Markstein Length Research Articles

Effects of unburnt reaction progress on stretch flame dynamics under elevated temperatures

Flames in practical combustors and engines are inevitably subject to the effects of upstream chemical reaction progress and stretch, induced by elevated thermodynamic conditions and flow non-uniformities, respectively. Recent shock tube experiments and simulation studies on flame propagation have shown that flame speed under engine-relevant conditions can be enhanced with non-negligible upstream chemical reaction progress, especially when low-temperature heat release is involved in the unburnt mixture. On the other hand, depending on the mixture equivalence ratio and diluents, nonequidiffusion (including the non-unity Lewis number effect and the preferential diffusion effect) can couple with the flame stretch to fundamentally affect the flame propagation, which can manifest as either facilitation or suppression. Depending on the transport property of a reacting mixture, there hence can be either inhibition or promotion from unburnt reaction progress and stretch effects on flame propagation. In the current work, through one-dimensional numerical simulations of transient planar and spherical flames of n-heptane/air under elevated thermodynamic conditions, the combined effects of upstream chemical reaction progress and stretch on flame propagation are investigated. Results show that for both lean and rich n-heptane/air mixtures, flame speed can be substantially promoted with reaction progress, while the rich mixtures can exhibit opposite stretch dependence after first-stage ignition. Different definitions of effective Lewis number are adopted to explain the change in Markstein length for spherical flame in lean and rich mixtures reformed by the low temperature chemistry. This work fills an important gap in laminar premixed flame research relevant to practical combustion systems and can provide useful insight into local turbulent flame behaviors and phi-sensitivity in engine combustion.

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.

On the flame transfer function models for laminar premixed conical and V- flames considering the stretch effect

This paper investigates a predictive model that considers the impact of stretch on the dynamic response of laminar premixed conical and V- flames; the flame stretch consists of two components: the flame curvature and flow strain. The steady and perturbed flame fronts are determined via the linearized G-equation associated with the flame stretch model. Parameter analyses of the effects of Markstein length L, flame radius R and unstretched flame aspect ratio β are also conducted. Results show that the flame stretch reduces the steady flame height, with this effect being more significant for larger Markstein lengths and smaller flame sizes. The effects of flame stretch on perturbed flames are evaluated by comparing the flame transfer function (FTF) considering the flame stretch and not. For flames of different sizes, the impact of flame stretch on FTF gain can be divided into three regions. When both β and R are relatively small, due to the decrease in steady flame height and the impact of flow strain, the FTF gain increases. As β and R gradually increase, the FTF gain of the conical flame oscillates periodically while the FTF gain of the V-flame decreases, primarily due to the flame curvature enhancing the flame front disturbance and the wrinkle counteracting effect. When β and R are large, a disruption in wrinkle counteracting effect ensues, leading to a significant increase in FTF gain. Furthermore, as the actual flame height is reduced, the flame stretch also reduces the FTF phase lag which is related to the disturbance propagation time from the flame root to the tip.

Study on flame propagation and inherent instability of hydrogen/ammonia/air mixture

To improve the safety and reliability of hydrogen energy, this paper studied the flame propagation and instability characteristics of hydrogen/ammonia/air mixture under different hydrogen contents, equivalence ratios and initial pressures. The results show that three typical phenomena are observed in the premixed hydrogen/ammonia/air flame propagation process under different experimental conditions. In a stable flame, the roughness of the flame surface does not increase, while during unstable flame expansion, the cracking of the flame surface splits and increases. In addition, under the condition of low equivalence ratio and hydrogen content, significant flame uplift is observed. With the increase in hydrogen content, the flame thickness and critical instability radius of the hydrogen/ammonia/air mixture decrease monotonously. The increase of hydrogen content mainly strengthens the hydrodynamic instability, leading to the weakening of flame stability. With the increase of the equivalence ratio, the Markstein length and the critical instability radius increase monotonically. The minimum value of flame thickness appears at the equivalence ratio of 1.0. The Lewis number is always less than 1 for poor combustion and greater than 1 for rich combustion. An increase in the hydrogen content only moves the Lewis number further away from 1. In rich burn, the thermal-diffusion is beneficial to flame stability, and the flame thickness increases with the increasing equivalence ratio. Under the combined effect of thermal-diffusion and hydrodynamic instability, the overall stability of the hydrogen/ammonia/air flame gradually increases with the increasing equivalence ratio. With the increase of initial pressure, the flame thickness decreases gradually. The critical instability radius and Markstein length both decrease rapidly with the increase of initial pressure.

Turbulent Flame Speed and Turbulent Markstein Length in the Near Field of a Harmonically Oscillating Flame

ABSTRACT This study investigates turbulent flame speed and turbulent Markstein length using analytic and numerical methods. Using asymptotic analysis on harmonically oscillating flames, explicit expressions for the un-stretched turbulent flame speed and the turbulent Markstein length are derived. Numerical simulations using the 3D level-set method were conducted on the parameters of laminar flame speed, amplitude of oscillation of the flame holder, turbulence intensity, and longitudinal length scale of the flow. Results show that with distance downstream the un-stretched turbulent flame speed increases parabolically and Markstein length increases linearly. The explicit expressions are consistent with simulation results and offer insight into flame dynamics in the near field. In the far field, the numerical simulations show saturation of the un-stretched turbulent flame speed and the turbulent Markstein length, which are not captured by the asymptotic analysis.

Experimental and numerical investigation of combustion characteristics of carbon-free NH3/H2 blends in N2O

A detailed experimental and numerical investigation of the combustion characteristics of carbon-free NH3/H2 blends in N2O was performed in the present work. The experiments with various hydrogen fractions (α = 0–1.0) and equivalence ratios (φ = 0.6–1.4) were performed at ambient pressure (1 atm) and temperature (283 K) in a standard 3.375-L cubic combustion vessel. The explosion behaviors (maximum explosion pressure Pmax and maximum pressure rise rate (dp/dt)max) were obtained from the pressure-time curves. Further, the combustion characteristics (e.g. laminar burning velocity (SL), thermal expansion ratio (σ), flame thickness (δ), effective Lewis number (Leeff) and Markstein length (Lb)) were calculated numerically by means of chemical kinetic analysis using Zhang's mechanism (Zhang et al., Combust. Flame, 2017, 182: 122–141). Besides, sensitivity analyses of the explosion pressure and laminar burning velocity were performed to identify the contribution of dominant elementary reactions. The results show that, for combustion supported by N2O, the flame instability and the potential of deflagration to detonation transition (DDT) should be considered for the evaluation of Pmax and (dp/dt)max. Moreover, the hydrodynamic instability of premixed H2/NH3/N2O flame reaches its peak near the stoichiometric ratio and increases monotonously with the hydrogen fraction. The diffusion-thermal instability is more significant in the leaner mixture, and it is not invariably enhanced with the increase of hydrogen fraction but is relevant to the composition of the mixture. In addition, the sensitivity analysis of explosion pressure demonstrates that R80 (N2O + H2N2+H2O) and R70 (N2O+(M) = N2+O (+M)) are the most dominant reactions contributing to the pressure rise. The sensitivity analysis of laminar burning velocity shows that R70 (N2O+(M) = N2+O (+M)), R71 (N2O + HN2+OH) and R80 (N2O + H2N2+H2O) are the most dominant reactions enhancing laminar burning velocity.

Study on the laminar combustion characteristics and kinetic of IC8H18/NH3 premixed flames

To promote the transition to a carbon neutral society, it is necessary to conduct a comprehensive research on the utilization of ammonia contained in gasoline engine. In the present study, the laminar burning velocity (SL), Markstein length (Lb) and flame instability of isooctane/ammonia (IC8H18/NH3)-air mixtures were analyzed at the initial temperatures (T) of 400 K and 470 K, initial pressure (P) of 1–4 atm, ammonia mole fraction (XNH3) of 0%-70% and an equivalence ratio (ϕ) ranging from 0.8 to 1.3. Also, the kinetic mechanism of IC8H18/NH3 as proposed by Cai et al. [Combust. Flame, 162, 2015] was further optimized and compared with the mechanism of CRECK and Cai. The results show that the difference between the Cai and present mechanisms is largely attributable to the variation in sensitivity of NH3 oxidation-related reaction to SL. In addition, it is widely known that SL is dependent on the initial temperature and pressure. In order to improve the applicability of IC8H18/NH3 fuel mixtures, their temperature (α) and pressure (β) coefficients were determined given varying ϕ and XNH3. Meanwhile, a mathematical correlation for SL was also conducted. The analysis of Lb was also conducted to obtain the results showing that Lb decreases with ϕ increasing, except for NH3-air flame. Moreover, as XNH3 increases, Lb varies between lean and rich flames. For lean-flames, NH3 addition leads to a significant decrease in Lb, while for rich-flames, NH3 addition causes a marginal increase in Lb. All these are attributed to the variation in Zel’dovich number (Ze), effective Lewis number (Leeff) and flame thickness (δ). Furthermore, the instability of flame suggests that the addition of ammonia enhances the stability of the flame due to the reduced molecular diffusion effect. Moreover, an increase in XNH3 contributes only to a slight decrease of critical Peclect number (Pecr), and a sharp rise in the critical flame radius (Rcr) results from the increase of flame thickness (δ).

Effect of H2 addition on laminar burning velocity of NH3/DME blends by experimental and numerical method using a reduced mechanism

Blending dimethyl ether (DME) into ammonia (NH3) can efficiently enhance the combustion of pure NH3, and has attracted increasing attention. Partial dissociation of NH3 can convert NH3/DME to NH3/DME/H2 mixtures. This work measured the Markstein length and laminar burning velocity of NH3/DME/air mixtures for different blend ratios (100/0, 80/20, 40/60, and 0/100) with various H2 additions (0%, 20%, and 40%) at ϕ = 0.7–1.7, 0.1 MPa, and 298 K using a spherical constant-volume combustion method. A reduced kinetics mechanism was developed and optimized based on own prior detailed mechanism. This model gives more accurate predictions for NH3, DME, NH3/DME, and NH3/DME/H2 compared to previous models in terms of laminar burning velocity, ignition delay time, and species concentration. The experimental and modeling results show that the measured Markstein length of NH3/DME/H2 blends as a function of equivalence ratio presents different states for different NH3/DME blend ratios. The existence of H2 can effectively increase the laminar burning velocity of NH3/DME/air mixtures. Relative increase in laminar burning velocity (ESL) shows a non-monotonic tendency with increasing equivalence ratio for different H2 additions. The peak value of ESL for NH3/DME/air mixtures with various H2 additions appears at around ϕ = 1.5. This can be attributed to the dominant effect of C-contain and N-contain reactions rather than HO reactions at the rich burn side. Similar to ESL, the free radicals that dominate laminar burning velocity may transit from H, O, and OH to CH and NH radicals at around ϕ = 1.3 ∼ 1.6. The thermal effect also contributes to the ESL and laminar burning velocity for rich fuel.

Laminar burning velocities and Markstein numbers for pure hydrogen and methane/hydrogen/air mixtures at elevated pressures

Spherically expanding flame propagations have been employed to measure flame speeds for H2/CH4/air mixtures over a wide range of H2 fractions (30 %, 50 %, 70 and 100 % hydrogen by volume), at initial temperatures of 303 K and 360 K, and pressures of 0.1, 0.5 and 1.0 MPa. The equivalence ratio (ϕ) was varied from 0.5 to 2.5 for pure hydrogen and from 0.8 to 1.2 for methane/hydrogen mixtures. Experimental laminar burning velocities and Markstein numbers for methane/hydrogen/air mixtures at high pressures, which are crucial for gas turbine applications, are very rare in the literature. Moreover, simulations using three recent chemical kinetic mechanisms (Konnov-2018 detailed reaction, Aramco-2.0-2016 and San Diego Methane detailed mechanism (version 20161214)) were compared against the experimentally derived laminar burning velocities. The maximum laminar burning velocity for 30 % and 50 % H2 occurs at ϕ = 1.1. However, it shifts to ϕ = 1.2 for 70 % H2 and to ϕ = 1.7 for a pure H2 flame. The laminar burning velocities increased with hydrogen fraction and temperature, and decreased with pressure. Unexpected behaviour was recorded for pure H2 flames at low temperature and ϕ = 1.5, 1.7 wherein ul did not decrease when the pressure increased from 0.1 to 0.5 MPa. Although, the measurement uncertainty is large at these conditions, the flame structure analysis showed a minimum decline in the mass fractions of the active species (H, O, and OH) with the rise in the initial pressure. Markstein length (Lb) and Markstein number (Mab and Masr) varied non-monotonically with hydrogen volume fraction, pressure and temperature. There was generally good agreement between simulations and experimentally derived laminar burning velocities, however, for experiments of rich-pure hydrogen at high initial pressures, the level of agreement decreased but remained within the limits of experimental uncertainty.

Acceleration characteristics of composite flame in the initial stage of gas/coal dust explosion and analysis of disaster intensification

To clarify the strengthening mechanism of gas/coal dust coupling explosion disasters, a 20 L spherical explosion test system, high-speed schlieren, and PIV analysis were used to study the acceleration characteristics and pressure change of composite flames in the initial stage of gas/coal dust explosion, and the flow field characteristics were also analyzed. The results showed that methane concentration had the most important influence on the formation and development of composite flame. When the methane concentration was 7.5% and 9%, the composite flame propagation showed obvious acceleration characteristics and developed spherically, the flame boundary was clear and smooth, with the whole flame being continuous and bright white. Meanwhile, the Markstein length of composite flame is positive, and increased with the increase of methane concentration and dust particle size but decreased with the increase of coal dust mass concentration, the Markstein length for 9% gas concentration was 7 times larger than 6% gas concentration, indicating the increase of gas concentration can obviously inhibit the instability of composite flame, which prolonged the residence time of coal dust particles in the composite flame, resulting in the enhancement of the pyrolysis of coal dust and improvement of explosion intensity. The PIV test results of 9% gas/coal dust explosion showed that due to the high initial explosion intensity, the negative pressure in the center of the composite flame made most of the coal dust particles undergo centripetal movement, and a large number of coal dust particles gathered around the front of the flame. At the same time, a large number of large vortices in opposite directions were formed around the front of the flame, which promoted rapid contact between the coal dust surface and oxygen and intensified the combustion reaction. The research results can provide guidance for the prevention and control of gas/coal dust composite explosion disasters.

Effect of hydrogen addition on the laminar burning velocity and the flame stability of n-dodecane reacting with air at elevated pressures

The effects of hydrogen addition on the premixed laminar burning characteristics of n-dodecane reacting in air were experimentally investigated using the freely expanding spherical flame method. The initial operating conditions were temperature = 425 K, pressure = 1 ˗ 4 bar, equivalence ratio = 0.8–1.4, and mole fraction of H2 in n-dodecane-H2 mixture = 0–40%. Nonlinear stretch extrapolation schemes were implemented to minimize the uncertainty in the estimation of unstretched flame speed as the binary fuel mixtures of n-dodecane and H2 had non-unity Lewis numbers. All experimental operating conditions were simulated in CHEMKIN using a planar flame model with three different reaction mechanisms. The statistical uncertainty estimated for the LBV was below ±6.93%. The LBV increased by three/two times at off–stoichiometric/stoichiometric mixtures as the mole fraction of hydrogen (XH2) increased from 0 to 40% in n-dodecane. The rich mixtures of n-dodecane-air (ϕ = 1.4) were unstable to thermo-diffusive effects due to the lower mass diffusivity of n-dodecane and Le < 1, in contrast, H2 blending to the n-dodecane by keeping the other parameters being same transformed it from an unstable to a stable one. The effective Lewis number and Markstein length decreased with an increasing XH2 in n-dodecane, indicating that the mixtures were stable to thermo-diffusive effects, but with further increase in XH2, there may be a transition in flame stability. H2 addition resulted in an earlier onset of hydrodynamic instability due to a reduction in the flame thickness. In addition, sensitivity analysis was performed to identify the key reactions responsible for the enhanced reactivity associated with H2 addition. The reaction pathway and emission analysis showed a significant reduction in CO2 and CO emissions. Finally, an increase of initial pressure decreased the LBV for all investigated mixtures.

Spherically expanding flame simulations using Cantera coupled to an unsteady Lagrangian formulation

A Lagrangian-based one-dimensional approach has been applied, using Cantera libraries, to study the dynamics of spherically expanding flames. Detailed reaction models have been employed to examine spherical flame propagation in hydrogen-air mixtures. In the first part, our approach was validated against laminar flame speed and Markstein number data from the literature. It was shown that the laminar flame speed was predicted within 5% on average but that significant discrepancies were observed for the Markstein number, especially for rich mixtures. This suggests that even application of detailed reaction models can lead to inaccurate prediction of the stability of rich flames, which can be detrimental to the prediction of flame acceleration and the deflagration to detonation transition process. In the second part, a detailed analysis of the thermo-chemical dynamics along the path of Lagrangian particles propagating in stretched flames was performed. For mixtures with negative Markstein lengths, it was found that at high stretch rates, the mixture entering the reaction-dominated period is less lean with respect to the initial mixture than at low stretch rates. This induces faster rates of chemical heat release and active radical production which results in higher flame propagation speed. Opposite effects were observed for mixtures with positive Markstein lengths, for which slower flame propagation was observed at high stretch rates compared to low stretch rates.

An investigation on the laminar flame propagation and auto-ignition characteristics of a coal-derived rocket kerosene-Part I: Experimental study

The energy structure of coal-rich and oil-poor in China, makes the conversion of coal to liquid fuels an attractive option to reduce petroleum dependence. In this study, the combustion characteristics of a newly developed coal-derived rocket kerosene were investigated to provide fundamental parameters for evaluating the feasibility and compatibility of the new fuel in potential applications. The laminar flame propagation properties of the fuel were studied at pressures of 1, 2 and 5 bar over a temperature range of 423–483 K for equivalence ratio of 0.7–1.8 in a constant volume combustion bomb, and the auto-ignition characteristics of the target fuel were then experimentally studied in a heated shock tube. Flame propagation process was studied based on the recorded flame morphology firstly. The unsteady transition from spark assisted ignition kernel propagation to normal flame was analyzed and was found to become less conspicuous in flames with stronger reaction intensity (higher initial pressure or temperature) and smaller Lewis number (higher equivalence ratio). The Markstein length was extracted by extrapolating the flame trajectories and used to investigate the flame instability. The results shows that the increase of pressure, equivalence ratio enhance the flame instability, but the impact of initial temperature shows no prominent pattern. The laminar burning velocity and ignition delay times were reported over a wide range to strengthen the understanding of combustion characteristics of the new fuel. A kinetic model was proposed to simulate the combustion chemistry of the fuel and kinetic analysis was performed to identify the key reactions driving the flame propagation and auto-ignition. The present study is hoped to provide insights into the evaluation of fuel application in the future and database support for further combustion reaction kinetic mechanism development.

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.

Laminar burning characteristics of ammonia and n-butanol blend fuels

Blending n-butanol with ammonia has been shown to effectively improve the combustion performance of ammonia and enable carbon–neutral combustion systems. This study investigated the laminar burning speed and Markstein length of n-butanol/ammonia/air mixtures at various equivalence ratios (0.8–1.4) using the spherical flame method with the nonlinear extrapolation model (NQ) in a cylindrical constant-volume combustion bomb. The initial temperatures varied from 393 K to 453 K, and the initial pressures ranged from 0.1 MPa to 0.6 MPa, with ammonia fractions ranging from 50% to 80%. Results illustrate that the unstretched laminar propagating speed and laminar burning speed have a negative correlation with the ammonia fraction and initial pressure, but are positively correlated with the initial temperature, with peak values occurring at aroundϕ = 1.0 orϕ = 1.1 under all initial conditions. Adiabatic flame temperature is negatively related to the ammonia fraction and positively correlated with the initial temperature, but varies minimally with increasing initial pressure. Hydrodynamic instability is negatively related to the ammonia fraction and initial temperature but positively correlated with the initial pressure. When xNH3=80%, the Markstein length Lb and Markstein number Ma firstly decrease and then slightly increase as the equivalence ratios increase. The results of Lb and Ma indicate that the ammonia addition plays a primary role in the flame instability of ammonia/n-butanol blends at both ϕ≤0.9 and ϕ≥1.3, while the effect of n-butanol matters more at ϕ=1.0-1.2 under xNH3=80%. Both Rcr and Pecr increase with increasing ammonia fractions under Tin = 423 K, Pin = 0.6 MPa andϕ = 1.4. The findings indicate that the burning flux has a negative correlation with the ammonia fraction, whereas it is positively related to the initial pressure and initial temperature, with peak values occurring at around ϕ=1.0 under all tested conditions.

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.

Propagating characteristics of spherical flames in H2-N2O mixtures

The laminar burning velocity and cellular instabilities of premixed flame in H2-N2O mixtures were studied experimentally at various equivalence ratios (φ) and initial pressures (p0). High-speed schlieren was used to record the morphology of the spherical flame. Based on the spherical flame method, the laminar burning velocity and Markstein length were derived. In addition, the Lewis number, flame thickness, thermal expansion and Zel’dovich number were evaluated numerically. The results show that both the laminar burning velocity and Markstein length increase with the equivalence ratio. The Lewis number (less than unity) increases with the equivalence ratio while the flame thickness exhibits an inverse evolution. Hence, the most unstable flames were found to be at φ= 0.4–0.6 due to the combined effects of the thermal-diffusion and hydrodynamic instabilities. At lower initial pressure, the flame front is smooth in spite of the diffusion-thermal instability. With the increase of initial pressure, both the diffusion-thermal instability and hydrodynamic instability dominate the cellular structure of the flame front. The flames undergo self-acceleration at φ= 0.4–0.6 and p0= 40 kPa, with the acceleration exponents close to 1.5 (the value of self-turbulization). Due to the local oscillation of velocity, the flame propagation speed decreases after the self-similar regime.

Ensemble-averaged kinematics of harmonically oscillating turbulent premixed flames

Direct numerical simulation (DNS) was used to characterize the kinematics of ensemble-averaged flame position in harmonically oscillating turbulent premixed flames. The flames are stabilized on a harmonically oscillating flame holder, and the fuel consists of methane and hydrogen at a volume ratio of 10:3 and is mixed with air at the stoichiometric ratio. Turbulence intensities of 3.7, 8.6, and 13.3% are simulated, and each turbulent flame is simulated for 20 forcing periods for ensemble averaging. A laminar flame speed calculation method is derived based on the G-equation framework; the new method reproduces the reference laminar flame speed and the laminar Markstein length. The turbulent flame speed shows strong linear correlation with the ensemble-averaged flame curvature. The turbulent flame speed at zero flame curvature increases proportional to the turbulence intensity, while the turbulent Markstein length is independent of turbulence intensity. When the flame is far from the oscillating flame holder, the correlations between the turbulent flame speed and the flame curvature exhibit either piecewise linear or power-law dependencies. At these locations, the turbulent Markstein length at high curvature is similar to the laminar Markstein length. When a power law model is applied, the turbulent flame speed dependency asymptotes to 0.7 powers of the ensemble-average flame curvature at the turbulence conditions considered here.

Influence of hydrogen and methane addition in laminar ammonia premixed flame on burning velocity, Lewis number and Markstein length

The use of Ammonia (NH3) and blends with either Methane (CH4) or Hydrogen (H2) obtained by in-situ NH3 cracking, seem to be promising solutions to partially or fully decarbonise our energy systems. To strengthen understanding of fundamental combustion characteristics of these NH3 blends, the outwardly propagating spherical flame configuration was employed to determine the flame speeds and Markstein lengths. The air/fuel mixtures were varied across a large range of compositions and equivalence ratios. In general addition of CH4 or H2 results in a linear and exponential increase in measured laminar burning velocity, respectively. Of the appraised mechanisms, Stagni and Okafor kinetics mechanisms yielded best agreement with NH3/H2 and NH3/CH4 flame speed measurements. With respect to measured Markstein length, for a fixed equivalence ratio, addition of CH4 to NH3 resulted in a linear reduction in stretch sensitivity for the tested conditions. For lean NH3/H2 flames, an initial decrease in Markstein length is observed up to 30–40% H2 addition, at which point any further addition of H2 results in an increase in Markstein Length, with a non-linear behaviour accentuated as conditions get leaner. Above stoichiometry similar stretch behaviour is observed to that of NH3/CH4. Different theoretical relationships between the Markstein length and Lewis Number were explored alongside effective Lewis Number formulations. For lean NH3/H2 mixtures, a diffusional based Lewis Number formulation yielded a favourable correlation, whilst a heat release model resulted in better agreement at richer conditions. For NH3/CH4 mixtures, a volumetric based Lewis Number formulation displayed best agreement for all evaluated equivalence ratios. For NH3/H2, changes in measured Markstein Length were demonstrated to potentially be the result of competing hydrodynamic and thermo-diffusive instabilities, with the influence of the thermo-diffusional instabilities reducing as the equivalence ratio increases. On the other hand, the addition of CH4 to NH3 results in the propensity of moderating hydrodynamic instabilities, resulting in a stabilising influence on the flame, reflected by increasing positive Markstein number values. Finally, a systematic analysis of the flame speed enhancements effects (kinetic, thermal, diffusive) of CH4 and H2 addition to NH3 was undertaken. Augmented flame propagation of NH3/CH4 and NH3/H2 was demonstrated to be principally an Arrhenius effect, predominantly through the reduction of the associated activation energy.