Post-flame Zone Research Articles

Structure and scalar correlation of ammonia/air turbulent premixed flames in the distributed reaction zone regime

Instantaneous structures of turbulent premixed ammonia/air flames on a piloted jet burner were investigated using simultaneous planar laser-induced fluorescence (PLIF) and Rayleigh thermometry measurements. Two-dimensional spatial distributions of temperature were simultaneously measured with those of the NH radical and the NO pollutant, respectively. Experiments were conducted for stoichiometric flames under five jet velocities. All flames are located in the distributed reaction zone regime of the Borghi-Peters diagram with the Karlovitz (Ka) number ranging from 274 to 4720. The NH PLIF images are used to characterize the fuel consumption layer of the reaction zones since the formation of NH is associated with the consumption of ammonia. The NH PLIF results show that under all flame conditions investigated, the NH layer remains thin in the proximity of the burner while it becomes progressively thickened and distorted by turbulence with increasing turbulent intensity and axial distance. For flames with Ka of 274, the NH layer essentially remains thin, while at Ka of 590 or higher, significant broadening of the NH layer is observed. Probability density functions (PDFs) of the NH layer thickness show that the NH layer can be broadened by 3 – 4 times as the flames are developed downstream. The broadening of the NH layer is considered to indicate that the flames are in the distributed reaction zone regime. The boundary between the thin-reaction zone regime and the distributed reaction zone regime occurs at a much larger Ka than that in methane/air flames. The broadening of the NH layer is due to the penetration of the turbulent eddies and the merging of flame branches. The latter occurs mainly near the flame tips. NO is shown to exist in a wide region in space, across the preheat zone, reaction zone, and postflame zone. NO formation occurs mainly in the reaction zone, however, it is transported by turbulence eddies to the preheat zone and by convection to the postflame zone. The temperature measurements indicate that the preheat zone is broadened in all flames investigated. The broadening of the preheat zone is moderately sensitive to the Ka number while it is more sensitive to the integral length scale of the flames.

Modeling the dual-fuel combustion of porous lycopodium particles and diesel using an analytical simulation framework

In this paper, a comprehensive analytical study is performed to assess the lycopodium-diesel dual-fuel combustion system in counter-flow premixed configuration. The system is modeled as multiple zones that are coupled together via proper boundary and jump conditions on interfaces. According to the respective reaction and transport phenomena in these zones, conservation equations of mass and energy are derived, non-dimensionalized, and solved by Matlab and Mathematica in an analytical way. The porosity of lycopodium particles and the thermal radiation from the reaction zone and the post-flame zones into the preheating zone are considered, in order to improve the realism and accuracy of the modeling. Experimental validation of the derived framework is first carried out with good agreement between our results and the data from the literature. Afterwards, the impacts of parameters such as reactants Lewis numbers, equivalence ratio, and critical strain rate on the flame structure, as well as lycopodium particles' combustion behavior in dual-fuel and single-fuel modes are evaluated and compared. It is found that dual-fuel combustion system enhances the vaporization and ignition of lycopodium particles resulting in a 13% increase in flame temperature. In addition, effects of thermal radiation and porosity of particles on both the flame temperature and position are quantified. The presence of biomass particle porosity and thermal radiation reduces the flame temperature and drive the flame front towards the fuel nozzle.

Experimental investigation of synthesis gas production in fuel-rich oxy-fuel methane flames

Combustion processes with pure oxygen (oxy-fuel) instead of air as an oxidant are attractive for e.g. high temperature thermal or thermochemical and gasification processes. The absence of nitrogen as an inert gas leads in such combustion processes to an increase in temperature and species concentrations. The scope of this study is the determination of axial profiles of major educts (CH4, O2) and major combustion products such as H2, H2O, CO and CO2 in flat fuel-rich premixed methane-oxygen flames (2.5 ≤ ϕ <3.0). A Heat-Flux-burner was used to stabilize a quasi-adiabatic one-dimensional flame at a preheating temperature of TP = 300 K and atmospheric pressure. Gas samples were taken at different HAB using a quartz probe and analyzed by a GC/MSD. The influence of the probe on determined species concentrations was investigated using CFD simulations. Additionally, one-dimensional calculations with detailed chemistry were performed using the GRI3.0 and CalTec2.3 mechanism. Their differences in regards to H2, H2O and CO concentrations were thoroughly investigated and could be explained by the detailed C2-chemistry of the CalTech2.3. The results of the analyzed gas samples show a rapid reduction and increase in the flame zone, of the educts, major products and higher hydrocarbons (C2H2, C2H4 and C6H6), respectively. Both mechanisms show a sharper gradient of the synthesis gas production in comparison to the experimental results, nevertheless a tendency towards the CalTech2.3 scheme is observed. In the post flame zone the determined species concentrations of H2 and CO2 correspond to the results of the GRI3.0, whereas in the case of H2O and CO the CalTech2.3 showed better performance. The calculations of higher hydrocarbons are in better agreement with higher equivalence ratio.

Applying heat flux method to laminar burning velocity measurements of NH3/CH4/air at elevated pressures and kinetic modeling study

Combustion of ammonia (NH3) blended fuels under elevated pressure conditions is critical for adopting this non-carbon fuel in the energy system for decarbonization. In the present work, laminar burning velocities of ammonia/methane(CH4)/air mixtures were measured using the heat-flux method at the pressure from 1 to 5 atm with the mixture equivalence ratios ranging from 0.6 to 1.6 and the mole fraction of NH3 ranging from 0 to 1.0. The relatively completed results obtained at elevated pressures were then used for validating and modifying the kinetic mechanisms (CEU-NH3-Mech 1.0) leading to a new version (CEU-NH3-Mech-1.1). Experimental results of NH3/H2/air in the present work, NH3/H2/CO/air mixtures measured on the same setup and reported in our previous works were also considered in the development of the kinetic mechanism. It was found that the CEU-NH3−Mech-1.1 can predict well the laminar flame speed, ignition delay time and species concentration in the ammonia oxidation at high temperatures for both NH3/CH4/air and NH3/H2/CO/air mixtures in a wide range of equivalence ratios and elevated pressures, including oxygen-enriched combustion conditions. The present experimental results also show that the value of pressure exponent (β) varies with the mole fraction of ammonia and behaves differently for the mixtures of ammonia blending into CH4 and H2. The kinetic and sensitivity analyses show that the sensitive reactions for β are weakly correlated to those for the laminar burning velocity, indicating that β can also work as a potential parameter for validating kinetic mechanisms. Ammonia content in the NH3/CH4/air mixtures determines the pressure exponent variation at over-rich equivalence ratios and reaction pathway variation in the post-flame zone. This work also clarifies the utilization of ammonia containing fuels in rich-lean combustion strategies.

Numerical Analysis on the Evolution of NH2 in Ammonia/hydrogen Swirling Flames and Detailed Sensitivity Analysis under Elevated Conditions

ABSTRACT Ammonia/hydrogen blends have received some attention toward the development of new technologies focused on gas turbine combustion systems, as doping of hydrogen in ammonia enhances flame speed and stability while decreasing ignition energy. One of the challenges of these blends relies on the appropriate computational modeling of their combustion properties in combination with the complex hydrodynamics inherent to flow control techniques such as swirling flows, which are known to be the main method of flame stabilization in current gas turbines. Moreover, it is well-known that large reaction kinetic models are difficult to employ in these computational analyses, thus increasing the difficulty of obtaining reliable methods for the design of new combustors. Therefore, this research analyses a reduced chemical reaction mechanism, namely Okafor’s mechanism, comparing its performance and accuracy against obtained experiments. Emission measurements and non-intrusive laser techniques (LDA) were employed to validate models running on CHEMKIN-PRO flow reactors and RANS Complex Chemistry. Once validated, the study identified the main contributors and reaction kinetics of NH2 and NO consumption, hence evaluating the process via production rates and sensitivity analysis of various important reactions. The results depicted positive correlation between NH2 formation and heat release, N2, H2O, N2O, NH, NNH, NO, O formation, whereas NH3, N2H3 and NO2 have shown negative correlation. Statistical correlations supported these findings but unfortunately were inconclusive to the impacts of vorticity and turbulence over the production/consumption of amidogen. Sensitivity analysis has shown NH2 radicals and atomic N to be the main contributors of NO formation in the flame zone, although most of the NO formed in the flame zone shown to be consumed at the post-flame zone due to the presence of NHx radicals and atomic N.

Chemical structure and laminar burning velocity of atmospheric pressure premixed ammonia/hydrogen flames

This paper presents experimental data on the flame structure of laminar premixed ammonia and ammonia/hydrogen flames at different equivalence ratios (φ = 0.8, 1.0 and 1.2) and the laminar flame speed of ammonia/hydrogen flames (φ = 0.7–1.5) at 1 atm. Experimental data were compared with modeling results obtained using four detailed chemical-kinetic mechanisms of ammonia oxidation. In general, all models adequately predict the flame structure. However, for the laminar burning velocity, this is not so. The main nitrogen-containing species present in the post-flame zone in significant concentrations are N2 and NO. Experimental data and numerical simulations show that the transition to slightly rich conditions enables to reduce NO concentration. Numerical simulation indicate that increasing the pressure rise also results into reduction of NO formation. However, when using ammonia as a fuel, additional technologies should be employed to reduce NO formation.

A derivation of temperature-based energy equation for LES of isochoric turbulent combustion with FDSGS model

In this study, with the aim of deriving a temperature-based energy equation and evaluating the fractal dynamic SGS (FDSGS) combustion model under an isochoric condition, modelling of the filtered energy conservation equation in the temperature form is considered and is tested in the contexts of a priori and a posteriori, comparing with a corresponding DNS of turbulent combustion in a constant volume vessel. Conditional averages of various terms in the filtered energy conservation equation in the temperature form show that three unclosed terms, the SGS scalar flux, the SGS pressure-dilatation and the heat release rate, are leading terms. New SGS models for the SGS pressure-dilatation and the heat release rate are proposed and are tested in a priori manner. The zeroth-order models are also compared. The heat release rate models are proposed for reaction zones and post-flame zones separately, since the present analysis revealed a substantial contribution of heat generated in the post-flame zones. In addition to the tests of the SGS models for the energy conservation equation, isochoric treatments for various flame quantities under varying mean pressure and unburnt temperature due to the isochoric condition are also proposed and are tested. LES using the FDSGS combustion model with the new SGS models and isochoric treatments is performed considering the same combustion conditions and configuration as the DNS. The LES with several other SGS combustion models are also performed and are compared. The performed LES show reasonable prediction of flame propagation for any of the SGS combustion models tested, although the prediction by the FDSGS combustion model shows closest to the DNS. Regardless of the SGS combustion models, the peak values of the mean pressure are close to the DNS value, which suggests that the proposed SGS models and isochoric treatments for the energy conservation equation work well.

Impact of post-flame processes on the hydrogen yield in partial oxidation of methane in the matrix reformer

Non-catalytic partial oxidation of methane in matrix reformers offers the possibility of producing hydrogen directly at sites of its consumption, in particular from local bioresources and non-traditional fossil hydrocarbons. The kinetics simulation of the process shows that it proceeds in two stages. In the flame zone, in the presence of oxygen, along with CO, H2, CO2 and H2O, methane pyrolysis products are formed. Then, in the post-flame zone, in the absence of oxygen, at 1400–1600 K the slow pyrolysis and steam conversion of the products occur. Kinetic analysis shows that the pyrolysis of methane into acetylene and the subsequent steam reforming of the latter proceeds much faster than the direct interaction of methane with H2O, which leads to decreasing methane and acetylene concertation and increasing H2 and CO concentration. It is necessary to optimize conditions in the prost-flame zone to increase hydrogen yield and decrease acetylene yield.

Hydrogen addition effect on NO formation in methane/air lean-premixed flames at elevated pressure

The present study investigates freely propagating methane/hydrogen lean-premixed laminar flames at elevated pressures to understand the hydrogen addition effect of natural gas on the NO formation under the conditions of industrial gas turbine combustors. The detailed chemical kinetic model which was used in the previous study on the NO formation in high pressure methane/air premixed flames was adopted for the present study to analyze NO formation of methane/hydrogen premixed flames. The present mechanism shows good agreement with experimental data for methane/hydrogen mixtures, including ignition delay times, laminar burning velocities, and NO concentration in premixed flames. Hydrogen addition to methane/air mixtures with maintaining methane content leads to the increase of NO concentration in laminar premixed flames due to the higher flame temperature. Methane/hydrogen/argon/air premixed flames are simulated to avoid the flame temperature effect on NO formation over a pressure range of 1–20atm and equivalence ratio of 0.55. Kinetic analyses shows that the N2O mechanism is important on NO formation for lean flames between the reaction zone and postflame region, and thermal NO is dominant in the postflame zone. The hydrogen addition leads to the increase of NO formation from prompt NO and NNH mechanisms, while NO formation from thermal and N2O mechanisms are decreased. Additionally, the NO formation in the postflame zone has positive pressure dependencies for thermal NO with an exponent of 0.5. Sensitivity analysis results identify that the initiation reaction step for the thermal NO and the N2O mechanism related reactions are sensitive to NO formation near the reaction zone.

Efficient emission modelling in lean premixed flames with pre-tabulated formation characteristics

It is practically important to accurately predict NOx and CO emissions in lean premixed flames for the development of fuel-efficient and low-emission combustion systems. In this study, the efficient modelling with pre-tabulated formation characteristics of NOx and CO is systemically investigated. Their formation characteristics for turbulent flames in the flamelet and thin reaction zone regimes are first quantified with one-dimensional adiabatic premixed flames through the incorporation of turbulence induced diffusion. Results show that the formation of NOx and CO can be characterized by two different stages, i.e., a rapid increase of both in flame front and the linear growth for NOx and exponential decay for CO in post-flame zone. An efficient NOx and CO modelling approach is formulated and demonstrated in a full-scale methane gas turbine combustor, in which species NOx and CO are transported and solved, with the source terms being modelled by pre-tabulated formation characteristics from one-dimensional premixed flames with detailed chemical kinetics. Realizable k-epsilon model and finite rate/eddy dissipation model in conjunction with a two-step global mechanism are employed to primarily predict the flow and flame characteristics. Results show that the predicted NOx and CO agree with experimental measurements over a wide range of equivalence ratios. The discrepancy in NOx emission can be accounted by fuel/air unmixedness. For CO, the rapid increase due to the incomplete CO oxidation resulting from its increasing characteristic oxidation time near lean blow-out (LBO) is correctly captured. It is further shown that the predicted CO emission is sensitive to combustion/kinetics model parameters and the accurate prediction of flame shape and dynamics is crucial to capture the trend of CO formation near LBO.

Research on the flame structure characteristics and NOx pathways of low swirl combustion

Low swirl combustion (LSC) technology has the advantage of ultralow NOx emissions, which is of great significance to the development of low-emission gas turbine engines in the future. To investigate the flow field and flame structure characteristics of LSC, a test rig of low swirl burner was designed and developed. Particle image velocimetry measurement results show that the location and size of the recirculation zone are different, and the flow field shows typical “W”- and “U”-shaped distributions under various swirling flow conditions. The self-luminous results of LSC show that there are three flame modes including attached flame, “W”-shaped flame, and “U”-shaped flame. To deeply understand NOx generation pathways, a chemical reactor network model was developed based on experiments and computational fluid dynamics simulations, and the effects of premixed gas components on NOx pathways were calculated by using Chemkin software. It was verified that the NOx production of the CH4 mixture mixed with H2, N2, and CO2 was mainly formed by the thermal NO pathway in the recirculation zone. The increase of H2 promotes the generation of NNH-type NOx in the main flame zone and inhibits prompt NOx. The addition of N2 and CO2 greatly promotes the generation of prompt NOx and at the same time inhibits NNH-type NOx. In addition, there is little prompt NOx formation in the post-flame zone.

Ultraviolet Absorption Cross-Sections of Ammonia at Elevated Temperatures for Nonintrusive Quantitative Detection in Combustion Environments.

Ammonia (NH3) is regarded as an important nitrogen oxides (NOx) precursor and also as an effective reductant for NOx removal in energy utilization through combustion, and it has recently become an attractive non-carbon alternative fuel. To have a better understanding of thermochemical properties of NH3, accurate in situ detection of NH3 in high temperature environments is desirable. Ultraviolet (UV) absorption spectroscopy is a feasible technique. To achieve quantitative measurements, spectrally resolved UV absorption cross-sections of NH3 in hot gas environments at different temperatures from 295 K to 590 K were experimentally measured for the first time. Based on the experimental results, vibrational constants of NH3 were determined and used for the calculation of the absorption cross-section of NH3 at high temperatures above 590 K using the PGOPHER software. The investigated UV spectra covered the range of wavelengths from 190 nm to 230 nm, where spectral structures of the transition of NH3 in the umbrella bending mode, v2, were recognized. The absorption cross-section was found to decrease at higher temperatures. For example, the absorption cross-section peak of the (6, 0) vibrational band of NH3 decreases from ∼2 × 10−17 to ∼0.5 × 10−17 cm2/molecule with the increase of temperature from 295 K to 1570 K. Using the obtained absorption cross-section, in situ nonintrusive quantification of NH3 in different hot gas environments was achieved with a detection limit varying from below 10 parts per million (ppm) to around 200 ppm as temperature increased from 295 K to 1570 K. The quantitative measurement was applied to an experimental investigation of NH3 combustion process. The concentrations of NH3 and nitric oxide (NO) in the post flame zone of NH3–methane (CH4)–air premixed flames at different equivalence ratios were measured.

Development and Application of an Efficient Chemical Reactor Network Model for Oxy-fuel Combustion

The excessive computational cost of computational fluid dynamics (CFD) simulations of complex reactors is still a barrier to investigations using detailed chemical kinetics. Chemical rector network modeling is a promising tool for affordable numerical investigations of novel combustor technologies, such as oxy-fuel combustion, using directly coupled detailed chemical kinetics. In this work, a novel chemical reactor network solver, NetSMOKE, which is part of the OpenSMOKE++ suite, is presented and discussed. The numerical model employed, together with the relative solution method, is explained, also exploiting a combined sequential-modular and equation-oriented approach for solving the global system of equations. The novel solver was employed to build a chemical reactor network that represents the Technische Universität Darmstadt oxy-fuel burner using a reduced number of ideal reactors with directly coupled detailed kinetic models for the first time. On the basis of previously available CFD simulations and measurements, the complex flow is accurately characterized and discretized into macrozones, facilitating the development of a simplified reactor network. Carbon monoxide emissions were analyzed in detail, supported by sensitivity analysis with respect to the reactor temperatures. The sensitivity analysis revealed that the post-flame zone is crucial for the overall CO emission. Therefore, on the basis of the sensitivity analysis, an iterative approach for refining the reactor network model is developed. The increase in the number of ideal reactors in targeted areas of the system allows for significant improvements with respect to CO predictions. The chemical reactor network provides good agreement with the experimental data, requiring only a limited increase in the overall computational cost. The presented tool offers a computationally efficient strategy to investigate and predict the behavior of complex reactors, including the emission of pollutants in combustion devices, allowing for the employment of state-of-the-art, detailed chemical kinetic mechanisms available from the literature.

Numerical and Experimental Investigations on Combustion Characteristics of Premixed Lean Methane–Air in a Staggered Arrangement Burner with Discrete Cylinders

Premixed combustion of lean methane–air in an artificial porous media burner with staggered alumina cylinders was experimentally and numerically performed. Numerical simulations were conducted at gas mixture velocities of 0.43–0.86 m/s and equivalence ratios of 0.162 and 0.243, respectively. Through comparison with experimental results, temperature distribution, peak temperature and flame propagation velocity are analyzed and discussed in detail. The numerical calculated temperature profile over the axis of the combustor coincided well with test data in the post-flame zone, however a certain deviation was found in the preheated zone. A two-dimensional flame shape was observed and the flame thickness was the size of cylinder diameter. The peak temperature increased with the gas mixture inlet velocity at the certain equivalence ratio, and its peak value was about 1.8–2.16 times higher than the adiabatic combustion temperature under the desired equivalence ratio, which indicates that super-adiabatic combustion was the case for all the numerical simulations. The flame propagating velocity had a positive correlation with the gas mixture inlet velocity.

Propagation of weakly stretched premixed spherical spray flames in localized homogeneous and heterogeneous reactants

Propagation of weakly stretched spherical flames in partially pre-vaporized fuel sprays is theoretically investigated in this work. A general theory is developed to describe flame propagation speed, flame temperature, droplet evaporation onset, and completion locations. The influences of liquid fuel and gas mixture properties on spherical spray flame propagation are studied. The results indicate that the spray flame propagation speed is enhanced with increased droplet mass loading and/or evaporation heat exchange coefficient (or evaporation rate). Opposite trends are found when the latent heat is high due to strong evaporation heat absorption. Fuel vapor and temperature gradients are observed in the post-flame evaporation zone of heterogeneous flames. The evaporation completion front location considerably changes with flame radius. For larger droplet loading and a smaller evaporation rate, the fuel droplet tends to complete evaporation behind the flame front. Flame bifurcation occurs with high droplet mass loading under large latent heat, leading to multiplicity of flame propagation speed, droplet evaporation onset, and completion fronts. The flame enhancement or weakening effects by the fuel droplet sprays are revealed by the enhanced or suppressed heat and mass diffusion process in the pre-flame zone. Besides, for heterogeneous flames, heat and mass diffusion in the post-flame zone also exists. The mass diffusion for both homogeneous and heterogeneous flames is enhanced with a decreased Lewis number. The magnitude of the Markstein length is considerably reduced with increased droplet loading. Moreover, post-flame droplet burning behind the heterogeneous flame influences the flame propagation speed and Markstein length when the liquid fuel loading is relatively low.

Kinetics modeling of NOx emissions characteristics of a NH3/H2 fueled gas turbine combustor

The two major challenges for ammonia combustion in gas turbines are its high NOx emission and low burning velocity. Recently, while H2 doping has been proved as an effective means of improving the NH3 burning velocity, there is a knowledge gap on the emissions characteristics of NH3/H2 fuel mixture in gas turbine conditions. This paper presents a kinetics modeling study on the NOx emission characteristics of a staged rich-lean combustor cofiring NH3/H2 mixture. A chemical reaction network was set up to model the staged combustor. Emission characteristics were obtained at various operating parameters, including equivalence ratio, residence time, and H2 doping ratio. The rich-burn stage equivalence ratio was found to be the primary factor influencing NOx emissions. The key to reduce NOx emissions is to balance the NO production and the unburnt NH3 in the rich-burn stage. Extending the residence time of its post-flame zone and increasing H2 doping ratio also benefit NOx reduction by decreasing unburnt NH3 content, but with a secondary impact. NO reaction pathway analyses showed that NO is formed mainly through the HNO pathway in the flame zone of the rich-burn stage, while NHi decomposition in the post-flame zone plays a key role in NO reduction. Depending on the NHi concentration, different mechanisms dominate NO reduction in the post-flame zone.

Analytical investigation of magnesium aerosol combustion based on the asymptotic model of flame structure

Due to shortage and environmental pollution of fossil fuels and environment-dependent production of renewable energies, the metal fuels are promising candidates for alternative energy sources. In this study, the micron-sized magnesium particles are considered as energy carrier and the flame propagation of lean premixed magnesium-air combustion is investigated under zero gravity condition. To analyze dust cloud combustion, an asymptotic model of flame structure is proposed. According to the combustion behavior of single particle, the flame structure consists of four different zones including preheat, liquid magnesium, vaporized magnesium and post-flame zones in which the melting, vaporization and flame occur instantaneously. Afterwards, the non-dimensional forms of governing equations including mass, energy and gaseous fuel mass fraction conservations and the appropriate boundary conditions are derived and analytically solved. Subsequently, as the important achievements of the present study, the explicit formulas are obtained for flame velocity, location and temperature. Eventually, the effects of involved parameters on combustion characteristics are examined. The results indicate that as the particle diameter enhances from 15 to 60 µm, the flame front moves to a place 2.5 times farther away. The flame temperature increases linearly with concentration, while it decreases with the inverse of square of the diameter.

Experimental and kinetic modeling study of NO formation in premixed CH4+O2+N2 flames

The nitric oxide (NO) formation in methane (CH 4 ) flames has been widely investigated, with quite a few kinetic mechanisms available in the literature. However, studies have shown that there are often discrepancies between the simulations using various mechanisms and the experimental results. To elucidate reactions leading to these discrepancies, experiments were designed to measure the NO formation in the post flame zone of CH 4 +O 2 +N 2 flames with the oxygen ratio, x O 2 = O 2 /(O 2 +N 2 ), varying from 0.2 to 0.27. The experiments were carried out on a heat flux burner at atmospheric pressure and 298 K using saturated Laser-induced fluorescence. The equivalence ratio, ϕ , was changed from 0.7 to 1.6. The corresponding laminar burning velocity, S L , for each condition was also measured using the heat flux method. A comparison was made for the present experimental data and simulation results using the Konnov, Glarborg, NOMecha 2.0, and San Diego mechanisms, and none of them well reproduced the new NO experimental data for all investigated conditions. Numerical analyses show that the increment of NO mole fraction in stoichiometric and fuel-lean flames when the x O 2 increases is mostly defined by the thermal-NO production, which is found to be over-predicted, especially by the Konnov and San Diego mechanisms. The rate constant of reaction NO+N = N 2 +O was derived as k = 1.529 × 10 13 T − 0.0027 exp ( 185.41 [ c a l / m o l e ] R T ) cm 3 / mol s over 225–2400 K temperature range. The rate constants of four reactions controlling CH mole fraction profiles and prompt-NO formation were updated based on the analysis of the literature data that yields an improved performance of the Konnov mechanism.

Experimental investigation of the pressure influence on flame structure of fuel-rich oxy-fuel methane flames for synthesis gas production

The structure of fuel-rich CH4/O2-flames at equivalence ratios in the range of 2.5≤ϕ≤2.9, a preheating temperature of TP=573K and pressure in the range of 1bara≤p≤5bara was studied experimentally and compared with numerical results using detailed reaction mechanisms. Two measurement techniques, invasive gas sampling (GC/MS) and optical in-situ TDLAS measurements (tunable diode laser absorption spectroscopy), were used to determine temperature and major combustion species (CH4, O2, H2, H2O, CO and CO2). The results showed that the composition of the synthesis gas in the post flame zone strongly depends on ϕ, while p has only a minor influence. Furthermore, the influence of ϕ and p on the phenomena of super-adiabatic flame temperatures (SAFT) was determined by experimentally and numerically investigating temperature and H2O concentration. The results showed similar trends as for the synthesis gas composition: a strong dependence of ϕ and a minor influence of pressure on the degree of significance of the SAFT phenomena.

Propagation of heterogeneous and homogeneous planar flames in fuel droplet mists

Propagation of one-dimensional planar flames laden with fuel droplets is theoretically investigated in this work. Localized homogeneous and heterogeneous flames are considered, characterized by different relative locations of dispersed droplets and propagating flame front. With the assumption of large activation energy for the chemical reaction, correlations describing flame propagation speed, flame temperature, evaporation onset front and completion front are derived. The influences of droplet and fuel properties on propagation of fuel-droplet-laden planar flames are studied. The results indicate that the flames are enhanced under fuel-lean conditions but weakened under fuel-rich conditions. Both effects are intensified with increased droplet mass loading. Fuel vapor and temperature gradient are observed in the post-flame evaporation zone of heterogeneous flames. Evaporation completion front location is considerably affected by the droplet diameter, but the evaporation onset front varies little with droplet properties. Flame bifurcation occurs with high droplet mass loading under fuel-rich mixture, leading to multiplicity of flame propagation speed as well as droplet evaporation onset and completion fronts. Sprayed fuels with larger latent heat of vaporization would weaken the flame enhancement effect under fuel-lean conditions, due to larger evaporation heat loss. Moreover, for homogeneous flames, evaporative heat loss only occurs in the pre-flame zone. However, for heterogeneous flames, heat loss from the post-flame evaporation zone becomes dominant for relatively large droplets. Besides, for fuel-lean mixtures laden with medium-sized fuel droplets, the propagation speed of heterogeneous flames increases with Lewis number, due to the enhanced fuel vapor diffusion in the post-flame zone.