The flow field behaviours of under-expansion jet flame in premixed hydrogen/air explosion venting

The short chemical reaction time and high heat release of hydrogen fuel make a fine propulsion performance. Hydrogen is recognized as an excellent new resource. However, the inherent hazard of combustion and explosion should not be ignored. In this paper, the formation of the under-expansion jet flame (UEJF) of premixed hydrogen/air in explosion venting was simulated by ANSYS Fluent, and the wave system structures of the UEJF were systematically analyzed. The changes of pressure, Mach number, and main gas mass fraction distribution in the formation of the UEJF structures were described. The results indicated that the pressure difference at the outlet of explosion venting tube induced the intersection of expansion wave and reflected wave, and the formation of an obvious negative pressure area in the flow field. Meanwhile, alternating changes of the pressure in the jet center area propagated forward with the flame, resulting in the generation of UEJF. The fluxion of airflow at the outlet of explosion venting tube can be regarded as Prandtl-Meyer flow. The expansion angle was a positive correlation to the Mach number and the pressure ratio of the internal and external of the explosion venting tube. The distribution of H₂O, O₂, and –OH on the Mach disk revealed that the residual hydrogen reacted with O₂ to produce secondary or multiple explosions. Therefore, the attention and research on the hazard of the under-expansion jet field should be strengthened.     

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.     

Laminar burning velocities and flame instabilities of diluted H2/CO/air mixtures under different hydrogen fractions

An experimental study was conducted using outwardly propagating flame to evaluate the laminar burning velocity and flame intrinsic instability of diluted H₂/CO/air mixtures. The laminar burning velocity of H₂/CO/air mixtures diluted with CO₂ and N₂ was measured at lean equivalence ratios with different dilution fractions and hydrogen fractions at 0.1 MPa; two fitting formulas are proposed to express the laminar burning velocity in our experimental scope. The flame instability was evaluated for diluted H₂/CO/air mixtures under different hydrogen fractions at 0.3 MPa and room temperature. As the H₂ fraction in H₂/CO mixtures was more than 50%, the flame became more unstable with the decrease in equivalence ratio; however, the flame became more stable with the decrease in equivalence ratio when the hydrogen fraction was low. The flame instability of 70%H₂/30%CO premixed flames hardly changed with increasing dilution fraction. However, the flames became more stable with increasing dilution fraction for 30%H₂/70%CO premixed flames. The variation in cellular instability was analyzed, and the effects of hydrogen fraction, equivalence ratio, and dilution fraction on diffusive-thermal and hydrodynamic instabilities were discussed.     

Explosion venting of hydrogen-air mixtures from a duct to a vented vessel

Experiments were conducted on the vented explosion of hydrogen–air mixtures from a 150-cm-long duct to a cylindrical vessel with a vent at the center of its side wall to investigate the effects of vent burst pressure and an obstacle in duct on the process of explosion venting. Turbulent pressure oscillation owing to a pressure wave moving back and forth in a duct and vessel was observed for unvented explosions. For explosion venting from duct to vessel, flame acceleration in duct much increases the explosion overpressure in vessel. The maximum explosion in duct is always higher than that in vessel, and both of them increase with an increase in the vent cover thickness. An obstacle installed in duct significantly affected the explosion overpressure, which first increased and then decreased with an increase in the blockage ratio. Three pressure peaks were distinguished in the external pressure-time histories, which were resulted form different pressure waves formed outside the vessel.     

The effect of hydrogen addition on premixed laminar acetylene–hydrogen–air and ethanol–hydrogen–air flames

In the present work, the laminar premixed acetylene–hydrogen–air and ethanol–hydrogen–air flames were investigated numerically. Laminar flame speeds, the adiabatic flame temperatures were obtained utilizing CHEMKIN PREMIX and EQUI codes, respectively. Sensitivity analysis was performed and flame structure was analyzed. The results show that for acetylene–hydrogen–air flames, combustion is promoted by H and O radicals. The highest flame speed (247 cm/s) was obtained in mixture with 95% H₂–5% C₂H₂ at λ = 1.0. The region between 0.95 < X_H2 < 1.0 was referred to as the acetylene-accelerating hydrogen combustion since the flame speed increases with increase the acetylene fraction in the mixture. Further increase in the acetylene fraction decreases the H radicals in the flame front. In ethanol–hydrogen–air mixtures, the mixture reactivity is determined by H, OH and O radicals. For X_H2 < 0.6, the flame speed in this regime increases linearly with increasing the hydrogen fraction. For X_H2 > 0.8, the hydrogen chemistry control the combustion and ethanol addition inhibits the reactivity and reduces linearly the laminar flame speed. For 0.6 < X_H2 < 0.8, the laminar flame speed increases exponentially with the increase of hydrogen fraction.     

On the effective Lewis number formulations for lean hydrogen/hydrocarbon/air mixtures

Decades of research have underlined the undeniable importance of the Lewis number (Le) in the premixed combustion field. From early experimental observations on laminar flame propagation to the most recent DNS studies of turbulent flames, the unbalanced influence of thermal to mass diffusion (i.e. Le ≠ 1), known as nonequidiffusion, has shed the light on a wide range of combustion phenomena, especially those involving stretched flames. As a result the determination of the Lewis number has become a routine task for the combustion community. Recently, the growing interest in hydrogen/hydrocarbon (HC) fuel blends has produced extensive studies that have not only improved our understanding of H₂/HC flame dynamics, but also, in its wake, raised a fundamental question: which effective Lewis number formulation should we use to characterize the combustion of hydrogen/hydrocarbon/air blends? While the Lewis number is unambiguously defined for combustible mixtures with a single fuel reactant, the literature is unclear regarding the appropriate equivalent formulation for bi-component fuels. The present paper intends to clarify this aspect. To do so, effective Lewis number formulations for lean (φ = 0.6 and 0.8) premixed hydrogen/hydrocarbon/air mixtures have been investigated in the framework of an existing outwardly propagating flame theory. Laminar burning velocities and burned Markstein lengths of H₂/CH₄, H₂/C₃H₈, H₂/C₈H₁₈ and H₂/CO fuel blends in air were experimentally and numerically determined for a wide range of fuel compositions (0/100% → 100/0% H₂/HC). By confronting the two sets of results, the most appropriate effective Lewis number formulation was identified for conventional H₂/HC/air blends. Observed deviations from the validated formulation are discussed for the syngas (H₂/CO) flame cases.     

Experimental study on the explosion behaviors of premixed syngas-air mixtures in ducts

Explosion characteristics of premixed syngas-air mixtures at room temperature and atmospheric pressure were experimentally reported when the explosion flame propagates in ducts with various heights (H) and lengths (L). The discussion was based on flame morphology and pressure dynamics. The ratio of L/H and the ratio of H₂/CO had a significant effect on the explosion flame behaviors as the explosion occurred in ducts. The structure of the explosion flame changes more drastically, as both the L/H ratio is large. The ratio of L/H affected the flame tip dynamics after the flame reached the duct wall, and the time of flame reaching the duct walls is divinable. For a given duct height, the shorter the duct length is, the faster flame propagates, and the maximum flame tip speed was higher as the duct length was small. For a given duct length, flame tip dynamics showed a nearly same development tendency, but the shorter the duct height, the faster the flame propagated. The venting pressure affected the overpressure dynamics, and the venting pressure increased with the increase of the L/H ratio and the H₂/CO. For a given duct height, the overpressure reached the maximum value almost at the same time, and the longer duct length resulted in the greater maximum overpressure. Finally, for a given duct length, the higher duct height caused the higher maximum overpressure.     

Numerical investigations on flashback dynamics of premixed methane-hydrogen-air laminar flames

Injecting hydrogen into the natural gas network to reduce CO₂ emissions in the EU residential sector is considered a critical element of the zero CO₂ emissions target for 2050. Burning natural gas and hydrogen mixtures has potential risks, the main one being the flame flashback phenomenon that could occur in home appliances using premixed laminar burners. In the present study, two-dimensional transient computations of laminar CH₄ + air and CH₄ + H₂ + air flames are performed with the open-source CFD code OpenFOAM. A finite rate chemistry based solver is used to compute reaction rates and the laminar reacting flow. Starting from a flame stabilized at the rim of a cylindrical tube burner, the inlet bulk velocity of the premixture is gradually reduced to observe flashback. The results of the present work concern the effects of wall temperature and hydrogen addition on the flashback propensity of laminar premixed methane-hydrogen-air flames. Complete sequences of flame dynamics with gradual increases of premixture velocity are investigated. At the flame flashback velocities, strong oscillations at the flame leading edge emerge, causing broken flame symmetry and finally flame flashback. The numerical results reveal that flashback tendency increase with increasing wall temperature and hydrogen addition rate.     

Turbulent flame topology and the wrinkled structure characteristics of high pressure syngas flames up to 1.0 MPa

The turbulent flame topology characteristics of the model syngas with two different hydrogen ratios were statistically investigated, namely CO/H₂ ratio at 65/35 and 80/20, at equivalence ratio of 0.7. The combustion pressure was kept at 0.5 MPa and 1.0 MPa, to simulate the engine-like condition. The model syngas was diluted with CO₂ with a mole fraction of 0.3 which mimics the flue gas recycle in the turbulent combustion. CH₄/air flame with equivalence ratio of 1.0 was also tested for comparison. The flame was anchored on a premixed type Bunsen burner, which can generate a controllable turbulent flow. Flame front, which is represented by the sharp increased interface of the OH radical distribution, was measured with OH-PLIF technique. Flame front parameters were obtained through image processing to interpret the flame topology characteristics. Results showed that the turbulent flames possess a wrinkled character with smaller scale concave/convex structure superimposed on a larger scale convex structure under high pressure. The wrinkled structure of syngas flame is much finer and more corrugated than hydrocarbon fuel flames. The main reason is that scale of wrinkled structure is smaller for syngas flame, resulting from the unstable physics. Hydrogen in syngas can increase the intensity of the finer structure. Moreover, the model syngas flames have larger flame surface density than CH₄/air flame, and hydrogen ratio in syngas can increase flame surface density. This would be mainly attributed to the fact that the syngas flames have smaller flame intrinsic instability scale l_i than CH₄/air flame. S_T/S_L of the model syngas tested in this study is higher than CH₄/air flames for both pressures, due to the high diffusivity and fast burning property of H₂. This is mainly due to smaller L_M and l_i. V_f of the two model syngas is much smaller than CH₄/air flames, which suggests that syngas flame would lead to a larger possibility to occur combustion oscillation.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

Studies on properties of laminar premixed hydrogen-added ammonia/air flames for hydrogen production

In order to evaluate the potential of burning and reforming ammonia as a carbon-free fuel in production of hydrogen, fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) for laminar premixed hydrogen-added ammonia/air flames were studied both experimentally and computationally. Freely (outwardly)-propagating spherical laminar premixed flames at normal temperature and pressure were considered for a wide range of global fuel-equivalence ratios, flame stretch rates (represented by the Karlovitz number) and the extent of hydrogen substitution. Results show the substantial increase of laminar burning velocities with hydrogen substitution, particularly under fuel-rich conditions. Also, predicted flame structures show that the hydrogen substitution enhances nitrogen oxide (NO_x) and nitrous oxide (N₂O) formation. At fuel-rich conditions, however, the amount of NO_x and N₂O emissions and the extent of the increase with the hydrogen substitution are much lower than those under fuel-lean conditions. These observations support the potential of hydrogen as an additive for improving the burning performance with low NO_x and N₂O emissions in fuel-rich ammonia/air flames and hence the potential of using ammonia as a clean fuel. Increasing the amount of added hydrogen tends to enhance flame sensitivity to stretch.     

Effects of nitrogen addition on vented hydrogen–air deflagration in a vertical duct

In this paper, experiments were performed to investigate the coupling effects of venting and nitrogen addition ratio (χ) on flame behavior and pressure evolution during hydrogen–air deflagration within and outside a 1-m-high vertical duct with a vent on its top. Experimental results reveal that χ has significant effects on the pressure–time histories in the duct. Helmholtz oscillations of the internal overpressure were observed in all tests, and acoustic type oscillations appears in the tests only for χ = 25% and 30%. For a certain χ, the maximum overpressure (Pmax) increased with the distance to the vent, i.e., the highest overall explosion overpressure was attained near the duct bottom; however, the difference in Pmax between various measuring points decreases with an increase in χ. In all tests, a pressure peak in the duct was observed shortly after external explosion. The maximum internal and external overpressure decreased as χ was increased.     

Deflagration-to-detonation transition in H2-CO-Air mixtures in a partially obstructed channel

In this study flame dynamics in homogeneous hydrogen-carbon monoxide-air (H₂-CO-air) mixtures are investigated in an explosion channel. The small scale test rig is a 6 m long channel and has a rectangular cross section of 300 x 60 mm. Obstacles with a blockage ratio of 60% and a spacing of 300 mm are placed in the first section of the channel. A 2.05 m long unobstructed section in the rear of the channel allows for investigation of sustainable propagating flames and detonations. The fuel composition is varied from 100/0 to 50/50 vol.-% H₂/CO mixtures. The overall fuel content ranges from 15 to 40 vol.-% in air aiming to obtain fast flames and deflagration-to-detonation transition (DDT). Flame speed and dynamic pressure data are evaluated. Results extend data obtained by Veser [1] and can be used for validation of numerical frameworks. Limits for fast flames and DDT in homogeneous H₂-CO-air mixtures at the given geometry are presented.     

Laser raman measurements of temperature and species concentration in swirling lifted hydrogen jet diffusion flames

Simultaneous spatially and temporally resolved point measurements of temperature, mixture fraction, major species (H₂, H₂O, O₂, N₂), and minor species (OH) concentrations are performed in unswirled (S_g = 0), low swirl (S_g = 0.12), and high swirl (S_g = 0.5) lifted turbulent hydrogen jet diffusion flames into still air. Ultraviolet (UV) Raman scattering and laser-induced predissociative fluorescence (LIPF) techniques are combined to make the multi-parameter measurements using a single KrF excimer laser. Experimental results are compared to the fast chemistry (equilibrium) limit, to the mixing without reaction limit, and to simulations of steady stretched laminar opposed-flow flames. It is found that in the lifted region where the swirling effects are strong, the measured chemical compositions are inconsistent with those calculated from stretched laminar diffusion flames or stretched partially premixed flames. Sub-equilibrium values of temperature, sub-flamelet values of H₂O, and super-flamelet values of OH are found in an intermittent annular turbulent brush of the swirled flame but not in the unswirled flame. Farther downstream of the nozzle exit (x/D ≥ 50), swirl has little effect on the finite-rate chemistry.     

A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH₄) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H₂–air and CH₄–H₂–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NO_x emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NO_x emissions in the CH₄ flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH₄ flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH₄ flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.     

Flame front structure and burning velocity of turbulent premixed CH4/H2/air flames

Flame front structure of turbulent premixed CH₄/H₂/air flames at various hydrogen fractions was investigated with OH-PLIF technique. A nozzle-type burner was used to achieve the stabilized turbulent premixed flames. Hot-wire anemometer measurement and OH-PLIF observation were performed to measure the turbulent flow and detect the instantaneous flame front structure, respectively. The hydrogen fractions of 0%, 5%, 10% and 20% were studied. Results show that the flame front structures of the turbulent premixed flames are wrinkled flame front with small scale convex and concave structures compared to that of the laminar-flame front. The wrinkle intensity of flame front is promoted with the increase of turbulence intensity as well as hydrogen fraction. Hydrogen addition promotes the flame intrinsic instability which leads to the active response of laminar flame to turbulence and results in the much more wrinkled flame front structure. The value of S_T/S_L increases monotonically with the increase of u′/S_L and hydrogen fraction. The increase of S_T/S_L with the increase of hydrogen fraction is mainly attributed to the diffusive-thermal instability effects represented by the effective Lewis number, Le_eff. A general correlation between S_T/S_L and u′/S_L is provided from the experimental data fitting in the form of S_T/S_L ∝ a(u′/S_L)ⁿ, and the exponent, n, gives the constant value of 0.35 for all conditions and at various hydrogen fractions.     

Hydrogen addition effects in a confined swirl-stabilized methane-air flame

The effect of hydrogen addition in methane-air premixed flames has been examined from a swirl-stabilized combustor under confined conditions. The effect of hydrogen addition in methane-air flame has been examined over a range of conditions using a laboratory-scale premixed combustor operated at 5.81 kW. Different swirlers have been investigated to identify the role of swirl strength to the incoming mixture. The flame stability was examined for the effect of amount of hydrogen addition, combustion air flow rates and swirl strengths. This was carried out by comparing adiabatic flame temperatures at the lean flame limit. The combustion characteristics of hydrogen-enriched methane flames at constant heat load but different swirl strengths have been examined using particle image velocimetry (PIV), micro-thermocouples and OH chemiluminescence diagnostics that provided information on velocity, thermal field, and combustion generated OH species concentration in the flame, respectively. Gas analyzer was used to obtain NO_x and CO concentration at the combustor exit. The results show that the lean stability limit is extended by hydrogen addition. The stability limit can reduce at higher swirl intensity to the fuel-air mixture operating at lower adiabatic flame temperatures. The addition of hydrogen increases the NO_x emission; however, this effect can be reduced by increasing either the excess air or swirl intensity. The emissions of NO_x and CO from the premixed flame were also compared with a diffusion flame type combustor. The NO_x emissions of hydrogen-enriched methane premixed flame were found to be lower than the corresponding diffusion flame under same operating conditions for the fuel-lean case.     

Effect of preferential diffusion and flame stretch on flame structure and laminar burning velocity of syngas Bunsen flame using OH-PLIF

By using OH-PLIF technique, experiments were conducted for laminar Bunsen flame of premixed CO/H₂/air mixtures with equivalence ratio ranging from 0.5 to 1.8. Reynolds number was varied from 800 to 2200, X_H2 = H₂/(H₂+CO) in the mixture was varied from 20% to 100% to study the effects of both preferential diffusion and flame curvature on flame structures and laminar flame burning velocities. Results showed that the combined effects of preferential diffusion and curvature gave an interesting phenomenon of the flame OH radical distributions on high hydrogen content flames. Furthermore, with the increase of H₂ fraction in fuel mixture, the effects of both preferential diffusion and flame curvature were increased. Interpretation of flame stretch effect on laminar burning velocity is also provided in this paper.     

Visible emission of hydrogen flames

The common misconception that hydrogen flames are not visible is examined. Examples are presented of clearly visible emissions from typical hydrogen flames. It is shown that while visible emissions from these flames are considerably weaker than those from comparable hydrocarbon flames, they are indeed visible, albeit at reduced light levels in most cases. Detailed flame spectra are presented to characterize flame emission bands in the ultraviolet, visible and infrared regions of the spectrum that result in a visible hydrogen flame. The visible blue emission is emphasized, and recorded spectra indicate that fine spectral structure is superimposed on a broadband continuum extending from the ultraviolet into the visible region. Tests were performed to show that this emission does not arise from carbon or nitrogen chemistry resulting from carbon-containing impurities (hydrocarbons) in the hydrogen fuel or from CO₂ or N₂ entrainment from the surrounding air. The spectral structure, however, is also observed in methane flames. The magnitude of the broadband emission increases with flame temperature in a highly nonlinear manner while the finer spectral structure is insensitive to temperature. A comparison of diffusion and premixed H₂ flames shows that the fine scale structure is comparable in both flames.     

Turbulent burning velocity of stoichiometric syngas flames with different hydrogen volumetric fractions upon constant-volume method with multi-zone model

Syngas has been widely concerned and tested in various thermo-power devices as one promising alternative fuel. However, little is known about the turbulent combustion characteristics, especially on outwardly propagating turbulent syngas/air premixed flames. In this paper, the outwardly propagating turbulent syngas/air premixed flames were experimentally investigated in a constant-volume fan-stirred vessel. Tests were conducted on stoichiometric syngas with different hydrogen volumetric fractions (X_H2, 10%–90%) in the ambience with different initial turbulence intensity (u'_rms, 0.100 m/s~1.309 m/s). Turbulent burning velocity was taken as the major topic to be studied upon the multi-zone model in constant-volume propagating flame method. The influences of initial turbulent intensity and hydrogen volumetric fraction on the turbulent flame speed were analysed and discussed. An explicit correlation of turbulent flame speed was obtained from the experimental results.