Laminar Flame Front Research Articles

Interactions between high hydrogen content syngas–air premixed flames and homogeneous isotropic turbulence: Flame thickening

This paper reports the findings from a three-dimensional direct numerical simulation conducted to investigate the turbulent flame behaviors of premixed high-hydrogen content syngas (with 50% hydrogen on a per mole basis) and air mixtures. To accomplish this, a laminar flame front is placed in a homogeneous isotropic decaying turbulence field composed of a syngas–air mixture at an equivalence ratio of 0.7 and allowed to evolve for 1.4 eddy turnover times. Homogeneous isotropic turbulence is generated using a helical forcing function in a cubic domain with a grid size of 256 × 256 × 256. The Reynolds number based on the Taylor microscale, Reλ, is 57 for the generated turbulence field. The laminar flame front is placed at the center of the domain. The premixture enters the domain at a velocity of 8 m/s and at an initial temperature of 800 K. The pressure remains constant at 1 atm. In addition to quantifying the spatial and temporal evolution of turbulent characteristics and flame structure, the study also focuses on identifying turbulence/flame interactions, specifically, the impact of these interactions on flame thickness. Energy transfer from small to large scales, i.e., a reverse cascade is observed as a result of energy release due to chemical reactions at the small scales that is transferred to larger scales. The increase in turbulent intensities due to chemical reactions correlates with flame thickening.

The ro-vibrational ν2 mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy.

We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν2 mode spectrum of methane (CH4). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000cm-1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH4 ν2 CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH4 chemistry is demonstrated in a laboratory CH4/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O2), carbon dioxide (CO2), and molecular hydrogen (H2), along with CH4. Fundamental physicochemical processes, such as H2 production via CH4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH4 v2 CRS thermometry, and we validate it against CO2 CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH4-rich environments, e.g., in plasma reactors for CH4 pyrolysis and H2 production.

Regime and morphology of polyhedral bunsen flames

Intrinsic flamefront cellular instabilities wrinkle, otherwise smooth, laminar flamefronts leading to excess surface area and, thus, burning rate. Since such instabilities increase flame propagation speed and burning rate without any external influence, such as turbulence, they are of both fundamental and practical interest. While extensively studied for freely propagating expanding or planar flames, cellular instabilities can also occur in burner-stabilized premixed flames. In this study, we present an experimental investigation of flamefront polyhedral instability occurring in premixed Bunsen flames for practically relevant lean hydrogen mixtures. Through systematic experiments, we delineate the effect of flame temperature, Lewis number, and flow rate on the stability and morphology of such flames. The results are presented as regime maps to clearly identify the conditions for which the flame manifests the instabilities and further illustrate how the transitional boundaries shift due to changes in experimental conditions. Moreover, the morphology of the flames was also investigated by identifying changes in the polyhedral structure as a function of operating conditions. Finally, we explain the observed dynamics of the polyhedral flames with the aid of analyzes of dispersion relation and residence time.

The Effect of Initial Conditions on the Laminar Flame Front Velocity in Gas Mixtures

In this paper, we consider the scatter of the laminar flame front velocities caused both by an error in the composition of the combustible mixture and by artificial initial disturbances. It is shown how the configuration of the initial perturbations of an initially flat front of a laminar flame in a gas mixture with the constant composition affects the velocity spread of expanding spherical flames. The effect of the error in the composition of the combustible mixture on the parameters that determine the flame front velocity was analyzed from the literature data on the spread of the flame propagation velocity. These parameters were recalculated for the possible variation in the mixture composition obtained based on the data on the accuracy of the equipment used in the experiments.

A correlation for turbulent combustion speed accounting for instabilities and expansion speed in a hydrogen-natural gas spark ignition engine

An analysis of the turbulent premixed combustion speed in an internal combustion engine using natural gas, hydrogen and intermediate mixtures as fuels is carried out, with different air-fuel ratios and engine speeds. The combustion speed has been calculated by means of a two-zone diagnosis thermodynamic model combined with a geometric model using a spherical flame front hypothesis. 48 operating conditions have been analyzed. At each test point, the pressure record of 200 cycles has been processed to calculate the cycle averaged turbulent combustion speed for each flame front radius. An expression of turbulent combustion speed has been established as a function of two parameters: the ratio between turbulence intensity and laminar combustion speed and the second parameter, the ratio between the integral spatial scale and the thickness of the laminar flame front increased by instabilities. The conclusion of this initial study is that the position of the flame front has a great influence on the expression to calculate the combustion speed. A unified correlation for all positions of the flame front has been obtained by adding one correction term based on the expansion speed as a turbulence source. This unified correlation is thus valid for all experimental conditions of fuel types, air–fuel ratios, engine speeds, and flame front positions. The correlation can be used in quasi-dimensional predictive models to determine the heat released in an ICE.

Влияние начальных условий на скорость фронта ламинарного пламени в газовых смесях

The scatter in laminar flame front speed caused by both an error in the composition of the combustible mixture and initial disturbances is reported. It's shown how the configuration of the initially planar front in laminar flame initial disturbances in a gas mixture of the same composition affects the scatter of speeds of expanding spherical flames. The experimental results previously obtained by the authors, demonstrating the scatter in the speed of the laminar flame front in an initially quiescent gas mixture of constant composition under the same conditions, are explained by integrating the Sivashinsky equation with various initial disturbances. The influence of combustible mixture composition errors on the parameters determining the speed of the flame front is analyzed. These parameters were recalculated for a possible scatter in the mixture composition, obtained based on data on the accuracy of the equipment used in previously published experiments.

Correlations for the laminar burning velocity and burned gas Markstein length of methane-air mixtures diluted with flue gases at high temperatures and pressures

Laminar burning velocity and burned gas Markstein length correlations have been developed from the measurements of spherically expanding methane-air flames under constant pressure at 1–5 bar, 373–473 K, and with 0–15% dilution. A dilution mixture consisting of 71.49% N2 + 19.01% H2O + 9.50% CO2 by mole, representing the actual principal residual concentrations from stoichiometric methane-air combustion, was used. Only equivalence ratios where the laminar burning velocity was greater than 15 cm/s were reported, due to the buoyancy effect limitation. The physical and chemical aspects of changes in the laminar burning velocity and flame front stability due to changes in temperature, pressure, equivalence and dilution ratios were discussed in detail. Experimentally measured laminar burning velocity data and a newly developed correlation in this study were compared with numerical results obtained from several chemical mechanisms as well as experimental data from the literature.

A New Analytic pdf for Simulations of Premixed Turbulent Combustion

A new reaction rate source term omega _m(c) for modelling of premixed combustion with a single progress variable c is proposed. omega _m(c) mimics closely the Arrhenius source term omega _A(c) for a large range of activation energies and density ratios while admitting analytic evaluation of many quantities of interest. The analytic flame profile c_m(xi ) very closely approximates the numerically integrated Arrhenius flame profiles c_A(xi ). An important feature of c_m(xi ) is that it is analytically invertible into a xi _m(c). Analytic estimates of the laminar flame Eigenvalue Lambda and of the Le dependence of the laminar flame speed s_L are derived, which are more accurate than classic results based on asymptotic analyses. The flamelet pdf p(c)=1/(Delta *c*(1-c^m)) for a flat laminar flame front in a LES cell of width Delta is derived. The exact mean of the reaction rate overline{omega (c)} is compared to a beta pdf result, which is shown to be inaccurate for large ratios of filter width to flame thickness Delta /delta _f and particularly for high activation energy flames. For multidimensional flame wrinkling we derive the exact relationship p(c)=p_{1D}(c)I(c)Xi (c) between the 3D pdf p(c), the 1D flat flame pdf p_{1D}(c), a correction factor I(c) for change of inner flame structure and a geometrical wrinkling factor Xi (c). We show that the c dependence of these quantities cannot be neglected for small Delta /delta _f. A simple model of a sinusoidally wrinkled flame front qualitatively demonstrates the effect of flame wrinkling on p(c). We show that for large Delta /delta _f, a wrinkling of the reaction zone almost constantly increases p(c) in the reaction zone by a wrinkling factor Xi ^* (defined for the surface of the isosurface of maximum heat release) while reducing it near c=0,1 as required for normalisation of p(c). The 1D p(c) evaluated using a reduced filter width Delta '=Delta /Xi ^* may be a good approximation of the wrinkled flame pdf for evaluation of overline{omega (c)} for such cases.

The Propagation of Flame Fronts through Inhomogeneously Magnetized Plasma

Abstract The effects of inhomogeneous magnetic fields on the propagation of magnetohydrodynamical (MHD) laminar flame fronts are investigated. This investigation is motivated by the occurrence of magnetized thermonuclear combustion in astrophysical systems. Magnetized thermonuclear burning occurs on the surfaces of neutron stars during Type I X-ray bursts, within the interiors of white dwarfs during SNe Ia, and during classical novae. Thermonuclear flames that propagate in these systems are expected to travel through inhomogeneous magnetic fields. We present the results of a series of 1.5-dimensional numerical simulations of magnetized flame propagation. A simplified flame model is used with one-step Arrhenius kinetics, an ideal gas equation of state, and constant thermal conductivity coefficients. Although idealized, the model allows for the opportunity to study the physics of the problem without the complexities of the nuclear kinetics of thermonuclear burning. We simulate the propagation of laminar flames through inhomogeneous magnetic media. A changing magnetic medium significantly alters the structure of the flame through the generation of an electric current. The electric current rotates the direction of the magnetic field within the flame and produces strong shear flows. Furthermore, for flames that conduct heat anisotropically and that propagate at an angle 0 < ψ ≲ π/2 to the magnetic field, the flame speed increases due to the nonuniform magnetic field. Naturally occurring flames in astrophysical systems may experience similar changes to their structure and speed that would influence the observational properties of these systems.

Effect of the initial pressures on evolution of intrinsically unstable hydrogen/air premixed flame fronts

Laminar premixed flame front may be wrinkled due to the hydrodynamic and diffusive-thermal instabilities. This may lead to the occurrence of the cellular structure and the self-acceleration. The lean unstable hydrogen/air premixed flame at various initial pressures are studied to clarify the effect of the initial pressure on the evolution of the unstable laminar flame. Linear and nonlinear development stages of the unstable flame are simulated and investigated separately. In the linear stage, the initial sinusoidal wave disturbance on the flame front will still keep its initial configuration. The growth rate increases firstly and then decreases with the increase of the wavenumbers. The effect of the self-acceleration on the unstable flame front will be stronger in the linear stage at the higher initial pressure, since there are larger thermal expansion and constant Lewis number for hydrogen/air premixed flame at higher pressure. There are little discrepancies for the calculated growth rates with those predicted by the revised dispersion relation. The nonlinear stage of the unstable flame propagation could be divided into two stages, the transitional and the stable nonlinear stages. In the transitional stage, the flame front cells splits, merges and moves all the time and the initial wavenumber has a great influence on the cell evolution process. With the evolution of the cell on the flame front, the cellular structure on the flame front will not change greatly with the initial wavenumbers in the stable nonlinear stage. The effect of self-acceleration due to the wrinkling of the flame front at this stage is weakened with the increase of the initial pressure. At the higher pressure, more wrinkled structures with smaller mean curvature are distributed on the flame front. At last, results show that the flame front will propagate faster for the larger computation domain. Based on the fractal theory, the fractal dimension of lean hydrogen/air premixed flame with the equivalence ratio of 0.6 at 0.5 MPa in the 2D domain is obtained and around 1.26.

Optimal neural network feature selection for spatial-temporal forecasting.

Neural networks, and in general machine learning techniques, have been widely employed in forecasting time series and more recently in predicting spatial-temporal signals. All of these approaches involve some kind of feature selection regarding what past data and what neighbor data to use for forecasting. In this article, we show extensive empirical evidence on how to independently construct the optimal feature selection or input representation used by the input layer of a feed forward neural network for the purpose of forecasting spatial-temporal signals. The approach is based on results from the dynamical systems theory, namely, nonlinear embedding theorems. We demonstrate it for a variety of spatial-temporal signals and show that the optimal input layer representation consists of a grid, with spatial-temporal lags determined by the minimum of the mutual information of the spatial-temporal signals and the number of points taken in space-time decided by the embedding dimension of the signal. We present evidence of this proposal by running a Monte Carlo simulation of several combinations of input layer feature designs and show that the one predicted by the nonlinear embedding theorems seems to be optimal or close to being optimal. In total, we show evidence in four unrelated systems: a series of coupled Hénon maps, a series of coupled ordinary differential equations (Lorenz-96) phenomenologically modeling atmospheric dynamics, the Kuramoto-Sivashinsky equation, a partial differential equation used in studies of instabilities in laminar flame fronts, and finally real physical data from sunspot areas in the Sun (in latitude and time) from 1874 to 2015. These four examples cover the range from simple toy models to complex nonlinear dynamical simulations and real data. Finally, we also compare our proposal against alternative feature selection methods and show that it also works for other machine learning forecasting models.

Computational Study of the Laminar Reaction Front Properties of Diluted Methanol–Air Flames Enriched by the Fuel Reforming Product

The current work investigates the flame structure and the propagation of premixed laminar flame fronts for mixtures of diluted methanol–air enriched by fuel reforming products under spark ignition engine conditions. Two engine concepts were investigated: one with external fuel reforming (EFR) and one with reformed exhaust gas recirculation (R-EGR). Here, the fuel reformate (or syngas) is a mixture of H2, CO, and CO2 with a CO selectivity of 6.5%, which is used to represent the products of methanol steam reforming over a Cu–Mn/Al foam catalyst. The simulations were exercised over a wide range of dilution level and unburned gas temperature at 40 bar with a skeletal chemical kinetic mechanism using zero/one-dimensional codes. Two types of dilution—air and EGR—were compared at the same fuel-to-charge equivalence ratio (ϕ′). The results showed that the knock limit for spark-ignited operation is extended with rising dilution levels, especially when diluted by an R-EGR mixture. Syngas addition also leads to a re...

Simulations of gasoline engine combustion and emissions using a chemical-kinetics-based turbulent premixed combustion modeling approach

This work presents a turbulent premixed combustion modeling approach which is based on chemical kinetics. In this approach, the smallest length scales are of the order of 0.1–1.0 mm for typical engine simulations with a Reynolds-averaged Navier–Stokes turbulence model and, after adaptive mesh refinement technology is used to consider the magnitude of the subgrid field, the Reynolds-averaged Navier–Stokes turbulent flow field can be well resolved. For solution of the flame front, an artificially thickened laminar flame concept is introduced to balance the computational accuracy and the computational cost. Around the artificially thickened laminar flame front, a special grid resolution strategy is designed, i.e. using much finer resolution in the normal direction of the flame front and typical adaptive mesh refinement resolution in the other two perpendicular directions. Then, chemical kinetics can be applied to the chemistry process which occurs in the flame front. To use this chemical-kinetics-based turbulent premixed combustion modeling approach better, a good chemical kinetics mechanism is very important. For this reason a practical primary-reference-fuel chemical kinetics mechanism is improved and validated in present work. The newly improved mechanism resolves several issues in the existing mechanisms, including unrealistically fast autoignition reactions and limited laminar flame speed validation. After reoptimization of those laminar-flame-speed-related reactions, the new mechanism can correctly compute the laminar flame speeds for a wide range of Ford spark ignition engines and for various operating conditions. Using this combustion modeling approach together with the new mechanism, simulations of the combustion and the emissions of several spark ignition engines for typical operating conditions were carried out. The simulated in-cylinder pressures, the simulated burn rates, and the simulated emissions including the brake specific carbon monoxide emissions, the nitrogen oxide emissions, and the unburned hydrocarbon emissions are compared with the experimental data, and very good agreement is found without tuning any model constants.

Devolatilization and volatiles reaction of individual coal particles in the context of FGM tabulated chemistry

The method of Flamelet Generated Manifolds (FGM) is coupled with a coal devolatilization model to perform transient simulations of a well-defined single coal particle combustion experiment for the first time. The gas phase chemistry is mapped onto a three-dimensional manifold controlled by the mixture fraction, a reaction progress variable and the enthalpy. A simulation of an electrically heated inert pyrolysis reactor is performed in order to evaluate transferability and applicability of experimentally obtained devolatilization kinetic parameters to Large Eddy Simulation (LES) codes for combustion configurations. Finally, the FGM modeling approach is applied to a premixed flat flame configuration, in which the coal particles cross a laminar flame front and are exposed to the hot gases. Numerical results regarding the volatiles reaction are compared to experimental findings. Particle and gas phase states are studied. Overall, good agreement between numerical results and experimental findings regarding the volatiles ignition range could be observed.

Analysis of dynamic models for large eddy simulations of turbulent premixed combustion

Dynamic procedures to automatically determine flame wrinkling factors from known resolved fields in large eddy simulations of turbulent premixed combustion are investigated from a priori tests processing a DNS database of a turbulent swirled flame. These flame wrinkling factors measure the ratio of total to resolved flame surfaces in the filtering volume and enter directly or indirectly into various flamelet combustion models through the sub-grid scale turbulent flame speed. They are usually modeled by algebraic expressions derived assuming equilibrium between turbulence motions and flame dynamics, a situation generally not reached during early stages of flame developments. Dynamic models then appear as a promising alternative to flame wrinkling factor or flame surface density balance equations to handle out-of-equilibrium situations. Attention is paid to three key requirements: (i) the correct prediction of propagating laminar flame fronts; (ii) the replacement of the averaging volume introduced to determine resolved and test-filtered flame wrinkling factors by a Gaussian operator easier to implement on unstructured meshes and/or massively parallel machines; (iii) the use of a local model parameter, evolving both in space and time. The two first requirements suggest basing the procedure on flame surface conservation instead of on chemical reaction rates. The saturated form of the Charlette et al. efficiency function [1], ΞΔ=(Δ/δl)β, where Δ is the filter width and δl the flame thickness, is found to be very well suited to dynamic determination of the model parameter β, easy to implement and very robust in practice, as confirmed by preliminary a posteriori tests.

Modelling of Landau–Darrieus and thermo-diffusive instability effects for CFD simulations of laminar and turbulent premixed combustion

An algebraic model is derived that accounts for the effects of non-resolved Landau–Darrieus and thermo-diffusive instabilities on the propagation speed of fully premixed laminar and turbulent flame fronts in the Large Eddy Simulation (LES) context provided that the laminar flame speed appears as a model parameter in the LES combustion model. The model is derived assuming fractal characteristics of flames which exhibit cellular structures due to instabilities. The smallest and largest unstable wavelengths are computed employing a dispersion relation for nominally planar flames. Values for the fractal dimension characterising the flame structures are taken from the literature. A phenomenological model accounts for the stabilising effect of strain. Based on experimental data, a correlation for a critical strain rate, which indicates the onset of instabilities, is formulated. To validate the new model which accounts for instabilities on the effective speed of laminar flame propagation, laminar expanding spherical methane–air flames at p = 5 bar and p = 10 bar are simulated in the LES context. Values for the fractal dimension, as proposed in the literature, are varied. The predicted flame propagation speed is in very good agreement with experimental data when applying a fractal dimension of about D = 2.06. The critical strain turns out to be a suitable parameter to indicate the onset of instabilities and to quantify the influence of instabilities. Simulations applying a second model proposed by Bradley and valid for spherically expanding flames show similar results. LES of turbulent Bunsen flames at 1, 5 and 10 bar, which are characterised by u′/s0L < 1, are performed to evaluate the derived instability model for turbulent flames. The simulated flames (from the Kobayashi database) have already been experimentally investigated in the context of Landau–Darrieus and thermo-diffusive instabilities. In agreement with conclusions from these investigations, for the considered cases the new model does not predict any onset of instabilities.

Flame Wrinkling Factor Dynamic Modeling for Large Eddy Simulations of Turbulent Premixed Combustion

Large eddy simulations (LES) of a turbulent jet premixed flame are performed using a dynamic formulation for the flame surface wrinkling factor entering the F-TACLES combustion model. The model parameter is automatically adjusted on the fly, taking advantage of the knowledge of the resolved scales in the flow field. Three cases are considered: a global formulation where the parameter is spatially uniform depending only on time and determined either from reaction rates or progress variable resolved fields, as well as a local determination where this parameter, estimated from resolved progress variable fields, varies with both location and time. The global formulation, in which the model parameter evolves only slightly around its mean value, provides similar results when setting this mean as a fixed non-dynamic value. On the other hand, a local parameter is found to increase from low values, corresponding to planar laminar flame fronts, to values larger than in the global case, as the flame is progressively wrinkled by turbulence motions when convected downstream. However, the three formulations lead to very similar results in terms of mean velocity and mass fraction profiles which then do not allow to discriminate between the different models. On the contrary, the flame response to inlet velocity modulation is found to depend on the model formulation, suggesting a possible important role in the prediction of combustion instabilities. Local parameter values, as well as local heat release rates, are clearly related to the large coherent structures developing in the flow, in agreement with previous observations.

Planar SiC MEMS flame ionization sensor for in-engine monitoring

A novel planar silicon carbide (SiC) MEMS flame ionization sensor was developed, fabricated and tested to measure the presence of a flame from the surface of an engine or other cooled surface while withstanding the high temperature and soot of a combustion environment. Silicon carbide, a ceramic semiconductor, was chosen as the sensor material because it has low surface energy and excellent mechanical and electrical properties at high temperatures. The sensor measures the conductivity of scattered charge carriers in the flame's quenching layer. This allows for flame detection, even when the sensor is situated several millimetres from the flame region. The sensor has been shown to detect the ionization of premixed methane and butane flames in a wide temperature range starting from room temperature. The sensors can measure both the flame chemi-ionization and the deposition of water vapour on the sensor surface. The width and speed of a premixed methane laminar flame front were measured with a series of two sensors fabricated on a single die. This research points to the feasibility of using either single sensors or arrays in internal combustion engine cylinders to optimize engine performance, or for using sensors to monitor flame stability in gas turbine applications.

Influence of pressure and temperature on laminar burning velocity and Markstein number of kerosene Jet A-1: Experimental and numerical study

Turbulent flames are utilised in almost all applications employing combustion. Thus, better understanding of the combustion process is the key for increasing the efficiency of fuel consumption and reducing the environmental footprint of future combustors. As the turbulent flame velocity depends on the laminar burning velocity and intensity of the turbulence, the laminar burning velocity can be considered as a crucial parameter for fuel characterisation. Since turbulent flame can be considered basically as wrinkled laminar flame (flamelet approach), the effect of stretch (caused by turbulence) on the flame propagation must also be considered. The Markstein number, which quantifies the response of a laminar flame to the stretch, can be employed for this purpose. Additionally, the Markstein number can be utilised as an indicator for laminar and turbulent flame front stability. The experimental data of the conventional liquid fuel – kerosene Jet A-1 – are scarce, especially at elevated pressure conditions. Identification of the parameters for elevated pressures is very important as such conditions are closer to real operational ones. Additionally, as the laminar velocity embodies fundamental information on the diffusivity, reactivity and exothermicity of a given mixture, it is often utilized for the validation of fuel reaction mechanisms.The combustion characteristics laminar burning velocity and Markstein number of kerosene Jet A-1 are investigated experimentally in an explosion vessel. For this purpose an optical laser method is employed based on the Mie-scattering of the laser light by smoke particles. Within this experimental study the influence of three crucial parameters: initial temperature, initial pressure and mixture composition on the burning velocity and Markstein number are investigated. The experiments are performed at five different pressures: 0.1, 0.2, 0.4, 0.6 and 0.8MPa; three different temperatures: 373, 423 and 473K; and for a range of equivalence ratios of 0.67–1.43. Along with the experiments, two different reaction mechanisms are used to evaluate their ability to predict the experimentally observed laminar burning velocities. The observed results are compared and discussed in detail.

Sustained propagation of ultra-lean methane/air flames with pulsed microwave energy deposition

An investigation of pulsed microwave energy addition to laminar methane/air flame fronts is undertaken. Microwave coupling efficiencies of up to 60% are measured in a microwave resonator with very low average power requirements (<30W). Pulsed microwave energy addition to laminar flame fronts is quantified using power absorption measurements and compared with filtered Rayleigh scattering temperature measurements of a laminar stagnation flame. Relative increases in nitric oxide concentrations are measured using a selective radar REMPI laser technique. For outwardly propagating methane/air flames, short microwave pulse bursts increase the effective flame speed up to 25%. With microwave pulse bursts, results show the ability to sustain deflagration fronts at ultra-lean equivalence ratios of 0.3 (compared to the 0.6 normal lean limit) with microwave energy deposition of 10% of the available thermal energy content of the fuel.