Experimental studies on the explosion indices in turbulent stoichiometric H2/CH4/air mixtures

Explosion characteristics of the stoichiometric hydrogen/methane/air mixtures with different hydrogen fractions (λ) and different turbulent intensities (u'_rms) in a fan-jet-stirred spherical explosion vessel. From the experimental results, it could be clearly found that both the maximum explosion overpressure (p_max) and the maximum rise rate of overpressure rose with the increase of u'_rms, but the major reasons to such rising were not totally the same. In turbulence, with the increase of λ, p_max declined but (dp/dt)_max rose, and such behaviours were mainly attributed to the completion on the variations between propagation speed and adiabatic explosion pressure. The explosion duration (t_c) was also measured, it rose with the increase of u'_rms and/or λ for the enhancement on propagation albeit such enhancement was attributed to different mechanism for different influence factors. The variations of deflagration index (K_G) indicated that the hazardous level of stoichiometric hydrogen/methane mixtures would become more hazardous in the presence of turbulence. Furthermore, the heat loss during the explosion also was calculated and analysed. The results reported in this article could provide more basic but important information to practical utilizations of hydrogen/methane blended fuels, especially on the safety protection strategies.     

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.     

Turbulent combustion evolution of stoichiometric H2/CH4/air mixtures within a spherical space

Turbulent combustion evolutions of stoichiometric H₂/CH₄/air mixtures were experimentally studied within a spherical constant-volume combustion vessel. A series of initial turbulent ambience (with the range of turbulence intensity from 0 to 1.309 m/s) and a series of hydrogen volumetric fraction (with the range from 0.3 to 0.9) were taken as the variables to studied the influences of turbulence intensity and the fuel composition on the turbulent combustion evolutions. The evolutions of explosion overpressure were studied upon the variations of maximal pressure, the influences of turbulence intensity mainly located at heat loss while the influences of fuel composition mainly located at adiabatic explosion. Subsequently, the evolutions of burnt mass were discussed, the competition between pressure rising and temperature rising induced by the heat release during combustion was considered as major influence mechanism. Then, the nexus between burning velocity and the related burnt mass rate were discussed, the variations regulations of maximal burning velocity brought by turbulence intensity and hydrogen volumetric fraction were analysed. Finally, the nexus between maximum burning velocity and heat loss was discussed.     

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.     

Autoignition and structure of nonpremixed CH4/H2 flames: Detailed and reduced kinetic models

The ignition dynamics and subsequent flame evolution of hydrogen-enriched methane mixtures are investigated numerically in a reacting vortex ring configuration. The CH₄/H₂ combustion is studied using a detailed reaction mechanism (GRI-Mech v3.0) and two augmented reduced mechanisms (11-step and 12-step). The main objective of this study is to identify the extent that the current reduced mechanisms can go in replicating the dynamics of the ignition process and flame structure in an unsteady nonpremixed configuration. The parameters of the numerical simulations are adjusted such that flame ignition occurs during either the formation or the postformation of the ring. The quasi-steady state assumption for O in the 12-step reduced kinetic model leads to shorter ignition delay times than those in the other kinetic models. For formation-phase ignition runs, the flame structure near the stoichiometric region is captured well by the 12-step model compared to GRI-Mech 3.0. For postformation ignition runs, the 12-step model predicts larger heat release rates and main species mole fractions compared to GRI-Mech 3.0. The 11-step model predicts well the ignition delay time. At later times the fuel-rich side of the flame predicted by this reduced mechanism exhibits differences from the detailed model. Counterflow diffusion flame results are used to further compare the fuel-rich chemistry for the detailed and augmented reduced kinetic models.     

Ignition transition in turbulent premixed combustion

Recently, Shy and his co-workers reported a turbulent ignition transition based on measurements of minimum ignition energies (MIE) of lean premixed turbulent methane combustion in a centrally-ignited, fan-stirred cruciform burner capable of generating intense isotropic turbulence. Using the same methodology, this paper presents new complete MIE data sets for stoichiometric and rich cases at three different equivalence ratios ? = 1.0, 1.2 and 1.3, each covering a wide range of a turbulent Karlovitz number (Ka) indicating a time ratio between chemical reaction and turbulence. Thus, ignition transition in premixed turbulent combustion depending on both Ka and ? can be identified for the first time. It is found that there are two distinct modes on ignition in randomly stirred methane–air mixtures (ignition transition) separated by a critical Ka where values of Ka_c ≈ 8–26 depending on ? with the minimum Ka_c occurring near ? = 1. For Ka < Ka_c, MIE increases gradually with Ka, flame kernel formation is similar to laminar ignition remaining a torus, and 2D laser tomography images of subsequent outwardly-propagating turbulent flames show sharp fronts. For Ka > Ka_c, MIE increases abruptly with Ka, flame kernel is disrupted, and subsequent randomly-propagating turbulent flames reveal distributed-like fronts. Moreover, we introduce a reaction zone Péclet number (P_RZ) indicating the diffusivity ratio between turbulence and chemical reaction, such that the aforementioned very scattering MIE data depending on Ka and ? can be collapsed into a single curve having two drastically different increasing slopes with P_RZ which are separated by a critical P_RZ ≈ 4.5 showing ignition transition. Finally, a physical model is proposed to explain these results.     

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 de?agration 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.     

Experimental study of shock wave propagation and its influence on the spontaneous ignition during high-pressure hydrogen release through a tube

An experimental study of shock wave propagation and its influence on the spontaneous ignition during high-pressure hydrogen release through a tube are measured by pressure transducers and light sensors. Results show that the pressure behind a shock wave first increases, and subsequently remains near constant value with an increase of the propagation distance. That is, a certain propagation distance is required to form a stable shock wave in the tube. In the front of the tube, the minimum value of pressure behind the shock wave (P_shock) required for spontaneous ignition decreases with the increase in axial distance to the diaphragm. However, the minimum P_shock remains nearly a constant value in the rear part of the tube. Moreover, the critical values of shock Mach number (M_S) for spontaneous ignition decrease with the increase in tube length. And the ignition delay time decreases with the increase of the M_S. As the ignition kernel grows in size to a flame, it propagates downstream along the tube with velocity greater than the theoretical flow velocity of the hydrogen-air contact surface. The flame propagation velocity relative to tube wall increases with M_S. When the self-sustained flame exits from the tube, a rapid non-premixed turbulent combustion is observed in the chamber. The combustion-wave overpressure increases with the increase of the M_S.     

Spark ignition transitions in premixed turbulent combustion

Recent discoveries and developments on the dynamic process of premixed turbulent spark ignition are reviewed. The focus here is on the variation of turbulent minimum ignition energies (MIE_T) against laminar MIE (MIE_L) over a wide range of r.m.s. turbulence fluctuation velocity (uʹ) alongside effects of the spark gap between electrodes, Lewis number, and some other parameters on MIE. Two distinguishable spark ignition transitions are discussed. (1) A monotonic MIE transition, where MIE_L sets the lower bound, marks a critical uʹ_c between linear and exponential increase in MIE_T with uʹ increased. (2) A non-monotonic MIE transition, where the lower bound is to be set by a MIE_T at some uʹ_c, stems from a great influence of Lewis number and spark gap despite turbulence. At sufficiently large Lewis number >> 1 and small spark gap (typically less than 1 mm), turbulence facilitated ignition (TFI), where MIE_T < MIE_L, occurs; then MIE_T increases rapidly at larger uʹ > uʹ_c because turbulence re-asserts its dominating role. Both phenomena are explained by the coupling effects of differential diffusion, heat losses to electrodes, and turbulence on the spark kernel. In particular, the ratio of small-scale turbulence diffusivity to reaction zone thermal diffusivity, a reaction zone Péclet number, captures the similarity of monotonic MIE transition, regardless of different ignition sources (conventional electrodes versus laser), turbulent flows, pressure, and fuel types. Furthermore, TFI does and/or does not occur when conventional spark is replaced by nanosecond-repetitively-pulsed-discharge and/or laser spark. The latter is attributed to the third lobe formation of laser kernel with some negative curvature segments that enhance reaction rate through differential diffusion, where MIE_L < MIE_T (no TFI). Finally, the implications of MIE transitions relevant to lean-burn spark ignition engines are briefly mentioned, and future studies are suggested.     

Influence of interface conditions on laser diode ignition of pyrotechnic mixtures: application to the design of an ignition device

This paper treats of numerical modelling which simulates the laser ignition of pyrotechnic mixtures. The computation zone is divided into two fields. The first is used to take account of the heat loss with the outside. It can represent an optical fibre or a sapphire protective porthole. The second field represents the reactive tablet which absorbs the laser diode's beam. A specific feature of the model is that it incorporates a thermal contact resistance R_c between the two computation fields. Through knowledge of the thermal, optical and kinetic properties, this code makes it possible to compute the ignition conditions. The latter are defined by the energy E₅₀ and the time t_i of ignition of any pyrotechnic mixture and for various ignition systems.This work was validated in the case of an ignition system consisting of a laser diode with an optical lens re-focussing system. The reactive tablet contains 62% by mass of iron and 38% by mass of KClO₄. Its porosity is 25.8%. After an evaluation of the laser's coefficient of absorption, the variations of the ignition parameters E₅₀ and t_i are studied as a function of the thermal contact resistance R_c. Temperature profiles are obtained as a function of time and for various values of the thermal contact resistance R_c. More fundamental observations are made concerning the position of the hot spot corresponding to priming. From this study, which concerns the heat exchange between the two media, several practical conclusions are given concerning the design of an ignition device. By evaluation of the thermal contact resistance R_c, comparison with test results becomes possible and the results of the computations are in reasonable agreement with the test measurements.     

A new correlation for turbulent mass transfer from liquid droplets

This paper presents a new correlation for mass transfer from single liquid droplets into a turbulent environment. Experiments were carried out under ambient room temperature and pressure. Homogeneous isotropic turbulence with zero-mean velocity was generated by eight identical electrical fans placed on the eight corners of a cubic chamber. The LDV technique was used to characterize the turbulence inside the chamber. The vaporization of fiber suspended droplets of five different n-alkanes and the bi-component droplet of n-heptane and n-decane mixtures subjected to varying turbulent kinetic energy is investigated by imaging techniques. For mono-component droplets the d²-law holds for all fuels and turbulent kinetic energies, and the vaporization rates increase with increasing the turbulent kinetic energy. Bi-component droplets exhibit a sequential vaporization behavior for all mixtures and turbulent kinetic energies. The instantaneous vaporization rates increase with increasing turbulence kinetic energy and increasing volume fraction of the highest volatility component. The proposed correlation predicts the vaporization rates of mono and bi-component n-alkane droplets subjected to isotropic turbulence with zero-mean velocity.     

Influence of heat release and turbulence on scalar dissipation rate in autoigniting n-heptane/air mixtures

Scalar dissipation rate χ is an important parameter in stratified charge combustion because it is a measure of the extent of fuel/air mixing in the flow field. In this work, the influence of heat release on the evolution of χ in laminar and turbulent autoigniting stratified n-heptane/air mixtures is studied. Multi-step kinetics is employed to model n-heptane oxidation. The pressure and temperature conditions selected are relevant to compression-ignited engines. Ignition is initiated in fuel-rich regions. Subsequently, an ignition front is observed to propagate from the initiation location to the final flame stabilization location. The influence of heat release on scalar dissipation rate is observed to be controlled by a balance of two competing effects: expansion of burned gases which decreases χ and increased laminar diffusivity due to combustion which increases χ. Behind the propagating ignition front, decrease in χ due to expansion is greater than its increase due to increased laminar diffusivity; therefore, the net effect of heat release on χ is to decrease it. The magnitude of decrease in χ due to expansion is observed to increase with increasing compositional gradients in the flow field. Ahead of the ignition front, the increased diffusivity effect is more pronounced than the expansion effect which results in an increase in χ. Turbulence, as expected, modifies the initially specified scalar dissipation rate to generate a range of scalar dissipation rates around the initial value. Turbulent mixing creates well-mixed spots with low χ, and turbulent strain creates regions with high χ. Ignition is observed in the regions with low χ, and the expansion effect due to heat release further decreases the scalar dissipation rate at such locations.     

Numerical study on the spark ignition characteristics of hydrogen-air mixture using detailed chemical kinetics

Hydrogen is a promising fuel and is expected to replace hydrocarbon fuels for its significant potentials to reduce the pollutants and greenhouse gases. It is very important to investigate Minimum ignition energy (MIE) on safety standards and ignition process of hydrogen-air mixtures. Even though the formation of flame kernels in quiescent hydrogen-air mixtures has been researched numerically and experimentally, the details of ignition mechanism have never been satisfactorily explained. In this study, the spark ignition of hydrogen-air mixture is investigated by using detailed chemical kinetics and considering the heat loss to the electrode. The purpose of this study is emphasized in the effects of the energy supply procedure, the radius of the spark channel, electrode size and electrode gap distance on the MIE. In addition, the effects of mixture temperature, electrode gap distance and electrode size on relationship between the equivalence ratio and the MIE are examined.     

Experimental study on the turbulent premixed combustion characteristics of 70%H2/30%CO/air mixtures

To investigate the effect of equivalence ratio and turbulence intensity on the combustion characteristics of syngas/air mixtures, experiments involving premixed combustion of 70% H₂/30% CO/air mixtures at various equivalence ratios and turbulence intensities were conducted in a turbulent combustion bomb at atmospheric temperature and pressure. The turbulent burning velocity and flame curvature were used to study turbulent combustion characteristics. The results show that the turbulent burning velocity grew nonlinearly as the equivalence ratio increased, while the normalized turbulent burning velocity tended to decrease. When the equivalence ratio was relatively low, the turbulence intensity was a greater determinant of the burning velocity. The normalized turbulent burning velocity increased as the turbulence intensity increased. Re and Da were found to be directly and inversely proportional to u’/u_L, respectively. A linear relationship was observed between u_T/u_L and ln Re. As the turbulence intensity increased or equivalence ratio decreased, the wrinkle degree of the flame front increased, and the maximum and minimum values of flame front curvature increased and decreased, respectively. Meanwhile, the range of the flame front curvature increased gradually. The proportion of components with smaller absolute value of flame front curvature gradually decreases.     

Effect of igniter type and number of igniters on vented deflagrations for near lean flammability limit hydrogen-air mixtures in a large scale rectangular volume

This paper examines the effect of igniter type (glow plug vs. spark igniter) and number of igniters on the dynamics of vented combustion under both initially quiescent and fan-induced turbulent conditions. These experiments are a subset of many series of tests performed in a 120 m³ large scale vented combustion test facility at the Canadian Nuclear Laboratories using near lean flammability hydrogen-air mixtures (8–12% H₂). One of the objectives of these studies was to have a better understanding of the effectiveness of deliberate ignition for mitigation of hydrogen during a postulated accident and to provide data for code validation. The test results of the current study show that the two types of ignition sources have no significant influence on the maximum combustion overpressure except that the initial burning rate is slightly faster using the spark igniter. Under either the quiescent or turbulent conditions, the maximum combustion overpressure always increases with an increase in the number of igniters, but under the current experimental conditions, the turbulent combustion overpressure with one igniter is always higher than quiescent combustion with multiple igniters.     

A shock tube study of iso-octane ignition at elevated pressures: The influence of diluent gases

The ignition of iso-octane/air and iso-octane/O₂/Ar (∼20% O₂) mixtures was studied in a shock tube at temperatures of 868-1300 K, pressures of 7-58 atm, and equivalence ratios Φ=1.0, 0.5, and 0.25. Ignition times were determined using endwall OH^∗ emission and sidewall piezoelectric pressure measurements. Measured iso-octane/air ignition times agreed well with the previously published results. Mixtures with argon as the diluent exhibited ignition times 20% shorter, for most conditions, than those with nitrogen as the diluent (iso-octane/air mixtures). The difference in measured ignition times for mixtures containing argon and nitrogen as the diluent gas can be attributed to the differing heat capacities of the two diluent species and the level of induction period heat release prior to ignition. Kinetic model predictions of ignition time from three mechanisms are compared to the experimental data. The mechanisms overpredict the ignition times but accurately capture the influence of diluent gas on iso-octane ignition time, indicating that the mechanisms predict an appropriate amount of induction period heat release.     

Experimental study of Markstein number effects on laminar flamelet velocity in turbulent premixed flames

Effects of turbulent flame stretch on mean local laminar burning velocity of flamelets, , were investigated experimentally in an explosion vessel at normal temperature and pressure. In this context, the wrinkling, At/Al, and the burning velocity, ut, of turbulent flames were measured simultaneously. With the flamelet assumption the mean local laminar burning velocity of flamelets, , was calculated for different turbulence intensities. The results were compared to the influence of stretch on spherically expanding laminar flames. For spherically expanding laminar flames the stretched laminar burning velocity, un, varied linearly with the Karlovitz stretch factor, yielding Markstein numbers that depend on the mixture composition. Six different mixtures with positive and negative Markstein numbers were investigated. The measurements of the mean local laminar burning velocity of turbulent flamelets were used to derive an efficiency parameter, I, which reflects the impact of the Markstein number and turbulent flame stretch—expressed by the turbulent Karlovitz stretch factor—on the local laminar burning velocity of flamelets. The results showed that the efficiency is reduced with increasing turbulence intensity and the reduction can be correlated to unsteady effects.     

Numerical investigation on the start-up of methane-fueled catalytic microreactors

Transient simulations have been performed in a plane-channel, methane-fueled microreactor made of either cordierite or FeCr alloy walls and coated with a platinum catalyst. A two-dimensional model for the flow domain was used, which included detailed catalytic and gas-phase chemical reaction mechanisms. In the solid wall, axial heat conduction and surface radiation heat transfer were accounted for. Simulations were performed by varying the inlet pressure, the solid wall thermal conductivity and heat capacity, the inlet velocity, and the equivalence ratio at fuel-lean stoichiometries. The effect of solid material properties as well as the impact of gas-phase chemistry and surface radiation on the ignition (t_ig) and steady-state (t_st) microreactor times has been assessed. An increase in inlet pressure from 1 to 5 bar induced a ∼50% reduction in both t_ig and t_st owing to the enhancement of the catalytic reactivity with rising pressure. A similar behavior was also attested when increasing the equivalence ratio from 0.4 to 0.6. Reactors with low wall thermal conductivity (cordierite material) exhibited shorter ignition times compared to higher thermal conductivity ones (FeCr alloy) due to the creation of spatially localized hot spots that promoted catalytic ignition. At the same time, the ceramic material required shorter times to reach steady-state. Higher inlet velocities reduced the time required for steady-state, however, at the cost of increased cumulative reactor emissions. Surface radiation heat transfer played a key dual role in the start-up process of low thermal conductivity channels. Radiation increased t_ig by removing heat away from the initial hot spot, but from the other side it decreased t_st due to a very efficient transfer of heat from the rear to the front of the reactor. Gas-phase chemistry elongated the steady-state times for both ceramic and metallic materials and impacted the emissions of catalytic microreactors.