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.     

Effect of different turbulence models on combustion and emission characteristics of hydrogen/air flames

This paper aims to present modeling results of hydrogen/air combustion in a micro-cylindrical combustor. Modeling studies were carried out with different turbulence models to evaluate performance of these models in micro combustion simulations by using a commercially available computational fluid dynamics code. Turbulence models implemented in this study are Standard k-ε, Renormalization Group k-ε, Realizable k-ε, and Reynolds Stress Transport. A three-dimensional micro combustor model was built to investigate impact of various turbulence models on combustion and emission behavior of studied hydrogen/air flames. Performance evaluation of these models was executed by examining combustor outer wall temperature distribution; combustor centerline temperature, velocity, pressure, species and NO_x profiles. Combustion reaction scheme with 9 species and 19 steps was modeled using Eddy Dissipation Concept model. Results obtained from this study were validated with published experimental data. Numerical results showed that two equation turbulence models give consistent simulation results with published experimental data by means of trend and value. Renormalization Group k-ε model was found to give consistent simulation results with experimental data, whereas Reynolds Stress Model was failed to predict detailed features of combustion process.     

Effect of elliptic burner geometry and air equivalence ratio on the nitric oxide emissions from turbulent hydrogen flames

An experimental study on turbulent hydrogen flames from circular and elliptic burners with varying degrees of premixedness (diffusion, fuel-rich, stoichiometric, and fuel-lean) is presented. Flame stability, visible flame height, flame radiation, global nitric oxide (NO) concentration, and inflame temperature and NO concentration profiles were measured. We found that the elliptic burner flames had lower liftoff velocity, were shorter, and radiated less heat to the surrounding as compared to circular burner flames. Global NO concentration decreased with an increase in air equivalence ratio for both circular and elliptic burner flames. Peak in-flame NO concentration along the flame centerline increased with a decrease in air equivalence ratio. Elliptic burner flames produced higher peak in-flame temperatures. Overall, the elliptic burner flames produced less peak NO as compared to circular burner flames at all air equivalence ratios except zero (diffusion flames) in accordance with the global emission measurements.     

Effect of differential diffusion on turbulent lean premixed hydrogen enriched flames through structure analysis

This study presents the flame structure influenced by the differential diffusion effects and evaluates the structural modifications induced by the turbulence, thus to understand the coupling effects of the diffusively unstable flame fronts and the turbulence distortion. Lean premixed CH₄/H₂/air flames were conducted using a piloted Bunsen burner. Three hydrogen fractions of 0, 30% and 60% were adopted and the laminar flame speed was kept constant. The turbulence was generated with a single-layer perforated plate, which was combined with different bulk velocities to obtain varied turbulence intensities. Quasi-laminar flames without the plate were also performed. Explicit flame morphology was obtained using the OH-PLIF. The curvature, flame surface density and turbulent burning velocity were measured. Results show that the preferential transport of hydrogen produces negatively curved cusps flanked with positively curved bulges, which are featured by skewed curvature pdfs and consistent with the typical structure caused by the Darrieus-Landau instability. Prevalent bulge-cusp like wrinkles remain with relatively weak turbulence. However, stronger turbulence can break the bulges to be finer, and induce random positively curved cusps, therefore to destroy the bulge-cusp structures. Evident positive curvatures are generated in this process modifying the skewed curvature pdfs to be more symmetric, while the negative curvatures are not affected seriously. From low to high turbulence intensities, the hydrogen addition always strengthens the flame wrinkling. The augmentation of flame surface density and turbulent burning velocity with hydrogen is even more obvious at higher turbulence intensity. It is suggested that the differential diffusion can persist and even be strengthened with strong turbulence.     

Numerical simulation of high-pressure hydrogen jet flames during bonfire test

Characteristics of high-pressure hydrogen jet flames resulting from ignition of hydrogen discharge during the bonfire test of composite hydrogen storage vessels are studied. Firstly, a 3-D numerical model is established based on the species transfer model and SST k − ω turbulence model to study the high-pressure hydrogen jet flow. It is revealed that under-expanded jets are formed after the high-pressure hydrogen discharging from the vessel. Secondly, the mathematical methods are adopted to study the high-pressure hydrogen jet flames. The effects of pressure, initial temperature and the nozzle diameter on the jet flames are investigated. The results show that the jet flame length increases with the increase of discharge pressure, but decreases with the increase of nozzle diameter and temperature difference between the filling hydrogen temperature and the environment temperature. Finally, the simulation models are established to study the characteristics of hydrogen jet flames in an open space. The effects of barrier walls on the distribution of jet flames are also studied. The results show that the barrier walls can greatly reduce the damage from hydrogen jet flames to testers and properties around.     

On scaling and mathematical modelling of large scale industrial flames

Gaseous flames and pulverized coal flames are considered to examine relationships between lab-scale flames, semi-industrial scale and industrial-scale flames. The experimental data spans the thermal input range from the lowest scale of 30 kW to the largest of 12 MW with several intermediate scales. The primary questions are whether effects observed in lab-scale flames are scalable to industrial applications and whether mathematical models developed on the basis of lab-scale data are directly applicable to industrial flames.It has been observed that disparity between the in-flame temperatures measured in lab-scale and in large-scale flames can be as large as 100–200 K due to different measurement techniques used. In lab-scale experiments one observes a strong interaction between turbulence and chemistry and the measured data is sensitive to small alterations to burner inputs and/or boundary conditions. The sensitivity almost disappears at large-scales since the convective mixings is the dominant (the slowest) mechanism. In other words, different effects are seen at small- and large-scales and different mechanisms are controlling. Although the paper is concerned with single flames, in our opinion, the conclusions are also relevant to gas turbines. Until genuine efforts are taken to develop a good understanding of combustion system scaling, the worlds of combustion science and combustion engineering will remain parted.     

Effect of hydrogen enrichment on flame broadening of turbulent premixed flames in thin reaction regime

An experimental study to identify the effect of hydrogen enrichment and differential diffusion on the flame broadening is conducted. Turbulent lean premixed flames in the Broadened Preheat–Thin Reaction (BP-TR) regime are obtained. The flames are stabilized on a Bunsen burner and CH₄/H₂/air mixtures are adopted with three hydrogen fractions of 0, 30% and 60%. The preheat zone and heat release zone are captured with the multi-species Planar Laser-Induced Fluorescence (PLIF) of OH and CH₂O radicals. Flame thicknesses of the preheat and heat release layers are measured. Results show broadened preheat zone and thin heat release layers for the flames, as predicted by the BP-TR regime. The preheat zone thickness can be increased to about 3–6 times compared to the laminar preheat thickness. An apparently decreased preheat zone thickness with hydrogen addition is observed. The differential diffusion is anticipated to locally thicken the heat release zone along the flame front. The mean heat release thickness is nearly not affected by the turbulence or hydrogen addition.     

Laminar burning velocities of rich near-limiting flames of hydrogen

In the present work, near-limiting hydrogen flames were investigated both experimentally and numerically. Very rich hydrogen + air flames were studied in a constant volume bomb equipped with a pressure sensor and a Schlieren system for optical registration of the flame front movement. The mixtures contained 70% and 75% of hydrogen, the rest being air. The measurements were conducted at pressures from 1 to 4 atm for 70% H₂ + air mixture and from 0.7 to 1.4 atm for 75% H₂ + air mixture. Two methods for determination of the laminar burning velocity were used: from the temporal evolution of the flame front movement and from the pressure records at nearly constant pressure. These methods were compared and discussed in terms of accuracy and implicit assumptions behind them. Markstein lengths were also extracted and compared with the literature by using different extrapolation models. An important role of the critical radius for extraction of the burning velocity and Markstein length is demonstrated. New experimental data are compared with three models for hydrogen combustion to elucidate the need for their further development.     

Analysis of jet flames and unignited jets from unintended releases of hydrogen

A combined experimental and modeling program is being carried out at Sandia National Laboratories to characterize and predict the behavior of unintended hydrogen releases. In the case where the hydrogen leak remains unignited, knowledge of the concentration field and flammability envelope is an issue of importance in determining consequence distances for the safe use of hydrogen. In the case where a high-pressure leak of hydrogen is ignited, a classic turbulent jet flame forms. Knowledge of the flame length and thermal radiation heat flux distribution is important to safety. Depending on the effective diameter of the leak and the tank source pressure, free jet flames can be extensive in length and pose significant radiation and impingement hazard, resulting in consequence distances that are unacceptably large. One possible mitigation strategy to potentially reduce the exposure to jet flames is to incorporate barriers around hydrogen storage equipment. The reasoning is that walls will reduce the extent of unacceptable consequences due to jet releases resulting from accidents involving high-pressure equipment. While reducing the jet extent, the walls may introduce other hazards if not configured properly. The goal of this work is to provide guidance on configuration and placement of these walls to minimize overall hazards using a quantitative risk assessment approach. The program includes detailed CFD calculations of jet flames and unignited jets to predict how hydrogen leaks and jet flames interact with barriers, complemented by an experimental validation program that considers the interaction of jet flames and unignited jets with barriers.     

Stability of lifted flames in centerbody burner

The centerbody burner was designed with the objective of understanding the coupled processes of formation, growth, and burn-off of soot through decoupling them using recirculation zones (RZs). Experimentally it was found that the sooting characteristics of the centerbody burner could be altered dramatically via simple changes in the operating conditions. One of the interesting operating regimes in which a flame lifts off and forms a column of soot was identified when oxygen in the annulus air jet was reduced sufficiently. This paper describes the numerical studies performed to aid the understanding of lifted flames in the centerbody burner. A time-dependent, axisymmetric, detailed-chemistry CFD model is used. Combustion and PAH formation are modeled using the Wang–Frenklach (99 species and 1066 reactions) mechanism, and soot is simulated using a two-equation model of Lindstedt. Calculations have predicted the structure of the lifted flame very well. Two RZs [outer (ORZ) and inner (IRZ)] are formed between the fuel and air jets. A diffusion flame that is lifted-off the centerbody plate anchors steadily to the outer periphery of the ORZ. A near-perfect match between the computed and measured flame lift-off heights is achieved. RZs transport soot that is formed in the flame toward the face of the centerbody and create the soot column. Ethylene and its lighter fuel fragments that are formed in the RZs diffuse toward the annulus air jet and establish a mixing layer with the inwardly diffusing oxygen. Heat diffusing away from the RZs initiates autoignition reactions in the mixing layer. A flame with a triple-flame-base structure becomes established at a location where the ignition-delay time matches the residence time. Soot that is transported into the RZs is found to have a significant effect on the flame lift-off height. Numerical experiments are performed to aid the understanding of the relationship between soot and flame lift-off. Radiation from the soot decreases the temperature, slows the autoignition process, and increases the lift-off height. Soot oxidation consumes O and OH radicals, slows the autoignition reactions, and increases the lift-off height.     

Exploratory studies of modeling approaches for hydrogen triple flames

A review of triple flame modeling is first presented, which demonstrates the need for additional work in this area. Building on previous methods described in the literature, a hybrid model that uses a weighted average of one-dimensional premixed and diffusion flamelet reaction rates has been proposed and evaluated for a hydrogen triple flame. Results indicated that some type of progress variable is needed for application of the diffusion flamelet contribution. Weighting the premixed flamelet reaction rate contribution at 100%, it is shown that peak temperatures between the model and a case employing detailed chemistry vary 7.5%, while heat release rate, flame speed, and mass fraction contours agree well.A second model, based on a library of reaction rates built from numerical studies which directly resolve the propagating triple flame has also been tested. Computational time for the baseline case is shown to be reduced by a factor of 3 ½ in comparison to use of detailed chemistry. The role of scalar dissipation rate as a necessary independent variable to the library has also been investigated using simulations with variable mixing layer thicknesses. Overall, it is found that large changes in local mixture fraction gradient cause rather small changes in propagation speed and total heat release rate of the hydrogen triple flame. This implies that such a model may be useful for CFD simulations that do not employ spatial resolution capable of resolving the triple flame itself.     

Flame speed and tangential strain measurements in widely stratified partially premixed flames interacting with grid turbulence

Methane-air partially premixed flames subjected to grid-generated turbulence are stabilized in a two-slot burner with initial fuel concentration differences leading to stratification across the stoichiometric concentration. The fuel concentration gradient at the location corresponding to the flame base is measured using planar laser induced fluorescence (PLIF) of acetone in the non-reacting mixing field. Simultaneous PLIF of the OH radical and particle image velocimetry (PIV) measurements are performed to deduce the flow velocity and the flame front. These flames exhibit a convex premixed flame front and a trailing diffusion flame, with flow divergence upstream of the flame, as indicated by the instantaneous OH–PLIF, Mie scattering images, and PIV data. The mean streamwise velocity profile attains a global minimum just upstream of the flame front due to expansion of a gases caused by heat release. The flame speed measured just upstream of the flame leading edge is normalized with respect to the turbulent stoichiometric flame speed that takes into account variations in turbulent intensity and integral length scale. The turbulent edge flame speed exceeds the corresponding stoichiometric premixed flame speed and reaches a peak at a certain concentration gradient. The mean tangential strain at the flame leading edge locally peaks at the concentration gradient corresponding to the peak flame speed. The strain varies non-monotonically with the flame curvature unlike in a non-stratified curved premixed flame. The mechanism of peak flame speed is explained as the competition between availability of hot excess reactants from the premixed flame branches to the flame stretch induced due to flame curvature. The results suggest that the stabilization of lifted turbulent partially premixed flames occurs through an edge flame even at a relatively gentle concentration gradient. The strain is also evaluated along the flame front; it peaks at the flame leading edge and decreases gradually on either side of the leading edge. The present results also show qualitatively similar trends as those of laminar triple flames.     

Numerical investigations on flashback dynamics of premixed methane-hydrogen-air laminar flames

Injecting hydrogen into the natural gas network to reduce CO₂ emissions in the EU residential sector is considered a critical element of the zero CO₂ emissions target for 2050. Burning natural gas and hydrogen mixtures has potential risks, the main one being the flame flashback phenomenon that could occur in home appliances using premixed laminar burners. In the present study, two-dimensional transient computations of laminar CH₄ + air and CH₄ + H₂ + air flames are performed with the open-source CFD code OpenFOAM. A finite rate chemistry based solver is used to compute reaction rates and the laminar reacting flow. Starting from a flame stabilized at the rim of a cylindrical tube burner, the inlet bulk velocity of the premixture is gradually reduced to observe flashback. The results of the present work concern the effects of wall temperature and hydrogen addition on the flashback propensity of laminar premixed methane-hydrogen-air flames. Complete sequences of flame dynamics with gradual increases of premixture velocity are investigated. At the flame flashback velocities, strong oscillations at the flame leading edge emerge, causing broken flame symmetry and finally flame flashback. The numerical results reveal that flashback tendency increase with increasing wall temperature and hydrogen addition rate.     

Acceleration of hydrogen/air flames in a cylindrical envelope

Flame front behavior during hydrogen/air deflagration in initially quiescent mixtures in cylindrical envelopes was experimentally studied using ionization gauges and infrared photography. Hydrogen/air mixtures with hydrogen content from 12% to 30% were filled in the polyethylene envelope of 4.5 m³ and ignited with exploding wire of 5 J energy. The dependences of the flame front position on time were obtained. Dynamics of the flame front was analyzed on harmonic instabilities development. The mechanisms for the creation of turbulence are discussed. Flame front acceleration is analyzed using Kolmogorov law.     

Studies on properties of laminar premixed hydrogen-added ammonia/air flames for hydrogen production

In order to evaluate the potential of burning and reforming ammonia as a carbon-free fuel in production of hydrogen, fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) for laminar premixed hydrogen-added ammonia/air flames were studied both experimentally and computationally. Freely (outwardly)-propagating spherical laminar premixed flames at normal temperature and pressure were considered for a wide range of global fuel-equivalence ratios, flame stretch rates (represented by the Karlovitz number) and the extent of hydrogen substitution. Results show the substantial increase of laminar burning velocities with hydrogen substitution, particularly under fuel-rich conditions. Also, predicted flame structures show that the hydrogen substitution enhances nitrogen oxide (NO_x) and nitrous oxide (N₂O) formation. At fuel-rich conditions, however, the amount of NO_x and N₂O emissions and the extent of the increase with the hydrogen substitution are much lower than those under fuel-lean conditions. These observations support the potential of hydrogen as an additive for improving the burning performance with low NO_x and N₂O emissions in fuel-rich ammonia/air flames and hence the potential of using ammonia as a clean fuel. Increasing the amount of added hydrogen tends to enhance flame sensitivity to stretch.     

The effects of hydrogen addition on soot formation in counterflow diffusion n-heptane flames

The effects of H₂ addition on soot formation are investigated in counterflow diffusion n-heptane flames. Three effects including chemical, thermal, and dilution are fully isolated and characterized by additions of H₂, He, and Ar. Soot volume fractions are measured using LE-calibrated LII technique, and flame temperatures are measured using OH-TLAF method along with a thermocouple. Numerical simulations are conducted with a detailed mechanism with soot model. The simulated soot volume fractions and flame temperatures are in good agreement with experimental data. The experimental results show that H₂ addition can greatly reduce the soot formation. It is also found that the chemical and dilution effects suppress soot formation, while the thermal effect with increasing flame temperature promotes soot formation. Kinetic analysis suggests that HACA growth rate could be the dominant factor that controls the final soot formation through the three effects due to H₂ addition.     

Characterization of high-pressure,underexpanded hydrogen-jet flames

Measurements were performed to characterize the dimensional and radiative properties of large-scale, vertical hydrogen-jet flames. This data is relevant to the safety scenario of a sudden leak in a high-pressure hydrogen containment vessel and will provide a technological basis for determining hazardous length scales associated with unintended hydrogen releases at storage and distribution centers. Jet flames originating from high-pressure sources up to 413 bar (6000 psi) were studied to verify the application of correlations and scaling laws based on lower-pressure subsonic and choked-flow jet flames. These higher pressures are expected to be typical of the pressure ranges in future hydrogen storage vessels. At these pressures the flows exiting the jet nozzle are categorized as underexpanded jets in which the flow is choked at the jet exit. Additionally, the gas behavior departs from that of an ideal-gas and alternate formulations for non-ideal gas must be introduced. Visible flame emission was recorded on video to evaluate flame length and structure. Radiometer measurements allowed determination of the radiant heat flux characteristics. The flame length results show that lower-pressure engineering correlations, based on the Froude number and a non-dimensional flame length, also apply to releases up to 413 bar (6000 psi). Similarly, radiative heat flux characteristics of these high-pressure jet flames obey scaling laws developed for low-pressure, smaller-scale flames and a wide variety of fuels. The results verify that such correlations can be used to a priori predict dimensional characteristics and radiative heat flux from a wide variety of hydrogen-jet flames resulting from accidental releases.     

Tubular premixed and diffusion flames: Effect of stretch and curvature

Tubular flames are ideal for the study of stretch and curvature effects on flame structure, extinction, and instabilities. Tubular flames have uniform stretch and curvature and each parameter can be varied independently. Curvature strengthens or weakens preferential diffusion effects on the tubular flame and the strengthening or weakening is proportional to the ratio of the flame thickness to the flame radius. Premixed flames can be studied in the standard tubular burner where a single premixed gas stream flows radially inward to the cylindrical flame surface and products exit as opposed jets. Premixed, diffusion and partially premixed flames can be studied in the opposed tubular flame where opposed radial flows meet at a cylindrical stagnation surface and products exit as opposed jets. The tubular flame flow configurations can be mathematically reduced to a two-point boundary value solution along the single radial coordinate. Non-intrusive measurements of temperature and major species concentrations have been made with laser-induced Raman scattering in an optically accessible tubular burner for both premixed and diffusion flames. The laser measurements of the flame structure are in good agreement with numerical simulations of the tubular flame. Due to the strong enhancement of preferential diffusion effects in tubular flames, the theory-data comparison can be very sensitive to the molecular transport model and the chemical kinetic mechanism. The strengthening or weakening of the tubular flame with curvature can increase or decrease the extinction strain rate of tubular flames. For lean H₂-air mixtures, the tubular flame can have an extinction strain rate many times higher than the corresponding opposed jet flame. More complex cellular tubular flames with highly curved flame cells surrounded by local extinction can be formed under both premixed and non-premixed conditions. In the hydrogen fueled premixed tubular flames, thermal-diffusive flame instabilities result in the formation of a uniform symmetric petal flames far from extinction. In opposed-flow tubular diffusion flames, thermal-diffusive flame instabilities result in cellular flames very close to extinction. Both of these flames are candidates for further study of flame curvature and extinction.     

Effects of ammonia substitution on extinction limits and structure of counterflow nonpremixed hydrogen/air flames

The potential of partial ammonia substitution to improve the safety of hydrogen use was evaluated computationally, using counterflow nonpremixed ammonia/hydrogen/air flames at normal temperature and pressure. The ammonia-substituted hydrogen/air flames were considered using a recent kinetic mechanism and a statistical narrow-band radiation model for a wide range of flame strain rates and the extent of ammonia substitution. The effects of ammonia substitution on the extinction limits and structure, including nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions, of nonpremixed hydrogen/air flames were investigated. Results show reduction of the high-stretch extinction (i.e., blow-off) limits, the maximum flame temperature and the concentration of light radicals (e.g., H and OH) with ammonia substitution in hydrogen/air flames, supporting the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use in nonpremixed hydrogen/air flames. For high-stretched flames, however, NO_x and N₂O emissions substantially increase with ammonia substitution even though ammonia substitution reduces flame temperature, implying that chemical effects (rather than thermal effects) of ammonia substitution on flame structure are dominant. Radiation effects on the extinction limits and flame structure are not remarkable particularly for high-stretched flames.