coefficient of reactants C₃H₈ and C₄H₁₀ and increases the maximum ROP of free radicals H, O, and OH. Meanwhile, the addition of hydrogen significantly influences the maximum ROP of the elementary reaction R2.     

Experimental study on explosion characteristics of hydrogen–propane mixtures

The explosion characteristics of a hydrogen–propane mixture under different initial temperatures and at different hydrogen addition, equivalence, and dilution ratios were studied using a 20-L apparatus. First, the effects of hydrogen addition and equivalence ratio were studied. The results showed that the maximum explosion pressure and the maximum rate of pressure rise first increased, then decreased from lean to an equivalence ratio of 1.6, and reached the maximum value at the equivalence ratio of 1.2. The maximum explosion pressure and the maximum rate of pressure rise increased with the increasing hydrogen proportion of the mixtures, and the time to the maximum explosion pressure decreased. Furthermore, the mixed fuel with the equivalence and hydrogen addition ratios of 1.0 and 0.4, respectively, was explored under nitrogen or carbon dioxide dilution. Carbon dioxide exhibited a stronger dilution effect than nitrogen. For the mixed fuel with the equivalence and hydrogen addition ratios of 1.0 and 0.4, respectively, the maximum explosion pressure saliently decreased with the increasing initial temperature in the range of 25 °C-120 °C. The variation trend of experimental results was consistent with theoretical predictions.     

天然气/氢气燃烧特性研究

在定容燃烧弹中研究了不同氢气掺混比例、燃空当量比和初始压力下的大然气／氢气混合气的燃烧特性,建立了适合用于容弹计算的准维双区模型。研究结果表明：在各种当量比和初始压力下,随着掺氢比例的增加,混合气的质量燃烧速率明显增加,燃烧持续期和火焰发展期娃著缩短。随着掺氢比例的增加,短的燃烧持续期所对应的当量比范围变宽,稀混合气和浓混合气条件下天然气掺氢对火焰发展期缩短的效果更明显。化学计量比附近（1．0—1．1）掺氢燃烧对燃烧最大压力值影响不大,浓混合气（燃空当量比大于1．1）和稀混合气燃烧时,随着掺氢比例的增加,最大燃烧压力值增加。     

Explosion characteristics of hydrogen–nitrogen–air mixtures at elevated pressures and temperatures

An experimental study on the combustion characteristics of nitrogen diluted hydrogen was conducted in a constant volume combustion vessel over a wide range of equivalence ratios and dilution ratios at elevated pressures and temperatures. The explosion characteristics such as the explosion pressure, the combustion duration, the maximum rate of pressure rise, the deflagration index and the normalized mass burning rate were derived. The result shows that a short combustion duration and higher normalized mass burning rate were presented with the increase of equivalence ratio. With the increase of initial temperature, the explosion pressure, the maximum rate of pressure rise and the deflagration index were decreased, and a shorter combustion duration and higher normalized mass burning rate were presented. With the increase of initial pressure, the explosion pressure, the maximum rate of pressure rise and the deflagration index increase, a shorter combustion duration and higher normalized mass burning rate were presented. Nitrogen dilution significantly reduces the normalized mass burning rate and the deflagration index and thus the potential of explosion hazards.     

Dynamic couplings of hydrogen/air flame morphology and explosion pressure evolution in the spherical chamber

This paper aims at exploring the dynamic couplings of flame morphology and explosion pressure evolution experimentally and theoretically. In the experiment, flame morphology and explosion pressure evolution under diffusional-thermal and hydrodynamic instability are recorded using high-speed schlieren photography and pressure transducer. In the theoretical calculation, the effects of cellular flame on the explosion pressure evolution are conducted using smooth flame, D = 2.0566, 2.1 and 7/3. The results demonstrate that the cellular flame formation of various equivalence ratios could be attributed to the fact Lewis number is less than unity on the lean side. The flame destabilization of Φ = 0.8 and 3.0 with increasing initial pressure is due to the decreasing flame thickness regardless of unchangeable thermal expansion ratio. Much smaller cells formation on the cellular flame surface as the explosion pressure rises could be attributed to the joint effect of the diffusional-thermal and hydrodynamic instability. Note that the explosion pressure evolution in spherical chamber is obviously underestimated assuming the flame surface is smooth during the hydrogen/air explosion. But the explosion overpressure is overpredicted significantly with D = 7/3. The theoretical overpressure with D = 2.1 is in satisfactory agreement with experimental results.     

Instabilities and soot formation in high-pressure, rich, iso-octane-air explosion flames: 1. Dynamical structure

Simultaneous OH planar laser-induced fluorescence (PLIF) and Rayleigh scattering measurements have been performed on 2-bar rich iso-octane-air explosion flames obtained in the optically accessible Leeds combustion bomb. Separate shadowgraph high-speed video images have been obtained from explosion flames under similar mixture conditions. Shadowgraph images, quantitative Rayleigh images, and normalized OH concentration images have been presented for a selection of these explosion flames. Normalized experimental equilibrium OH concentrations behind the flame fronts have been compared with normalized computed equilibrium OH concentrations as a function of equivalence ratio. The ratio of superequilibrium OH concentration in the flame front to equilibrium OH concentration behind the flame front reveals the response of the flame to the thermal-diffusive instability and the resistance of the flame front to rich quenching. Burned gas temperatures have been determined from the Rayleigh scattering images in the range 1.4???1.9 and are found to be in good agreement with the corresponding predicted adiabatic flame temperatures. Soot formation was observed to occur behind deep cusps associated with large-wavelength cracks occurring in the flame front for equivalence ratio ??1.8 (C/O?0.576). The reaction time-scale for iso-octane pyrolysis to soot formation has been estimated to be approximately 7.5-10 ms.     

Explosion characteristics of n-decane/hydrogen/air mixtures

Hydrogen can be used in conjunction with aviation kerosene in aircraft engines. To this end, this study uses n-decane/hydrogen mixtures to investigate the explosion characteristics of aviation kerosene/hydrogen in a constant volume combustion chamber with different hydrogen addition ratios (0, 0.2, 0.4), wide effective equivalence ratios (0.7–1.7), an initial temperature of 470 K, and initial pressures of 1 and 2 bar. The results show that the explosion pressure and explosion time decrease linearly with increasing hydrogen addition ratio. The effect of initial pressure is also discussed. A comparison of the adiabatic explosion pressures indicates that the hydrogen addition effect varies at different initial pressures and effective equivalence ratios owing to heat loss. In addition, the maximum pressure rise rate and deflagration index increase with increasing hydrogen concentration, which is more obvious for rich mixtures and high hydrogen concentrations.     

Effect of hydrogen addition on the laminar premixed combustion characteristics the main components of natural gas

With the increasing use of natural gas, improving the thermal efficiency and reducing emissions has become the major goals in its combustion. The objective of the present work is to investigate the effect of hydrogen addition on the combustion characteristics of natural gas. A ChemkinⅡ/Premix Code with the detailed chemical reaction mechanisms was employed with the Soret effect taken into account in all the calculations. With the mole fraction of hydrogen in the fuel varied from 0 to 40% at different initial temperatures (298–500 K) and pressures (1–8 atm), the results showed that the laminar burning velocities (LBVs) and the adiabatic flame temperatures of the C₁C₄ four alkanes increased with increasing hydrogen-doping ratio. The LBV and the adiabatic flame temperature of methane displayed the maximum increase with the hydrogen-doping ratio. Additionally, the generation of active radicals H, O, and OH during the combustion process was strongly correlated with the LBV. The sensitivity of the flame temperature in four alkane fuels present in the natural gas at the maximum temperature gradient was analyzed. At a constant hydrogen-doping ratio, the LBV and the adiabatic flame temperature increased significantly with the increasing initial temperature. With increasing the pressure, the LBV gradually decreased while the adiabatic flame temperature increased.     

Experimental and numerical investigation of explosive behavior of syngas/air mixtures

In this study, the explosive behavior of syngas/air mixtures was investigated numerically in a 3-D cylindrical geometric model, using ANSYS Fluent. A chamber with the same dimensions as the geometry in the simulation was used to investigate the explosion process experimentally. The outcome was in good agreement with experimental results for most equivalence ratios at atmospheric pressure, while discrepancies were observed for very rich mixtures (? > 2.0) and at elevated pressure conditions. Both the experimental and simulated results showed that for syngas/air mixture, the maximum explosion pressure increased from lean (? = 0.8) to an equivalence ratio of 1.2, then decreased significantly with richer mixtures, indicating that maximum explosion pressure occurred at the equivalence ratio of 1.2, while explosion time was shortest at an equivalence ratio of 1.6. Increasing H₂ content in the fuel blends significantly raised laminar burning velocity and shortened the explosion time, thereby increasing the maximum rate of pressure rise and deflagration index. Normalized peak pressure, the maximum rate of pressure rise and the deflagration index were sensitive to the initial pressure of the mixture, showing that they increased significantly with increased initial pressure.     

Effect of initial pressure,temperature and equivalence ratios on laminar combustion characteristics of hydrogen enriched natural gas

In this study, the flame propagation characteristics of premixed natural gas–hydrogen–air mixtures were studied in constant volume combustion bomb by using the high-speed schlieren photography system. The flame radius, laminar flame propagation speed and the flame stretch rate were obtained under different initial pressure, temperature, equivalence ratios and hydrogen fractions. Meanwhile, the flame stability and their influencing factors were obtained by analyzing the Markstein length and the flame propagation schlieren photos under various combustion conditions. The results show that the stretched laminar propagation speed increases with the increase of the initial temperature and hydrogen fraction of the mixture, and will decreases with the increase of the initial pressure. Meanwhile, according to the Markstein length and the flame propagation pictures, the flame stability decreases with the increase of the temperature and hydrogen fraction, and the slight flaws occurred at the early stage; at larger flame radius, the flame stability is more sensitive to the variation of the initial temperature and hydrogen fraction than to that of initial pressure and equivalence ratio.     

Effect of nitrogen on deflagration characteristics of hydrogen / methane mixture

In order to study the influence of nitrogen on the deflagration characteristics of premixed hydrogen/methane, the explosion parameters of premixed hydrogen/methane within various volume ratios and different dilution ratios were studied by using a spherical flame method at room temperature and pressure. The results are as follows: The addition of nitrogen makes the upper limit of explosion of hydrogen/methane premixed gas drop, and the lower limit rises. For explosion hazard (F-number), hydrogen/methane premixed fuel with a hydrogen addition ratio of 10% has the lowest risk, and nitrogen has a greater impact on the dangerous degree of hydrogen and methane premixed gas whose hydrogen addition ratio does not exceed 30%. In terms of flame structure, the spherical flame was affected by buoyancy instability as the percentage of nitrogen dilution increased, but the buoyancy instability gradually decreased as the percentage of hydrogen addition increased. The addition of diluent gas reduces the spreading speed of the stretching flame and reduces the stretching rate in the initial stage of flame development. The laminar flame propagation velocity calculated by the experiment in this paper is consistent with the laminar flow velocity of the hydrogen/methane premixed gas calculated by GRI Mech 3.0. Considering the explosion parameters such as flammability limit, laminar combustion rate and deflagration index, when hydrogen is added to 70%, it is the turning point of hydrogen/methane premixed fuel.     

Combustion characteristics of a closed cycle diesel engine with different intake gas contents

This paper is to carry out a numerical simulation of combustion characteristics for a closed cycle diesel engine (CCDE) with different intake gas contents under different engine speeds and equivalence ratios. The numerical simulation used Kiva3V-Release2 code as the main program by modifying some subroutines containing different intake gas contents of oxygen, argon, and nitrogen and their thermodynamic properties. The results show that the pressure will increase earlier if the percentage of argon is higher when we use argon to replace nitrogen, and the ignition delay will be shorter, too. The higher in-cylinder temperature results from a higher concentration of argon.     

On a porous medium combustor for hydrogen flame stabilization and operation

In this study, the characteristics of hydrogen flame stabilization in porous medium combustor were investigated. The flame was observed in a quartz tube. The porous medium was oxide-bonded silicon carbide (OB-SiC) or aluminum oxide (Al₂O₃) with 60 PPI and 30 PPI pore size distributions. The results indicated that under a low equivalence operation, the flame would transform from surface combustion to interior combustion with an increased heating value. Under a high equivalence ratio, both interior combustion and flashback transition existed at the same time. The thermal conductivity of silicon carbide is higher than that of aluminum oxide. Thus, interior combustion region was more extensive under a low equivalence ratio operation with a high premixed gas velocity. Flashback was apparent for Al₂O₃ under high an equivalence ratio with low a premixed gas velocity. Consequently, hydrogen flame stability could be controlled by the pore size distribution and thermal conductivity of the porous media, input heating value and input equivalence ratio.     

Flame characteristics of premixed H2-air mixtures explosion venting in a spherical container through a duct

The explosion venting duct can effectively reduce the hazard degree of a gas explosion and conduct the venting energy to the safe area. To investigate the flame quantitative propagation law of explosion venting with a duct, the effects of hydrogen fraction and explosion venting duct length on jet flame propagation characteristics of premixed H₂-air mixtures were analyzed through experiment and simulation. The experiment results under initial conditions of room temperature and 1 atm show that when hydrogen fraction was high enough, part of the unburned hydrogen was mixed with air again to reach an ignitable concentration, resulting in the secondary combustion was easier produced and the duration of the secondary flame increased. With the increase of venting duct length, the flame front distance and propagation velocity increased. Meanwhile, the spatial distribution of pressure field and temperature field, and the propagation process and mechanism of the flame venting with a duct were analyzed using FLUENT software. The variation of the pressure wave and the pressure reflection oscillation law in the explosion venting duct was captured. Therefore, in the industrial explosion venting design with a duct, the hazard caused by the coupling of venting pressure and venting flame under different fractions should be considered comprehensively.     

Investigating the explosion hazard of hydrogen produced by activated aluminum in a modified Hartmann tube

Metallic powders exposed to water are sources of hydrogen gas that may result in an explosion hazard in the process industries. In this paper, hydrogen production and flame propagation in a modified Hartmann tube were investigated using activated aluminum powder as fuel. A self-sustained reaction of activated aluminum with water was observed at cool water and room temperatures for all treatments. One gram of Al mixed with 5 wt% NaOH or CaO resulted in a rapid rate of hydrogen production and an almost 100% yield of hydrogen generation within 30 min. The flame structures and propagation velocity (FPV) of released hydrogen at different ignition delay times were determined using electric spark ignition. Flame structures of hydrogen were mainly dependent on hydrogen concentration and ignition delay time, likely due to different mechanisms of hydrogen generation and flame propagation. As expected, FPVs of hydrogen in the Hartmann tube increased with ignition delay time. However, the FPV of upward flame propagation was much larger than that of downward flame propagation due to the effect of spreading acceleration at the explosion vent. Once ignited, the FPV of upward flame propagation reached 31.3–162.5 m/s, a value far larger than the 7.5–30 m/s for downward flame propagation. Hydrogen explosion caused by the accumulation of wet metal dust can be far more dangerous than an ordinary hydrogen explosion.     

Explosion venting of rich hydrogen-air mixtures in a small cylindrical vessel with two symmetrical vents

The safety issues related to explosion venting of hydrogen-air mixtures are significant and deserve more detailed investigations. Vented hydrogen-air explosion has been studied extensively in vessels with a single vent. However, little attention has been paid to the cases with more than one vent. In this paper, experiments about explosion venting of rich hydrogen-air mixtures were conducted in a small cylindrical vessel with two symmetrical vents to investigate the effect of vent area and distribution on the pressure buildup and flame behavior. Experimental results show that venting accelerates the flame front towards the vent but has nearly no effect on the opposite side. The maximum internal overpressure decreases while the maximum external flame length increases with the increase of the vent area. Two pressure peaks can be identified outside the vessel, which correspond to the external explosion and the following gas jet, respectively. Compared with the case of single vent, the use of two vents with same total vent area leads to nearly unchanged maximum internal and external overpressure but much smaller external flame length.