Diffusive-thermal Instability Research Articles

Numerical study of external explosion in vented hydrogen explosions

In the current study, numerical study of vented hydrogen explosions was performed utilizing computational fluid dynamics (CFD) software GASFLOW-MPI. A turbulent combustion model based on Schmidt correlation was formulated. Within this model, flame instabilities resulted from two intrinsic effects, Hydrodynamic instability, and Landau-Darrieus and Thermal-Diffusive instabilities were incorporated. The numerical simulation results revealed the mechanism of overpressure evolution inside and outside the vessel. Notably, the mechanism of the overpressure peak induced by the external explosion was revealed. The effects of turbulence models on overpressure time profiles were investigated. Moreover, it was determined that heat transfer, arising from thermal radiation and convection, exerts only a negligible influence on the maximum internal overpressure. Subsequently, the performance of GASFLOW-MPI in simulating vented hydrogen explosions for different ignition locations (center and rear ignitions) and varying hydrogen concentrations (22%-38%) was assessed against experimental data. Comparative analysis revealed a close agreement between the predicted results and experimental data. Furthermore, the competency of GASFLOW in simulating medium-scale vented hydrogen explosions was validated against experimental data.

Dynamics of fuel-lean and stoichiometric methane-air explosion venting: Experiments and simulations

The computational explosion venting analyzer (EVA), a zero-dimensional (0D) model based on the conservation of mass and energy, is being developed to model centrally- and rear-ignited explosions. For this purpose, a series of explosion venting experiments in a cylinder with vent areas of 132.7, 86.6, 67.9 cm2 (corresponding to the vent area ratios of K=0.47,0.31,0.24, respectively) is performed. Two equivalence ratios of ϕ=0.8 and 1 are considered to represent the fuel-lean and stoichiometric methane-air mixtures, respectively. The dynamics of explosions is studied through the observation of flame propagation and pressure measurements. In rear ignition experiments, laminar, so-called “finger flame” propagation is observed, while in the case of center ignition, a flame initially expands spherically and then is pulled by the vent, acquiring a half-elliptical and half-spherical shape. The peak pressures obtained from rear ignition exceed their counterparts in the center ignition experiments. The EVA is compared with the experimental matrix. No turbulence is implemented in stoichiometric simulations, and slight turbulence has been accounted for in the lean mixture simulations. It is found that the large vent, generally, imposes more disturbances on the flame shape and the fuel-lean mixtures are more prone to the diffusional-thermal instabilities. It is shown that such a simple numerical tool, as the EVA is, can estimate a complicated problem such as pressure evolution resulted from a vented gas explosion.

The stability and morphology of laminar premixed hydrogen/air flames under various gravity conditions

In the present work, direct numerical simulations (DNS) were employed to understand the stability and morphology of laminar lean premixed hydrogen/air flames under various gravity conditions. The combined effect of Darrieus–Landau (DL) instability, thermal-diffusive (TD) instability and Rayleigh–Taylor (RT) instability was studied quantitatively for both the initial linear regime and the long-term nonlinear regime, with a particular focus on the effect of RT instability. In the linear regime, it was found that the RT instability has a negligible effect on the linear growth rate of flames with an equivalence ratio of 0.4, but the effect is significant when the equivalence ratio decreases to 0.3. The performance of various models for the dispersion relation was discussed, including a theoretical model considering the effects of all instabilities, which captures well the trend of the simulated results in the cases with unity Lewis numbers. The hydrogen/air flames with an equivalence ratio of 0.3 was investigated comprehensively in the nonlinear regime. For the cases with unity Lewis numbers, the RT-unstable flame exhibits a similar single-cusp structure as the RT-neutral flame, but with a deeper cusp and larger consumption speed as a result of the destabilizing effect of RT instability. The RT-stable flame gradually returns to a planar flame shape owing to the stabilizing effect of RT instability. For the cases with non-unity Lewis numbers, the effect of domain size on the flame evolution was investigated, showing a domain width of 200δT is enough to obtain a domain-independent result, where δT is the laminar flame thickness. It was found that the cases initialized with multimode perturbations reach the quasi-steady state faster, while the flame statistics are consistent with those of the cases initialized with single mode perturbations. The effect of RT instability was explored by comparing the results from various gravity conditions. The flame consumption speed is the largest in RT-unstable flames, and the smallest in RT-stable flames. The variation in flame consumption speed is due to the difference in the flame surface area, while the local reactivity and flame structure are not influenced by the RT instability. Further fractal analysis reveals that although the RT instability causes significant differences in flame surface area, the characteristic fractal dimension remains constant for flames under different gravity conditions.Novelty and significanceIn the present work, direct numerical simulations were employed to understand the stability and morphology of RT-unstable, RT-neutral and RT-stable laminar lean premixed hydrogen/air flames. For the first time, the effect of DL, TD and RT instabilities was quantitatively examined for both the linear regime and the nonlinear regime. A theoretical model considering various instabilities was compared with the simulated dispersion relations in the linear regime. In the non-linear regime, the flame surface area and corresponding flame consumption speed are influenced by the RT instability, while the flame stretch factor remains unaffected. A fractal analysis was conducted in laminar premixed hydrogen/air flames under the influence of various instabilities. Remarkably, it was discovered that the characteristic fractal dimension remains constant for the cases with and without the RT instability.

The premixed flame structure and combustion mechanism of clean hydrogen mixed with multi-component natural gas

Hydrogen is widely taken for the most potential clean energy for the moment. Currently, pipeline transport of natural gas mixed with hydrogen is one of the primary approaches of hydrogen transportation. However, compared to pure natural gas (CH4), the explosion and combustion of natural gas mixed with hydrogen may lead to more severe consequences, and its reliability needs to be considered. Existing research of combustion and explosion using methane instead of natural gas have yet to be considered for application to real multi-component natural gas. To address this issue, real multi-component natural gas is configured, and the premixed flame and combustion characterization parameters of natural gas/H2/air are obtained through experiment. The chemical reaction kinetics of natural gas/H2/air mixtures are analyzed using Chemkin Pro software. These results prove that the laminar burning (LB) velocity varies between 0.33 m/s and 2.34 m/s by adding H2. The primitive reactions R1 (H2 + OH → H + H2O) and R3 (H + O2 → O + OH) predominantly contribute to enhancing the LB velocity. The Markstein lengths rang from −0.19 mm to −0.049 mm, most lewis numbers are less than 1, the flame thicknesses and critical destabilization radii are reduced. Additionally, hydrodynamic instability and thermal-diffusion instability are strengthened by adding the hydrogen. As for smaller hydrogen content, the average size of flame cell from about 8 mm program first rises and then falls cyclic fluctuations, fluctuation amplitude gradually reduces, fluctuation frequency accelerates. In larger hydrogen content, this fluctuation amplitude is reduced and the average size of flame cell decreases. In addition, H2 enhances the explosive strength of natural gas. Finally, methane instead of natural gas may exaggerate the velocity of laminar burning and exaggerate or weaken the ignition time delay of natural gas with different hydrogen contents. The current research might provide crucial information for the safe transport of hydrogen mixed with natural gas.

Combined effects of ignition position and hydrogen ratio on vented CH4/H2/air explosions

Experiments on the effects of ignition position and hydrogen ratio (χ) on the explosions of CH4/H2/air mixtures were experimentally investigated in a vented cylindrical vessel. Relevant experiments were conducted utilizing ignition sources situated at the rear (rear ignition), the center of the vessel (central ignition), or near the vent (front ignition) with χ varying from 0 to 1.0. Two types of cellular structures on the flame surface, owing to the diffusional-thermal instability and acoustically enhanced combustion, respectively, were observed, and the latter resulted in acoustic oscillations of the overpressure within the vessel and a unique overpressure peak p2 with amplitude sensitive to both ignition position and χ. The effects of ignition position and χ on the build-up of the internal pressure were not significant when χ ≤ 0.15. The maximum explosion overpressure in the vessel (pmax) under the explosions of rear and central ignitions (RI and CI) increased monotonically as χ was increased from 0.3 to 1.0, but a nonmonotonic trend was found in the explosions of front ignition (FI). CI could be regarded as the worst-case scenario when 0.45≤χ ≤ 1.0 except for χ = 0.7, because FI resulted in the highest pmax at χ = 0.7. A pressure peak outside the vent (pext) caused by the combustion expansion of the combustible cloud could be distinguished when χ ≥ 0.45 in the explosions of RI and CI. The amplitude of pext increased with an increase in χ. Rear ignition always led to the highest pext when χ > 0.6.

Deflagration characteristics of N2 diluted propanal/air mixtures at elevated pressures

Hydrocarbon fuels, in addition to being direct sources of energy, also serve as crucial raw materials for numerous industrial processes. In this study, the oxidation of propanal to propionic acid under elevated pressures has garnered our attention due to the substantial heat release during the mixing of propanal and oxidants, posing considerable hazards to production. Fortunately, the introduction of N2 effectively mitigates these risks by reducing the laminar burning speed (SL) and peak deflagration pressure (Pm), and even preventing the ignition of mixtures. On this basis, the deflagration characteristics of propanal/N2/air mixtures were investigated in a 20 L spherical vessel at 348 K under various equivalent ratios (φ, 0.5 - 2.0), dilution rates (D, 0 - 0.5) and initial pressures (P0, 0.1 - 1.0 MPa). The combustion characteristics of propanal/N2/air were also analyzed through the 1-D laminar flame speed model, and the sensitivity analysis was performed to elucidate the effect of φ, D and P0 on the combustion of propanal. Results reveal that the addition of N2 stabilizes the flame propagation, and the Pm of propanal/N2/air mixtures decreases with the increasing D, with a sudden drop observed at a certain D level. However, the reduction of SL does not undergo an abrupt decrease across the entire dilution rate range. Furthermore, Pm exhibits a linear relationship with P0 at lower D, but this linear trend disappears once D approaches the critical suppression concentration. Interestingly, the critical suppression concentration of N2 increases with P0 at fuel-rich conditions, yet remains unaffected by P0 at fuel-lean conditions. Near the critical suppression concentration, a limiting burning speed (Slim) emerges, below which the flame is incapable of spontaneous propagation. Notably, Slim is not a fixed value, but varies according to the composition and initial state of the mixture. For a given gas component, an increase in P0 leads to a decline in Slim, while for gases with varying components, Slim is observed to be lower under fuel-rich conditions compared to fuel-lean scenarios. Also, both the addition of N2 and the augmentation of P0 significantly reduce the SL. When SL is comparable to the gas rise speed due to natural convection, the flame propagation becomes vulnerable to buoyancy instability. Meanwhile, the flame propagation may also be influenced by thermal diffusion instability and hydrodynamic instability. Under the combined effect of these instabilities, the flame is prone to be unstable. More importantly, when the flame contacting the upper wall of the is in a critical state where it can or cannot propagate downward, the irregularity of the disturbed flame may introduces uncertainty into the subsequent propagation, resulting in markedly different Pm even among mixtures with identical compositions.

An experimental study on the flame instability of NH3/DME blends with H2 addition

It is of significance to investigate the effect of H2 on the combustion of NH3/DME blends since NH3 can partially decompose to produce hydrogen (H2) in the combustion process. However, few studies were conducted on NH3/DME/H2 mixtures, especially for flame instability. This work presents an experimental study of the flame instability of NH3/DME blends with H2 addition. Experiments were conducted using the spherical constant-volume combustion method at φ = 0.7 – 1.4, Tu = 298 K. The influences of NH3/DME blend ratio (BNH3/DME = 80/60, 60/40, 40/60, and 20/80), initial pressure (Pu = 0.1 – 0.5 MPa), and H2 fraction (XH2 = 0%, 20%, and 40%) on the flame instability were examined in detail. The results show that increasing either DME fraction or initial pressure can promote the hydrodynamic instability of NH3/DME/air flames because of the decreased flame thickness. For both XH2 = 0% and 20%, the cellular flame instability with BNH3/DME = 40/60 and 20/80 increases with increasing equivalence ratio. For XH2 = 40%, the flame instability shows a similar trend with BNH3/DME = 20/80, but the most unstable flame appears around stoichiometric combustion with BNH3/DME = 40/60. This is due to the compound effects of the diffusive-thermal imbalance and hydrodynamic instability for lean or rich fuel, while the diffusive-thermal instability can be neglected for stoichiometric combustion. Therefore, an effective control of H2 or DME addition can help enhance the combustion performance of the fuel while reducing the occurrence of dangerous events caused by potential flame instabilities. In addition, critical parameters, i.e., critical flame radius rcr, critical Peclet number Pecr, and critical Karlovitz stretch factor Kacr were analyzed to quantify the onset of flame instability. This work provides insights into the flame instabilities of NH3/DME with H2 addition.

Studies on properties of laminar premixed oxygen-enriched NH3/H2/O2/N2 flames at elevated temperature and pressure

In this work, the effects of oxygen enrichment coefficient (Ω = 0.21 − 0.42) and initial temperature (Tu = 298 − 448 K) on the laminar burning properties of NH3/H2/O2/N2 flames are systematically investigated by using the spherical flame propagation method at equivalence ratios (Φ = 0.8 − 1.4) and initial pressure (Pu = 3 atm). LBV is analyzed by obtaining experimental data. In order to analyze flame instability, thermal diffusion and hydrodynamic instability are separated by linear stability theory. The thermal and chemical effects of oxygen enrichment on LBV and its effect on heat release rate are analyzed by numerical simulation. The results show that the laminar burning velocity (LBV) exhibits a positive correlation with both the oxygen enrichment coefficient and the initial temperature. The temperature index (α) exhibits a non-monotonic variation in response to the initial temperature of the NH3/H2/O2/N2 flame. Leeff is insensitive to temperature and positively correlated with the oxygen enrichment coefficient. The NH3/H2/O2/N2 flame is mainly affected by hydrodynamic instability when the oxygen enrichment coefficient is large. The peak NHRR scales positively with the oxygen enrichment coefficient. Oxygen enrichment helps to contribute to the overall net reaction rate.

Evaluation of thermal radiation and Lewis number effects on oscillatory thermal-diffusive instabilities of counterflow premixed flame fed with moisty biomass particles

In this study, a thermal-diffusive model using a sinusoidal function for the oscillation term is presented. The model is employed to evaluate the pulsating behavior in a premixed counterflow flame fed with biomass particle cloud considering Lewis number and thermal radiation effects. In order to scrutinize the pulsating characteristics of a flame, time-related governing equations are written for the assumed multi-zone system and analytically solved utilizing Matlab and Mathematica software. To enforce continuity, suitable matching conditions for the temperature and heat flux at the interfaces are derived. Time-dependent characteristics of burning velocity, flame temperature, flame front position, reactants temperature, and mass distributions are presented, taking into account effective Lewis number, thermal radiation, and strain rate influences. An acceptable comparison with numerical simulations affirms the reliability of the mathematical procedure. The obtained results indicated that the pulsating instability of biomass-fueled premixed flames in counterflow arrangements significantly relevant not only to the effective Lewis number but also to thermal radiation transferred from the reaction zone to unburnt reactants. Furthermore, it is found that in counterflow premixed flames, strain rate, in addition to Lewis number and thermal radiation, can be the key to controlling the individual flame oscillation modes.

Study of CH4-H2-CO-Air premixed combustion characteristics based on fractal dimension and flame crack method

The laminar combustion characteristics of CH4-H2-CO-Air are studied in a constant volume combustion chamber with different hydrogen content ratios and the initial conditions of 300 K and 0.2 MPa by schlieren technology. The influence laws of flame instability are analyzed through the influence laws of hydrogen content ratio and equivalence ratio (Φ) on the fractal dimensions of the flame contour and the ratio of the total flame face cracks' lengths to the flame radius (L/R), as well as the influence laws of hydrodynamic instability and thermal-diffusive instabilities on the flames. The results show that L/R is more sensitive to flame instabilities than flame front fractal dimensions due to a wider sampling range. After R exceeds a certain value, for lean and rich combustion conditions with the same distance from the stoichiometric ratio, the L/R is greater during lean combustion, and as the equivalence ratio gradually increases from 0.6 to 1.4, the effect of increasing hydrogen content (Hydrogen mole fraction ranges from 60% to 66.67%) on flame instability gradually shifts from promotion to suppression. This study has certain reference value for the combustion application of biomass synthesis gas.

Experimental study on the explosion characteristics of ammonia-hydrogen-air mixtures

Ammonia-hydrogen blended fuel has drawn substantial attention as an environmentally friendly, carbon-free fuel because it effectively overcomes the inherent disadvantages of pure ammonia in combustion performance, such as a high ignition energy, slow combustion rate, and low combustion efficiency. While recent research on the combustion performance of hydrogen-ammonia blends has advanced, there remains a scarcity of studies addressing the explosion characteristics of these fuels. In this paper, a series of experiments conducted within a standard 20-L spherical combustion chamber to investigate the explosion characteristics of ammonia-hydrogen-air premixed gases under diverse conditions, spanning from low to high pressures (0.02–0.3 MPa) and from fuel-lean to fuel-rich scenarios (0.7–1.5), are discussed. The results reveal that the explosion characteristics are pronouncedly enhanced by hydrogen addition. The maximum explosion pressure (Pmax) and maximum pressure rise rate ((dP/dt)max) increase, while the explosion time (texp) decreases with increasing hydrogen concentration. Pmax and (dP/dt)max exhibit linear increases as the initial pressure rises and reach their peaks at an equivalence ratio of 1.1; texp displays nonmonotonic variations with increasing initial pressure and equivalence ratio. Hydrogen addition remarkably increases the hydrodynamic instability and thermal-diffusion instability of the reactive mixture, enhances the thermal-mass diffusion on the flame front, and promotes flame deformation and cellular structure formation, resulting in an improvement of the flame propagation and compression effect on the unburnt reactant before the flame. Research into the explosion characteristics provides valuable insights for the safe storage and transportation of ammonia-hydrogen blended fuels. These results indicate avenues for further enhancing the detonation characteristics and explosion performance of ammonia-hydrogen blended fuels.

Diffusive-thermal instabilities in unstrained H2O-diluted syngas diffusion flames

A new version of the unstrained diffusion flame burner that can be operated with gaseous fuels containing high vapor content is introduced. Being a good approximation of the classical chambered diffusion flame solution, the flames generated are nominally unstrained, unlike common research burners where hydrodynamic effects are significant. This permits quantitative comparison with theoretical models that are often based on this simple configuration, and paves the way for fundamental experimental studies with vaporized fuels. In this paper, the capabilities of the new burner design are exploited to study diffusive-thermal instabilities (DTIs) in H2O-diluted H2-CO-CH4-CO2 mixtures. H2O dilution can be significant in biomass-derived syngas mixtures that are not cooled prior to combustion, and that are often burned directly to lower losses as waste heat and pollutant emission in practical combustors. Flammability limits are first presented for a broad range of fuel blends, where the destabilizing effect of H2O dilution is discussed. Instability maps in terms of the Damköhler number are then provided to illustrate the different types of superimposed cellular-pulsating instabilities that onset from the simultaneous presence of H2 with high diffusivity, and CO/CH4 with much lower mobility. The characteristics of these peculiar instabilities are highly dependent on the H2O dilution fraction, which increases both the fuel blend and oxidizer Lewis numbers. The degree of cellularity superimposed in the pulsating multi-fuel flame is reduced at higher water content, as the number of observed cells decreases. The opposite effect is observed on the pulsation frequency, which increases at higher water concentrations.

Cellular instabilities of outwardly propagating spherical hydrogen-oxygen flames using a soap bubble method

Explosion hazards associated with hydrogen-oxygen mixtures remain a significant challenge for the energy industry, despite hydrogen garnering attention as a carbon emissions-free alternative energy source. While several studies have been conducted on hydrogen-air explosions, research on hydrogen-oxygen explosions has been limited. In the current study, we employed a spherical soap bubble method to experimentally investigate the flame propagation behavior of an unconfined hydrogen-oxygen explosion. The experimental values of the laminar burning velocity were in agreement with its calculated values and were notably higher than those observed hydrogen-air mixtures. In the case of hydrogen-oxygen mixtures, a spherical expanding flame is accelerated due to the presence of Darrieus-Landau and diffusive-thermal instabilities. Furthermore, we compared the correlation between the critical Pѐclet number and the critical Karlovitz number with the Markstein number for hydrogen-oxygen mixtures to results obtained with propane-oxygen and methane-oxygen mixtures.

Effects of cryogenic temperature on turbulent premixed hydrogen/air flames

As a carbon-free fuel, hydrogen (H2) is of increasing importance in the development of low-emission engines. Due to a low volumetric energy density, H2 is preferably stored at cryogenic temperatures (Tu≤100 K). In this context, it is indispensable to investigate the combustion behavior of hydrogen at such low temperatures. Although some studies focused on the ignition and detonation of hydrogen, investigations about premixed H2/air flame propagation interacting with turbulence at cryogenic temperatures are rather scarce. In this work, stoichiometric turbulent premixed H2/air flames are studied at cryogenic temperature (Tu=100K) and normal temperature (Tu=300K), using three-dimensional direct numerical simulations with detailed chemistry and transport. It is found that at cryogenic temperature, dimensionless turbulent flame speed and flame surface area increase significantly due to Darrieus–Landau instability (DLI) induced by a large expansion ratio. Since the effective Lewis numbers of the two cases are close to unity, the diffusive-thermal instability (DTI) is negligible in the cases. Furthermore, it is found that there are substantial differences in the peaks of HO2 and H2O2 mass fractions between the two cases, probably due to smaller local flame curvatures at cryogenic temperature. Moreover, the results indicate that the flame response to stretch is not sensitive to the change of the initial temperature. A larger fractal inner cutoff scale is found at Tu=100K, suggesting that the flame exhibits more large-scale flame wrinkling than that at Tu=300K due to the impact of DLI. All these facts lead to the conclusion that cryogenic temperature can significantly promote large-scale flame wrinkling, increase turbulent flame speed and flame surface area, and affect intermediate species distribution. This suggests that combustion of cryogenic H2 may have a high risk of flashback.

Three-dimensional diffusive-thermal instability of flames propagating in a plane Poiseuille flow

The three-dimensional diffusive-thermal stability of a two-dimensional flame propagating in a Poiseuille flow is examined. The study explores the effect of three non-dimensional parameters, namely the Lewis number Le, the Damköhler number Da, and the flow Peclet number Pe. Wide ranges of the Lewis number and the flow amplitude are covered, as well as conditions corresponding to small-scale narrow (Da≪1) to large-scale wide (Da≫1) channels. The instability experienced by the flame appears as a combination of the traditional diffusive-thermal instability of planar flames and the recently identified instability corresponding to a transition from symmetric to asymmetric flame. The instability regions are identified in the Le-Pe plane for selected values of Da by computing the eigenvalues of a linear stability problem. These are complemented by two- and three-dimensional time-dependent simulations describing the full evolution of unstable flames into the non-linear regime. In narrow channels, flames are found to be always symmetric about the mid-plane of the channel. Additionally, in these situations, shear flow-induced Taylor dispersion enhances the cellular instability in Le<1 mixtures and suppresses the oscillatory instability in Le>1 mixtures. In large-scale channels, however, both the cellular and the oscillatory instabilities are expected to persist. Here, the flame has a stronger propensity to become asymmetric when the mean flow opposes its propagation and when Le<1; if the mean flow facilitates the flame propagation, then the flame is likely to remain symmetric about the channel mid-plane. For Le>1, both symmetric and asymmetric flames are encountered and are accompanied by temporal oscillations.

Propagation characteristics of lean turbulent premixed ammonia–hydrogen flames

Direct numerical simulations (DNS) of fuel-lean turbulent premixed NH3-H2-N2-air flames are analyzed to investigate propagation and flame structural characteristics under fixed velocity and length ratios. To comprehensively assess the impact of diffusive-thermal imbalances on hydrogen–enriched ammonia flames, additional solutions with unity-Lewis-number transport were analyzed and compared with those obtained using the mixture-averaged transport model. The increase of H2 fraction in the fuel leads to elevated mean turbulent flame speed and stretch factor, indicating the impact of thermal-diffusive instability. The turbulent flame speed of the 60%NH3-25%H2-15%N2-air flame displays pronounced oscillations, a phenomenon absent in other mixtures considered in the current study. This behavior is attributed to the preferential diffusion of H2 mixed with the low-reactive NH3 in moderate quantities, resulting in higher generation of flame elements extending into the product side and dynamic evolution of H2. The flame structure analysis, in terms of conditional averages, revealed a distinctive variation in H2 and H atom distributions. The flames with a higher H2 fraction (40%NH3-45%H2-15%N2-air) produced a second peak of H2O2 in the trailing edge region, indicating additional production in the intense reaction zone. Additionally, in the 60%NH3-25%H2-15%N2-air flame, the reaction rate of H2 exhibited a unique behavior, with H2 being produced in the intermediate flame zone and rapidly consumed in the reaction zone, differing from other cases.

Flame propagation characteristics and thermal radiation hazards of methane-hydrogen-mixed cloud explosion in unconfined area: Experiment research and theoretical modeling

Under an equivalency ratio of 1.1, this manuscript studies the fireballs of methane-combustible gas clouds with varying hydrogen doping ratios (5 %, 10 %, 20 %, 30 %, 50 %, and 75 %). It then modifies the semiempirical-theoretical fireball heat radiation model based on flame size. Under 5%–30 % mixing ratio, flame growth characteristics are more comparable, and peak flame propagation speed is 18 m/s, whereas the mixing ratios of 50 % and 75 % are higher, take 0.1s to complete the accelerated process, and reach 26.7 and 36.7 m/s, respectively. In flames including methane hydrogen-doped mixtures, thermal diffusion instability and hydrodynamic instability are common. With an increase in the hydrogen doping ratio, the flame self-acceleration index shows a steady rising trend. Additionally, heat flux meter peak heat radiation values increase with hydrogen blending ratio, while the time to peak decreases. These findings align with the developmental process of flame morphology. Based on the observed flame development characteristics, a semi-empirical-theoretical model of heat radiation from the fireball is simplified. The results show that as the hydrogen doping ratio increases, the safety zone must be moved further away.

Confined deflagration of propanal/air in the presence of intrinsic instability at elevated pressures

The partial oxidation process is a chemical technique with a high risk of deflagration, which involves directly mixing of fuels with oxidizers at elevated conditions. Under non-standard conditions, the deflagration behaviors of fuels are often unavailable to the general public, which hampers progress in further basic research. In this study, the deflagration behaviors of propanal-air mixtures at 75 °C and 0.1–1.0 MPa were investigated, and the effect of initial pressure (P0) and equivalence ratio (φ) on the deflagration characteristics of propanal was discussed. To explain the similarities and differences in the deflagration characteristics under various conditions, the buoyancy instability, hydrodynamic instability and thermal-diffusion instability suffered by the flames were also analyzed. Results show that the pressure generated by the burned mixture is a cubic function of time, and the state of flame propagation can be determined by the monotonicity of the second-order derivative of pressure. Moreover, the peak deflagration pressure (Pm) is more sensitive to changes in φ on the fuel-lean side, but Pm/P0 at the fuel-rich condition is markedly higher than Pm/P0 at the fuel-lean condition. At all φ, an increase in P0 effectively shortens the ignition time and reduces the minimum ignition energy. Nevertheless, there are discernible differences in the variation patterns of the deflagration time (θm) with P0 at various φ. Specifically, θm gradually prolongs with increasing P0 at fuel-lean conditions, whereas it tends to be the same at different P0 and φ = 1. Furthermore, under fuel-rich conditions, θm progressively shortens as P0 increases. The decrease in θm is attributed to the increasing flame speeds in the presence of intrinsic instability, and the self-accelerating flame yields pressure waves, which travel faster than the flame. Then a series of reflected pressure waves are formed after the pressure waves strike the vessel wall, interacting with the flame front and resulting in the pressure oscillation. Moreover, increasing P0 advances the pressure oscillation and augments the intensity of pressure oscillation.

Turbulent Flame Speed and Flame Characteristics of Lean Premixed H2-CH4 Flames at Moderate Pressure Levels

Abstract Carbon dioxide emissions in gas turbine power generation can be reduced by adding an increasing amount of hydrogen to the existing natural gas-fueled combustion systems. To enable safe operation, more insight on how H2 addition affects turbulent flame speed and other important flame characteristics is needed. In this work, the investigation of hydrogen addition effects on certain flame properties has been carried out in a high-pressure axial-dump combustor at gas turbine relevant conditions. OH planar laser induced fluorescence (PLIF) was applied to retrieve flame front contours and turbulent flame speed. The results show that as the concentration of hydrogen in the fuel mixture increases, turbulent flame speed and flame characteristics change drastically. Two main regimes can be identified: From 0 to 50% vol. Hydrogen, the turbulent flame speed increases weakly in an almost linear fashion, while from 50% vol. to 100% vol. the trend sharply changes and the higher reactivity of hydrogen, combined with a lower Lewis number, cause thermal-diffusive instability and preferential diffusion effects to become increasingly strong, leading to very high burning rates. The presented results help to understand and to define the relevant modifications that are necessary to successfully operate gas turbine combustor systems with high H2 content fuels.

Propagation and extinction of premixed H2[sbnd]O2[sbnd]N2 edge-flames

The effects of equivalence ratio (ϕ), global strain rate (σ) and mixture strength on the shapes and propagation rates (Uedge) of premixed edge-flames in H2O2N2 mixtures were investigated experimentally using a counterflow slot-jet apparatus. Results are presented for both single (premixed gas against cold inert gas) and twin (premixed gas against premixed gas) configurations. Both advancing (Uedge > 0) and retreating edge-flames (Uedge < 0) were characterized. Flame images show that for lean mixtures where H2 is the deficient species and thus the effective Lewis number (Leeff) is less than unity, the leading edge-flame has a stronger burning intensity than the trailing strained premixed flame. For mixtures with low ϕ (thus low Leeff) at near-extinction conditions corresponding to sufficiently high or low σ, transitions from continuous to broken structures were observed due to diffusive-thermal instabilities. Such transitions enable flames to survive under conditions where extinction would have occurred without transition. The combined effects of ϕ, adiabatic flame temperature and σ were found to correlate well in terms of the effect of a non-dimensional strain rate (ε) on a scaled propagation rate Ũ.