Flame Stretch Research Articles

Self-similar propagation and flame acceleration of hydrogen-rich syngas turbulent expanding flames

In order to investigate the flame self-similar propagation and flame acceleration characteristics of hydrogen-rich syngas turbulent expanding flames, the turbulent combustion experimental studies of H2/CO/air mixtures with XH2 = 55%-95%, φ = 0.6–1.0, P = 1 bar-3 bar, u’=0.809–2.533 were conducted in a fan-stirred turbulent combustion chamber with an inner diameter of 380 mm. Results show that the normalized flame propagation speed ST/SL and turbulent flame Reynolds number ReT,flame could be represented by power-law correlations. With the increase of hydrogen fraction from 55% to 95%, the power exponent decreases from 0.413 to 0.301. The increase of hydrogen fraction makes the flame acceleration weaker because that the enhancing of laminar flame speed is more obvious than the differential-diffusion effect by decreasing Le. With the decrease of equivalence ratio from 1.0 to 0.6, the power exponent increases from 0.304 to 0.411 in this study. The relative high power exponent under low equivalence ratio is due to the coupling between differential-diffusion and the flame stretch on the local wrinkled flame structures. In addition, the influence of turbulence and flame intrinsic instabilities on flame acceleration propagation process of hydrogen-enriched syngas were studied. The acceleration exponent was extracted to quantitatively investigate the flame self-acceleration propagation in the transition stage, and the relationships between acceleration exponent and turbulence intensity (u’), Kovasznay number (Kz), turbulence integral length scale (LT), flame thickness (δ), thermal expansion ratio (ρu/ρb), effective Lewis number (Leeff) were analyzed

Stretch Effects in Large Eddy Simulation of Turbulent Premixed Bunsen Flames

ABSTRACT The stretch effects in large eddy simulation (LES) of a turbulent lean propane-air premixed Bunsen flame in the wrinkled flamelet regime are investigated. A simple approach to model the subgrid-scale flame stretch is proposed. It is based on the analysis which shows that, when the surface-filtered strain rate in a subgrid-scale stretch model is approximated using the volume-filtered velocity field solved for in LES, heat release and associated gas expansion in the filtered flame brush tend to artificially alter the wrinkling of resolved flame fronts. To minimize such artifacts, it is suggested that the volume-filtered strain rate on the unburned side of the filtered flame brush be used to approximate the surface-filtered strain rate and projected through the filtered flame brush. The results show the importance of mitigating the artificial heat release effects when considering the strain effects in LES. The relative importance of the curvature stretch is also investigated in terms of the mean and local effects. For a Bunsen flame with a positive Markstein number, the mean flame curvature effect tends to increase the total burning rate by enhancing the laminar flame speed near the flame tip, while the local flame curvature effect tends to decrease the total burning rate by suppressing the wrinkling of the resolved flames. It is found that the competition between the two makes the overall curvature effects not influence the total burning rate much for the flame investigated here, as compared with the strain effects.

Investigation on the transition of a propagating planar flame

A novel method to find the burning velocity and stretch rate of a premixed flame out of flame surface area has been developed by an image processing technique. The study is tailored for the control of precarious burning during combustion based on an automatic flame detection method. The flame propagation of premixed LPG-Air mixture is carried out in a tube of 30 mm diameter at the equivalence ratio of 1.2. A visible transition of tulip flame is observed towards the finger-shaped flame during initial propagation, which is envisaged to be triggered by the hydrodynamic instability. However, the flame is driven by stabilizing effect of diffusion and stretch. The flame structure is experimentally captured by an image processing method using Python 3.6 software to analyse the burning behaviour. The laminar burning velocity of an unstretched propagating flame has been found in congruence with the non-propagating steady flame at exit. The laminar burning velocity of steady flame is calculated by the volumetric flame formation rate. Based on the study of flame stretch rates, and flame propagation velocities at stationary burned conditions, the burning velocity of axially propagating laminar planar flame is found to be closer agreement with the higher stretched radially propagating spherical flame. The Markstein number is found to be 20.734, which divulges the presence of significant intrinsic diffusional effects in planar flame than the spherical flame during transition. The laminar and turbulent burning velocities are obtained to be accurate, respectively, at different equivalence ratios and turbulent intensities. The transition is also achieved and compared numerically by the Flamelet Generated Manifold method. The laminar burning velocity of LPG flamelet is observed to be precise at 75% propane composition and ϕ≤ 1.09. A higher laminar burning has been ascertained in lean mixtures due to the longer transition time.

A study of flame dynamics and structure in premixed turbulent planar NH3/H2/air flames

Ammonia (NH3) has received considerable attention as a near future carbon-free synthetic fuel due to its economic storage/transportation/distribution, and its potential to be thermally decomposed to hydrogen (H2). To promote the low burning velocity and heat of combustion of ammonia, one viable option is to enrich pure ammonia with hydrogen. In this study, two quasi direct numerical simulations (quasi-DNS) with detailed chemistry and the mixture-averaged transport model are examined to study stoichiometric planar ammonia/hydrogen/air flames under decaying turbulence. The reactants temperature and pressure are set to 298 K and 1 atm, respectively. The initial turbulent Karlovitz number is changed from 4.3 to 16.9, implying that all the test conditions are located within the thin reaction zones combustion regime. The results indicate that the density-weighted flame displacement speed ([Formula: see text]), on average, is higher than the unstrained premixed laminar burning velocity ([Formula: see text]) value for both test cases. This suggests that the flame elements propagate faster than their laminar flame counterpart. With increasing the Karlovitz number, the turbulent burning velocity and the wrinkled flame surface area increase by about 35%. Furthermore, the mean flame stretch factor defined as the ratio of the turbulent to the laminar burning velocity divided by the ratio of the wrinkled to the unwrinkled flame surface area is equal to 1.08. This indicates that the local flamelet velocity value, on average, is higher than the unstrained premixed laminar burning velocity. In addition, the results show that the mean value of the local equivalence ratio for the turbulent conditions is higher than its laminar counterpart due to the preferential diffusion of hydrogen and turbulent mixing. Furthermore, the net production rate of hydrogen is shown to be negatively correlated with the flame front curvature suggesting that the local burning rate is intensified in positively curved regions.

Experimental and numerical investigation on H2-fueled thermophotovoltaic micro tube with multi-cavity

To enhance H2-fueled combustion and improve energy efficiency, a micro burner with multi-cavity is proposed, and experimental and numerical investigations are carefully conducted. Effects of equivalence ratio, cavity settings and operating conditions on flow field, species distribution, combustion characteristics and heat transmission are analyzed and discussed. The results show that combustion stability and mean outer wall temperature Tm of the burner with cavity are improved, and the highest Tm can be obtained at equivalence ratio Φ = 0.9. Moreover, the lower inlet-step, which strongly affects the flow field, is beneficial to enhance the heat transmission of the combustor with cavity because of a smaller entropy generation. The setting of cavities in combustion chamber affects the flame stretch and anchoring because of the flow recirculation behind the step/fins, resulting in the enhanced heat transfer of gas-wall and a higher radiation temperature in the burner setting with multi-cavity. It can be further improved via the modification of cavity setup, such as step length and cavity numbers. Besides, the burner with three cavities achieves the highest Tm and radiation power at a higher flow rate, such as the highest power 35.61 W at Vin = 5 m/s and Φ = 0.9.

Flammability Limits: A Comprehensive Review of Theory, Experiments, and Estimation Methods

Flammability limits play an important role in combustion research, industrial applications, and fire safety. This article provides a comprehensive review of recent developments in the fundamental understanding of flammability limits and their experimental determination as well as estimation methods for pure fuels and fuel mixtures. The article begins with a discussion of the importance and challenges of determining flammability limits. It then presents the theoretical, computational, and experimental methods available to understand the mechanism of flammability limits and to quantify them. The experimental setups using cylindrical and spherical vessels to determine the flammability limits are discussed. The effects of buoyancy, thermal radiation, and flame stretch are examined. The relationship between the fundamental flammability limits and the extinction limits of stretched flames via strain and radiation is presented. The effects of initial temperature, pressure, mixtures of different fuels, and diluents are examined, and available estimation methods are presented. Finally, the flammability limits of renewable and alternative fuels are addressed and strategies for estimating the flammability limits of these fuels are presented.

Ammonia and hydrogen blending effects on combustion stabilities in optical SI engines

Ammonia is a promising energy carrier and carbon-free fuel, which is getting more attention today when fossil energy is increasingly exhausted. However, ammonia combustion in the IC engine suffers from combustion instabilities and misfiring issues. Hydrogen is also carbon-free energy and can be directly obtained by ammonia catalytic reforming. The addition of a small amount of hydrogen can effectively alleviate the weakness of ammonia combustion, while ammonia and hydrogen blending effects on combustion stabilities and their critical conditions remain unclear. In this work, the effects of hydrogen addition on ammonia combustion characteristics were experimentally investigated in an optical SI engine with high compression ratios. The results show that the indicated thermal efficiency first increases and then decreases with the elevation of hydrogen-ammonia energy ratios (χ), with an optimal value at χ≈7.5 %. Further hydrogen addition can aggravate indicated thermal efficiency due to the large heat transfer loss caused by high-temperature hydrogen combustion. Combustion visualizations show that there is a critical threshold of hydrogen-ammonia energy ratio (χ=10.0–12.5 %), around which engine combustion characteristics are changed significantly. When χ < 10 %, the effect of hydrogen addition on the ammonia flame speed becomes more pronounced since the hydrogen substantially improves the stability of the early-stage combustion. When χ > 12.5 %, hydrogen shows an obvious influence on the initial flame formation of ammonia, which is attributed to the low flame stretch sensitivity, facilitating early flame development. In addition, flame propagation when χ < 10 % is more sensitive to temperature than flame propagation with χ > 12.5 %. The current research demonstrates that proper ammonia-hydrogen energy ratios are important for stable combustion and thermal efficiency improvement.

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.

Direct Numerical Simulation of Partial Fuel Stratification Assisted Lean Premixed Combustion for Assessment of Hybrid G-Equation/Well-Stirred Reactor Model

Abstract Partial fuel stratification (PFS) is a promising fuel injection strategy to stabilize lean premixed combustion in spark-ignition (SI) engines. PFS creates a locally stratified mixture by injecting a fraction of the fuel, just before spark timing, into the engine cylinder containing homogeneous lean fuel/air mixture. This locally stratified mixture, when ignited, results in complex flame structure and propagation modes similar to partially premixed flames and allows for faster and more stable flame propagation than a homogeneous lean mixture. This study focuses on understanding the detailed flame structures associated with PFS-assisted lean premixed combustion. First, a two-dimensional direct numerical simulation (DNS) is performed using detailed fuel chemistry, experimental pressure trace, and realistic initial conditions mapped from a prior engine large-eddy simulation (LES), replicating practical lean SI operating conditions. DNS results suggest that the conventional triple flame structure is prevalent during the initial stage of flame kernel growth. Both premixed and nonpremixed combustion modes are present with the premixed mode contributing dominantly to the total heat release. Detailed analysis further reveals the effects of flame stretch and fuel pyrolysis on flame displacement speed. Based on the DNS findings, the accuracy of a hybrid G-equation/well-stirred reactor (WSR) combustion model is assessed for the PFS-assisted lean operation in the LES context. It is found that the G-equation model qualitatively captures the premixed branches of the triple flame, while the WSR model predicts the nonpremixed branch of the triple flame. Finally, potential needs for improvements to the hybrid G-equation/WSR modeling approach are discussed.

Enhancing ammonia combustion using reactivity stratification with hydrogen addition

In this paper, we propose a new method to enhance the burning rate of ammonia through reactivity stratification by using the binary fuel of ammonia and hydrogen. It is found that stratification from the high-reactivity fuel, hydrogen, to the low-reactivity fuel, ammonia, can substantially increase the flame speed of the low-reactivity, ammonia/air mixture. Moreover, the enhancement can sustain for a certain period even when the flame is entirely within the original low-reactivity mixture, indicating a memory effect of the flame. Such sustaining effect is found to be caused mainly by ammonia decomposition and the associated hydrogen production initiated by the hydrogen and active radicals. The reactions of NHx+H as well as reactions involving HNO and N2H2 were found to be important for this process. The length scale of the flame speed sustenance is further investigated in terms of the stratification thickness, pressure, flame stretch and stoichiometry. Furthermore, the flammability limits of the low-reactivity fuel are found to be substantially extended by the reactivity stratification. These findings suggest a strategy to facilitate the direct combustion application of ammonia.

Flow strain and curvature Markstein numbers of edge flame in the counterflow configuration

Based on the counterflow configuration, the Markstein numbers of one-dimensional premixed flame and two-dimensional edge flame of hydrogen-air system are numerically studied and compared with each other. By varying the inlet velocity and mixing layer thickness, the proportional relationship between flame stretch caused by flow strain (Kas) and by flame curvature (Kac) disappears, and therefore two corresponding Markstein numbers Mas and Mac at different flame markers (or iso-contours of temperature) within flame front could be obtained through linear fitting between normalized flame displacement speed Sd∗ and bivariate (Kas,Kac). With the flame marker shifting from unburnt to burnt side, Mac profile of edge flame exhibits a decreasing trend while Mas profiles of premixed flame and edge flame both display an increasing trend with its sign changing from negative to positive. Moreover, a good agreement of Mas profiles between two kinds of flames is observed while the discrepancy begins to appear when normalized temperature θ ≥5, which should be attributed to the shift of flame marker from premixed branch into diffusion branch rather than the decrease of heat release rate. The comparison between simulation and theory reveals differences of Mas values except for 2.5 < θ < 4 and those of Mac values except for θ = 2.5, which might be accounted for a moderate Zel'dovich number Ze, i.e. 3.9, based on detailed hydrogen-air chemical mechanism. Therefore, the “larger activation energy” assumed in asymptotic theory may not be valid for hydrogen-air flames whose equivalence ratio is 1.6 in this study.

Analysis of Lewis number effects on dynamic response of laminar premixed flames

The dynamic response of premixed flames with different fuel Lewis numbers is crucial to understand for the design of safe combustion chambers with wide operating range. Due to preferential diffusion effects associated with fuel Lewis number for lean mixtures, a premixed flame responds differently to flame stretch. These responses could manifest in the flame’s response to small perturbations. In order to investigate this effect, in this study, lean premixed hydrogen, methane and propane flames at the same adiabatic unstretched flame speed are studied using numerical simulations. Steady unperturbed flames show significant differences in flame stand-off distances, flame height, flame stabilization limits, burner temperature and recirculation zone length. Flames are perturbed using a step function excitation and noticeable changes in the gain and phase of the flame transfer function are found even when the flame burning velocity and the inlet flow speed are kept the same. Maximum gain of the response is found to increase with the mixture Lewis number and is shown to be directly dependent on the flame tip burning strong or weak. Phase difference at low frequencies is found to depend on the average of the time scales of the recirculation vortex which in turn depends on the flame burning stronger or weaker under the influence of positive stretch at the flame base, axial convective wave and perturbation travelling along the flame front. Time for the flame to settle to a new position after the step excitation is found to correlate well with the sum of time scales of vortex, flame tangential speed and flame tip speed. Overall, it is found that hydrogen flame responds the fastest to the perturbations and exhibits smaller increase in gain compared to methane and propane.

Flame speed in diffusion dominated premixed flames

Development of a premixed flame theory that includes the effects of flame stretch and curvature has been at the forefront of combustion research. Diffusion dominated flames such as highly curved flame balls present a challenging flame structure that has not been included in current flame stretch theory so far. In such flames, the relationship between consumption speed and negative displacement speed usually marks the boundary of what flame stretch theory can predict. In this work, our objective is to derive a general formulation which naturally includes this relationship. We use flamelet equations derived using a mass based stretch rate and show that if the diffusion flux at the unburnt side is not ignored, as is normally done in flame stretch theory, a formulation that can describe the propagation of different types of premixed flames can be derived. Based on the thin reaction zone assumption, solutions from theory are verified against numerical results for 1D ideal flame balls. Further verification is done for multi-dimensional ball-like flames, where both convection and diffusion dominated regions are present for highly curved flames. It is shown that the extended theory is able to predict the flame kinematics in a better way by describing the diffusion dominated flame propagation as well.

The effects of flame orientation and mean shear on flame stretch in highly turbulent premixed flames using DNS

In the present work, direct numerical simulation data of a laboratory-scale high Karlovitz number (Ka) jet flame with a strong mean shear and a high Ka freely propagating planar flame without a mean shear were analyzed to explore the influence of flame orientation and mean shear on flame stretch in highly turbulent flames. Three flame fronts, i.e. the leading, in-plane and trailing front, were identified for the jet flame. The leading and trailing front is defined as the flame normal being facing the downstream and upstream regions, respectively, while the flame normal of the in-plane front is near the cross-stream plane. It was found that the flame stretch is positive in the leading front, and is negative in the trailing front of the jet flame. As for the planar flame, the statistics of the flame stretch are similar in various flame fronts. The components of the flame stretch are analyzed. The normal vector of the leading front is aligned with the most compressive strain rate in the jet flame, resulting in a large tangential strain rate, while the tangential strain rate of the trailing front is significantly lower. In contrast, the alignment characteristics and the strain rates of the planar flame are not sensitive to the flame orientation. The value of the stretch due to curved propagating flame front is large and negative in the trailing front of the jet flame due to the negative correlation between the displacement velocity and curvature, which is weak in the leading and in-plane front. The correlation between the displacement velocity and curvature is, however, similar in various fronts of the planar flame. Overall, it is suggested that the effects of flame orientation and mean shear should be accounted for in understanding turbulence-flame interactions in practical combustion devices.

Elucidating the challenges in extracting ultra-slow flame speeds in a closed vessel—A CH[formula omitted]F[formula omitted] microgravity case study using optical and pressure-rise data

Refrigerants with a low global warming potential (GWP) possess mild flammability. Hence, a fundamental understanding of their combustion characteristics is required to assess their fire-hazardous potential. The laminar burning velocity is one fundamental property to describe these under safety evaluation aspects. However, measuring laminar burning velocities for slow-burning components, such as low-GWP alternatives, is challenging. This is due to influencing effects such as buoyant flame deformation, stretch, radiation, and confinement, especially at the flammability limits. In the present study, we investigated near-limit flames of the representative refrigerant difluoromethane (CH2F2) with nitrogen-enriched oxidizer mixtures to elucidate the potentials and limitations at ultra-slow flame speeds of two widely used flame speed measurement methods in the closed vessel configuration: the optical flame speed measurement in the early quasi-isobaric regime and the flame speed determination from the pressure-rise during the later phase of the isochoric combustion process. Experiments were performed under terrestrial- and microgravity, conducted in a specially designed setup suitable for drop tower applications, using both methods in parallel. Recommendations for the extraction of flame speed data are derived from the non-buoyant reference investigations under microgravity and transferred to ultra-slow flames at terrestrial gravity. Near-limit flames under microgravity were found to be significantly affected by stretch effects for the optical method so that flame speed extrapolations to unstretched conditions are prone to errors. Stretch also affects the pressure method’s lower evaluation limit, and errors can be estimated by combining flame speed data from the pressure method and Markstein lengths from optical experiments.

Turbulent flame speed and morphology of pure ammonia flames and blends with methane or hydrogen

Ammonia appears a promising hydrogen-energy carrier as well as a carbon-free fuel. However, there remain limited studies for ammonia combustion especially under turbulent conditions. To that end, using the spherically expanding flame configuration, the turbulent flame speeds of stoichiometric ammonia/air, ammonia/methane and ammonia/hydrogen were examined. The composition of blends studied are currently being investigated for gas turbine application and are evaluated at various turbulent intensities, covering different kinds of turbulent combustion regimes. Mie-scattering tomography was employed facilitating flame structure analysis. Results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount. It is less pronounced with increasing methane addition, analogous to the behavior displayed in the laminar regime. The turbulent to laminar flame speed ratio increases with turbulence intensity. However, smallest gains were observed at highest hydrogen content, presumably due to differences in the combustion regime, with the mixture located within the corrugated flamelet zone, with all other mixtures positioned within the thin reaction zone. A good correlation of the turbulent velocity based on the Karlovitz and Damköhler numbers is observable with the present dataset, as well as previous experimental measurements available in literature, suggesting that ammonia-based fuels may potentially be described following the usual turbulent combustion models. Flame morphology and stretch sensitivity analysis were conducted, revealing that flame curvature remains relatively similar for pure ammonia and ammonia-based mixtures. The wrinkling ratio is found to increase with both increasing ammonia fraction and turbulent intensity, in good agreement with measured increases in turbulent flame speed. On the other hand, in most cases, the flame stretch effect does not change significantly with increasing turbulence, whilst following a similar trend to that of the laminar Markstein length.

Experimental investigation on reactivity-controlled compression ignition (RCCI) combustion characteristic of n-heptane/ammonia based on an optical engine

Ammonia as a carbon-free fuel used in IC engines always suffers from large cyclic variations and high NOx emissions. Ammonia blending with high-reactivity fuels is recognized as an effective attempt to overcome these shortcomings. In this work, for the first time, the combustion and emission characteristics of n-heptane/ammonia were investigated under reactivity-controlled compression ignition (RCCI) conditions in an optical engine. An intake adapter was introduced to modify turbulent flow motion and n-heptane was directly injected to regulate mixture reactivity based on energy ratios. Particle image velocimetry (PIV) was employed for in-cylinder flow measurement, high-speed photography and instantaneous pressure acquisition were used for combustion evolutions, and Fourier transform infrared (FTIR) spectroscopy was adopted for NOx and NH3 emissions. The results show both mixture reactivity and turbulence have significant impacts on ammonia combustion. Specifically, n-heptane addition manifesting mixture reactivity dominates combustion processes at early-injection conditions, while the turbulence with increasing intensity becomes significant at late-injection conditions. Visualization images show that the initiation of initial flame kernels is controlled by the local distribution of n-heptane fraction, and that high n-heptane fraction at early injection or low n-heptane fraction at late injection conditions tends to induce homogeneous combustion. Flame stretch analysis confirms the promotion of mixture reactivity and turbulence motion on turbulent flame propagation and identifies that a high n-heptane fraction with swirl-tumble motion exhibits lower flame stretch sensitivity, especially for late-injection scenarios. Besides, low mixture reactivity with early injection and high mixture reactivity with late injection strategies can achieve high-efficiency combustion. However, when considering NOX/NH3 emissions, a high n-heptane fraction at late injection with swirl-tumble motion has the most potential to achieve the compromise between thermal efficiency and emissions. The present work demonstrates that with optimized fuel injection and flow arrangement, ammonia blending with high-reactivity fuels can achieve high-efficient combustion while maintaining NOx/NH3 emissions within reasonable levels.

Memory effects of local flame dynamics in turbulent premixed flames

A premixed and thermo-diffusively unstable turbulent hydrogen-air flame-in-a-box case is simulated in conjunction with the flame particle tracking (FPT) method. The flame is located in the flamelet regime. The focus lies on the assessment of memory effects in local flame dynamics. By tracking flame particles on an iso-surface of the flame during flame-turbulence interaction, the time history of flame speed and flame stretch can be recorded for each point on the flame iso-surface in a Lagrangian reference frame. The results reveal a time delay between the local flame speed and flame stretch signal, showing that previous values of flame stretch affect currently observed values of flame speed. Furthermore, by choosing flame particles whose trajectories are dominated by single frequencies, the time delay can be quantified. While plotting instantaneous values of flame speed and flame stretch results in a large scattering for turbulent flames, a quasi-linear correlation can be achieved by shifting the time signal of flame stretch according to the time delay. The time delay itself depends on the local flow time scale, which is expressed as a local Damköhler number. There is, however, an important difference between consumption and displacement speed. While most analyses in the literature are limited to the flame displacement speed, the flame consumption speed is evaluated for each flame particle in this work as well, which shows a strong correlation with the local equivalence ratio even at unsteady conditions. As the flame particles move toward regions with more negative flame stretch, the consumption speed decreases as the flame locally extinguishes. At the same time, the diffusive component of the displacement speed increases, as the tangential component of the diffusive flux increases in regions with strong negative flame curvature.

A study of propagation of spherically expanding flames at low pressure using direct measurements and numerical simulations

The outwardly propagating spherical flame (OPF) method is widely used to measure the laminar burning velocity (LBV). However, there are still numerous challenges to perform unambiguous measurements for extreme conditions, mainly for high and low (sub-atmospheric) pressures. In this work, the density weighted displacement speeds relative to burned (Sd,b˜) and fresh gases (Sd,u˜) are considered to report LBV for a large range of sub-atmospheric pressure (from 1 to 0.3 bar), equivalence ratio and fuels (methane and n-decane). These stretched flame speeds are obtained from experiments by using advanced optical diagnostics and the spherical flame propagation is simulated by the code A-SURF with different kinetic schemes to perform a direct comparison with the experimental results. It is shown that Sd,b˜ is significantly affected by the pressure decrease and becomes strongly non-linear with flame stretch. This is explained by the finite flame thickness structure and qualitatively modeled by the finite thickness expression (FTE) model used for extrapolation at zero stretch. However, assumptions (equilibrium and static burned gases) made to report Sd,b˜ are no longer valid and leads to strong under-prediction of LBV from extrapolation especially at low pressures. The shape of Sd,u˜ is less impacted by the pressure decreases and a linear behavior with flame stretch is globally found even for strong sub-atmospheric conditions. However, the temperature of the fresh gas isotherm where Sd,u˜ is extracted is not rigorously equal to the fresh gas’s temperature, and a correction factor is required to get accurate LBV. Therefore, a direct comparison of Sd,u˜ instead of extrapolated LBV between experimental and numerical data seems necessary to validate kinetic schemes at low pressures. A brief discussion is conducted on the validation of FFCM-1 and GRI3.0 for methane and Dryer mechanisms for n-decane at low pressures.