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.     

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 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.     

Effect of concentration,obstacles, and ignition location on the explosion overpressure of hydrogen-air in a closed-vessel

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.     

Investigating the explosion of ethyl acetate in the presence of hydrogen

The explosion characteristics of ethyl acetate/hydrogen-air (EA/H₂/air) mixtures have been investigated in this document using a constant volume combustion vessel. The experiments were executed using the initial pressure (1, 2, and 3 bar) and temperature (358 K) over a wide range of equivalence ratios. Hydrogen was mixed with compressed air in the volumetric proportion of 0%, 4%, 8%, and 12% and added to EA. The experimental pressure data were refined to determine the explosion characteristics. The explosion characteristics of EA/H₂/air using different H₂ fractions were compared. The study indicated that there was a slight difference between the peak explosion pressures of 4%, 8%, and 12% H₂ fraction, however, the explosion with H₂ of 12% had the maximum pressure rise rate and deflagration index when the explosion was evaluated in terms of the pressure rise rate and deflagration index. The explosion time at 8% and 12% hydrogen was relatively shorter than that of 4%.     

Laminar burning characteristics of ammonia/hydrogen/air mixtures with laser ignition

Ammonia, as a zero-carbon fuel, is drawing more and more attention. The major challenge of using ammonia as a fuel for the combustion engines lies in its low chemical reactivity, and therefore more fundamental researches on the combustion characteristics of ammonia are required to explore effective ways to burn ammonia in engines. In this study, the laminar burning characteristics of the premixed ammonia/hydrogen/air mixtures are investigated. In the experiment, the laser ignition was used to achieve stable ignition of the ammonia/air mixtures with an equivalence ratio range from 0.7 to 1.4. The propagating flame was recorded with the high-speed shadowgraphy. Three different processing methods were introduced to calculate the laminar burning velocity with a consideration of the flame structure characteristics induced by the laser ignition. The effects of initial pressure (0.1 MPa–0.5 MPa), equivalence ratio (0.7–1.4), hydrogen fraction (0–20%) on the laminar burning velocity were investigated under the initial ambient temperature of 360 K. The state-of-the-art kinetic models were used to calculate the laminar burning velocities in the CHEMKIN-pro software. Both the simulation and experimental results show that the laminar burning velocity of the ammonia mixtures increases at first, reaches the peak around ϕ of 1.1, and then decreases with the equivalence ratio increasing from 0.7 to 1.4. The peak laminar burning velocities of the ammonia mixture are lower than 9 cm/s and are remarkably lower than those of hydrocarbon fuels. The laminar burning velocity of the ammonia mixture decreases with the increase of the initial ambient pressure, and it can be drastically speeded up with the addition of hydrogen. While the models except for those by Miller and Bian can give reasonable predictions compared to the experimental results for the equivalence ratio from 0.7 to 1.1 in the ammonia (80%)/hydrogen (20%)/air mixtures, all the kinetic models overpredict the experiments for the richer mixtures, indicating further work necessary in this respect.     

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.     

Hydrogen peroxide for improving premixed methane–air combustion

In this study, the effects of hydrogen peroxide on laminar, premixed, methane–air flames at atmospheric pressure and temperature were investigated using CHEMKIN III and GRI 3.5 mechanism. The range of fuel/air equivalence ratio (φ) was varied from 0.6 to 1.2, and the amount of hydrogen peroxide was altered from 0% to 20% volumetric fraction of the methane–hydrogen peroxide (air excluded) mixture. The burning velocity was found to increase with increasing hydrogen peroxide addition, with a relatively larger increase for the fuel-richer mixtures (ΔS_u up to 15 cm/s for φ≈1.2). The adiabatic flame temperature rose with hydrogen peroxide addition, and the temperature rise per unit hydrogen peroxide addition was more significantly (ΔT up to 100 K) for the leaner mixtures. For the same mixture stoichiometry, adding hydrogen peroxide also increased CO concentration and NO_x emissions somewhat. Accordingly, the benefits of adding hydrogen peroxide to the combustion conditions considered here can be best realized by burning leaner mixtures.     

Explosion mechanism of aluminum powder mixed with low-concentration hydrogen

This paper examines dust explosion characteristics and mechanisms for aluminum powder (Alp) in four different low-concentration hydrogen environments (0%, 1%, 2%, and 4 vol%). The aluminum powder's combustion characteristics were determined and the minimum ignition temperature and minimum ignition energy were found to be 690 °C and 45 mJ, respectively. A 20-L apparatus was used to evaluate how the explosive characteristics varied under different hydrogen concentrations. The explosion pressure generated by 4 vol% H₂ was ca. 8.3% stronger than an explosion in the air, and the explosion pressure rise rate showed an increase of 48%. In addition, the time to maximum rate of pressure rise was shortened in the mixed Alp/hydrogen experiment. The experimental residues were observed, and from this it was inferred that the explosion mechanism changed when a low concentration of H₂ was present.     

Numerical and experimental study of the influence of CO2 dilution on burning characteristics of syngas/air flame

In this study, effect of carbon dioxide dilution on explosive behavior of syngas/air mixture was investigated numerically and experimentally. Explosion in a 3-D cylindrical geometry model with dimensions identical to the chamber used in the experiment was simulated using ANSYS Fluent. The simulated results showed that after ignition, the flame front propagated outward spherically until it touched the wall, like the propagating flame observed in the experiment. Both experimental and simulated results presented a same trend of decreasing the maximum explosion pressure and prolonging the explosion time with CO₂ dilution. The results showed that for CO₂ additions, the maximum explosion pressure decreased linearly and the explosion time increased linearly, while the maximum rate of pressure rise decreased nonlinearly, which can be correlated to an exponential equation. In addition, both results showed a good agreement for syngas/air flame with CO₂ addition up to 20% in volume. However, larger discrepancies were observed for higher levels of CO₂ dilutions. Of the three diluents tested, carbon dioxide displayed the strongest effect in reducing explosion hazard of syngas/air flame compared to helium and nitrogen. Chemical kinetic analysis results showed that maximum concentration of major radicals and net reaction rates of important reactions drastically decreased with CO₂ addition, causing a reduction of laminar flame speed.     

Laminar burning velocities and flame stability analysis of hydrogen/air premixed flames at low pressure

An experimental and numerical study on laminar burning velocities of hydrogen/air flames was performed at low pressure, room temperature, and different equivalence ratios. Flames were generated using a small contoured slot-type nozzle burner (5 mm × 13.8  mm). Measurements of laminar burning velocity were conducted using the angle method combined with Schlieren photography. Numerical calculations were also conducted using existing detailed reaction mechanisms and transport properties. Additionally, an analysis of the intrinsic flame instabilities of hydrogen/air flames at low pressure was performed. Results show that the behavior of the laminar burning velocity is not regular when decreasing pressure and that it depends on the equivalence ratio range. The behavior of the laminar burning velocity with decreasing pressure can be reasonably predicted using existing reaction mechanisms; however changes in the magnitude of the laminar burning velocity are underestimated. Finally, it has been found experimentally and proved analytically that the intrinsic flame instabilities are reduced when decreasing the pressure at sub-atmospheric conditions.     

Flame front evolution and laminar flame parameter evaluation of buoyancy-affected ammonia/air flames

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 NH₃ with OH. The NO concentration decreases significantly with the increase of ER.     

Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

In this work, the explosion behavior of stoichiometric CH₄/O₂/N₂/CO₂ and H₂/O₂/N₂/CO₂ mixtures has been studied both experimentally and theoretically at different CO₂ contents and oxygen air enrichment factors. Peak pressure, maximum rate of pressure rise and laminar burning velocity were measured from pressure time records of explosions occurring in a closed cylindrical vessel. The laminar burning velocity was also computed through CHEMKIN–PREMIX simulations.     

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.     

Experimental and numerical study of the laminar burning velocity of NH3/H2/air premixed flames at elevated pressure and temperature

Ammonia (NH₃) is a carbon-free fuel that shows great research prospects due to its ideal production and storage systems. The experimental data of the laminar burning velocity of NH₃/H₂/air flame at different hydrogen ratios (X_H2 = 0.1–0.5), equivalent ratios (φ = 0.8–1.3), initial pressures (P = 0.1–0.7 MPa), and initial temperatures (T = 298–493 K) were measured. The laminar burning velocity of the NH₃/H₂/air flame increased upon increasing the hydrogen ratios and temperature, but it decreased upon increasing the pressure. The equivalent ratio of the maximum laminar burning velocity was only affected by the proportion of reactants. The equivalence ratio value of the maximum laminar burning velocity was between 1.1 and 1.2 when X_H2 = 0.3. The chemical reaction kinetics of NH₃/H₂/air flame under four different initial conditions was analyzed. The less NO maximum mole fraction was produced during rich combustion (φ > 1). The results provide a new reference for ammonia as an alternative fuel for internal combustion engines.     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

Effects of hydrogen on combustion characteristics of methane in air

The explosion process of multi-component gas mixture is extremely complex and may cause serious disaster effects. The safety issue concerning explosion of multi-component gas mixture is urgent to be investigated on account of its wide range of applications. In current work, series of experiments were performed in a 20 L spherical explosion vessel at initial conditions of 1 atm and 293 K, involving methane–hydrogen/air mixtures. The proportion of hydrogen in fuels varied from 0% to 100%. It was observed that peak temperature is always behind the peak pressure in arrival time whatever the fuel equivalence is. Experimental values of peak overpressure are lower than adiabatic ones due to heat loss. It was also founded that the hydrogen addition can raise explosion pressure and temperature in experiment but slightly decrease that in adiabatic condition, and both the increase in experiment and the decrease in adiabatic show a linear correlation versus the proportion of hydrogen. Hence the deviation between the experimental results and the adiabatic results decreases as the hydrogen proportion rises. Moreover, the positive effect of hydrogen addition on (dp/dt)_max is very slight at low hydrogen proportion, while the effect becomes much more pronounced at higher hydrogen contents, showing an exponential growth. For each fuel composition throughout all experiments, the peak overpressure, peak temperature and (dp/dt)_max concerning fuel equivalence ratios of 0.6, 1 and 1.5 follow a same rule: Ф = 1 is the highest, followed by Ф = 1.5 and Ф = 0.6. Finally, the MIEs of gaseous methane–hydrogen/air mixtures at a fuel equivalence ratio of 1.5 were measured as a function of hydrogen proportion. It shows a sharp decrease as the fraction of hydrogen in fuel rises, from 118 mJ for methane–air to 0.12 mJ for hydrogen–air. It is also observed that the MIE of multi-component gas mixtures can be approximately figured as the linear weighted sum of the MIE of each component; the weighting factor is respectively the volume fraction of each component. This can be considered as a universal method to obtain the MIE for a specific multi-component gas.     

Experimental and numerical study on laminar burning velocities and flame instabilities of hydrogen–air mixtures at elevated pressures and temperatures

Experimental and numerical study on hydrogen–air flames at elevated pressures and temperatures was conducted. Meanwhile, the calculation is extended to initial pressure and temperature up to 8.0 MPa and 950 K, respectively. Laminar burning velocities and Markstein lengths were obtained at the elevated pressures and temperatures. Sensitivity analysis and flame structure were also analyzed. The results show good agreement between the computed results and experimental data. The study shows that laminar burning velocities are increased with the increase of initial temperature, and they decrease with the increase of initial pressure. With the increase of initial pressure, advancement of the onset of cellular instability is presented and Markstein length is decreased, indicating an increase of flame instability with the increase of initial pressure. The study shows insensitivity of flame instability to initial temperature. Laminar burning velocity is depended on the competition between the main chain branching reactions and chain termination reaction. The chain branching reactions are the temperature-sensitive reaction, while the termination reaction is the temperature-insensitive reaction. Through the extraction of the overall reaction orders, it is demonstrated that with increasing pressure, the overall reaction orders give a decreasing trend and then increasing trend. This behavior suggests an analogy to three explosion limits of hydrogen/oxygen mixtures. Numerical study also shows that the suppression (or enhancement) of overall chemical reaction with the increase of initial pressure (or temperature) is closely linking to the decrease (or increase) of H, O and OH mole fractions in the flames. Strong correlation is existed between burning velocity and maximum radical concentrations of H and OH radicals in the reaction zone of premixed flames. On the basis of the numerical data, an empirical formula for laminar burning velocity is correlated for the hydrogen–air premixed mixture at elevated pressures and temperatures. The correlated laminar burning velocities are in good agreement with the known experimental results and simulated results with CHEMKIN. The correlation can be used in the calculation of laminar burning velocities at evaluated pressures and temperatures.     

Effects of hydrogen and steam addition on laminar burning velocity of methane–air premixed flame: Experimental and numerical analysis

Effects of hydrogen enrichment and steam addition on laminar burning velocity of methane–air premixed flame were studied both experimentally and numerically. Measurements were carried out using the slot burner method at 1 bar for fresh gases temperatures of 27 °C and 57 °C and for variable equivalence ratios going from 0.8 to 1.2. The hydrogen content in the fuel was varied from 0% to 30% in volume and the steam content in the air was varied from 0 to 112 g/kg (0–100% of relative humidity). Numerical calculations were performed using the COSILAB code with the GRI-Mech 3.0 mechanism for one-dimensional premixed flames. The calculations were implemented first at room temperature and pressure and then extended to higher temperatures (up to 917 K) and pressures (up to 50 bar). Measurements of laminar burning velocities of methane–hydrogen–air and methane–air–steam agree with the GRI-Mech calculations and previous measurements from literature obtained by different methods. Results show that enrichment by hydrogen increases of the laminar burning velocity and the adiabatic flame temperature. The addition of steam to a methane–air mixture noticeably decreases the burning velocity and the adiabatic flame temperature. Modeling shows that isentropic compression of fresh gases leads to the increase of laminar burning velocity.     

Laminar burning velocities of rich near-limiting flames of hydrogen

In the present work, near-limiting hydrogen flames were investigated both experimentally and numerically. Very rich hydrogen + air flames were studied in a constant volume bomb equipped with a pressure sensor and a Schlieren system for optical registration of the flame front movement. The mixtures contained 70% and 75% of hydrogen, the rest being air. The measurements were conducted at pressures from 1 to 4 atm for 70% H₂ + air mixture and from 0.7 to 1.4 atm for 75% H₂ + air mixture. Two methods for determination of the laminar burning velocity were used: from the temporal evolution of the flame front movement and from the pressure records at nearly constant pressure. These methods were compared and discussed in terms of accuracy and implicit assumptions behind them. Markstein lengths were also extracted and compared with the literature by using different extrapolation models. An important role of the critical radius for extraction of the burning velocity and Markstein length is demonstrated. New experimental data are compared with three models for hydrogen combustion to elucidate the need for their further development.