Unstretched Laminar Burning Velocity Research Articles

Laminar burning velocity of lithium-ion battery thermal runaway vent gas in air

In this study, a spherically propagating flame of vent gas generated during the thermal runaway of lithium-ion batteries with different states of charge (SOC) was experimentally and numerically investigated. The vent gas is mainly composed of H2, CO, CO2, and some hydrocarbons. With increasing SOC, the CO2 fraction tends to decrease while the H2 and CO fractions tend to increase. Combustion experiments were conducted using a closed vessel, and the combustion progression was captured using a high-speed Schlieren image system to measure the unstretched laminar burning velocity, Su0. In addition, the combustion was computationally estimated under the experimental conditions by performing 1D planar flame simulations using Chemkin-Pro with the Aramco Mech 1.3 reaction model. Remarkably, the calculated values of Su0 closely matched the experimental ones, demonstrating good agreement. The results showed that Su0 tended to increase with increasing SOC. Furthermore, the effects of the chemical reaction mechanism, which plays a pivotal role in driving this increase in Su0 with increasing SOC, were investigated. Specifically, the heightened SOC levels promoted the H2 branching reaction led to an increased concentration of OH radicals in the mixture. This consequently accelerated the rate-determining reaction for the oxidation of hydrocarbons, leading to an increase in Su0. As an effect of the hydrocarbons in the vent gas mixtures, the number of moles of CO increased slightly near the flame because the rate of CO production in the flame zone was more dominant than that of consumption under the condition of SOC 50.

Experimental and simulation study on the combustion characteristics of ammonia/n-dodecane mixtures

Laminar burning velocity is a crucial characterization parameter in fuel combustion progress. Investigating the combustion properties is significant to improve combustion efficiency. This paper employs a constant volume combustion bomb and high-speed ripple shadow camera technology to explore the influence of equivalence ratios (0.8–1.4) and ammonia energy shares (0%–40%) on the laminar burning velocity of n-dodecane/ammonia mixture at an ambient temperature of 400 K and a pressure of 1 bar. The results illuminated that the unstretched laminar burning velocity of n-dodecane increased from 46 cm/s at ϕ = 0.8 to 61 cm/s at ϕ = 1 and then decreased to 32 cm/s at ϕ = 1.4. When the ammonia energy ratio reaches 40%, the maximum laminar burning velocity experiences a reduction of 44% because the blended ammonia inhibited the formation of H and O radicals based on kinetic analysis in the fast reaction zone. With the increase of ammonia ratio, the Markstein length of mixed burned gas gradually increases. Particularly under a 1.4 equivalent ratio condition, the Markstein length increases from 0.2 mm to 0.8 mm, indicating that ammonia components significantly enhance flame stability. To gain a deeper understanding of the influence mechanisms of laminar burning velocity, a simplified Yao-Otomo (Y-O) model with 92 species and 355 reactions is proposed, coupling the mechanisms of n-dodecane and ammonia. The Y-O model has demonstrated a predictive trend that is consistent with more detailed WUT-NH3 model (176 species and 2839 reactions), and the Y-O model reduces the amount of computation to save calculation time, which has a higher engineering applicability. The turbulent kinetic energy (TKE) induced by premixed fuel in engine cylinders is estimated through comprehensively analyzing the laminar burning velocity and flame thickness characteristics. The findings indicate a 45% reduction in the peak value of TKE as the ammonia energy ratio increases.

Experimental and numerical analysis of ammonia/hydrogen combustion under artificial exhaust gas recirculation

This paper investigates the potential of exhaust gas recirculation in hydrogen/ammonia mixtures. The effects of artificial exhaust gas recirculation were evaluated on flame morphology and unstretched laminar burning velocities experimentally, while the flame stability and NO emissions were simulated using previous kinetic models. The experiments and numerical simulations were performed for various equivalence ratios, initial temperatures, pressures, exhaust gas recirculation rates, and compositions. Results show that recirculation of exhaust gases can enhance flame stability by diminishing the hydrodynamic instabilities while negatively affecting the flame laminar burning velocities. This negative impact, which is more sensible at non-stoichiometric conditions, reduces the flame speed between 19% and 58% based on the flue gas content of the combustible mixture. Despite this reduction, recirculating the exhaust gases can considerably reduce the high NO emissions of hydrogen/ammonia/air mixtures. Indeed, adding flue gases can decrease the maximum NO concentrations up to 45% at near stoichiometric conditions. The effects are even more significant in fuel-rich mode, diminishing the NO emissions up to 84% at an equivalence ratio of 1.3. Finally, these evaluations support the idea of exhaust gas recirculation application for using hydrogen/ammonia blends as a clean fuel while considering the expenses of burning rate reduction.

Onset of flame acceleration in propane–oxygen mixtures

In this study, the onset of flame acceleration because of Darrieus–Landau and diffusive–thermal instability in propane–oxygen mixtures was investigated using experiments and analysis. Using the soap bubble method, the unstretched laminar burning velocity Su0 and critical flame radius rc at which flame acceleration occurs of the propane-oxygen mixture were evaluated. The experimental values of Su0 and critical Péclet number Pec agreed qualitatively with the calculated values, and Pec tended to decrease with increasing equivalence ratios. In addition, the dependency of Pec on the Markstein number Mab and the critical Karlovitz number Kac on Mab for the propane–oxygen flame was compared with that for other fuel–air mixtures. The comparison revealed that the decreasing trend of Kac with Mab for the propane-oxygen flame was qualitative consistent with other fuel-air mixtures, in contrast to the different slopes of the Pec-Mab relationship.

A study on the effect of different diluents on laminar burning velocity and flame stability of oxy-n-dodecane mixtures at elevated pressures and temperatures

N-dodecane is one of the primary surrogate species of many commercial liquid fuels. The objective here was to measure the unstretched laminar burning velocity (LBV) and burned gas Markstein length (Lb) of oxy-n-dodecane mixtures having high-flame temperatures safely by adding a third party inert species using the spherically outwardly propagating flame (SPF) method. (100-Z)% (n-C12H26+(18.5 O2/ϕ)) +Z% (N2/CO2/H2O) mixtures were analysed at 400–450 K, 1–4 bar, ϕ=0.6–1.4, and Z: (a) N2=55–75%, (b) CO2/ H2O = 65%. N2 diluted mixtures were subjected to predominant thermal effects, whereas H2O/ CO2 diluted mixtures were affected by all the real-time effects like thermal & chemical, radiation and thermo-diffusive effects. The planar flame model in CHEMKIN was used to simulate LBV using You, JetsurF2.0, and PoliMi-1410 mechanisms. At 65% dilution, mixtures diluted with: (a) N2 had the highest LBV due to the high-flame temperature, (b) CO2 suppressed the LBV the most due to the thermal and chemical effects. Predicted LBV with You and JetsurF2.0 mechanisms showed an excellent agreement with the present measurements. PoliMi's kinetic scheme always over-predicted the LBV in the rich mixtures. The effect of thermal radiation on the LBV of CO2/H2O diluted mixtures was quantified using a freely propagating planar flame model with YaoSK54 mechanism and SNBCK radiation model. The diluted oxy-fuel mixtures generated stable flames towards thermo-diffusive effects at all the studied conditions. At a given thermodynamic condition, the flame's stability to the thermo-diffusive effects was the highest for the mixtures diluted with steam and it was followed by N2 and CO2 due to their respective Lewis numbers. If the flame temperature attained 2150 K or lower, the diluted oxy-n-dodecane mixtures at ϕ=1.4 were affected by thermo-diffusive instability. The sensitivity of the chain termination reaction H+OH+M↔H2O+M showed a transition from negative to positive at 0% N2 case.

Analysis of explosion and laminar combustion characteristics of premixed ammonia-air/oxygen mixtures

Ammonia is suggested to be a potential carbon-free energy source because of its high volumetric energy density and has attracted attention recently. However, systematic investigations into the explosion characteristics and flame instability of ammonia are still lacking, particularly for low-pressure ammonia-oxygen mixture. The use of low-pressure ammonia-oxygen mixtures could not only benefit for increasing engine's efficiency but also enhance its operational safety. In this study, explosion experiments of premixed ammonia-air/oxygen mixtures are carried out in a 20-L spherical chamber. By adjusting the initial pressure (P0) and equivalence ratio (ϕ), the maximum explosion pressure (Pmax) and maximum rate of pressure rise ((dP/dt)max) are measured. The laminar flame characteristics are analysed simultaneously. The results show that for the ammonia-air mixture, Pmax increases as P0 increases and reaches the peak at ϕ = 1.1; the unstretched laminar burning velocity (SL), Markstein length (Lb), and laminar flame thickness (δ) decrease as the initial pressure increases and reach the peak at ϕ = 1.1. For the ammonia-oxygen mixture, the explosion parameters linearly increase as P0 increases, while the maximum values of the parameters are obtained at ϕ = 1.0; the unstretched laminar burning velocity increases first and then gradually decreases as P0 increases; the maximum SL is 1.14 m/s at P0 = 0.1 MPa and ϕ = 1.0, which is 18 times that of the premixed ammonia-air mixture under the same condition. The schlieren images demonstrate that the buoyancy instability has significant impacts on the premixed ammonia-air flame propagation, while it has negligible impacts on the premixed ammonia-oxygen flame propagation. The effect of hydrodynamic instability on flame propagation could be observed in both the premixed ammonia-air flame at ϕ > 1.0 and the premixed ammonia-oxygen flame at P0 > 0.1 MPa and ϕ < 1.0. In addition, it is proposed that for the premixed ammonia-air flame, the buoyancy instability would remarkably influence the flame propagation when the unstretched laminar burning velocity is < 5 cm/s. The fundamental research on the combustion characteristics of ammonia-air and ammonia-oxygen in this study can provide an experimental reference for the later development of chemical reaction mechanisms, especially for the mechanism suitable for ammonia combustion in oxygen-rich environments. Meanwhile, the combustion characteristics of the ammonia-oxygen mixture at ultra-low pressure also provide a data basis for the possibility of using ammonia fuel as a propellant for hypersonic vehicles in the future.

Effects of hydrogen content on laminar burning velocity and Markstein length in outwardly propagating spherical SNG–air premixed flames

As the hydrogen content of the five synthetic natural gas (SNG) fuels fixed with the Wobbe index increases, the unstretched laminar burning velocity and Markstein length also increase. For the addition of hydrogen to natural gas pipelines, an experiment was conducted using up to 19% hydrogen gas (SNG H19). By comparing the error rates with the measured laminar burning velocities and the four reaction mechanisms, Aramco 2.0 was selected as the optimized mechanism for the SNG mixtures. The unstretched laminar burning velocities were compared with those of Aramco 2.0 for SNG–air premixed flames under elevated pressures up to 0.5 MPa for equivalence ratios (ϕ) of 0.7–1.4. The results showed that the measured laminar burning velocities were consistent with the numerical values under lean and stoichiometric conditions; however, the deviations increased under rich conditions as the hydrogen content increased. The laminar burning velocities estimated using the four mixing laws were evaluated under elevated pressure and rich conditions. Consequently, the results predicted by mass-fraction mixing and Q/k models were in good agreement with the measured values. The Markstein length decreases linearly as the hydrogen content increases, but all Markstein lengths are positive; therefore, no instability occurs in the SNG–air premixed flames. The measured Markstein lengths were compared with the theoretical Markstein lengths and reasonably traced well to the isothermal surface at 600 K. When the hydrogen content reaches up to SNG H19, the theoretical Markstein lengths overpredict the results, except for ϕ values of 0.7 and 1.4.

Minimum ignition energy of hydrogen-air mixtures at ambient and cryogenic temperatures

The ignition and combustion of hydrogen in air is considered more hazardous compared to other fuels due to the lower minimum ignition energy (MIE) and the wider flammability range. Spark discharge is the most common type of electrostatic ignition hazard. There is a need in validated safety engineering tools to accurately calculate MIE in a wide range of temperatures from atmospheric to cryogenic which are characteristic for hydrogen systems and infrastructure. Current MIE assessment methodologies rely on the availability of experimental data on quenching distance and/or laminar burning velocity and thus are mostly empirical correlations. This prevents their application beyond the limited number of experimental data, i.e. to arbitrary composition of the hydrogen-air mixture at arbitrary temperatures including cryogenic. This work aims at the development of a model able to accurately predict MIE for hydrogen-air mixtures with arbitrary initial composition and temperature. Cantera and Chemkin software are used to calculate the properties and unstretched laminar burning velocity of hydrogen-air mixtures. The flame thickness is found to well represent the critical flame kernel in the suggested model. The model is validated against experimental data on MIE for mixtures at ambient and cryogenic (down to 123 K) temperatures. Results show that the effect of flame stretch and preferential diffusion shall be considered to accurately predict MIE for lean hydrogen-air mixtures, which was not possible for previous models.

Determination of Unstretched Laminar Burning Velocity by Simultaneous Measurements of Flame Radius and Pressure-Time Trace Using Constant Volume Method

ABSTRACT Values of laminar burning velocity obtained by the constant volume method suffer from the inaccuracies introduced by the semi-analytical burned gas mass fraction model and flame stretch. A cylindrical combustion chamber with full optical access was developed in the present study, enabling the photography of the flame until the flame touches the inner wall of the combustion chamber. This enables direct visualization of the flame, thus eliminating the dependency on burned gas mass fraction models. The values of laminar burning velocity obtained by simultaneous measurement of flame radius and pressure-time data using the constant volume method are not stretch-free. This paper also develops a methodology for stretch correction based on the determined laminar burning velocity. Experiments were performed on mixtures of methane and air at initial pressures (0.8–1.05 bar), initial temperatures (293–320 K) and equivalence ratios (0.8–1.2). Stretch-corrected laminar burning velocity for lean, stoichiometric, and rich methane-air flames has been obtained in the present study, and the results obtained are validated against available experimental data and 1-D flame computations. It is expected that the proposed method significantly reduces the inaccuracies in laminar burning velocity obtained by the constant volume method.

Development of a comprehensive laminar burning velocity and flame instability profile of refined producer gas (H2:CO:CH4) – Air mixtures at elevated pressures

Unstretched laminar burning velocity (LBV) and intrinsic instabilities of Refined producer gas (H2:CO:CH4)-Air mixtures were systematically investigated at 300 K, 1–4 bar and ϕ = 0.8–1.2 using freely expanding spherical flame method. In H2/CO/CH4 rich mixtures, LBV increased with increase in CO (and reduction in CH4)/H2 (and reduction in CH4)/H2 (and reduction in CO) at any given equivalence ratio, while peak LBV occurred at ϕ = (1.2 and remained at 1.2)/(1.2 and shifted to 1.1)/(1.1 and remained at 1.1). Computed unstretched LBV using GRI Mech 3.0 and FFCM mechanisms deviated from measurements with initial pressure. From the comprehensive susceptibility analysis (to instabilities), the composition H2:CO:CH4 = 0:1:1 had the highest resilience towards thermo-diffusive and hydrodynamic instabilities. Refined producer gas with higher mole fractions of H2 were vulnerable to intrinsic instabilities, while increment in CH4 suppressed the susceptibility to hydrodynamic instability and increment in CO suppressed the thermo-diffusive instability.

Development of laminar burning velocity measurement system in premixed flames with hydrogen-content syngas or strong oxidizer conditions in a slot burner

This study presents the experimental determination of laminar burning velocity for critical premixed flames with multiple fuels or strong oxidizers. Experiments were conducted with a slot burner under methane/air, methane/nitrous oxide, and syngas (CH4/CO/H2)/air premixed flames conditions with varying equivalence ratios from 0.8 to 1.4. With the flame surface area determined from the Schlieren measurement system and stretch effect corrected by Markstein length, unstretched laminar burning velocity can be garnered according to the conservation of mass. First, the experimental results of methane/air premixed flame velocities were validated by comparing with one-dimensional unstretched burning velocities through numerical simulations with GRI 3.0 mechanism. The experimental results in the equivalence ratio ranging from 0.85 to 1.2 demonstrated errors less than 3.8%. Then, the validated burning velocity measuring techniques were implemented under methane/nitrous oxide and syngas/air premixed flame conditions. Finally, appropriate chemical mechanisms, such as USM or UGM, can be validated for the numerical simulation of critical premixed flames via this proposed laminar burning velocity measuring technique.

Assessment of Damköhler's hypotheses in the thin reaction zone regime using multi-step chemistry direct numerical simulations of statistically planar turbulent premixed flames

The effects of the definition of the reaction progress variable and equivalence ratio on the validity of Damköhler's hypotheses for turbulent premixed flames belonging to the thin reaction zone regime have been studied using multi-step chemistry direct numerical simulations of statistically planar CH4–air premixed flames with equivalence ratios of 0.8 and 1.0. Although CH4–air premixed flames with equivalence ratios of 0.8 and 1.0 have effective Lewis numbers close to unity, local differential diffusion effects can play a non-negligible role in determining the turbulent burning velocity and flame surface area in all cases. However, the augmentations of burning rate and flame surface area under turbulence do not occur in equal proportion, but their ratio remains of the order of unity. This conclusion holds irrespective of the definition of the reaction progress variable for the cases considered here. Damköhler's second hypothesis, which relates the ratio of turbulent burning velocity and the unstretched laminar burning velocity to the ratio of turbulent diffusivity and molecular diffusivity, has been found not to hold in the sense of equality, but it is valid in an order of magnitude sense for all choices of reaction progress variable definition. The findings of the current analysis indicate that Damköhler's first and second hypotheses should only be interpreted in an order of magnitude sense in the thin reaction zone regime even when the effective Lewis number remains close to unity.

Investigation on the effects of steam/CO2/N2 on the flame suppression and flame stability of the methane-oxygen mixtures at elevated thermodynamic conditions

This work aimed to study the dilution effect of steam, carbon dioxide, and nitrogen on the unstretched laminar burning velocity and burned gas Markstein length of premixed (100-Z) % (CH4 + (2O2)/ϕ) + Z%(H2O/N2/CO2) mixtures at 300–453 K, 1–4 bar, equivalence ratio (ϕ)- 0.6–1.4, and Z was varied from 40 to 60% for steam and kept constant at 50% for CO2/N2. Expanding spherical flames method was used. Nonlinear stretch extrapolation schemes were used to extract the burning velocity and Markstein length. All conditions were simulated using the 1D planar flame model in CHEMKIN with GRIMech3.0 and FFCM-1 reaction mechanisms. Present mixture formulation led to the comparison of the absolute dilution effect of each diluent at an identical mixture and thermodynamic conditions. Addition of 50% diluents significantly suppressed the flame propagation. It was the highest for CO2 (87% as compared to no dilution) due to the simultaneous reduction in flame temperature and its kinetic effect, and it was followed by steam (69%) and N2 (61%), and their suppression effect was primarily due to the reduction in the flame temperature. All the studied mixtures were stable to preferential diffusional instability as their: (a) Markstein length was positive and (b) the effective Lewis number of the mixtures were well higher than the critical Lewis number. Predicted LBV with GRIMech3.0 compared well with the experiments than FFCM-1. As the LBV increased beyond 150 cm/s, predicted LBV of both GRIMech3.0 and FFCM-1 showed significant deviation with the measurements.

A comprehensive study on laminar burning velocity and flame stability of oxy-producer gas mixtures. Part-1: Gas mixture composition and flame stability analysis based on Lewis number

The objective was to understand the effect of change in the composition of combustible (CC)/ non-combustible components (NCC) on the unstretched laminar burning velocity (LBV), burned gas Markstein length (Lb) and Lewis number (Le) of premixed producer gas (PG)-oxygen mixtures. Design of composition and flame stability aspects based on Le are presented in part-1. PG comprised of CC(CH4:CO:H2) + NCC(CO2:N2) and the extent of variation was identified from 800 compositions reported in 123 research works. In this work, forty-five PG mixtures were studied where CC:NCC varied from 25:75–45:55, CO2:N2 from 0:100–100:0 and CH4:CO:H2 from 25:25:50–50:25:25. Spherical flame method was used to measure the LBV at 1 bar, 300 K and ϕ = 0.8 in a 4.5L spherical chamber. Validation experiments were performed with (50%H2 + 50%CO)-air mixtures and it compared well with the literature and computational data. Unstable mixtures identified by effective (volume-based model) and critical Lewis numbers corroborated well with the experimental burned gas Markstein length.

Effects of Initial Temperature on the Deflagration Characteristics and Flame Propagation Behaviors of CH4 and Its Blends with C2H6, C2H4, CO, and H2

An experimental study was performed on pressure evolution and flame propagation for CH4 and its blends with C2H6, C2H4, CO, and H2 over its entire flammable range for systems with various concentrations (0.4–2.0 %) and initial temperatures (298–373 K) of closed-vessel deflagration. The peak explosion pressure (Pmax) and the maximum pressure rise rate (dp/dt)max were observed and analyzed. In the outwardly spherically propagating flame method, the unstretched laminar burning velocity (Ul) was obtained from the flame radius with time. Within the range of the experiments, the results show that Pmax decreases linearly and that (dp/dt)max first increases and then decreases with increasing initial temperature at a constant initial concentration. When a gas mixture is added to a 9.5 % CH4–air mixture, Pmax and (dp/dt)max exhibit decreasing trends at a constant initial temperature. A nonlinear regression formula was obtained, and this formula can be used to predict Pmax at different temperatures and concentrations. The kinetic model using two detailed mechanisms (GRI-Mech 3.0, FFCM-1) is compared with the experimental results using linear and nonlinear models of stretch extrapolation. The laminar burning velocities (LBVs) measured by FFCM-1 are closer to the experimental value than those measured by GRI-Mech 3.0. The Ul values of CH4 and its blends with C2H6, C2H4, CO, and H2 increase with an increase in the initial temperature. Based on the analysis of experimental values, the temperature exponent (α) is derived to predict the LBVs of the mixture at an elevated temperature under atmospheric pressure.

Measurements of laminar burning velocity and Markstein length in outwardly-propagating spherical SNG-air premixed flames at elevated pressures

A study has been conducted to measure unstretched laminar burning velocities and burnt Markstein lengths for outwardly-propagating spherical SNG-air premixed flames at elevated pressures. The range of flame radius and extrapolation models (LM, NM I and NM II) were first optimized for CH4-air premixed flames at normal temperature. The presently measured unstretched laminar burning velocities yielded the minimal errors when Aramco 2.0 and NM II were adopted for the optimized range of flame radius. The capability of predicting Markstein length with a composite model was investigated by using different definition of flame thickness and non-dimensional thermal conductivity for four iso-temperature surfaces. The theoretical Markstein lengths with appropriately defined flame thickness and non-dimensional thermal conductivity best trace the presently measured ones for the iso-temperature surface of 600 K.Unstretched laminar burning velocities against equivalence ratio were measured and compared with the predicted ones with Aramco 2.0 for SNG-air premixed flames. Except 0.1 MPa, the predicted ones were deviated appreciably from measured data when chamber pressure increases. The capability of predicting unstretched laminar burning velocities against equivalence ratio was improved via modifying several kinetic data based on sensitivity analysis to Aramco 2.0 Mechanism. The capability of predicting Markstein lengths was also tested for SNG-air premixed flames at elevated pressures up to 0.7 MPa. Choosing the iso-temperature surface of 600 K excellently traced the presently measured data when flame thickness and non-dimensional thermal conductivity are appropriately defined, while several issues are raised up as future works.

Effects of stretch and radiation on the laminar burning velocity of R-32/air flames

Experiments are performed for outwardly propagating spherical flames of R-32/air mixtures at constant pressure. A high-speed video camera captures shadowgraph images from which the flame radius as a function of time is extracted. Direct numerical simulations with an optically thin radiation model and detailed kinetics are performed to predict the flame radius and study the effects of stretch and radiation on the inferred laminar burning velocity. The simulations accurately predict the observed flame radius as a function of time. Analyses of the simulation results show that both stretch and radiation effects are important, occur concurrently, and make extraction of the unstretched adiabatic laminar burning velocity from the experimental data difficult.

Experimental laminar burning velocity of syngas from fixed-bed downdraft biomass gasifiers

Laminar burning velocity is considered as being one of the fundamental properties of a premixed flame and reliable data are constantly required for practical applications. Although it is possible to find an extensive amount of experimental laminar burning velocity data for fuels containing one or two components, data for fuels with three or more components are scarce. The goal of this study is to help fill this gap by providing experimental laminar burning velocity data for a fuel mixture with five components (H2:CO:CO2:CH4:N2). The fuel composition utilized is proposed as surrogates of the fuel provided by a downdraft gasifier, the most common and the most efficient type of gasifier. Experimental measurements were carried out for different fuel-to-air equivalence ratios (0.88 < ϕ < 1.74) at atmospheric conditions, 954 mbar and 298 K. The method utilized was the conical-flame surface using OH PLIF images (Planar-Laser-Induced Fluorescence imaging of OH). The area method provides a good approximation of the unstretched laminar burning velocity. Experimental data were compared with the simulated results obtained from a CHEMKIN chemical kinetics software. The highest experimental laminar flame speed of a downdraft syngas air mixture was 0.3491 m s−1 and occurred when ϕ ∼1.3.

Laminar flame speed, Markstein length, and cellular instability for spherically propagating methane/ethylene–air premixed flames

An experimental study on laminar flame speed, Markstein length, and the onset of cellular instability was conducted by varying the equivalence ratio and ethylene/methane mixing ratio in spherically propagating premixed flames at ambient temperature and elevated pressures up to 0.8 MPa. Unstretched laminar burning velocities were first validated for methane − air flames by optimizing the range of the flame radius in testing linear and non-linear extrapolation models, and subsequently comparing the results with those simulated using four kinetic mechanisms. Based on the results, unstretched laminar burning velocities were determined for premixed flames of methane/ethylene mixture fuels. The predictability of theoretical Markstein lengths was appreciated by adopting a composite solution of the heat-release-weighted Lewis number and the temperature-dependent Zel'dovich number. Measured Markstein lengths were compared with those predicted based on a composite model for laminar flame speeds against flame radius. Depending on the fuels (methane or methane/ethylene mixture), pressure, and equivalence ratio, the predictability of the model varied. For methane − air flames, cellular instabilities were not observed within the observation window at pressures up to 0.6 MPa. Cell formation, caused by hydrodynamic instability, was enhanced by an increase in the ethylene ratio and chamber pressure. Theoretical critical flame radii for the onset of cellular instability predicted by the composite model were consistent with the measured ones for both lean and rich mixtures.

Measurement and modelling of the laminar burning velocity of methane-ammonia-air flames at high pressures using a reduced reaction mechanism

Ammonia and blends of ammonia with methane are gaining increased interest as fuels for gas turbine applications and hence optimized reduced reaction mechanisms for the fuels are required for the development of combustors. However, there is a scarcity of measured data on the laminar burning velocity of the fuels for optimizing and validating reaction mechanisms especially at high pressures. In this study, an extensive set of measurements of the unstretched laminar burning velocity and Markstein lengths of CH4NH3-air flames at high pressures are reported for the first time and an optimized reduced reaction mechanism is proposed. The experiments were conducted in a constant volume chamber for various ammonia heat fractions in the fuel ranging from 0 to 0.30, equivalence ratios ranging from 0.7 to 1.3, and mixture pressures ranging from 0.10 to 0.50 MPa. The reduced reaction mechanism was developed from a detailed reaction mechanism for CH4NH3-air flames by Okafor et al., and optimized against the present measurements and data in the literature. It is shown that the reduced mechanism models the unstretched laminar burning velocity of NH3/air and CH4NH3-air flames with high fidelity at all studied conditions. It also models satisfactorily NH3, NO and CO concentration in CH4NH3O2N2 oxidation in a laminar flow reactor. Furthermore, the reduced mechanism is demonstrated to predict NO, OH, and NH profiles in a premixed stagnation NH3-air flame satisfactorily. The experimental measurements were also used to validate selected detailed reaction mechanisms. It was found that the significant over-prediction of NO production from NH3 oxidation by GRI Mech 3.0 is primarily due to the influence of the reaction NH+H2OHNO+H2, which in fact may not be important in fuel NO chemistry.