Stability and characteristics of NH3/CH4/air flames in a combustor fired by a double swirl stabilized burner

Compared to hydrocarbons, ammonia's low reactivity and higher NO_x emissions limit its practical application. Consequently, its implementation in combustion systems requires a different combustor geometry, by adapting existing systems or developing new ones. This study investigates the flame stability, NO emissions, and flame structure of NH₃/CH₄/air premixed flames in a novel combustor comprising a double swirl burner. A lean premixed CH₄/air mixture of equivalence ratio, Φ_out, was supplied to the outer swirl, while a NH₃/CH₄/Air mixture fed the inner swirl. The molar fraction of NH₃ in the inner fuel blend, x_NH3, was varied from 0 (pure CH₄) to 1 (pure NH₃) over far-lean to far-rich inner stream equivalence ratio, Φ_in. This new burner's stability map was established in terms of Φ_in versus x_NH3 for different Φ_out. Then, NO emissions were measured versus Φ_in for various x_NH3 and Φ_out. Finally, based on the NO emissions, eight flames were down-selected for in-flame measurements, which included temperature and OH-PLIF. The stability measurements revealed that increasing x_NH3 modifies the stability map by increasing the lean blowout limits and narrowing the flashback region. At Φ_out ≥ 0.6, a stable flame was achieved for a pure inner NH₃/air mixture. Low NO emissions were achieved in this burner configuration at x_NH3=1 by either enriching or far-leaning Φ_in. Enriching Φ_in led to a steep decrease in NO concentrations. However, to achieve low NO concentrations, precise control of Φ_out was needed. At Φ_in=1.4, 220 ppm NO at Φ_out=0.7 versus 690 at Φ_out=0.6 was measured. Moreover, substantially enriching Φ_in>1.2 led to a slight decrease in measured NO. Generally, the OH-PLIF images revealed a conical OH-layer at the burner exit. Certain flame conditions created OH-pockets inside the conical structure or formed a V-shaped OH-layer far downstream. This change in flame structure was found to impact NO emissions strongly.     

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

Ammonia has widely attracted interest as a potential candidate not only as a hydrogen energy carrier but also as a carbon free fuel for internal combustion engines, such as gas turbines. Because ammonia contains a nitrogen atom in its molecule, nitrogen oxides (NO_x) and other pollutants may be formed when it burns. Therefore, understanding the fundamental product gas characteristics of ammonia/air laminar flames is important for the design of ammonia-fueled combustors to meet stringent emission regulations. In this study, the product gas characteristics of ammonia/air premixed laminar flames for various equivalence ratios were experimentally and numerically investigated up to elevated pressure conditions. In the experiments, a stagnation flame configuration was employed because an ammonia flame can be stabilized by using such a configuration without a pilot flame. The experimental results showed that the maximum NO mole fraction was about 3,500 ppmv, at an equivalence ratio of 0.9 at 0.1 MPa. The NO mole fraction decreased as the equivalence ratio increased. In addition, the maximum value of the NO mole fraction decreased with an increase in mixture pressure. Furthermore, it was experimentally clarified that the simultaneous reduction of NO and unburnt ammonia can be achieved at an equivalence ratio of about 1.06, which is the target equivalence ratio for emission control in rich-lean two-stage ammonia combustors. Comparison of experimental and numerical results showed that even though the reaction mechanisms employed have been optimized for predicting the laminar burning velocity of ammonia/air flames, they failed to satisfactorily predict the measured species in this study. Sensitivity analysis was used to identify elementary reactions that control the species profiles but have negligible effects on the burning velocity. It is considered that these reaction models need to be updated for accurate prediction of product gas characteristics of ammonia/air flames.     

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.     

Effects of CO2 dilution on partially premixed CH4-air flames in swirl and bluff body stabilized combustor

The effect of CO₂ dilution on the flame characteristics and pollutant emission of a partially premixed CH₄-air flame in a confined bluff body and swirl influenced flowfield is investigated using optical and laser diagnostic methods. The non-premixed burner produced a converging-diverging flowfield at the burner exit and a lifted flame is produced at all test cases, with an upstream movement of the flame with decreasing global equivalence ratios (?_g). Based on variations in ?_g, two flame stabilization modes – bluff body influenced and swirl stabilized – with a transition mode in-between is observed for the cases with (flame FB) and without dilution (flame FM). The characteristics of the heat release zone are influenced by dilution, with the FB flames being longer and also less intense when compared to FM flames. Pollutant measurement at 30 mm downstream from the combustor exit highlighted the ultra-low NO_x capability of the IIST-GS2 burner. CO₂ dilution leads to a reduction in NO_x emission due to both thermal and chemical effects. For ?_g ≥ 0.7 extreme low levels of CO and unburned hydrocarbons (UHC) are observed for both cases. For ?_g ≤ 0.6 the dramatic increase of both CO and UHC maybe due to the lower flame temperatures and shorter flame zone residence times, respectively.     

Cavity ring-down measurements of seeded NO in premixed atmospheric-pressure H2/air and CH4/air flames

Quantitative aspects of using cavity ring-down absorption spectroscopy near 226 nm for measurements of NO mole fractions in premixed atmospheric-pressure flames are discussed. Measurements in methane–air flames showed strong broadband absorption near 226 nm by hot CO₂ molecules, precluding using the cavity ring-down method in these flames at atmospheric pressure. In hydrogen–air flames, the broadband absorption at this wavelength was substantially lower. Absorption cross sections derived from non-seeded cavity ring-down spectra suggest that absorption by water is the major contribution to the background in these flames. The detectability limit for NO by cavity ring-down measurements in hydrogen–air flames using the current setup is estimated to be 10 ppm. Effects of the cold boundary layer on the measured NO mole fraction were accounted for by measuring the radial distributions of temperature and NO mole fraction using coherent anti-Stokes Raman scattering and laser-induced fluorescence (LIF), respectively. Measurements performed in seeded stoichiometric and lean hydrogen–air flames showed no reburning at temperatures above 1750 K, demonstrating the adequacy of using these flames for calibration of LIF measurements. At lower temperatures, the mole fraction of NO in the hot gases was up to 30% lower than that expected from the degree of seeding in the cold gases. PACS 42.62.Fi; 42.68.Ca; 82.33.Vx     

Effect of fuel mixture fraction and velocity perturbations on the flame transfer function of swirl stabilized flames

A novel methodology is developed to decompose the classic Flame Transfer Function (FTF) used in the thermo-acoustic stability analysis of lean premix combustors into contributions of different types. The approach is applied, in the context of Large Eddy Simulation (LES), to partially-premixed and fully-premixed flames, which are stabilized via a central recirculation zone as a result of the vortex breakdown phenomenon. The first type of decomposition is into contributions driven by fuel mixture fraction and dynamic velocity fluctuations. Each of these two contributions is further split into the components of turbulent flame speed and flame surface area. The flame surface area component, driven by the pure dynamic velocity fluctuation, which is shown to be a dominant contribution to the overall FTF, is also additionally decomposed over the coherent flow structures using proper orthogonal decomposition. Using a simplified model for the dynamic response of premixed flames, it is shown that the distribution of the FTF, as obtained from LES, is closely related to the characteristics of the velocity field frequency response to the inlet perturbation. Initially, the proposed method is tested and validated with a well characterized laboratory burner geometry. Subsequently, the method is applied to an industrial gas turbine burner.     

Direct numerical simulations and analysis of three-dimensional n-heptane spray flames in a model swirl combustor

Three-dimensional n-heptane spray flames in a swirl combustor are investigated by means of direct numerical simulation (DNS) to provide insight into realistic spray evaporation and combustion as well as relevant modeling issues. The variable-density, low-Mach number Navier–Stokes equations are solved using a fully conservative and kinetic energy conserving finite difference scheme in cylindrical coordinates. Dispersed droplets are tracked in a Lagrangian framework. Droplet evaporation is described by an equilibrium model. Gas combustion is represented using an adaptive one-step irreversible reaction. Two different cases are studied: a lean case that resembles a lean direct injection combustion, and a rich case that represents the primary combustion region of a rich-burn/quick-quench/lean-burn combustor. The results suggest that premixed combustion contribute more than 70% to the total heat release rate, although diffusion flame have volumetrically a higher contribution. The conditional mean scalar dissipation rate is shown to be strongly influenced, especially in the rich case. The conditional mean evaporation rate increases almost linearly with mixture fraction in the lean case, but shows a more complex behavior in the rich case. The probability density functions (PDF) of mixture fraction in spray combustion are shown to be quite complex. To model this behavior, the formulation of the PDF in a transformed mixture fraction space is proposed and demonstrated to predict the DNS data reasonably well.     

Quantitative one-dimensional single-pulse multi-species concentration and temperature measurement in the lift-off region of a turbulent H2/air diffusion flame

In this article, we report on one-dimensional single-pulse measurements of temperature and major-species concentration (O₂, N₂, H₂O and H₂) in a turbulent H₂/air jet diffusion flame using Raman and Rayleigh scattering of KrF* excimer-laser radiation. Spatial resolution of 0.5 mm along a 6mm long line has been obtained, with reasonable error limits for mole fraction ( = 5 % for N₂ detection) and temperature (T = 8 %) determination at flame temperatures. We present various profiles showing the composition and temperature along a line at different heights in the flame with particular emphasis on the lift-off region (i.e. lowx/D). In this zone, temperature and mixture fraction can be determined simultaneously — from a single laser pulse — in a spatial region extending from unburnt gas in the center of the jet across the flame front into the cool air of the surrounding atmosphere. This allows for the first time the systematic study of the shape and width of the high-temperature region and the corresponding concentration and temperature gradients. The comparison of averaged data and scatter plots with previous pointwise measurements shows good agreement.     

Simultaneous Raman/LIF measurements of major species and NO in turbulent H2/air diffusion flames

A single-pulse spontaneous Raman scattering apparatus, based on a flashlamp-pumped dye laser, was used to determine the concentrations of the major species and the temperature in turbulent H₂/N₂/air jet diffusion flames. The concentrations of nitric oxide were simultaneously measured by Laser-Induced Fluorescence (LIF) after excitation of theA ^{2 +}–X ² transition with a Nd: YAG-pumped dye laser. Some fundamentals of the employed methods, including the calibration procedure, quenching corrections, and accuracy are discussed. Besides a detailed study of the experimental technique, a main goal of the presented investigations was the generation of comprehensive data sets of high accuracy from well-defined turbulent flames which allow for a quantitative comparison with model calculations. Two flames with different fuel dilution and Reynolds numbers were investigated in a pattern of typically 100 measuring locations each comprising 300 single shots. In addition, four flames with different flow velocities but same fuel composition were compared with respect to their temperature and NO concentration profiles. The results show that differential diffusion plays an important role in these flames, especially near the flame base, where the temperature is increased above the adiabatic flame temperature and deviations from adiabatic equilibrium are large. The correlations between NO and mixture fraction and NO and temperature reveal characteristic features of the different flames.     

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.     

Lewis number effects on laminar and turbulent expanding flames of NH3/H2/air mixtures at elevated pressures

This study clarifies the effects of Lewis number (Le) on laminar and turbulent expanding flames of NH₃/H₂/air mixtures. The laminar burning velocity (S_L) and turbulent burning velocity (S_T) were measured using a medium-scale, fan-stirred combustion chamber with ammonia/hydrogen molar ratio (NH₃/H₂) of 50/50 and 80/20 under the maximum pressures of 5 atm. The lean laminar flame with NH₃/H₂ = 50/50 is significantly accelerated by the diffusional–thermal instability, which dominated the trend of S_T,c=0.1 with the equivalence ratio (ϕ). The lean normalized turbulent burning velocity (S_T/S_L) increases with the decrease of hydrogen content due to the weakening effects of S_L. However, the S_T/S_L reaches peak with hydrogen volumetric content less than 20% due to effects made by diffusional–thermal instability than S_L did. The turbulent flame of NH₃/H₂/air mixtures is characterized by self-similar acceleration propagation, and propagation with Le < 1 is faster. A modified correlation considering the effects of Le was proposed, as (d<r>/dt)/σS_L = 0.118(Re_T,flameLe⁻²)^0.57, which was able to predict not only the self-similar propagation of NH₃/H₂/air but also the previous syngas/air flames. The Kobayashi correlations modified by three kinds of Le power exponents were used to clarify the effects of Le by comparing their fitting parameters and predictive powers on experimental data and literature data. Similar pre-factors, power exponents and the goodness of fit (R²) were obtained with Le ranging from 0.58 to 1.62, which suggested that the determination of Le power exponent had no significant effect on the prediction accuracy of the S_T/S_L trend with data of Le near unity. This might be attributed to the fact that the variation ranges of the dimensionless number that characterizes the experimental conditions is much larger than that of the Le.     

Experimental and theoretical investigations on sonoluminescence under dual frequency conditions

The multibubble sonoluminescence (MBSL) intensities from water exposed to the simultaneous ultrasonic irradiation from 20 kHz (fixed at 6.3 W) and 355 kHz (variable power) ultrasound sources have been compared to the MBSL from the individual ultrasound sources under the same power conditions. A synergistic enhancement of the sonoluminescence (SL) signal, >30-fold, at low powers (4.6 W) of the higher frequency was observed. At a higher acoustic power level (15.8 W) the dual frequency operation produced a decrease in the SL signal. These results are in agreement with previously reported data [P. Ciuti, N.V. Dezhkunov, A. Francescutto, F. Calligaris, F. Sturman, Ultrasonics Sonochem. 10 (2003) 337; N.V. Dezhkunov, J. Eng. Phys. Therm. 76 (2003) 142] under similar experimental conditions. Numerical single bubble (SB) dynamics calculations have been used to help interpret the experimental results. It is suggested that the observed effects are caused by a combination of changes to the peak collapse temperature of individual bubbles as well as to changes in the active bubble population.     

Spatially resolved absolute concentration and fluorescence-lifetime determination of H2CO in atmospheric-pressure CH4/air flames

Using laser-induced fluorescence (LIF), spatially resolved concentration profiles of formaldehyde (H₂CO) were obtained in the preheating zone of atmospheric-pressure premixed CH₄/air flames stabilized on the central slot of a multiple-slot burner similar in construction to domestic boilers. The isolated pQ₁(6) rotational line (339.23 nm) in the 2¹ ₀4¹ ₀ vibronic combination transition in the ?¹A₂- ¹A₁ electronic band system around 339 nm was excited in the linear LIF intensity regime. For a quantification of quenching effects on the measured LIF signal intensities, relative fluorescence quantum yields were determined from direct fluorescence lifetime as a function of height above the slot exit. Absolute H₂CO number densities in the flames were evaluated from a calibration of measured LIF signal intensities versus those obtained in a low-pressure sample with a known H₂CO vapor pressure. Peak concentrations in the slightly lean and rich flames reached (994±298) and (174±52) ppm, respectively. Received: 25 September 2000 / Published online: 30 November 2000     

2D computations of FREI with cool flames for n-heptane/air mixture

Two-dimensional axisymmetric numerical simulation reproduced flames with repetitive extinction and ignition (FREI) in a micro flow reactor with a controlled temperature profile with a stoichiometric n-heptane/air mixture, which have been observed in the experiment. The ignition of hot flame occurred from consumption reactions of CO that was remained in the previous cycle of FREI. Between extinction and ignition locations of hot flames, several other heat release rate peaks related to cool and blue flames were observed for the first time. After the extinction of the hot flame, cool flame by the low-temperature oxidation of n-heptane appeared first and was stabilized in a low wall temperature region. In the downstream of the stable cool flame, a blue flame by the consumption reactions of cool flame products of CH₂O and H₂O₂ appeared. After that, the hot flame ignition occurred from the remaining CO in the downstream of the blue flame. Then after the next hot flame ignition, the blue flame was swept away by the propagating hot flame. Soon before the hot flame merged with the stable cool flame, the hot flame propagation was intensified by the cool flame. After the hot flame merged with the stable cool flame, the hot flame reacted with the incoming fresh mixture of n-C₇H₁₆ and O₂.     

Properties of nanocrystalline TiO2 synthesized in premixed flames stabilized on a rotating surface

The properties of TiO₂ nanoparticles synthesized using the flame stabilized on a rotating surface method (FSRS) are investigated. The method uses a laminar, premixed, stagnation flame, combining particle synthesis and film deposition in a single step. The current study examines the effects of flame properties on particle characteristics. Synthesized particles were characterized using X-ray diffractometry, Transmission Electron Microscopy and UV–vis spectrometry in order to quantify the effects of equivalence ratio and precursor loading on particle size, crystallinity and optical band-gap. Results show that flame stoichiometry significantly affects crystal phase, but it has little to no effect on particle size and light absorption band edge. In addition, precursor loading impacts both the particle size and the crystal phase. The study demonstrates the potential of the FSRS method for producing tailored nanoscale TiO₂ particles for a variety of applications.     

Experimental and theoretical evidence for long-lived molecular hydrogen anions H2- and D2-

The existence of (metastable) molecular hydrogen anions H2(-), D2(-), and H3(-) is demonstrated. These anion species were produced by sputtering of TiH2 and TiD2 targets with Cs+ ions and were identified by accelerator mass spectrometry. From the respective flight times through the spectrometer, lifetimes for H2(-) and D2(-) of at least 3 micros and 4 micros, respectively, can be inferred. Theoretical calculations within the nonlocal resonance model predict the existence of highly rotationally excited anions with lifetimes in the micros range. It is proposed that in sputtering molecular hydrogen species with high rotational and vibrational excitation are formed that are stable on the time scale of the experiment.