Laminar Flame Properties Research Articles

Advancements in Sustainable Aviation Fuels: Impact of Nano-Additives and Ammonia-Based Strategies On Emissions

Abstract Our study investigates the impact of ammonia- and water-based nano-particulate additives on the combustion characteristics of Jet-A1 aviation fuel, using a 300-kW liquid swirl combustor. Experiments were conducted at two global equivalence ratios (Φ = 0.24 and Φ = 0.40), focusing on laminar flame speed (LFS) and flame properties through chemiluminescence imaging and modal analysis techniques. The primary objective was to understand how these nano-additives modulate flame dynamics and internal chemical reactions, alongside evaluating the environmental implications of combustion alterations. Results showed that integrating urea and water additives into the fuel matrix affected LFS, enhancing it at the lower equivalence ratio but having detrimental effects at the higher ratio. Modal analysis revealed a notable stabilizing influence on flame behavior, especially under leaner fuel conditions. The addition of water and urea influenced combustion chemistry and spray patterns, leading to more uniform sprays and more complete combustion. Chemiluminescence imaging demonstrated higher emission intensity of NH2* radicals compared to NH* radicals, varying with the global equivalence ratio. The data indicated a significant reduction in NOx emissions, particularly at lower equivalence ratios, accompanied by a slight increase in CO2 and CO emissions. This study highlights the potential of ammonia- and water-based nano-additives to enhance the combustion performance and environmental outcomes of Jet-A1 aviation fuel, with the trade-off of increased CO2 and CO emissions requiring further consideration.

Synergistic inhibition of methane/air explosions by NaHCO3 particles with a bimodal size distribution

To reveal the inhibitory effect of stoichiometric methane/air explosions, NaHCO3 with a bimodal distribution were selected as inhibitors. A 5 L vertical transparent duct was used. All the results revealed that Blend82, whose mass fraction contained 80% OP500–600 and 20% OP200–220 NaHCO3, exhibited the optimal inhibitory effect. For the bimodal distribution of NaHCO3, assessing the inhibitory effect solely on the basis of a single particle size parameter is unreliable. Synergistic effects between particles of different sizes should be considered when the interaction index IS < 1. Multiple size classes of bimodal NaHCO3 were proposed for use in response surface methodology to predict the inhibitory effect of methane/air flames. Furthermore, we employed a detailed chemical kinetic model to quantify the endothermic and chemical effects of NaHCO3 on the laminar burning velocity of methane/air flames and established a good relationship between the explosion parameters and laminar flame properties.

Heat release and flame scale effects on turbulence dynamics in confined premixed flows

As industry transitions to a net-zero carbon future, turbulent premixed combustion will remain an integral process for power generating gas turbines, aviation engines, and high-speed propulsion due to their ability to minimize pollutant emissions. However, accurately predicting the behavior of a turbulent reacting flow field remains a challenge. To better understand the dynamics of premixed reacting flows, this study experimentally investigates the effects of combustion heat release and flame scales on the evolution of turbulence in a high-speed, confined bluff-body combustor. The combustor is operated across a range of equivalence ratios from 0.7 to 1 to isolate the role of chemical heat release, flame speed, and flame thickness on the evolution of turbulence as the flow progresses from reactants to products. High-speed particle image velocimetry and CH* chemiluminescence imaging systems are simultaneously employed to quantify turbulent flame and flow dynamics. The results notably demonstrate that the flame augments turbulence fluctuations as the flow evolves from reactants to products for all cases, which opposes most simulations of premixed turbulent reactions. Notably, turbulence fluctuations increase monotonically with the heat of combustion and corresponding turbulent flame speed. Spatial profiles of turbulence statistics are conditioned on the mean flame front, and nondimensionalizing the turbulence profiles using laminar flame properties is shown to collapse all conditions onto a single curve. The resulting nondimensional profile confirms that turbulence dynamics scales with the heat of combustion and was used to develop a novel correlation to predict the increase in turbulent fluctuations across the premixed flame. A Reynolds averaged Navier Stokes decomposition is also explored to further characterize the effects of combustion heat release on the dominant mechanisms of turbulent energy transport. The cumulative results can guide modeling capabilities to better predict flame and flow dynamics and accelerate design strategies for premixed turbines with carbon-free fuels.

A universal Karlovitz number to predict the lean blowoff limits of stabilized premixed flames

A Karlovitz number formulation is defined to predict the stability and lean blowoff (LBO) limits of premixed bluff-body flames. The Karlovitz number relies on a newly defined flame timescale as a representative chemical time, where the flame timescale is identified from experimental measurements of LBO of premixed, propane-air, bluff-body flames in the literature. The timescale is first defined as a function of the extinction strain rate across the premixed flame, and satisfies a criterion for LBO to occur when Ka ≥ 1. Theoretical analysis shows that the defined chemical timescale based on the extinction strain rate can be recast using unstretched laminar flame properties, and reveals a coupling between premixed flame (τpf) and extinction strain rate timescales (τesr) commonly used in literature. The new chemical timescale and the coupling between τpf and τesr is explored for a variety of premixed fuel-air mixtures including propane, methane, ethylene, ammonia, and hydrogen. The timescale definition is found to uphold for all fuels except for hydrogen, which required a Lewis number correction to account for the effects of enhanced mass diffusion. The timescale is also validated against a variety of LBO data within literature. When coupled with a flow timescale, defined as recirculation zone residence time, the chemical timescale is able to collapse the LBO data to a nominally uniform Karlovitz number near unity for a range of flameholder geometries and Reynolds numbers. However, adjusting the flow timescale to estimate the inverse of the flame strain rate at LBO shifted the compiled data to a collapsed value of Ka ≈ 1, thereby realizing a unified Karlovitz (or Damköhler) theory to predict LBO. The advantage of the new Karlovitz model is its ability to provide an a priori method to predict the stability and LBO limits of premixed flames based on appropriately defined timescales.

Laminar flame properties correlations for H2/C3H8 mixtures at high temperature and pressure conditions

Laminar burning velocity (SL) and flame thickness correlations for H2, C3H8 and their mixtures at autoignition conditions for the study of ignition regime were developed. The thermodynamic conditions covered by the correlations are 750–1100 K for the unburned temperature and 5–60 atm for the pressure, typical of weak/mixed ignition regime occurrence. The data for fitting the correlation were generated using a kinetic model with a detailed reaction mechanism. A new definition of energy mixture fraction as a weighting factor in Le Chatelier's mixing rule for H2/C3H8 was proposed to SL calculations. The expression for the flame thickness calculation was developed considering the correction by the radical diffusion from the reactive zone to the preheating zone. Experimental tests and Schlieren image recording inside the combustion chamber from a Rapid Compression Machine were carried out to evaluate the Sankaran Prediction criteria (Sap) capability to predict the ignition regime. Average absolute relative residual values of 4.36% and 0.91% and the total points with an accuracy of 10% were 92.57% and 99.43% evaluating the test data group in the proposed correlations were obtained for H2 and C3H8, respectively. The new version of energy mixture fraction allows an increase in the amount of data with an accuracy of 10% from 36.4%, 34.3% and 19.2% to 96.4%, 96.6% and 75.6 % for a content of 75%, 80% and 95 % of H2 in volume, respectively. The Sap values are in agreement with observations in the RCM, where deflagrative flame fronts were identified prior to volumetric autoignition, confirming the weak/mixed ignition regime. The proposed formulations are suitable to be used in the Sap calculations and ignition regime prediction as well as to determinate the SL and flame thickness at autoignition conditions.

Improved Correlations for the Unstretched Laminar Flame Properties of Mixtures of Air with Iso-octane and Gasoline Surrogates TRF86 and TRF70

&lt;div&gt;Laminar flame properties embody the fundamental information in flame chemistry and are key parameters to understanding flame propagation. The current study focuses on two parameters: the unstretched laminar flame speed (LFS) and &lt;i&gt;ϕ&lt;sub&gt;m&lt;/sub&gt; &lt;/i&gt; (the equivalence ratio at which the LFS reaches its maximum). Most existing correlations for LFS are either only applicable within a narrow range of conditions or built on a large number of coefficients. Few correlations are available for &lt;i&gt;ϕ&lt;sub&gt;m&lt;/sub&gt; &lt;/i&gt;. Thus, the objectives of the current study are to provide accurate, while concise, correlations for both properties for a wide range of working conditions in internal combustion (IC) engines, including dilution effects. The original results were obtained for iso-octane and gasoline surrogates from one-dimensional (1D) simulations for a range of 300–950 K for unburned temperature, 1–120 bar for system pressure, 0.6–1.4 for equivalence ratio, and 0–0.5 for diluent mass fraction, and then were correlated using an improved power law method and an improved Arrhenius form method. Comparisons with the literature show that the predicted LFSs from both methods and &lt;i&gt;ϕ&lt;sub&gt;m&lt;/sub&gt; &lt;/i&gt;s are close to the experimental measurements for a wide range of conditions. The predicted dilution factor has a similar trend with others, but fewer coefficients are needed. Overall, the improved Arrhenius form is recommended to calculate the LFS for future use, considering its lower standard errors. The experimental measurements at very high temperatures and pressures are limited, and thus the predictions under these conditions need further validation.&lt;/div&gt;

Flame Transfer Functions for Turbulent, Premixed, Ammonia-Hydrogen-Nitrogen-Air Flames

Abstract Ammonia is a promising hydrogen and energy carrier but also a challenging fuel to use in gas turbines, due to its low flame speed, limited flammability range, and the production of NOx from fuel-bound nitrogen. Previous experimental and theoretical work have demonstrated that partially dissociated ammonia (NH3/H2/N2 mixtures) can match many of the laminar flame properties of methane flames. Among the remaining concerns pertaining to the use of NH3/H2/N2 blends in gas turbines is their thermoacoustic behavior. This paper presents the first measurements of flame transfer functions (FTFs) for turbulent, premixed, and NH3/H2/N2-air flames and compares them to CH4-air flames that have a similar unstretched laminar flame speed and adiabatic flame temperature. FTFs for NH3/H2/N2 blends were found to have a lower gain than CH4 FTFs at low frequencies. However, the cutoff frequency was found to be greater, due to a shorter flame length. For both CH4 flames and NH3/H2/N2 flames, the confinement diameter was found to have a strong influence on peak gain values. Chemiluminescence resolved along the longitudinal direction shows a suppression of fluctuations when the flame first interacts with the wall followed by a subsequent recovery, but with a significant phase shift. Nevertheless, simple Strouhal number scalings based on the flame length and reactant bulk velocity at the dump plane result in a reasonable collapse of the FTF cutoff frequency and phase curves.

Extinction strain rates of premixed ammonia/hydrogen/nitrogen-air counterflow flames

Chemical energy vectors will play a crucial role in the transition of the global energy system, due to their essential advantages in storing energy in form of gaseous, liquid, or solid fuels. Ammonia (NH3) has been identified as a highly promising candidate, as it is carbon-free, can be stored at moderate pressures, and already has a developed distribution infrastructure. As a fuel NH3 has poor combustion properties that can be improved by the addition of hydrogen, which can be obtained energy-efficiently by partially cracking ammonia into hydrogen (H2) and nitrogen (N2) prior to the combustion process. The resulting NH3/H2/N2 blend leads to significantly improved flame stability and resilience to strain-induced blow-out, despite similar laminar flame properties compared to equivalent methane/air flames. This study reports the first measurements of extinction strain rates, measured using the premixed twin-flame configuration in a laminar opposed jet burner, for two NH3/H2/N2 blends over a range of equivalence ratios. Local strain rates are measured using particle tracking velocimetry (PTV) and are related to the inflow conditions, such that the local strain rate at the extinction point can be approximated. The results are compared with 1D-simulations using three recent kinetic mechanisms for ammonia oxidation. By relating the extinction strain rates to laminar flame properties of the unstretched flame, a comparison of the extinction behaviour of CH4 and NH3/H2/N2 blends can be made. For lean mixtures, NH3/H2/N2-air flames show a significant higher extinction resistance in comparison to CH4/air. In addition, a strong non-linear dependence between the resistance to extinction and equivalence ratio for NH3/H2/N2 blends is observed.

Experimental and numerical study of the effects of N2 dilution and CH4 addition on the laminar burning characteristics of syngas

The study of combustion characteristics of complex biomass syngas under changing conditions is helpful for its practical utilization. In this paper, the effects of N2 dilution (dilution rates from 0% to 60%) and CH4 addition (mixing ratios from 0% to 20%) on the laminar flame properties of H2/CO/N2 syngas were researched. The laminar burning velocities (LBVs) were obtained by constant volume combustion chamber system. Five common mechanisms were used for numerical simulation using the CHEMKIN software package and the results were compared with the measurement data. The Li mechanism that was well matched with the experimental data was selected for kinetic analysis. The results indicate that the LBVs of H2/CO/N2/air mixtures decrease due to the dilution of N2. The addition of CH4 decreases the LBVs of H2/CO/N2/CH4/air mixtures and the LBVs of fuel-rich mixtures decrease faster. Sensitivity analysis illustrates that R29 is the most important branching reaction affecting LBV in H2/CO/N2/air premixed flame, the net reaction rate of all branched reactions and the mole fraction of H and OH in the mixture decrease due to the increase of N2 content. After adding CH4, the dominant branching reaction becomes R1. Due to the increase of CH4 addition ratio, the concentrations of H and O decrease, but the concentrations of OH and CH3 increase slightly. The chemical effect of adding CH4 increases LBV, but the physical effect decreases LBV. The reduction in the proportion of chemical effect is the reason for the faster reduction of LBV in the fuel-rich region. Finally, the formation of NO was analyzed in detail using the GRI 3.0 mechanism. The results show that N2 dilution reduces the mole fraction of NO, but CH4 addition increases the mole fraction of NO. Pathway analysis shows that N2 dilution and CH4 addition decrease and increase the effect of thermal mechanism on NO generation, respectively.

Laminar burning velocity, Markstein length, and cellular instability of spherically propagating NH3/H2/Air premixed flames at moderate pressures

Blending hydrogen (H2) into ammonia (NH3) as a carbon-free fuel has attracted increased attention. However, fundamental experimental data on laminar combustion characteristics of NH3/H2/air mixtures are rather rare for moderate pressure (0.5 - 2.0 atm). In this work, experiments of laminar flame properties of NH3/H2/air mixtures were performed using the spherical flame propagation method in a constant-volume chamber at initial temperature Tu = 298 K. Numerical simulations with various kinetics mechanisms were conducted and compared to the experiments. Effects of equivalence ratio (ϕ = 0.8 - 1.4), hydrogen ratio (XH2= 0.2 - 1.0), and initial pressure (Pu = 0.5 - 2.0 atm) were scrutinized. The results show that laminar burning velocity increases with increasing hydrogen ratio due to the enhancement of both thermal and chemical kinetics effects. The influence of pressure on laminar burning velocity depends on hydrogen concentration, i.e., a negative effect for XH2< 0.8, while a positive effect for pure H2. In particular, the laminar burning velocity is insensitive to pressure variation at XH2= 0.9. The Markstein length increases monotonically with the increase of equivalence ratio, and decreases with initial pressure from 0.5 to 1.5 atm, but slightly increases with increasing the initial pressure from 1.5 to 2.0 atm. Flame morphology shows that the NH3-H2 flames suffer from cellular instabilities except Pu = 0.5 atm. Cellular instability arises at lean burn because of the combined effects of thermal-diffusive and hydrodynamic instabilities. Increasing hydrogen ratio or initial pressure can promote the global flame instability caused by hydrodynamic destabilizing mode. In addition, the simulations show considerable discrepancies between different reaction mechanisms and experiments for certain ranges of initial conditions, and thus the current measurements can be useful for improving and validating kinetics mechanisms.

Improved Correlations for the Unstretched Laminar Flame Properties of Iso-Octane/Air Mixtures

Improved Correlations for the Unstretched Laminar Flame Properties of Iso-Octane/Air Mixtures

Experimental study and proposed power correlation for laminar burning velocity of hydrogen-diluted methane with respect to pressure and temperature variation

The aim of the present work is to contribute to the better understanding of the combustion process and the laminar flame properties of methane/hydrogen-air flames at elevated temperatures and pressures. The heat flux method provides an accurate and direct measurement of laminar burning velocities (LBV) at elevated temperatures, while the constant volume chamber method provides measurements at elevated pressures. In the present work, a database of more than 250 experimental points for the range of temperature (298–373 K) and pressure conditions (1–5 bar) for mixtures up to 50% hydrogen in methane was generated using these two methods. Comparison with the sparse literature data shows quite good agreement. A power-law correlation for temperature and pressure is proposed for methane/hydrogen-air mixtures, which has a practical application in estimating the LBV of a natural gas/hydrogen mixture intended to replace pure natural gas in different processes. The power-law temperature exponent, α, and the pressure exponent, β, show inverse trends. The former decreases almost linearly and the latter increases approximately linearly when the hydrogen content is increased. The power-law exponents are highly affected by the mixture equivalence ratio, ϕ, showing a parabola like trend. However, for the pressure exponent this trend becomes almost linear for 50% H2 in the mixture. The power-law correlation has been validated against experimental data for a wide range of temperature (up to 573 K), pressure (1–7.5 bar), equivalence ratios (ϕ between 0.7 and 1.3) and H2 contents up to 50%.

Experimental investigation of unconfined turbulent premixed bluff-body stabilized flames operated with vapourised liquid fuels

The structure of turbulent unconfined bluff-body flames of vapourised liquid fuels was investigated at conditions far from and close to blow-off with high-speed (5 kHz) OH-PLIF imaging and 10 Hz CH2O-PLIF imaging. Four different fuels were considered: ethanol, heptane, and two different kerosene blends (a conventional Jet-A and an alcohol-to-jet kerosene, respectively denoted as A2 and C1 following the USA National Jet Fuels Combustion Programme. OH-PLIF images of ethanol flames far from blow-off display a high intensity of OH-LIF signal along the shear layer. In contrast, the OH-LIF signal was evenly distributed throughout the recirculation zone (RZ) of the heptane and kerosene flames. Regardless of the fuel used, close to blow-off the flame becomes shorter with peak OH-LIF signal intensities lying inside the RZ. All four fuels showed a decrease in flame surface density (Σ2D) and broadening of the 2-D curvature PDFs as their blow-off limits were approached. An increase in local turbulent consumption speed was observed in the downstream region as the flames approached blow-off. No significant variation in Σ2D, curvature PDF, and local turbulent consumption speed was observed between the different fuel types. The average CH2O-layer thickness was larger than the computed laminar flame value by a factor of two and six for conditions far from and close to blow-off, respectively. Moreover, heptane and kerosene flames showed more pockets of CH2O-LIF signal within the RZ as compared to ethanol, suggesting that considerably more partially-combusted fluid enters the RZ of the former than the latter. High-speed particle image velocimetry was performed to measure the local velocity fields and place various regions of the flame on the turbulent premixed regime diagram. It was observed that, regardless of fuel type, conditions close to blow-off occupy the same region on the regime diagram. However, the fact that the fuel type results in differences in some structural features near blow-off suggests that flames produced with heavy hydrocarbon fuels involve chemistry effects at blow-off that are not fully characterized by laminar flame properties.

A comparison of the blow-out behavior of turbulent premixed ammonia/hydrogen/nitrogen-air and methane–air flames

Ammonia has been identified as a promising energy carrier that produces zero carbon dioxide emissions when used as a fuel in gas turbines. Although the combustion properties of pure ammonia are poorly suited for firing of gas turbine combustors, blends of ammonia, hydrogen, and nitrogen can be optimized to exhibit premixed, unstretched laminar flame properties very similar to those of methane. There is limited data available on the turbulent combustion characteristics of such blends and important uncertainties exist related to their blow-out behavior. The present work reports experimental measurements of the blow-out limits in an axisymmetric unconfined bluff-body stabilized burner geometry of NH3/H2/N2-air flame, comprised of 40% NH3, 45% H2, and 15% N2 by volume in the “fuel” blend. Blow-out limits for the NH3/H2/N2-air flames are compared to those of methane–air flames. OH PLIF and OH chemiluminescence images of the flames just prior to blow-out are presented. Furthermore, two large-scale Direct Numerical Simulations (DNS) of temporally evolving turbulent premixed jet flames are performed to investigate differences in the turbulence-chemistry interaction and extinction behavior between the NH3/H2/N2-air and methane–air mixtures. The experiments reveal that the blow-out velocity of NH3/H2/N2-air flames is an order of magnitude higher than that of methane–air flames characterized by nearly identical unstretched laminar flame speed, thermal thickness and adiabatic flame temperature. Results from the DNS support the experimental observation and clearly illustrate that a methane–air mixture exhibits a stronger tendency towards extinction compared to the NH3/H2/N2-air blend for identical strain rates. Furthermore, the DNS results reveal that, even in the presence of intense sheared turbulence, fast hydrogen diffusion into the spatially distributed preheat layers of the fragmented and highly turbulent flame front plays a crucial role in the enhancement of the local heat release rate and, ultimately, in preventing the occurrence of extinction.

Burning Velocity of Turbulent Methane/Air Premixed Flames in Subatmospheric Environments.

The aim of our work was to study turbulent premixed flames in subatmospheric conditions. For this purpose, turbulent premixed flames of lean methane/air mixtures were stabilized in a nozzle-type Bunsen burner and analyzed using Schlieren visualization and image processing to calculate turbulent burning velocities by the mean-angle method. Moreover, hot-wire anemometer measurements were performed to characterize the turbulent aspects of the flow. The environmental conditions were 0.85 atm, 0.98 atm, and 295 ± 2 K. The turbulence–flame interaction was analyzed based on the geometric parameters combined with laminar flame properties (which were experimentally and numerically determined), integral length scale, and Kolmogorov length scale. Our results show that the effects of subatmospheric pressure on turbulent burning velocity are significant. The ratio between turbulent and laminar burning velocities increases with turbulence intensity, but this effect tends to decrease as the atmospheric pressure is reduced. We propose a general empirical correlation as a function between ST/SL and u′/SL based on the experimental results obtained in this study and the equivalence ratio and pressure we established.

Transient dynamics of radiative extinction in low-momentum microgravity diffusion flames

Transient dynamics of growing diffusion flames produced by a low-momentum fuel flow emanating at the flat surface has not yet been properly addressed. The flames of interest resemble properties of laminar jet flames and the candle or droplet flames produced by a point fuel source. However, due to the considerable size of the fuel source, these flames appear to be dissimilar to either of the two cases. The practical importance of studying the stability and dynamics of these flames stems from the need to scrutinize the flammability of condensed fuels in normal and zero gravity. This paper presents comprehensive 3D simulations of the low-momentum zero- and normal gravity diffusion flames generated by the BRE (Burning Rate Emulator) burner. The scenarios of interest represent the subset of the ongoing orbital experiments in which flame development in the normal atmosphere is considered. The experimental observations have shown that these flames are prone to the local extinction followed by fluctuating instability and complete quenching. Such behavior was numerically predicted in the earlier “blind” simulations and is now replicated for the orbital test conditions. Time to flame extinction predicted in this work agrees with the experimental observations. Extinction occurs due to the excessive radiative losses when the instantaneous radiative fraction grows up to 60–70% of the current heat release rate. The excessively high level of the radiative losses is facilitated by the long residence time in the low-momentum microgravity flame. The transient dynamics of flame disintegration after the local extinction onset is analyzed. The hook-like (bibrachial) structure of flame edge is predicted after the extinction onset. When the re-ignition occurs, a classical triple (tribrachial) flame develops and propagates along the stoichiometric surface.

Estimating flammability limits through predicting non-adiabatic laminar flame properties

Lower and upper flammability limits (LFL and UFL) are widely and effectively used as simple safety boundaries for preventing gas-mixture ignition, and predicting them for new compounds would be valuable, especially the LFLs for low-flammability non-C/H/O species. Prediction with mechanistic flame models uses the idea that as these limits are reached, the flame speed and temperature drop below a threshold of flame feasibility because of radiant heat loss. Using methane as a reference study because its mechanisms and flammability are well characterized, non-adiabatic laminar flame speeds and profiles of temperature, flame structure, and chemiluminescent OH* and CH* are calculated using a modification of the hydrocarbon kinetics model of Hashemi et al. (2016), executed in CHEMKIN and Cantera. LFL prediction is emphasized here; large-hydrocarbon and soot radiant losses have been proven to be necessary for accurate UFL values (Bertolino et al., 2019) but were not included in this work. Property dependences on concentration are compared to published flammability limits of 5–15% methane concentration for methane/air mixtures at T0= 298 K and 1, 5, and 10 atm. At 1 atm, the LFL occurs at a laminar flame speed of 2.7 cm/s, and at the UFL, flame speeds between 4.5 and 6 cm/s correspond to the limit. Similarly, adiabatic temperature of 1450 K in a fuel-lean environment and 1560–1670 K in a fuel-rich environment correlate with the flammability limits. The ASTM standard test for flammability uses visual detection of a flame; ultraviolet chemiluminescence of OH* radical at limits of less than mole fraction of 10−3 is shown to reflect the methane LFL, and the equivalence-ratio dependence of OH* should resemble that of the visible C2* emission. Effects of pressure and challenges for modeling are discussed.

The reactivity of hydrogen enriched turbulent flames

The use of hydrogen enriched fuel blends, e.g. syngas, offers great potential in the decarbonisation of gas turbine technologies by substitution and expansion of the lean operating limit. Studies assessing explosion risks or laminar flame properties of such fuels are common. However, there is a lack of experimental data that quantifies the impact of hydrogen addition on turbulent flame parameters including burning velocities and scalar fluxes. Such properties are here determined for aerodynamically stabilised flames in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Ret=314±19) using binary H2/CH4 and H2/CO fuel blends. The binary H2/CH4 fuel blend is varied from α=XH2/(XH2+XF)= 0.0, 0.2 and 0.4–1.0, in steps on 0.1, and the binary H2/CO fuel blend from α=0.3−1.0 also in steps of 0.1. The equivalence ratio is adjusted between the mixture specific lower limit of local flame extinction and the upper limit of flashback. The flames are characterised using PIV measurements combined with a flame front detection algorithm. The study quantifies the impact of hydrogen enrichment on (i) turbulent burning velocity (ST), (ii) turbulent transport and (iii) the rate of strain acting on flame fronts. Scaling relations (iv) that correlate ST with laminar flame properties are evaluated and (v) flow field data that permits validation of computational models is provided. It is shown that CH4 results in a stronger inhibiting effect on the reaction chemistry of H2 compared to CO, that turbulent transport and burning velocities are strongly correlated with the rate of compressive strain and that scaling relationships can provide reasonable agreement with experiments.

Experimental study of turbulent syngas/methane/air flames at a sub-atmospheric condition

The aim of this work was determined turbulent burning velocities of air-syngas-methane flames at sub-atmospheric conditions using the angle method and Schlieren imaging. We analyzed a high hydrogen content syngas that can be obtained with a Conoco-Phillips coal gasification process. Equivalence ratios evaluated here correspond to lean combustion conditions: 0.8-1.0. Experiments were carried out at room temperature of 297 K and 849 mbar. The chemical-turbulence interaction was evaluated considering geometric parameters, laminar flame properties, and turbulence length scales. It was found that the turbulent burning velocity and the ratio between turbulent and laminar burning velocities increases with the turbulence intensity. Additionally, the addition of syngas to methane increases the laminar and turbulent burning velocity.

Laminar flame properties of C1-C3 alkanes/hydrogen blends at gas engine conditions

The use of fuel blending is encouraged in order to achieve more flexibility in gas engines. In order to design such engines effectively, relevant information about the laminar flame speeds and laminar flame thickness are necessary. Hydrodynamic and thermo-diffusive instabilities at gas engine conditions prevent the acquisition of reliable data experimentally. One-dimensional numerical simulations with detailed chemistry can be a solution. A huge database of laminar flame speeds is generated covering a broad range of gas engine applications, pressure (p) 0.1–20 MPa, fresh gas temperature (Tu) 300–1100 K, air-fuel equivalence ratio (λ) 0.9–2.5, methane 100–60 vol%, ethane 0–40 vol%, propane 0–40 vol%, hydrogen 0–30 vol% and exhaust gas recirculation (EGR) 0–30 m%. The detailed reaction mechanisms GRI 3.0 and AramcoMech 1.3 are used for the generation of flame speed data for the mentioned conditions. A laminar flame speed correlation for 100% hydrogen extending up to elevated pressure and temperature conditions is developed. A blending law based on Le Chatelier’s rule is investigated. It is observed that the HC-ratio has a very determining effect for the laminar flame properties of different C1-C3 alkane blends. Based on this observation, it was possible to derive a very efficient correlation for both laminar flame speed and laminar flame thickness for the group of natural gas blends with methane, ethane, propane and hydrogen, and as well as including relevant EGR, which corresponds within 7% accuracy to the calculated database of about 73,000 points. The developed laminar flame speed correlation is incorporated in an engine process simulation code. It is validated with the measured in-cylinder pressure traces from the single cylinder research engine experiments for different gas blends and EGR ratios.