Extinction Strain Rate Research Articles

An experimental and modeling study on extinction strain rate in C2HX flames with varied oxygen content

The extinction strain rates (ESR) is an important physical-chemical properties of fuels. Therefore, an experiment study on ESR is crucial for studying emission reduction techniques. In this work, the ESR and laminar burning velocity (LBV) of C2HX fuels under a wide range of conditions are determined experimentally. Moreover, a series of the reduced mechanisms for predicting ESR and LBV of C2 hydrocarbons (C2H2, C2H4, C2H6) is provided, because the research of mechanism reduction seldom pay attention to ESR and LBV. The contribution of this work is therefore the improvement of understanding of the oxidation characteristics of light C2 hydrocarbons and to provide reduced skeletal mechanisms for those fuels with different levels of accuracy for their interaction with reduced oxygen content. For these gas mixtures, the extinction strain rates (ESR) and the laminar burning velocity (LBV), two important physical-chemical properties, were investigated in a counterflow and a heat flux burner setup under atmospheric conditions (1 bar and 298 K). The LBV of the premixed flames, as well as the ESR of the non-premixed C2Hx/O2/N2 flames, are determined experimentally and compared to the numerical predictions of a detailed mechanism (USC II) and the seven corresponding reduced skeletal mechanisms. The present study highlights the non-linear influence on the ESR of C2 hydrocarbons that were found with decreasing fuel content and decreasing oxygen content in either case. Since C2H2 has a significantly higher reactivity than C2H4 and C2H6, this is manifested in lower ESR and higher LBV for the same conditions. The recently developed species-targeted global sensitivity analysis (STGSA) is used to develop reduced mechanisms, showing that they can reproduce the ESR trends and are within the determined measurement uncertainty in a wide range of operating conditions. In addition, these developed significantly reduced mechanisms remain good at predicting the laminar burning velocity. The research generated a unique experimental dataset of the ESR and LBV of C2Hx which was also used to develop reduced mechanisms. Sensitivity analysis for the ESR and LBV gives insight into the difference between ESR and LBV calculations.

Numerical Simulation and Field Experimental Study of Combustion Characteristics of Hydrogen-Enriched Natural Gas

For the safe and efficient utilization of hydrogen-enriched natural gas combustion in industrial gas-fired boilers, the present study adopted a combination of numerical simulation and field tests to investigate its adaptability. Firstly, the combustion characteristics of hydrogen-enriched natural gas with different hydrogen blending ratios and equivalence ratios were evaluated by using the Chemkin Pro platform. Secondly, a field experimental study was carried out based on the WNS2-1.25-Q gas-fired boiler to investigate the boiler’s thermal efficiency, heat loss, and pollutant emissions after hydrogen addition. The results show that at the same equivalence ratio, with the hydrogen blending ratio increasing from 0% to 25%, the laminar flame propagation speed of the fuel increases, the extinction strain rate rises, and the combustion limit expands. The laminar flame propagation speed of premixed methane/air gas reaches the maximum value when the equivalence ratio is 1.0, and the combustion intensity of the flame is the highest at this time. In the field tests, as the hydrogen blending ratio increases from 0% to nearly 10% with the increasing excess air ratio, the boiler’s thermal efficiency decreases as well as the NOx emission. This indicates that there exists a tradeoff between the boiler thermal efficiency and NOx emission in practice.

Flame characteristics and NO emission behaviors of methane/ammonia counterflow premixed flames with opposed N2 or air stream

Behaviors of methane/ammonia counterflow premixed flames and NO emission are numerically and experimentally investigated with opposed N2 or air stream by varying strain rate (ag) and equivalence ratio (ϕ). The premixed stream is 80% methane and 20% ammonia by volume mixed with air considering the utilization of ammonia by blending hydrocarbon fuels. The opposed stream is either N2 or air considering the potential application in gas turbines having single- or two-stage combustion. The simulations are conducted by increasing ag up to flame extinction for a specified ϕ and an experiment is conducted to evaluate various kinetic mechanisms. The extinction strain rate increases monotonically with ϕ with opposed air stream (AS), while increases and then decreases with opposed nitrogen stream (NS). The extinction mechanisms are explained by adopting the concepts of the local equilibrium temperature and heat loss ratio. For lean and rich mixtures with NS and lean mixtures with AS, the extinction occurs due to comparable effects of incomplete reaction and heat loss from the flame to the downstream burnt region. While for rich mixtures with AS, the extinction is attributed mainly to incomplete reaction. The contribution of thermal NO as compared with prompt NO in NO production is weak for all cases with NS and AS. The emission index (EINO) for lean and rich mixtures with NS and for lean mixtures with AS decreases monotonically with the increase of strain rate for all ϕ. Increasing equivalence ratio at a specified ag decreases NO production for lean and rich mixtures with NS and lean mixtures with AS. While for rich mixtures with AS, the EINO increases monotonically with ag. From the reaction pathway analysis, for lean mixtures, the effect of increasing ag on the reduction of NO production is attributed by weakening NH-related reaction paths, while NH2-related reaction paths change little. When ϕ increases for a specified ag with NS, NO production is reduced by weakening NH2-related reaction paths and strengthening NH-related reaction paths. For rich mixtures with AS, increasing ag increases NO production by strengthening NH2-related reaction paths.

Mechanisms Leading to Stabilization and Incomplete Combustion in Lean CH4/H2 Swirling Wall-Impinging Flames

Abstract In gas turbines, confined highly turbulent flames unavoidably propagate in the vicinity of a relatively cool combustor liner, affecting both the local flame structure and global operation of the combustion system. In our recent work, we demonstrated, using simultaneous [OH] × [CH2O] planar laser-induced fluorescence (PLIF) and stereo-particle image velocimetry (stereo-PIV), that lean CH4/H2 flames at a high Karlovitz number can present a highly broken structure near wall, highlighted by a diffuse CH2O cloud, which suggests local quenching and incomplete oxidation. Such high Karlovitz numbers were achieved using an inclined plate, which substantially extended the lean flammability of the low swirl flames. Yet, how a cooled wall acting as a heat sink played a conducive role in stabilizing high Ka flames remains unanswered. In addition, the origin of the CH2O cloud is also unclear. Hence, in this work, we look to better understand the stabilization mechanisms for lean and ultralean flames on the same configuration, and how they may change with a parametric variation of plate incident angle, plate-nozzle distance, and bulk velocity up to the critical values that lead to flame blow off. The results show that the impinging swirling flow creates a low speed region that helps hold the flame, while the wall prevents mixing with ambient cold air. The production of diffuse CH2O, which indicates the occurrence of local quenching, is associated with a mean strain rate K beyond the extinction strain rate (ESR) Ke. For CH4 flames, most of the reaction zones reside within |K|/Ke<1; for 70% H2 flames at ϕ=0.4, the reaction zones are highly broken and scattered in a large area, where |K|/Ke<8, the interspace of which is fully filled by CH2O. In other words, high H2 fraction flames appear to be more robust to persistent strain rate, thus extending their stability envelope. However, these flames can subsist as highly broken flames featuring strong incomplete combustion.

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.

Development of a novel subgrid combustion model for buoyant turbulent diffusion flames

A novel approach is presented to incorporate finite rate reactions in modeling of turbulent diffusion flame, with the existence of unresolved flamelets inside the CFD grid cells taken into account. This approach, called SCM (subgrid combustion model), is applied in large eddy simulations of the UMD line burner flames. With the global kinetic parameters fitted for the measurement data and detailed simulations of autoignition in stoichiometric methane-air mixture, as well as for the extinction strain rate in the opposed non-premixed jets, the SCM model is shown to replicate the mean temperature profiles in unsuppressed flame with mean total heat release of 50 kW. For flame suppression via dilution of the oxidizer flow by nitrogen, the lift-off and extinction of non-anchored flame are predicted in agreement with the experimental data. The SCM is designed as a tool to allow for the finite rate kinetics effects in large eddy simulations of buoyant turbulent diffusion flames, with the focus on weakly turbulent regions, in which applicability of conventional combustion models is limited.

Numerical analysis of hydrogen-oxygen hydrothermal combustion: Laminar counterflow diffusion flames

Hydrogen hydrothermal combustion is a promising technology in the field of energy conversion. The H2–O2 hydrothermal combustion is comprehensively investigated using a one-dimensional laminar counterflow diffusion flame model. The introduction of the real-fluid thermodynamic and transport properties approach corrects the temperature and location of the flame. Effects of strain rate, pressure, inlet temperature, and fuel concentration on flame structure and extinction limit are examined. The increase in strain rate and pressure reduces the maximum temperature and flame thickness, while the inlet temperature and fuel concentration have the opposite effect. The extinction strain rate ranges from 30 s−1 to 5200 s−1 at inlet temperatures of 773 K–973 K and fuel concentrations of 10 mol%–50 mol%, indicating that the hydrothermal flame stability is much worse than gas-phase combustion. The unity Lewis number assumption underpredicts the maximum flame temperature by nearly 100 K at any strain rate.

Numerical analysis of a nanosecond repetitively pulsed plasma-assisted counterflow diffusion flame

A computationally efficient model is proposed to analyze plasma-assisted combustion using nanosecond repetitive pulsed (NRP) plasmas. The NRP plasma discharge is placed in the oxidizer stream of a counter-flow diffusion flame. The effect of changing the flow rate and the pulse repetition frequency (PRF) of a continuous NRP plasma discharge on the temperature and species profiles of a counter-flow diffusion flame is investigated numerically. The results confirm that oxygen atom and nitrogen vibrational states are the most important species to enhance combustion. The results also show that kinetic effects are much more significant for higher PRF and lower pulse voltage. In addition, when steady plasma profiles are used instead of unsteady plasma profiles, the extinction strain rates increase by 25.8%, 21.1%, and 10.8% for PRF equal to 1, 2, and 4 kHz, respectively.

Forced swirl tubular flame under different equivalence ratios and oxygen mole fractions

The swirl tubular flame has been widely used in combustors owing to its unique merits of high combustion efficiency, uniform flame temperature and inherent safety to avoid flashback in the rapidly mixed (RM) mode. However, it also suffers from combustion instability, which would deteriorate the system performance. To explore the mechanism behind, the influences of acoustic forcing disturbances on both the premixed (PM) and rapidly mixed flame characteristics are experimentally investigated, in which the equivalence ratio (Φ) and the oxygen mole fraction (β) are also adjusted to examine the flame responses. Firstly, the measured combustion regimes show that there is an additional flame liftoff region for the RM flames in the mid-acoustic frequency range compared with the PM flames. In addition, the forced RM swirl tubular flame present similarly low-pass filtering with the PM flame, but has a 10 Hz smaller critical frequency for the flame extinction. Finally, the forced flame is found to be lifted off when Φ or β is low. By increasing Φ or β, the lifted flame evolves to the anchored flame, and the transition process is studied via the Proper Orthogonal Decomposition analysis and the 1D opposed-flow flame analysis. The results show that, for both of PM and RM tangential tubular swirl flames, the lifted flame would become anchored when the energy fraction of the axial mode is smaller than 5% of total energy and the extinction strain rate is larger than 1000 s−1.

Pressure effects on reactivity and extinction of n-dodecane diffusion cool flame

Low-temperature combustion at high pressure has gained increasing interest owing to the growing demand for clean and efficient propulsion and energy systems. In particular, cool flame structure, extinction limits, and fuel reactivities at high pressure are critical in affecting the engine performance at near-limit conditions. This paper investigates the effects of pressure on the cool flame extinction limit, structure, radical index, reactivity, and oxygen concentration dependence by experiments, analysis, and modeling. A large n-alkane, namely n-dodecane (n-C12H26), is selected to study its diffusion cool flame dynamics and reactivity in a high-pressure counterflow burner up to 10 atm. The experimental results show that higher pressure increases the cool flame extinction strain rates, and that the pressure-weighted extinction strain rate (aP) is proportional to the square of pressure, P2. A scaling analysis explains the relationship between the dependence of flame structure, heat release rate, and pressure-weighted cool flame strain rate on pressure. Furthermore, radical indexes at different pressures are measured by isolating the thermal and transport effects from the chemical contribution to diffusion cool flame extinction. The radical index clearly shows that the low-temperature reactivity increases with pressure. In addition, due to the critical role of multiple oxygen addition reactions in low-temperature chemistry, the relationship between the cool flame extinction limit and the oxygen concentration is explored. It is found that the cool flame extinction limits are proportional to the nth power of the oxygen concentration, [O2]n, and increasing pressure leads to stronger extinction limit dependence (larger n) on the oxygen concentration. The present experiment and detailed kinetic analysis show clearly that increasing pressure promotes the low-temperature chemistry including the oxygen addition reactions, while the scaling analysis explains well the experimental results.

On the binary diffusion coefficients of n-alkanes in He/N2

The binary diffusion coefficients of fuel molecules in bath gasses are key parameters for accurate predictions of flame properties such as extinction strain rates. In combustion simulations, they are often calculated using the Hirschfelder-Bird-Spotz (HBS) equation, which requires the availability of effective Lennard-Jones parameters, i.e. collision diameters and well depths, which can be obtained from the intermolecular potentials of two interacting species by applying an orientation-averaging rule. Besides the HBS equation, the Green-Kubo formula can be used to obtain binary diffusion coefficients, using velocity autocorrelation and cross-correlation functions from molecular dynamics simulations. In this work, we propose a new orientation-averaging rule, called VolW-σ-ε, based on the characteristic of intermolecular potentials, and evaluate different approaches to computing the diffusion coefficients of n-alkanes in He/N2 mixtures. Several orientation-averaging rules are evaluated and compared with experimental data of n-alkanes in He and N2 and the results of molecular dynamics simulations. Finally, using the binary diffusion coefficients of n-dodecane in N2 evaluated by the VolW-σ-ε method, we reproduce previously reported experimental extinction strain rates reported for counterflow n-dodecane/N2 versus O2 jets.

A systematic analysis of chemical mechanisms for ethylene oxidation and PAH formation

Accurate prediction of soot formation and evolution remains a formidable challenge due to the complex interaction between gas-phase composition and solid-phase particles. Recent studies have shown that the choice of gas-phase mechanisms is of primary importance in affecting predictability. In this work, a systematic analysis of ethylene (C2H4) combustion mechanisms denoted as KAUST, Stanford, Aachen, Polimi, ABF, DLR/UT, Naples, Caltech, and SJTU, which have been widely used in the soot community, is performed to investigate their differences in fundamental chemistry, polycyclic aromatic hydrocarbon (PAH) chemistry, and soot prediction. It is found that the nine mechanisms exhibit large differences even in predicting canonical combustion properties (e.g., ignition delay time, laminar flame speed, and extinction strain rate), indicating significant variations in the fundamental chemistry. This is due to the fact that although most mechanisms share very similar dominant fuel-oxidation reactions, there are notable differences in the rate coefficients of sensitive reactions used in these mechanisms. Owing to the uncertainties in the fundamental chemistry, the predictions of C2H2 from the nine mechanisms show significant differences, which contributes to the differences in soot precursor prediction, in conjunction with the difference in benzene (A1) formation pathways demonstrated by the element flux analysis of the C atom. Furthermore, it is found that PAHs containing two rings play a dominant role in soot formation for most mechanisms. Moreover, it is observed that while Caltech and SJTU significantly under estimate soot formation, they reasonably reproduce its sensitivity to strain rate. These results indicate that despite substantial advances in the development of C2H4 oxidation and PAH formation chemistry, the various existing mechanisms lead to significant differences in predicting soot concentrations which can be traced back to considerable differences in both fundamental chemistry and PAH chemistry. This suggests that the fundamental chemistry should be calibrated or improved before further development of PAH chemistry and soot models.

Effect of H2 addition on the local extinction, flame structure, and flow field hydrodynamics in non-premixed bluff body stabilized flames

We examined the effect of hydrogen (H2) enrichment on the primary fuel methane (CH4) in a canonical non-premixed bluff-body stabilized burner operating under typical central jet-dominated flame mode. In the chosen mode of operation, globally, the flow field and flame feature three important successive spatial zones: the recirculation zone, the neck zone, and the jet-like flame zone. The flame is exposed to a higher stretch rate in the neck zone in such a configuration and eventually undergoes local extinction. Such local extinction and subsequent re-ignition/reconnection of broken flame branches have substantial implications for the hydrodynamic instability of the coaxial annular air shear layer. It is well known that H2 addition increases the flame extinction strain rate (κext) and thus alters the local extinction phenomenon. To understand this, we performed experiments at 0%, 10%, 20%, 30%, 50%, 80%, and 100% hydrogen proportion in the H2-CH4 blend. High repetition rate (5 kHz) Particle Image Velocimetry and OH Planar Laser Induced Fluorescence (PLIF) measurements are simultaneously implemented to gain quantitative insight into the flow field and flame structure. A detailed analysis performed over the instantaneous OH–PLIF datasets reveals the absence of local extinctions in flames with H2 enrichment >30% due to an increased extinction strain rate (κext). Furthermore, it is found that H2 enrichment plays a significant role in the reconnection/re-ignition of the broken flame branches formed during the local extinction. For instance, a high reconnection probability is observed in flames with an H2 addition of ≥20%. Consequently, variations in the mean reaction zone height are witnessed for different H2 enrichment levels. Further analysis of the influence of variation in reaction zone height on flow field hydrodynamics is explored using Proper Orthogonal Decomposition (POD) and Continuous Wavelet Transform (CWT). The results obtained from POD and CWT indicated the suppression of vortex shedding at the annular air shear layer for H2 addition greater than 20% and irregular wrinkling of flame fronts. Thus, they quantified the beneficial effect of H2 addition in turbulent flame stabilization.

Effects of inlet flow non-uniformities on thermochemical structures and quasi-one-dimensional simulation of sooting counterflow diffusion flames

Counterflow flames are routinely used for investigating fundamental flame and fuel properties such as laminar flame speeds, autoignition temperature, extinction strain rate, and chemistries of soot formation. The primary merit of counterflow flame is that the essentially two-dimensional configuration can be mathematically treated as a one-dimensional problem with certain assumptions made; this dimensional reduction is much beneficial for computational costs, which are critical for the investigation of complex chemistries such as those of soot formation. In this work, we performed a comprehensive investigation on the performance of the 1D modeling by comparing the results with experimental measurements and the more rigorous 2D models. We focused on the effects of inlet flow uniformities, which are frequencies assumed in the 1D model but challenging to realize in experiments. Parametric studies on the effects of nozzle flow rates, nozzle separation distances, and curtain flow rates on inlet flow uniformities and the 1D modeling were performed. The results demonstrated the importance to specify actual velocity boundary conditions, either obtained from experiments or from two-dimensional modeling to the 1D model. An additional novel contribution of this work is a quantitative presentation of the fact that the presence of the curtain flow would exert a notable influence on the core counterflow by modifying the radial distribution of the nozzle exit velocity although the effects can be accounted for by using the correct velocity boundaries in the quasi-1D model. This work provides recommendation for various geometry and operational parameters of the counterflow flame to facilitate researchers to select proper burner configuration and flow conditions that are amiable for accurate 1D modeling.

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

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

On the combined effects of compositional inhomogeneity and ammonia addition to turbulent flames of ethylene

This paper is part of a broader program aimed at investigating the effects of co-firing clean fuels such as ammonia or hydrogen with hydrocarbons. The focus is on soot formation as well as flame stability in turbulent mixed-mode combustion, which is highly relevant in practical combustors. Ammonia substitution for nitrogen results in reduced flame stability, and this is correlated to differences in flame speed and extinction strain rate. While it is known that the addition of ammonia suppresses soot, visual inspection of compositionally inhomogeneous flames of ethylene-ammonia indicates a reduction in ammonia's ability to suppress soot formation. Measurements of soot volume fraction and laser-induced fluorescence in selected UV and visible bands are made along the centreline in selected flames to test this hypothesis. Experimental results are then compared to simulations in laminar diffusion flames, stratified counterflow flames, and partially premixed flames. All results confirm the soot-inhibiting ability of ammonia. Increasing inhomogeneity, leading to higher centreline mixture fractions, enhances soot formation, and the level of enhancement is greater for flames with ammonia than without. Moreover, it is found that partial premixing is ultimately responsible for determining the amount of soot formed as opposed to stratification of fuel mixtures near the pilot.

The impact of N2 micro-jets on the V-to-M flame shape transition in a premixed swirl burner

Injection of N2 through micro-jets located on the dump plane of a lean premixed swirl stabilized combustor is investigated as a new method for mitigating combustion instabilities. This study focuses on the chemical and fluid dynamic processes by which the N2 micro-jets impact the flame dynamics. An experimental and numerical investigation is performed to characterize the combustion instability during the V-to-M flame shape transition in a swirl burner fueled with premixed CH4/air, at an equivalence ratio of 0.62. Reasonable agreements have been found between the experimental measurements and simulation results. Both of them present that the flame changes from V-shape to M-shape periodically, and a low-frequency instability around 10 Hz is observed accordingly. It is confirmed that intermittent flame extinction in the outer recirculation zone (ORZ) is the source of the combustion instability. Furthermore, injection of N2 through micro-jets located on the combustor dump plane, into the outer recirculation zone, results in a stable V shape flame. It is clearly seen that the ORZ dilution can eliminate the combustion instability without inhibiting the combustion efficiency. A special focus is placed on the impact of the diluent injection on the local flame-flow interaction. The nitrogen micro-jets increase the local nitrogen concentration by 7% on average, lowering the flame speed and extinction strain rates by 27% and 17% respectively. Moreover, the micro-jets increase the turbulence intensity in the ORZ, leading to a significant increase in the Karlovitz number and transferring the local combustion regime from the thin reaction zone regime to the broken reaction zone regime. Hence, the nitrogen micro-jets impact on both the turbulence and the chemical reaction rates prevents flame propagation into the ORZ and results in a stable flame.

Simultaneous stereo‐PIV and OH×CH2O PLIF measurements in turbulent ultra lean CH4/H2 swirling wall‐impinging flames

We present experimental results from turbulent low-swirl lean H2/CH4 flames impinging on an inclined, cooled iso-thermal wall, based on simultaneous stereo-PIV and OH×CH2O PLIF measurements. By increasing the H2 fraction in the fuel while keeping Karlovitz number (Ka) fixed in a first series of flames, a fuel dependent near-wall flame structure is identified. Although Ka is constant, flames with high H2 fraction exhibit significantly more broken reaction zones. In addition, these high H2 fraction flames interact significantly more with the wall, stabilizing through the inner shear layer and well inside the near-wall swirling flow due to a higher resistance to mean strain rate. This flame-wall interaction is argued to increase the effective local Ka due to heat loss to the wall, as similar flames with a (near adiabatic) ceramic wall instead of a cooled wall exhibit significantly less flame brokenness. A second series of leaner flames were investigated near blow-off limit and showed complete quenching in the inner shear layer, where the mean strain rate matches the extinction strain rate extracted from 1D flames. For pure CH4 flames (Ka ≈ 30), the reaction zone remains thin up to the quenching point, while conversely for the 70% H2 flames (Ka ≈ 1100), the reaction zone is highly fragmented. Remarkably, in all near blow-off cases with CH4 in the fuel, a large cloud of CH2O persists downstream the quenching point, suggesting incomplete combustion. Finally, ultra lean pure hydrogen flames were also studied for equivalence ratios as low as 0.22, and through OH imaging, exhibit a clear transition from a cellular flame structure to a highly fragmented flame structure near blow-off.

Extinction and NO formation of ammonia-hydrogen and air non-premixed counterflow flames

Green ammonia, produced using renewable energy, is a promising carbon-free energy vector and fuel. This work studies combustion of ammonia-hydrogen fuel mixtures with air in counterflow diffusion flame experiments and provides an improved kinetic mechanism for modeling ammonia combustion. The extinction strain rate is measured for a range of 0 to 15% hydrogen in the fuel blend. The flame structure is also investigated with quantitative laser-induced fluorescence (LIF) measurements of nitric oxide (NO) for the same hydrogen concentrations and strain rate range from 26 to 134 s−1. For these conditions, NO concentration increases with both strain rate and fuel hydrogen content. The previously published kinetic model developed by the authors is used to perform one-dimensional flame simulations of the experimental setup and conditions, and results are compared to three other recently published ammonia mechanisms. None of the selected models satisfactorily predict both the measured extinction strain rate and flame NO concentration. The models mainly fail to predict extinction strain rate at higher H2 fraction and NO formation at the highest experimental strain rates and H2 fraction. The reaction rate parameters for some of the key reactions in the published model developed by authors were updated to improve agreement with experimental results. The updated model results are closely aligned with extinction strain rate measurements, and have improved prediction of flame NO concentration. The model reveals that the reactions from the NH2 and NH sub-mechanism are sensitive in predicting the extinction strain rate as well as NO. In particular, the reaction NH+NO=N2O+H had significant impact on NO predictions.

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.