Interactions of hydrogen and nitric oxide in outwardly propagating spherical flame: Insight into non-hydrocarbon NOX reduction mechanism

Co-firing ammonia (NH₃) and hydrogen (H₂) or H₂-rich fuel and partially cracking NH₃ are promising non-carbon combustion techniques for gas turbines and marine engines, raising a growing need to understand the interactions of H₂ and nitric oxide (NO) as well as the non-hydrocarbon nitrogen oxides (NO_X) reduction mechanism under flame conditions. In this work, the outwardly propagating spherical flame method was used to investigate the laminar flame propagation of H₂/NO and H₂/NO/nitrogen (N₂) mixtures at initial pressure (P_u) of 2 atm, initial temperature (T_u) of 298 K and equivalence ratios of 0.2-1.4. The laminar burning velocities (LBVs) of H₂/NO mixtures are generally 5-10 times lower than those of H₂/air mixtures, while the dilution of N₂ can dramatically inhibit the laminar flame propagation. A kinetic model of H₂/NO combustion was constructed and validated against the new data in this work and other types of experimental data in literature. The modeling analyses reveal that NO+H=N+OH becomes the most important chain-branching reaction in H₂/NO reaction system, while the LBV data of H₂/NO mixtures in this work can provide highly sensitive validation targets for the kinetics in H₂ and NO interactions. Furthermore, the NO reduction to N₂ mainly proceeds through NO+N=N₂+O under various H₂/NO ratios, and NO+O=N+O₂ is found to have a significant contribution to NO reduction under NO-rich conditions.     

Characterizing ammonia and nitric oxide interaction with outwardly propagating spherical flame method

With the growing attention on ammonia (NH₃) combustion, understanding NH₃ and nitric oxide (NO) interaction at temperatures higher than DeNOx temperature region or even flame temperature becomes a new research need. In this work, the outwardly propagation spherical flame method was used to investigate the laminar flame propagation of NH₃/NO/N₂ mixtures and constrain the uncertainties of the specific kinetics. The present experiments were conducted at initial pressure of 1 atm, temperature of 298 K and equivalence ratios from 1.1 to 1.9. A kinetic model of NH₃/NO combustion was updated from our previous work. Compared with several previous models, the present model can reasonably reproduce the laminar burning velocity data measured in this work and speciation data in literature. Based on model analyses, the interaction of NH₃ and NO was thoroughly investigated. As both the oxidizer and a carrier of nitrogen element, NO frequently reacts with different decomposition products of NH₃ including NH₂, NH and NNH, and converts nitrogen element to the final product N₂. It is found that the laminar burning velocity experiment of NH₃/NO/N₂ mixtures using the outwardly propagating spherical flame method can provide highly sensitive validation targets for the kinetics in NH₃ and NO interaction.     

An experimental and modeling study of ammonia with enriched oxygen content and ammonia/hydrogen laminar flame speed at elevated pressure and temperature

Laminar flame speeds of ammonia with oxygen-enriched air (oxygen content varying from 21 to 30 vol.%) and ammonia-hydrogen-air mixtures (fuel hydrogen content varying from 0 to 30 vol.%) at elevated pressure (1–10 bar) and temperature (298–473 K) were determined experimentally using a constant volume combustion chamber. Moreover, ammonia laminar flame speeds with helium as an inert were measured for the first time. Using these experimental data along with published ones, we have developed a newly compiled kinetic model for the prediction of the oxidation of ammonia and ammonia-hydrogen blends in freely propagating and burner stabilized premixed flames, as well as in shock tubes, rapid compression machines and a jet-stirred reactor. The reaction mechanism also considers the formation of nitrogen oxides, as well as the reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. The experimental results from the present work and the literature are interpreted with the help of the kinetic model derived here. The experiments show that increasing the initial temperature, fuel hydrogen content, or oxidizer oxygen content causes the laminar flame speed to increase, while it decreases when increasing the initial pressure. The proposed kinetic model predicts the same trends than experiments and a good agreement is found with measurements for a wide range of conditions. The model suggests that under rich conditions the N₂H₂ formation path is favored compared to stoichiometric condition. The most important reactions under rich conditions are: NH₂+NH=N₂H₂+H, NH₂+NH₂=N₂H₂+H₂, N₂H₂+H=NNH+H₂ and N₂H₂+M=NNH+H+M. These reactions were also found to be among the most sensitive reactions for predicting the laminar flame speed for all the cases investigated.     

Dilution effects analysis of opposed-jet H2/CO syngas diffusion flames

This paper reported the analysis of dilution effects on the opposed-jet H₂/CO syngas diffusion flames. A computational model, OPPDIF coupled with narrowband radiation calculation, was used to study one-dimensional counterflow syngas diffusion flames with fuel side dilution from CO₂, H₂O and N₂. To distinguish the contributing effects from inert, thermal/diffusion, chemical, and radiation effects, five artificial and chemically inert species XH₂, XCO, XCO₂, XH₂O and XN₂ with the same physical properties as their counterparts were assumed. By comparing the realistic and hypothetical flames, the individual dilution effects on the syngas flames were revealed. Results show, for equal-molar syngas (H₂/CO = 1) at strain rate of 10 s^?1, the maximum flame temperature decreases the most by CO₂ dilution, followed by H₂O and N₂. The inert effect, which reduces the chemical reaction rates by behaving as the inert part of mixtures, drops flame temperature the most. The thermal/diffusion effect of N₂ and the chemical effect of H₂O actually contribute the increase of flame temperature. However, the chemical effect of CO₂ and the radiation effect always decreases flame temperature. For flame extinction by adding diluents, CO₂ dilution favours flame extinction from all contributing effects, while thermal/diffusion effects of H₂O and N₂ extend the flammability. Therefore, extinction dilution percentage is the least for CO₂. The dilution effects on chemical kinetics are also examined. Due to the inert effect, the reaction rate of R84 (OH+H₂ = H+H₂O) is decreasing greatly with increasing dilution percentage while R99 (CO+OH→CO₂+H) is less affected. When the diluents participate chemically, reaction R99 is promoted and R84 is inhibited with H₂O addition, but the trend reverses with CO₂ dilution. Besides, the main chain-branching reaction of R38 (H+O₂→O+OH) is enhanced by the chemical effect of H₂O dilution, but suppressed by CO₂ dilution. Relatively, the influences of thermal/diffusion and radiation effects on the reaction kinetics are then small.     

Laminar flame propagation of acetone and 2-butanone at normal to high pressures: Insight into fuel molecular structure effects of ketones

This work reports an experimental and kinetic modeling investigation on the laminar flame propagation of acetone and 2-butanone at normal to high pressures. The experiments were performed in a high-pressure constant-volume cylindrical combustion vessel at 1–10 atm, 423 K and equivalence ratios of 0.7–1.5. A kinetic model of acetone and 2-butanone combustion was developed from our recent pentanone model [Li et al., Proc. Combust. Inst. 38 (2021) 2135–2142] and validated against experimental data in this work and in literature. Together with our recently reported data of 3-pentanone, remarkable fuel molecular structure effects were observed in the laminar flame propagation of the three C₃C₅ ketones. The laminar burning velocity increases in the order of acetone, 2-butanone and 3-pentanone, while the pressure effects in laminar burning velocity reduces in the same order. Modeling analysis was performed to provide insight into the key pathways in flames of acetone and 2-butanone. The differences in radical pools are concluded to be responsible for the observed fuel molecular structure effects on laminar burning velocity. The favored formation of methyl in acetone flames inhibits its reactivity and leads to the slowest laminar flame propagation, while the easiest formation of ethyl in 3-pentanone flames results in the highest reactivity and fastest laminar flame propagation. Furthermore, the LBVs of acetone and 3-pentanone exhibit the strongest and weakest pressure effects respectively, which can be attributed to the influence of fuel molecular structures through two crucial pressure-dependent reactions CH₃ + H (+M) = CH₄ (+M) and C₂H₄ + H (+M) = C₂H₅ (+M).     

Characterizing methane and nitric oxide interaction in oxygen-free outwardly propagating spherical flame

Co-firing methane (CH₄) and ammonia (NH₃) has attracted growing concerns as a feasible greenhouse gas reduction strategy in gas turbine-based power generation, which raises the need to better understand the interaction of methane and nitric oxide (NO) under flame conditions. In this work, laminar flame propagation of CH₄/NO mixtures at initial pressure (P_u) of 1 atm, initial temperature (T_u) of 298 K and equivalence ratios of 0.4–1.8 was experimentally investigated using a constant-volume combustion vessel. Laminar burning velocities (LBVs) and Markstein lengths were experimentally determined. A kinetic model of CH₄/NO combustion was developed with rate constants of several important reactions updated, presenting reasonable predictions on the measured LBVs of CH₄/NO mixtures. The modeling analyses reveal that the reduction of NO can proceed through two mechanisms, i.e. the hydrocarbon NO reduction mechanism and non-hydrocarbon NO reduction mechanism. Among the two mechanisms, the non-hydrocarbon NO reduction mechanism which includes reactions NO+H = N+OH, NO+O = N + O₂ and NO+N = N₂+O has a higher contribution to NO reduction at the equivalence ratio of 0.6, while the hydrocarbon NO reduction mechanism with hydrocyanic acid (HCN) as the key intermediate plays a more important role at the equivalence ratio of 1.8. NO+H = N+OH and CH₃+NOHCN+H₂O are found to be the two most sensitive reactions to promote the flame propagation, while the LBVs measured in this work are demonstrated to provide strong constraint for these reactions. Furthermore, previous CH₄/O₂/NO oxidation data measured in flow reactor and rapid compression machine were also simulated, which provides extended validation of the present model over wider conditions.     

An experimental and modeling study on auto-ignition of ammonia in an RCM with N2O and H2 addition

Deep insights into the combustion kinetics of ammonia (NH₃) can facilitate its application as a promising carbon-free fuel. Due to the low reactivity of NH₃, experimental data of NH₃ combustion can only be obtained within a limited range. In this work, nitrous oxide (N₂O) and hydrogen (H₂) were used as additives to investigate NH₃ auto-ignition in a rapid compression machine (RCM). Ignition delay times for NH₃, NH₃/N₂O blends, and NH₃/H₂ blends were measured at 30 bar, temperatures from 950 to 1437 K. The addition of N₂O and H₂ ranged from 0 to 50% and 0 to 25% of NH₃ mole fraction, respectively. Time-resolved species profiles were recorded during the auto-ignition process using a fast sampling system combined with a gas chromatograph (GC). An NH₃ combustion model was developed, in which the rate constants of key reactions were constrained by current experimental data. The addition of N₂O affected the ignition of NH₃ primarily through the decomposition of N₂O (N₂O (+M) = N₂ + O (+M), R1) and direct reaction between N₂O and NH₂ (N₂H₂ + NO = NH₂ + N₂O, R2). The rate constant of R2 was constrained effectively by experimental data of NH₃/N₂O mixtures. Two-stage ignition behaviors were observed for NH₃/H₂ mixtures, and the corresponding first-stage ignition delay times were reported for the first time. Experimental species profiles suggested the first-stage ignition resulted from the consumption of H₂. The oxidation of H₂ provided extra HO₂ radicals, which promoted the production of OH radicals and initiated first-stage ignition. Reactions between HO₂ radicals and NH₃/NH₂ dominated the first-ignition delay times of NH₃/H₂ mixtures. Moreover, the first-stage ignition led to the fast production of NO₂, which acted as a key intermediate and affected the following total ignition. Consequently, the reaction NH₂ + NO₂ = H₂NO + NO (R3) was constrained by total ignition delay times.     

Experimental and numerical study of product gas and N2O emission characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow

In order to achieve carbon neutrality, the use of ammonia as a fuel for power generation is highly anticipated. The utilization of a binary fuel consisting of ammonia and hydrogen can address the weak flame characteristics of ammonia. In this study, the product gas characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow were experimentally and numerically investigated for various equivalence ratios for the first time. A trade-off relationship between NO and unburnt ammonia was observed at slightly rich conditions. At lean conditions, NO reached a maximum value of 8,700 ppm, which was larger than that of pure ammonia/air flames. The mole fraction of nitrous oxide (N₂O) which has large global warming potential rapidly increased around the equivalence ratio of 0.6, which was attributed to the effect of a decrease in flame temperature downstream of the reaction zone owing to heat loss to the stagnation wall. To understand this effect further, numerical simulations of ammonia/hydrogen/air flames were conducted using the stagnation flame model for various equivalence ratios and stagnation wall temperatures. The results show that the important reactions for N₂O production and reductions are NH +NO = N₂O + H, N₂O + H = N₂ + OH, and N₂O (+M) = N₂ + O (+M). A decrease in flame temperature in the post flame region inhibited N₂O reduction through N₂O (+M) = N₂ + O (+M) because this reaction has a large temperature dependence, and thus N₂O was detected as a product gas. N₂O is reduced through N₂O (+M) = N₂ + O (+M) in the post flame region if the stagnation wall temperature is sufficiently high. On the other hand, it was clarified that an increase in equivalence ratio enhances H radical production and promotes N₂O reduction by H radical through the reaction of N₂O + H = N₂ + OH.     

Characterizing the fuel-specific combustion chemistry of acetic acid and propanoic acid: Laminar flame propagation and kinetic modeling studies

In order to study the combustion chemistry of carboxyl functionality, the laminar burning velocity of acetic acid/air and propanoic acid/air mixtures was investigated in a high-pressure constant-volume cylindrical combustion vessel at 423 K, 1 atm and equivalence ratios of 0.7–1.4. Experimental results reveal that the flame propagation of propanoic acid flame is much faster than that of acetic acid flame, especially under rich conditions, and the laminar burning velocity of propanoic acid/air mixtures peaks at richer conditions than that of acetic acid. The present theoretical calculations for the isomerization and decomposition of propanoic acid radicals indicate that the primary radical products are HOCO, H and C₂H₅, while those in acetic acid flame are CH₃ and OH based on previous studies. A kinetic model of the two acids was developed mainly based on previous and the present theoretical calculation results. It could reasonably capture the measured laminar burning velocities of acetic acid/air and propanoic acid/air mixtures in this work, as well as the previous experimental data in literature. Based on the present model, CH₃- and ketene-related pathways play an important role in acetic acid flames. Under rich conditions, ketene is mostly converted to CH₃ via CH₂CO+HCH₃+CO, and the chain-termination reaction of CH₃+H(+M)=CH₄(+M) is enhanced, which strongly inhibits the propagation of rich acetic acid flames. In contrast, C₂H₅ and ethylene chemistry play an important role in propanoic acid flames. Rich conditions promote the decomposition of C₂H₅, yielding ethylene and H, which can facilitate the flame propagation. This can explain the shift of the peak laminar burning velocity of propanoic acid/air mixtures towards a slightly richer condition compared with that of acetic acid/air mixtures.     

Investigation of flame structure and burning intensity of partially premixed methane enrichment of syngas using OH-PLIF and kinetic simulation

Various experiments were conducted to study the combustion characteristics of partially premixed methane enrichment of syngas by using the OH-PLIF technique. Experiments were conducted on a co-flow burner, and the methane concentration (X_CH4 = CH₄/(H₂+CO+CH₄)) was varied from 0 to 20%, the overall equivalence ratio was varied from 0.4 to 1.2 and the inner equivalence ratio was varied from 1.5 to 3.5. Kinetic simulation was conducted by using OPPDIF module of CHEMKIN-Pro software. Results show that an increase in X_CH4 and ?_overall weakens the OH signal intensity. Adding methane into the fuel greatly increases the height of the inner flame front, and the increase of methane concentration has a negative effect on flame propagation speed. Meanwhile, simulation results remain consistent with the experiments. The main OH radical production reaction changes from R46: H+HO₂ = 2OH to R38: H+O₂ = O+OH when methane concentration contained in the fuel mixture increases. Sensitivity analysis also indicates that reaction which plays a dominant effect on temperature changes with the increase of methane concentration.     

Combustion properties of H2/N2/O2/steam mixtures

The present work reports new experimental and numerical results of the combustion properties of hydrogen based mixtures diluted by nitrogen and steam. Spherical expanding flames have been studied in a spherical bomb over a large domain of equivalence ratios, initial temperatures and dilutions at an initial pressure of 100 kPa (T_ini = 296, 363, 413 K; N₂/O₂ = 3.76, 5.67, 9; %Steam = 0, 20, 30). From these experiments, the laminar flame speed $S_L^0$, the Markstein length L’, the activation energy Ea and the Zel'dovich β number have been determined. These parameters were also simulated using COSILAB^® in order to verify the validity of the Mével et al. [1] detailed kinetic mechanism. Other parameters as the laminar flame thickness δ and the effective Lewis number Le_eff were also simulated. These new results aim at providing an extended database that will be very useful in the hydrogen combustion hazard assessment for nuclear reactor power plant new design.     

A skeletal mechanism for flame inhibition by trimethylphosphate

On the basis of a multi-step kinetic mechanism for flame inhibition by organophosphorus compounds including more than 200 reactions, a skeletal mechanism for flame inhibition by trimethylphosphate was developed. The mechanism consists of 22 irreversible elementary reactions, involving nine phosphorus-containing species. Selection of the crucial steps was performed by analysing P-element fluxes from species to species and by calculating net reaction rates of phosphorus-involving reactions versus the flames zone. The developed mechanism was validated by comparing the modelling results with the measured and simulated (using the starting initial mechanism) speed and the chemical structure of H₂/O₂, CH₄/O₂ and syngas/air flames doped with trimethylphosphate. The mechanism was shown to satisfactorily predict the speed of H₂/O₂/N₂ flames with various dilution ratios, CH₄/air and syngas/air flames doped with trimethylphosphate. The skeletal mechanism was also shown to satisfactorily predict the spatial variation of H and OH radicals and the final phosphorus-containing products of the inhibitor combustion. Further reduction of the skeletal mechanism without modification of the rate constants recommended in the starting mechanism was shown to result in noticeable disagreement of the flame speed and structure.     

NH3NO interaction at low-temperatures: An experimental and modeling study

The present work provides new insight into NH₃NO interaction under low-temperature conditions. The oxidation process of neat NH₃ and NH₃ doped with NO (450, 800 ppm) was experimentally investigated in a Jet Stirred Flow Reactor at atmospheric pressure for the temperature range 900–1350 K. Results showed NO concentration is entirely controlled by DeNO_x reactions in the temperature range 1100–1250 K, while NH₃NO interaction does not develop through a sensitizing NO effect, for these operating conditions.A detailed kinetic model was developed by systematically updating rate constants of controlling reactions and declaring new reactions for N₂H₂ isomers (cis and trans). The proposed mechanism well captures target species as NO and H₂ profiles. For NH₃NO mixtures, NO profiles were properly reproduced through updated DeNO_x chemistry, while NH₂ recombination reactions were found to be essential for predicting the formation of H₂. The role of ammonia as a third-body species is implemented in the updated mechanism, with remarkable effects on species predictions. For neat NH₃ mixture, the reaction H+O₂(+M)=HO₂(+M) was crucial to predict NO formation via the reaction NH₂+HO₂H₂NO+OH.     

Structure and propagation of two-dimensional,partially premixed,laminar flames in diesel engine conditions

We investigate the influence of inflow velocity (V_in) and scalar dissipation rate (χ) on the flame structure and stabilisation mechanism of steady, laminar partially premixed n-dodecane edge flames stabilised on a convective mixing layer. Numerical simulations were performed for three different χ profiles and several V_in (V_in = 0.2 to 2.5m/s). The ambient thermochemical conditions were the same as the Engine Combustion Network’s (ECN) Spray A flame, which in turn represents conditions in a typical heavy duty diesel engine. The results of a combustion mode analysis of the simulations indicate that the flame structure and stabilisation mechanism depend on V_in and χ. For low V_in the flame is attached. Increasing V_in causes the high-temperature chemistry (HTC) flame to lift-off, while the low-temperature chemistry (LTC) flame is still attached. A unique speed S_R associated with this transition is defined as the velocity at which the lifted height has the maximum sensitivity to changes in V_in. This transition velocity is negatively correlated with χ. Near $V_{in}=S_R$ a tetrabrachial flame structure is observed consisting of a triple flame, stabilised by flame propagation into the products of an upstream LTC branch. Further increasing the inlet velocity changes the flame structure to a pentabrachial one, where an additional HTC ignition branch is observed upstream of the triple flame and ignition begins to contribute to the flame stabilisation. At large V_in, the LTC is eventually lifted, and the speed at which this transition occurs is insensitive to χ. Further increasing V_in increases the contribution of ignition to flame stabilisation until the flame is completely ignition stabilised. Flow divergence caused by the LTC branch reduces the χ at the HTC branches making the HTC more resilient to χ. The results are discussed in the context of identification of possible stabilisation modes in turbulent flames.     

Darrieus - Landau instability,growing cycloids and expanding flame acceleration

A premixed flame, propagating away from a point ignition source into an unlimited domain displays an increasing flame speed after the flame size has grown beyond a transition radius. Experiments by Gostintsev et al are described by the relation R = R₁ + At^3/2, where t is the time from ignition and, where S_L is the flame burning velocity and is the thermal diffusivity. The non-dimensional function a() is determined from the experimental results to be equal to 0.002², where is the density ratio across the flame.

In the present work, two-dimensional Lagrangian simulations of flame propagation also display a radial growth with a 3/2 power-law behaviour. This is a potential flow model - no vorticity is included. Hence, the Darrieus - Landau hydrodynamic instability by itself can generate flame acceleration. The numerical results are summarized by the relation R = R1+(2/40)L(SLt/L)3/2, where L is a reference length and is the volume production ratio, = - 1. Equating the zone of velocity jump in the numerical scheme with the temperature jump in hydrocarbon flames allows a definition of an effective thermal diffusivity in the numerical work as n = 0.0081SLL. With this relation, the radial growth is given as, in good agreement with the experimental result and the numerical results of Filyand et al.     

Further studies of the reaction kernel structure and stabilization of jet diffusion flames

The structure of axisymmetric laminar jet diffusion flames of ethane, ethylene, acetylene, and propane in quasi-quiescent air has been studied numerically in normal earth gravity (1g) and zero gravity (0g). The time-dependent full Navier–Stokes equations with buoyancy were solved using an implicit, third-order accurate numerical scheme, including a C₃-chemistry model and an optically thin-media radiation model for heat losses. Observations of the flames were also made at the NASA Glenn 2.2-Second Drop Tower. For all cases of the fuels and gravity levels investigated, a peak reactivity spot, i.e., reaction kernel, was formed in the flame base, thereby holding a trailing diffusion flame. The location of the reaction kernel with respect to the burner rim depended inversely on the reaction-kernel reactivity or velocity. In the C₂ and C₃ hydrocarbon flames, the H₂–O₂ chain reactions were important at the reaction kernel, yet the CH₃ + O → CH₂O + H reaction, a dominant contributor to the heat-release rate in methane flames studied previously, did not outweigh other exothermic reactions. Instead of the C₁-route oxidation pathway in methane flames, the C₂ and C₃ hydrocarbon fuels dehydrogenated on the fuel side and acetylene was a major hydrocarbon fragment burning at the reaction kernel. The reaction-kernel correlations between the reactivity (the heat-release or oxygen-consumption rate) and the velocity, obtained previously for methane, were developed further for various fuels in more universal forms using variables related to local Damköhler numbers and Peclet numbers.     

An experimental and kinetic modeling study of a premixed nitromethane flame at low pressure

A premixed nitromethane/oxygen/argon flame at low pressure (4.67 kPa) has been investigated using tunable vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. About 30 flame species including hydrocarbons, oxygenated and nitrogenous intermediates have been identified by measurements of photoionization efficiency spectra. Mole fraction profiles of the flame species have been determined by scanning burner position at some selected photon energies. The results indicate that N₂ and NO are the major nitrogenous products in the nitromethane flame. Compared with previous studies on nitromethane combustion, a number of unreported intermediates, including C₃H₄, C₄H₆, C₄H₈, C₂H₂O, C₂H₄O, CH₃CN, H₂CNHO, C₃H₃N and C₃H₇N, are observed in this work. Based on our experimental results and previous modeling studies, a detailed oxidation mechanism including 69 species and 314 reactions has been developed to simulate the flame structure. Despite some small discrepancies, the predictions by the modeling study are in reasonable agreement with the experimental results.     

Laminar burning velocities of 2-methyltetrahydrofuran at elevated pressures

The laminar burning velocities (LBVs) and cellular instability of 2-methyltetrahydrofuran (2-MTHF) were investigated at the unburned temperature of 423 K and pressures from 1 to 10 atm in a cylindrical constant-volume vessel. The LBVs of 2-MTHF/air flame exhibit a notably dropping with increasing pressure. The cellular instability analysis indicates that the critical flame radius of 2-MTHF/air mixture monotonically increases with increasing pressure and the flame surface suffers more badly cellularity under higher pressures. The critical flame radius exhibits non-monotonic variation versus ? and the most unstable flames appear at ? ≈ 1.3. It is observed that the measured Markstein length of 2-MTHF/air mixture decreases with increasing ? and P_u, leading to an earlier formation of wrinkling and cracks with respect to preferential-diffusional instability. Further investigation found that by using a mixture of 14.2% oxygen with 85.8% helium in place of air as bath gas at 10 atm can effectively suppress the cellular instability. Two recently developed models were used to simulate the experimental results and explore the chemical kinetic effects on LBV. Reaction path analysis reveals that the most consumption of 2-MTHF/air at stoichiometric conditions is through the abstraction of H-atom to form radical C₅H₉O-5. While the competitiveness of decomposition by CC scission yielding CH₃ and tetrahydrofuran radical is relatively weak. Sensitivity analysis illustrates that small-species reactions show a controlling effect on LBV. The increasing pressure leads to an evident increase in the sensitivity coefficient of the recombination reaction H + O₂ (+M)=HO₂ (+M). The reduction of H atom concentration will cause competition to the initiation reaction H + O₂ = O+OH. This could lower the overall oxidation rate and reduce the burning velocity.     

Dynamic behavior of diffusion flame interacting with a large-scale vortex by laser imaging techniques

Instantaneous and simultaneous measurements of two-dimensional temperature and OH-LIF profiles by combining Rayleigh scattering with laser induced fluorescence (LIF) were demonstrated in a nitrogen-diluted hydrogen (H₂ 30% + N₂ 70%) laminar normal diffusion flame interacting with a large-scale vortex by oscillating central fuel flow or in an inverse diffusion flame by oscillating central airflow. The dynamic behavior of the diffusion flame extinction and reignition during the flame–vortex interaction processes was investigated. The results obtained are described as follows. (1) The width of the reaction zone decreases remarkably, and a decrease in flame temperature and OH-LIF is seen with increasing central airflow in an inverse diffusion flame. OH-LIF increases, and temperature does not change with increasing central fuel flow in a normal diffusion flame. The computations predict the experimental results well, and it is revealed that flame temperature characteristics result from the preferential diffusion of heat and species, which induces excess enthalpy or on enthalpy deficit, and an increase or decrease in H₂ mole fraction in the flame. (2) When a large velocity fluctuation is given to the central flow, the temperature and the OH-LIF at the reaction zone become thin at the convex and circumferential part of the vortex where a high temperature layer exists, and the temperature at the reaction zone is lowered in the inverse flame and the normal flame. (3) The width and temperature of the reaction zone interacting with the vortex recover quickly to that of the laminar steady flame after the vortex passing in the normal flame, but the recovery to that of the steady flame after the vortex passing is delayed in the inverse flame. (4) When a remarkably large velocity fluctuation is given to the central airflow in the inverse flame, thinning of temperature and reaction zone starts at the convex and circumferential part of the vortex, resulting in a and flame extinction completely occurs at the tail part of the vortex and makes the pair of edge flames. The outside edge flame reignites and connects with the upstream reaction zone. The inside edge flame finally extinguishes as the supply of fuel is interrupted by the outside edge flame.     

Experimental and kinetic modelling studies of flammability limits of partially dissociated NH3 and air mixtures

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH₃ in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH₃ dissociation was varied from 0 to 25% (0 to 20%v/v H₂ in the fuel mixture with a fixed H₂/N₂ ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH₃ in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH₃ dissociation. At 25% NH₃ dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH₃ in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH₃. The production of key radicals including OH, H, O, and NH₂ was enhanced in the presence of H₂ and thus the conversion of NH to NO and then NO to N₂ near LFL and the conversion of NH₂ and NO to N₂ near UFL leading to wider flammability limits.