Computational investigation of non-premixed hydrogen-air laminar flames

Laminar diffusion hydrogen/air flames are numerically investigated. Detailed and global mechanisms are compared. NO formation is modelled by full nitrogen chemistry and the extended Zeldovich mechanism. A satisfactory agreement between the present predictions and the experiments of other authors is observed. Significance of different ingredients of mathematical modelling is analyzed. Minor roles of thermal diffusion and radiation, but a significant role of buoyancy is observed. It is observed that the full and quasi multi-component diffusion deliver the same results, whereas assuming Le = 1 to a remarkable difference. NO emissions logarithmically increase with increasing residence time. NO is the dominating nitrogen oxide. Its share increases with residence time, whereby NO₂ and N₂O show a reverse trend. It is observed that the NNH route plays a remarkable role in NO formation, where the share of the Zeldovich mechanism increases with residence time from about 20% to 85%, within the considered range.     

Aromatic formation pathways in non-premixed methane flames

A kinetic mechanism, previously developed and successfully applied to the prediction of the formation of benzene and aromatics in different flame conditions, was applied to assess the importance of the various benzene and aromatic formation pathways in non-premixed flames. Four sets of data were tested: the methane flame and the same flame doped with toluene, ethylbenzene, and tert-butylbenzene, as studied by Anderson and co-workers. The model predicts, with good accuracy, the growth of hydrocarbons and the formation of benzene and aromatic species. The modeling shows that in the undoped methane flame, benzene formation is controlled by propargyl radical combination. Acetylene addition to C4 radicals contributes a moderate amount, whereas toluene decomposition is insignificant. The predictions are almost unaffected by the fulvene pathway. Benzene is strongly perturbed by dopant addition to methane. Predictions agree quite well with benzene concentrations in the undoped flame and agree with the increase in benzene concentration when alkylbenzenes are added. Key reactions leading to the formation of naphthalene are the propargyl addition to benzyl radicals, and, to a lesser extent, the hydrogen-abstraction acetylene-addition mechanism. Cyclopentadienyl radical combination, which is the dominant route in premixed and partially premixed flames, is insignificant in these flame conditions.     

Flame structures and behaviors of opposed flow non-premixed flames in mesoscale channels

An opposed flow non-premixed flame (OFNPF) in a narrow channel was chosen as a model of a non-premixed flame in a mesoscale combustion space or micro-combustor. The stabilization limits and behaviors of methane-air flames and propane-air flames were compared for various experimental parameters such as flow velocity, nozzle distance, nozzle width, channel gap, and fuel dilution. Flames could be stabilized in a wide range of strain rates (0.9–150 s⁻¹) and dilution ratios (∼80% nitrogen at the fuel side). The flame extinction limits were classified into three types and their mechanisms were investigated: higher-strain-rate (HSR) extinction limit determined by the flame stretch, lower-strain-rate (LSR) extinction limit determined by the conductive or convective heat loss from the flame, and fuel-dilution-ratio (FDR) extinction limit determined by the decrease in the heat release rate from the flames. The HSR extinction limits in mesoscale channels could be explained with a modified strain rate, and the LSR extinction limits could be explained by employing a premixed quenching theory in which the heat loss through the dead space near the wall was considered as a major extinction mechanism. Finally, the variation of the extinction limits with the FDR in both the HSR and the LSR conditions could be explained with a modified global reaction rate in which the variations in flame temperature and species concentrations were reflected. This study provides an essential model for the stabilization and extinction of non-premixed flames in mesoscale combustion spaces.     

Characteristics of non-premixed oxygen-enhanced combustion: II. Flame structure effects on soot precursor kinetics resulting in soot-free flames

A detailed computational study was performed to understand the effects of the flame structure on the formation and destruction of soot precursors during ethylene combustion. Using the USC Mech Version II mechanism the contributions of different pathways to the formation of benzene and phenyl were determined in a wide domain of Z_st values via a reverse-pathway analysis. It was shown that for conventional ethylene-air flames two sequential reversible reactions play primary roles in the propargyl (C₃H₃) chemistry, namely

(1)     

PDF model for NO calculations with radiation and consistent NO–NO2 chemistry in non-premixed turbulent flames

A consistent method to derive the parameterized NO source term from a given flamelet library is presented. The approach was tested by conducting PDF simulations and comparing computed solutions of the Sandia D flame with measurements. The parameterization is based on laminar flamelets with additional parameters for NO formation and radiation. To obtain feedback of the NO concentration on the NO source term, evolution to equilibrium is calculated with a complex chemical mechanism (including nitrogen chemistry), whereas the concentration of all non-nitrogen species are parameters obtained from flamelet profiles. With this method, which requires an appropriate initialization of the nitrogen containing species, unrealistically large NO source terms are avoided, which is a shortcoming of previously devised methods.In addition, to properly account for radiative heat losses, an additional progress variable was introduced. Simulation results obtained with our PDF method are in excellent agreement with experimental data. While the NO concentration values appear too high (by approximately 30% at the exit of the burner), if radiation effects are ignored, their match is very good, if radiation effects are taken into account.     

Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor

Effects of H₂-enrichment on structures of CH₄/air turbulent swirling premixed flames affected by high intensity turbulence in a gas turbine model combsutor are investigated by conducting direct numerical simulations. Two stoichiometric mixture conditions, of which volume ratio of CH₄:H₂ = 50:50 and 80:20, are simulated by considering a reduced chemistry (25 species and 111 reactions). Results showed qualitatively different flame shapes and reaction zone characteristics between the cases. For the higher H₂-ratio case, the flame is stabilized both in the inner and outer shear layers. For the lower H₂-ratio case, the flame is stabilized only in the inner shear layer and extinction occurs in the outer shear layer. Comparison of the reaction zone characteristics with unstrained and strained laminar flames in phase space showed that H₂ mass fraction for the lower H₂-ratio case and reaction rate profiles for both cases deviate from the corresponding laminar values. Analysis of fuel species conservation equation suggests that the turbulent transports are substantially influential to determine local and global flame structures. These findings would be useful for designing practical H₂-enriched gas turbine combustor in the aspect of flame structures under high intensity turbulence.     

Effect of high hydrogen enrichment on the outer-shear-layer flame of confined lean premixed CH4/H2/air swirl flames

In this study, we investigated the H₂-induced transition of confined swirl flames from the “V” to “M” shape. H₂-enriched lean premixed CH₄/H₂/air flames with H₂ fractions up to 80% were conducted. The flame structure was obtained with Planar Laser-Induced Fluorescence (PLIF) of the OH radical. Flow fields were measured with Particle Image Velocimetry (PIV). It was observed that the flame tip in the outer shear layer gradually propagated upstream and finally anchored to the injector with the hydrogen fractions increase, yielding the transition from the “V” to “M” flame. We examined the flame structures and the flame flow dynamics during the transition. The shape transition was directly related to the evolution of the corner flame along the outer shear layer. With H₂ addition, the outer recirculation zone first appeared downstream where the corner flame started to propagate upstream; then, the recirculation zone expanded upward to form a stable “M” flame gradually. The flow straining was observed to influence the stabilization of the outer shear layer flame significantly. This study can be useful for the understanding of recirculation-stabilized swirling flames with strong confinement. The flame structure and the flow characteristics of flames with a high H₂ content are also valuable for model validation.     

Effect of hydrogen addition on NOx formation in high-pressure counter-flow premixed CH4/air flames

A laboratory-scale laminar counterflow burner was used to investigate NO formation in high pressure premixed CH₄/H₂/air flames. New experimental results on NO measurements by LIF were obtained at high pressure in CH₄/H₂/air flames with H₂ content fixed at 20% in the fuel at pressures ranging from 0.1 to 0.7 MPa and an equivalence ratio progressively decreased from 0.74 to 0.6. The effects of hydrogen addition, equivalence ratio and pressure are discussed. These results are satisfactorily compared to the simulations using two detailed mechanisms: GDFkin®3.0_NOmecha2.0 and the mechanism from Klippenstein et al., which are the most recent high-pressure NOx formation mechanisms available in the literature. A kinetic analysis based on Rate of Production/Rate of Consumption and sensitivity analyses of NO is then presented to identify the main pathways that lead to the formation and consumption of NO. In addition, the effect of hydrogen addition on NO formation pathways is described and analysed.     

The effect of hydrogen addition on biogas non-premixed jet flame stability in a co-flowing air stream

An investigation of the stability limits of biogas jet non-premixed (diffusion) flames in a co-flowing air stream was conducted. The stability limits were determined experimentally for two different methane–carbon dioxide mixtures that represent the typical biogas composition. Moreover, the effect of jet nozzle diameter was also investigated. It was found that with the presence of a significant amount of CO₂ in the fuel, the stability limits were very low and the flames can only be stabilized over a very small range of co-flowing air velocities. As expected, an increase in carbon dioxide concentration resulted in the narrowing of the region for stable flames. However, it was shown that the flame stability of such mixtures can be enhanced very significantly over a much wider range of co-flowing air velocities by introducing a small amount of hydrogen into the fuel. Results obtained in the current experimental setup indicate that an increase in the stability limits by approximately four-fold when 10% (by vol.) of hydrogen is added under the same operating conditions. The effect of the addition of hydrogen on the enhancement of biogas stability is most significant with a 10% initial addition. The degree of enhancement diminishes with further increases in hydrogen addition from 10% to 30%.     

Combined experimental and numerical study on the extinction limits of non-premixed H2/CH4 counterflow flames with varying oxidizer composition

Chemical reaction mechanisms with detailed kinetics are an important topic in combustion science and an essential prerequisite for the accurate modeling of reactive flows in combustors. Besides isolating and studying individual reactions, the development of reaction mechanisms is often based on well-defined experimental observables, such as the laminar burning velocity and the ignition delay time. While many optimization targets are associated with premixed combustion, the extinction strain rate (ESR) of non-premixed flames in the counterflow configuration is another well-defined experimental observable which, however, often receives less attention. In order to reduce the scarcity of corresponding datasets for the emerging fuel hydrogen and its blends with methane, this work reports ESR measurements for H₂, CH₄/H₂ and CH₄ counterflow diffusion flames considering a variation of the oxygen content in the oxidizer stream between 14 % and 21 %. The experimental investigation is complemented by calculations with a 1D counterflow model utilizing a temperature-control continuation method in order to determine the extinction limits numerically. The simulations are performed with six different well-established chemical reaction mechanisms. It is shown from both, experimental and numerical results, that with the substitution of CH₄ by H₂ the ESR increases and further, that the ESR decreases with a reduction of the oxygen content in the oxidizer stream. In addition, decreasing flame temperatures are observed at extinction as the H₂ content increases. Overall, all mechanisms are able to qualitatively recover the trends found for varying H₂ contents, fuel mole fraction, and oxygen content in the oxidizer. However, significant quantitative deviations are observed between the numerical results regarding the ESR values and the deviations are larger than for other important flame characteristics, such as the laminar burning velocity. The results suggest that the ESR could be a useful optimization target for further improving chemical reaction mechanisms which underlines the importance of datasets such as the one presented in this work.     

A DNS study of hydrogen/air swirling premixed flames with different equivalence ratios

Hydrogen/air swirling premixed flames with different equivalence ratios are studied using direct numerical simulation. A fourth-order explicit Runge–Kutta method for time integration and an eighth-order central differencing scheme for spatial discretization are used to solve the full Navier–Stokes (N–S) equation system. A 9 species 19-step reduced mechanism for hydrogen/air combustion is adopted. The flames are stabilized with the help of a recirculation zone characterizing a high swirling flow. The vortex structures of the swirling premixed flames are presented. The flame structures are investigated in terms of the flame front curvature and tangential strain rate probability density functions (pdfs). The local flamelet temperature profiles are also extracted randomly along the flame front and compared with the corresponding laminar flame temperature profile. In order to study preferential diffusion effects, direct numerical simulation of two additional freely propagating planar flames in isotropic turbulence is conducted. Preferential diffusion effects observed in the planar flames are suppressed in the swirling flames. Further analysis confirms that the coherent small-scale eddies play important roles in the interactions between turbulence and the flame front. They are able to change the dynamic properties of the flame font and lead to enhanced burning intensity in the flame front with negative curvature for both stoichiometric and fuel-lean flames.     

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.     

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.     

Influence of CH4/air injection location on non-premixed flame blow-off limits in a novel micro-combustor

Stable flame is still one of the challenging issues in the micro-combustors. Besides, the premixed flame may flash back under the large equivalence ratio, and the non-premixed combustion is an effective way to avoid the flame flash-back issue. In this work, a novel non-premixed CH₄/air micro-combustor with the flame holder, the backward-facing steps and the separated preheating channels is developed to avoid the flame flash-back issue and extend the flame blow-off limits. This paper investigates the influence of two different CH₄/air injection locations on the flame blow-off limits numerically and profoundly reveals the mechanisms in this micro-combustor. Results show that this novel configuration can considerably anchor the flame without the flame flash-back issue and significantly extend flammability owning to the flow & heat recirculations. Compared with the “Air_(out)/CH_4(in)” injection location, the “Air_(in)/CH_4(out)” injection location exhibits a comparatively larger size of recirculation, causing a more vital anchorage ability. In addition, after the preheating in the preheating channels, the CH₄/air mixture of the “Air_(in)/CH_4(out)” injection location presents a higher temperature level than that of the “Air_(out)/CH_4(in)” injection location. Consequently, compared to the “Air_(out)/CH_4(in)” injection location, the larger blow-off limits are attained for the “Air_(in)/CH_4(out)” injection location. These results provide new insights into the stable non-premixed flame and give valuable guidance for designing the similar non-premixed micro-combustor.     

Study on stretch extinction limits of CH4/CO2 versus high temperature O2/CO2 counterflow non-premixed flames

Extinction limits of counterflow non-premixed flames with normal and high temperature oxidizers were studied experimentally and numerically for development of new-type oxygen-enriched mild combustion furnace. Extinction stretch rates of CH₄/CO₂ (at 300 K) versus O₂/CO₂ flames at oxygen mole fractions of 0.35 and 0.40 and oxidizer temperatures of 300 K, 500 K, 700 K and 1000 K were obtained. Investigation was also conducted for CH₄/N₂ (at 300 K) versus air (O₂/N₂) flames at the same oxidizer temperatures. An effect of radiative heat loss on stretch extinction limits of oxygen-enriched flames and air flames was investigated by computations with optical thin model (OTM) and adiabatic flame model (ADI). The results show influence of radiative heat loss on stretch extinction limits was not significant in relative high fuel mole fraction regions. The extinction curve of the oxygen-enriched flames with oxygen mole fraction of 0.35 was close to that of the air flames at the oxidizer temperature of 300 K. However, the extinction curve of air flames with high temperature oxidizer was comparable with that of oxygen-enriched flames with oxygen mole fraction of 0.40. Scaling analysis based on asymptotic solution of stretch extinction was applied and it was found that stretch extinction limits can be expressed by two terms. The first term is total enthalpy flux of fuel stream based on thermo-physical parameters. The second term is a kinetic term which reflects an effect of the chemical reaction rate on stretch extinction limits. OH radicals which play important roles in chain propagating and main endothermic reactions were used to represent the kinetic term of both oxygen-enriched and air flames. The global rates of OH formation in these two cases were compared to understand the contribution of kinetic term to stretch extinction limits. Variation of extinction curves of oxygen-enriched flames and air flames was well explained by the present scaling analysis. This offers an effective approach to estimate stretch extinction limits of oxygen-enriched flames based on those of air flames at the same oxidizer temperature.     

The role of turbulent fluctuations on radiative emission in hydrogen and hydrogen-enriched methane flames

A theoretical analysis is reported in the present work to quantify the increase of radiative emission due to turbulence for hydrogen and hydrogen-enriched methane diffusion flames burning in air. The instantaneous thermochemical state of the reactive mixture is described by a flamelet model along with a detailed chemical mechanism. The shape of the probability density function (pdf) of mixture fraction is assumed. The results show that turbulent fluctuations generally contribute to reduce the Planck mean absorption coefficient of the medium, in contrast with the blackbody emissive power, which is significantly increased by turbulence. If the turbulence level is relatively small, the influence of turbulence on the absorption coefficient is marginal. Otherwise, fluctuations of the absorption coefficient of the medium should be taken into account. The scalar dissipation rate and the fraction of radiative heat loss have a much lower importance than the turbulence intensity on the mean radiative emission.     

Numerical analysis on influence of hydrogen peroxide (H2O2) addition on the combustion and emissions characteristics of NH3/CH4-air (O2/N2)/ H2O2 mixture

In this work, extensive chemical kinetic modeling is performed to analyze the combustion and emissions characteristics of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures at different replacement percentages of air with hydrogen peroxide (H₂O₂). This work is comprehensively discusses the ignition delay time, flame speed, heat release rate, and NOx & CO emissions of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures. Important intermediate crucial radicals such as OH, HO₂, HCO, and HNO effect on the above-mentioned parameters is also discussed in detail. Furthermore, correlations were obtained for the laminar flame speed, NO, and CO emissions with important radicals such as OH, HO₂, HCO, and HNO. The replacement of air with H₂O₂ increases flame speed and decreases the ignition delay time of the mixture significantly. Also, increases the CO and NOx concentration in the products. The CO and NOx emissions can be controlled by regulating the H₂O₂ concentration and equivalence ratios. Air replacement with H₂O₂ enhances the reactions rate and concentration of intermediate radicals such as O/H, HO₂, and HCO in the mixture. These intermediate radicals closely govern the combustion chemistry of the NH₃/CH₄– O₂/N₂/H₂O₂ mixture. A linear correlation is observed between the flame speed and peak mole fraction of OH + HO₂ radicals, and 2nd degree polynomial correlation is observed for the peak mole fraction of NO and CO with HNO + OH and HCO + OH radicals, respectively.     

The effect of hydrogen enrichment on the forced response of CH4/H2/Air laminar flames

Hydrogen-enrichment of conventional natural gas mixtures is an actively-explored strategy for reducing pollutant emissions from combustion. This study investigates the effect of hydrogen enrichment on the unsteady flame response to perturbations, with a view to understanding the implications for thermoacoustic stability. The Level Set Approach for kinematically tracking the flame front was applied to a laminar conical premixed methane/hydrogen/air flame subjected to 2D incompressible velocity perturbations. For hydrogen enrichment levels ranging from 0% to 80% by volume, the resulting unsteady heat release rate of the flame was used to generate the Flame Describing Functions (FDFs). This was performed across a range of perturbation frequencies and levels at ambient pressure. The mean heat release rate of the flame was fixed at $\overline{\stackrel{?}{Q}}$$=2.69\text{k}\text{W}$ and the equivalence ratio was set to ? = 1.08 for all hydrogen enrichment levels. Hydrogen-enrichment was found to shift the FDF gain drop-off to higher frequencies, which will increase propensity to thermoacoustic instability. It also reduced the effective flame time delay. Sensitivity analyses at ? = 0.8 revealed that the changes in FDF were driven predominantly by the flame burning speed, and were insensitive to changes in Markstein length.