Rich Hydrogen Air Mixtures Research Articles

Suppression of hydrogen-air explosions by isobutene with special molecular structure

Effects of isobutene (i-C4H8) with the unique molecular structure on hydrogen explosions are investigated in a 14 L spherical chamber at 100 kPa and 298 K. The results reveal that for lean hydrogen-air mixtures, the maximum explosion overpressure, maximum pressure rise rate, and deflagration index show a trend from rise to decline. However, for stoichiometric and rich hydrogen-air mixtures, pressure parameters decrease monotonously with increasing i-C4H8. The chemical suppression mechanism of i-C4H8 on hydrogen explosions is revealed in detail by numerical simulation. The relevant analyses show that i-C4H8 can not only change the chain reaction path and cut off the hydrogen-oxygen reaction, but also strengthen the consumption of oxygen and H radicals. This work will provide a novel theoretical foundation for hydrogen explosion prevention and control.

Effect of hydrogen concentration on the vented explosion of hydrogen–air mixtures in a 5-m-long duct

A series of vented hydrogen-air explosion experiments were carried out in an end-vented rectangular tube, and the effect of hydrogen concentrations varying from 10% to 50% on the flame evolution and the maximum overpressure inside and outside the duct were investigated. Experimental results reveal that the flame behavior was strongly affected by hydrogen concentration: the flame speed at the vent exit first increased and then decreased as hydrogen concentration was increased; a tulip flame and two bright fireballs were respectively observed within and outside the duct in some tests. Two dominant pressure peaks and two types of oscillations could be distinguished in the internal pressure curves, corresponding to the rupture of the membrane, venting of the burned gas, Helmholtz, and acoustic oscillations, respectively. Under the current experimental conditions, the pressure peak with a low amplitude owing to the rupture of membrane dominated the external pressure–time histories for hydrogen concentrations less than 25%. However, when the hydrogen concentration ranged from 30% to 50%, the pressure peak caused by the second external explosion become dominant. Besides, the influence of the external explosions on the internal pressure peak structures was discussed. As hydrogen concentration was increased, the maximum internal overpressure first increased and then decreased, whereas the maximum external overpressure increased monotonically. In this study, the maximum reduced overpressures calculated by Molkov’s best-fit model agreed well with the experimental data for near-stoichiometric and rich hydrogen-air mixtures.

Suppression of hydrogen-air explosions by hydrofluorocarbons

This study examines the suppression effectiveness of fluorinated species CHF3 and C2HF5 on the premixed hydrogen-air explosions experimentally and numerically. The results demonstrate that as for stoichiometric and rich hydrogen-air mixtures, both CHF3 and C2HF5 can effectively reduce thermal expansion ratio and increase flame thickness, and then reduce the influence of hydrodynamic instability on the flame acceleration. Laminar burning velocity, the maximum of explosion pressure, the maximum rate of pressure rise and the absolute value of pressure impulse all decrease with increasing suppressant concentration at various equivalent ratios. The maximum of explosion pressures for stoichiometric flames is decreased by 11.54 % with 2 % CHF3 added, and decreased rapidly by 40.39 % with 2 % C2HF5 added. The hydrogen-rich mixtures cannot be ignited when C2HF5 content reaches 10 %. Numerical simulations have confirmed that both CHF3 and C2HF5 can effectively reduce the concentration of active radicals with the decrease order of H > OH > O. C2HF5 is more effective in suppressing explosions than CHF3 by converting more H atoms to HF formation through the elementary reactions CHF2 + H <=> CHF + HF, CF2:O + H <=> CF:O + HF, and CF2 + H = CF + HF.

Specific features of the combustion of rich hydrogen air mixtures

Specific features of the combustion of rich hydrogen air mixtures

Turbulence influence on explosion characteristics of stoichiometric and rich hydrogen/air mixtures in a spherical closed vessel

Explosion always occur in turbulent ambience, but few researches focusing on the effects of turbulence on explosion characteristics has been performed in the current. In the present investigation, the effects of initial turbulence on the explosion characteristics of stoichiometric and rich hydrogen-air mixtures were experimentally studied and analyzed on the basis of explosion pressure traces. Maximum explosion pressure, explosion duration, maximum rate of pressure rise, fast explosion duration, deflagration index, and the averaged heat flux through the wall during explosion were systematically examined. The results indicated that, turbulence will reduced the corresponding equivalence ratio to maximum rate of pressure rise but won’t change the corresponding equivalence ratios to maximum explosion pressure and two explosion durations. In turbulent ambience, two explosion durations will decline for the accelerated propagation of pressure wave, the values of maximum explosion pressure and maximum rate of pressure rise will be raised for the reduced heat flux through the vessel’s wall during explosion. In addition, turbulence raises the value of deflagration index, namely it raises the hazardous level.

Critical slot size for deflagration initiation by hot products discharge into hydrogen–air atmospheres

This computational study addresses deflagration initiation of lean, stoichiometric, and moderately rich hydrogen–air mixtures by the sudden discharge of a hot planar jet of its adiabatic combustion products. The objective is to determine the minimum slot size required for ignition, a relevant quantity of interest for safety and technological applications concerning the accidental ignition of hydrogen. For sufficiently small jet velocities, the numerical solution of the problem requires integration of the two-dimensional Navier–Stokes equations for chemically reacting ideal-gas mixtures, supplemented by standard descriptions of the molecular transport terms and a reduced chemical–kinetic mechanism suitable for hydrogen–air combustion. The computations provide the variation of the critical slot size for hot-jet ignition with both the jet Reynolds number and the equivalence ratio of the mixture. In particular, it is seen that, while the Reynolds number exerts only a relatively weak effect on the ignition process, the influence of the equivalence ratio is much more pronounced, with the smallest slot widths found for stoichiometric or slightly-rich conditions. The numerical results show three different ignition modes, with the flame developing from a clearly identified ignition kernel located either at the core of the leading vortex pair (mode 1), at the symmetry plane near the leading edge of the starting jet (mode 2), or at the jet stem connecting the jet exit with the starting vortex (mode 3).

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.

Investigation of rich hydrogen–air deflagrations in models with detailed and reduced kinetic mechanisms

The structure, flame speed and stability of freely propagating combustion waves in rich hydrogen–air mixtures is investigated numerically using models with detailed and reduced two-step kinetic mechanisms. The critical conditions for the onset of diffusive-thermal pulsating instabilities are found in the equivalence ratio vs pressure parameter plane in each case. The oscillating solutions emerging as a result of these instabilities are investigated. It is demonstrated that the models with reduced kinetics are able to qualitatively (models with constant density) and quantitatively (models with variable density) predict the flame characteristics and stability as compared to the models with detailed reaction mechanism.

Effect of burst pressure on vented hydrogen-air explosion in a cylindrical vessel

Effect of vent burst pressure on internal pressure and flame evolution is experimentally investigated during explosion venting of rich hydrogen-air mixtures with equivalence ratio of 2 in a cylindrical vessel with a neck. Experimental results show that four pressure peaks are observed at the vessel exit under low vent burst pressures, corresponding to the following four successive stages: the vent failure, the venting of the burned gases, the maximum production rate of burned gas in vessel and the suction of gases into vessel. But under high vent burst pressure, the second and third pressure peaks disappear and the first one becomes dominant. The fourth pressure peak due to suction is kept around several kilopascals. The pressure in vessel is always characterized by single pressure peak which increases with the increase of the vent burst pressure. Under low vent burst pressures, the oscillation of internal flame due to flame-acoustic interaction results in oscillation of pressure rise rate in about 2000 Hz, and the oscillation nearly disappears under high vent burst pressures. The external flame speed does not decrease monotonously as the increase of distance away from the vent, and the maximum length of external flame is nearly independent of vent burst pressure.

Fundamental study on accidental explosion behavior of hydrogen-air mixtures in an open space

In this study, the flame propagation behavior and the intensity of the blast wave by an accidental explosion of a hydrogen-air mixture in an open space were measured simultaneously using the soap bubble method. The results show that the flame in lean hydrogen-air mixtures propagated by spontaneous flame instabilities. The flame in rich hydrogen-air mixtures propagated smoothly in the early stage, and was intensively wrinkled and accelerated in the later stage by different type of instabilities. The flame wrinkling in the later stage of rich hydrogen flame is generated when the flame approaches the non-uniformity transition region of concentration distribution. The intensity of the blast wave of hydrogen/air mixtures is strongly affected by the acceleration of the flame propagation by these spontaneous flame disturbances.

Analysing the stability of premixed rich hydrogen–air flame with the use of two-step models

In this paper we investigate the thermal-diffusive instability of premixed adiabatic flames in rich hydrogen–air mixtures at normal ambient conditions. Several models which feature the same two-step global kinetics with chain-branching and recombination reaction steps are considered. These global kinetic steps are assumed to be controlled by different elementary reactions. The flame speed and structure are investigated numerically by using the shooting-relaxation algorithms. The stability is studied by means of the Evans function method and by direct integration of the governing partial differential equations. It is demonstrated that the two-step models are capable of accurately predicting the speed and structure of combustion waves as well as flame stability and frequency of pulsations of unsteady combustion waves.

The “turbulent flame speed” of wrinkled premixed flames

The determination of the turbulent flame speed is a central problem in combustion theory. Early studies by Damköhler and Shelkin resorted to geometrical and scaling arguments to deduce expressions for the turbulent flame speed and its dependence on turbulence intensity. A more rigorous approach was undertaken by Clavin and Williams who, based on a multi-scale asymptotic approach valid for weakly wrinkled flames, derived an expression that apart from a numerical factor recaptures the early result by Damköhler and Shelkin. The common denominator of the phenomenological and the more rigorous propositions is an increase in turbulent flame speed due solely to an increase in flame surface area. Various suggestions based on physical and/or experimental arguments have been also proposed, incorporating other functional parameters into the flame speed relation. The objective of this work is to extend the asymptotic results to a fully nonlinear regime that permits to systematically extract scaling laws for the turbulent flame speed that depend on turbulence intensity and scale, mixture composition and thermal expansion, flow conditions including effects of curvature and strain, and flame instabilities. To this end, we use a hybrid Navier–Stokes/front-capturing methodology, which consistently with the asymptotic model, treats the flame as a surface of density discontinuity separating burned and unburned gases. The present results are limited to positive Markstein length, corresponding to lean hydrocarbon–air or rich hydrogen–air mixtures, and to wrinkled flames of vanishingly small thickness, smaller that the smallest fluid scales. For simplicity we have considered here two-dimensional turbulence, which although lacks some features of real three-dimensional turbulence, is not detrimental when using the hydrodynamic model under consideration, because the turbulent flame retains its laminar structure and its interaction with turbulence is primarily advective/kinematic in nature.

Influence of the Darrieus-Landau instability on the propagation of planar turbulent flames

The propagation of premixed flames in weak two-dimensional homogeneous turbulent flows is studied numerically via a hybrid Navier–Stokes/front capturing methodology within the context of a hydrodynamic model, which treats the flame as a surface of density discontinuity separating the burnt and unburnt gases. The focus is the influence of the Darrieus-Landau instability on the turbulent flame, which has been recognized recently to have a dramatic effect on its structure and the turbulent flame speed. Such instability, controlled by a parameter inversely proportional to the Markstein length, can be triggered in a laboratory setting by variations in system pressure or in fuel type and composition. Particular attention in this study is devoted to the influence of the Darrieus-Landau instability on a turbulent, statistically planar flame. Results are therefore limited to positive Markstein length corresponding to lean hydrocarbon–air or rich hydrogen–air mixtures. We show that, although the planar flame under similar but laminar conditions is stable, it is nonetheless affected by the instability in the presence of a turbulent incident flowfield. The turbulent flame speed is observed to exhibit, in addition to the effect of thermal expansion, a nontrivial dependence on the instability parameter and on the turbulence integral scale both effects modulating, in the weak turbulence regime, the well established quadratic dependence of turbulent flame speed on turbulence intensity.

Propagation of wrinkled turbulent flames in the context of hydrodynamic theory

We study the propagation of premixed flames in two-dimensional homogeneous isotropic turbulence using a Navier–Stokes/front-capturing methodology within the context of hydrodynamic theory. The flame is treated as a thin layer separating burnt and unburnt gases, of vanishingly small thickness, smaller than the smallest fluid scales. The method is thus suitable to investigate the flame propagation in the wrinkled flamelet regime of turbulent combustion. A flow-control system regulates the mean position of the flame and the incident turbulence intensity. In this context we study the individual effects of turbulence intensity, turbulence scale, thermal expansion, hydrodynamic strain and hydrodynamic instability on the propagation characteristics of the flame. Results are obtained assuming positive Markstein length, corresponding to lean hydrocarbon–air or rich hydrogen–air mixtures. For stable planar flames we find a quadratic dependence of turbulent speed on turbulence intensity. Upon onset of hydrodynamic instability, corrugated structures replace the planar conformation and we observe a greater resilience to turbulence, the quadratic scaling being replaced by scaling exponents less than one. Such resilience is also confirmed by the observation of a threshold turbulence intensity below which the propagation speed of corrugated flames is indistinguishable from the laminar speed. Turbulent speed is found to increase and later plateau with increasing thermal expansion, this affecting the average flame displacement but not the mean flame curvature. In addition, turbulence integral scale is also observed to affect the propagation of the flame with the existence of an intermediate scale maximizing the turbulent speed. This maximizing scale is smaller for corrugated flames than it is for planar flames, implying that small eddies that will be unable to significantly perturb a planar front could be rather effective in perturbing a corrugated flame. Turbulent planar flames, and more so corrugated flames, were observed to experience a positive mean hydrodynamic strain, which was explained in terms of the overwhelming mean contribution of the normal component of strain. The positive straining causes a decrease in the mean laminar propagation speed which in turn can decrease the turbulent speed. The effect of the flame on the incident turbulent field was examined in terms of loss of isotropy and vorticity destruction by thermal expansion. The latter can be mitigated by a baroclinic vorticity generation which is enhanced for corrugated flames.

Promotion and inhibition of oxidation of rich hydrogen-air mixtures by nitric oxides (NO and NO2) during adiabatic self-ignition

The tracer method was used to numerically study the effect of nitric oxides (NO and NO2) on the oxidation of rich hydrogen-air mixtures during adiabatic self-ignition at low and high initial temperatures and a pressure of 0.1 MPa. At low temperatures, the added NO interacts with HO2 to form NO2, and NO2 then interacts with H to form NO. When NO2 is added at the same temperatures, a two-stage mechanism takes place: NO formed by the reaction NO2 + H is not involved in the reaction until NO2 is almost completely consumed. In the temperature range 900–1200 K, NO2 inhibits self-ignition through participation in the reaction with H, leading to the replacement of part of the completely branched chain H → (O, OH) → 3H by the unbranched chain H → OH → H. At low initial temperatures, NO effectively promotes hydrogen oxidation due to replacement of the unbranched chain H → HO2 → H2O2 → OH → H by a chain with branching.

Concentration limits of combustion in rich hydrogen–air mixtures in the presence of inhibitors

An approximate analytical method is proposed for the analysis of data on the limits of flame propagation in hydrogen–air mixtures at atmospheric pressure in the presence of small quantities of active additives (inhibitors).

Effect of added reactive agents on the flame propagation velocity in rich hydrogen-air mixtures

An approximate analytical method for evaluating the efficiency of the action of an inhibitor on the velocity and propagation limits of a flame in rich hydrogen-air mixtures with small amounts of added propylene and isobutylene inhibitors is proposed. The method is based on the model of a narrow reaction zone and the distinct features of the branching chain mechanism of hydrogen oxidation reactions. Using this method, it is shown that the occurrence of flame propagation limits at higher concentrations of added reactive agents (inhibitors) is caused by the existence of a positive feedback between the flame front velocity and the number of active combustion sites, which break down in the inhibitor-added reaction. According to this feedback, the action of the inhibitor decreases the flame temperature and flame velocity.

On the mechanism of inhibition of a hydrogen flame by propane

Numerical simulations demonstrated that small additives of propane to rich hydrogen-air mixtures suppress the formation of HO2 and OH in the low-temperature region of the flame zone, thereby causing a substantial decrease in the laminar flame speed. In the low-and high-temperature regions, propane interacts predominantly with OH and H, respectively. In the flame zone, propane is completely converted to CO, CO2, CH4, C2H2, H2, and H2O, being oxidized concurrently with hydrogen at that.

DARRIEUS–LANDAU AND THERMO-ACOUSTIC INSTABILITIES IN CLOSED VESSEL EXPLOSIONS

Experiments involving a spherical explosion bomb are reported, in which Darrieus–Landau thermo-diffusive, D-L,T-D, flame instabilities interacted with primary and secondary, self-excited, thermo-acoustic oscillations. Explosions with central ignition demonstrated that rich i-octane and lean hydrogen-air mixtures generated strong pressure oscillations, a consequence of their negative Markstein numbers. Utilizing dual wall ignitions, the structures of high pressure flames were studied using appropriate optical techniques. The conditions that gave rise to the greatest increase in the rate of combustion were strong initial D-L,T-D, flame instabilities and a high rate of change of the heat release rate, sufficient to generate strong secondary pressure oscillations. These, in turn, generated Rayleigh-Taylor instabilities that further wrinkled the flames. The bomb was equipped with four fans which showed that an rms turbulent velocity in excess of about 0.6 m/s was sufficient to reduce, and almost eradicate, the effect of these instabilities on the flame speed.

Spiral waves in expanding hydrogen–air flames: Experiment and theory

We report herein the first experimental observation of spiral waves over propagating flame surfaces in rich hydrogen–air mixtures at elevated pressures up to 40 atm, conducted in a specially designed, optically accessible, constant-pressure combustion chamber. The observed spiral waves are a manifestation of the large Lewis number instability, exhibiting behaviors such as clockwise/counterclockwise rotation, meandering, and fast radial wave speeds that are similar to patterns often observed in other excitable media, for example the Belousov–Zhabotinsky reaction. In addition, these spiral waves also exhibit features that seem to be characteristic of combustion systems, such as the transition criterion for diffusional-thermal pulsating instability, and their confinement within the hydrodynamic cells that also develop over such high-pressure flames of much reduced flame thicknesses. A diffusional-thermal theory was developed that successfully describes the observed spiral patterns.