Flame Radius Research Articles

Theoretical prediction of hydrogen cloud explosion overpressure considering self-accelerating flame propagation

On the basis of revealing the effects of equivalence ratio on flame morphology, radius and maximum explosion overpressure, this work was aimed at establishing a theoretical model to predict hydrogen cloud explosion overpressure by considering self-accelerating flame propagation. The results indicated that the decreasing order of flame propagation velocity is Φ=2.0, Φ=1.0 and Φ=0.8. For Le＜1.0 and Le＞1.0, the cellular structures could be formed on the flame surface, which would increase flame surface area and result in self-accelerating flame propagation. When the equivalence ratio was fixed, the positive maximum explosion overpressure and absolute value of negative maximum explosion overpressure continue to decrease as the distance between pressure senor and ignition source increases. As the equivalence ratio changes, there are some differences for positive maximum explosion overpressure and absolute value of negative maximum explosion overpressure at the fixed distance. The absolute value of negative maximum explosion overpressure was relatively higher than positive maximum explosion overpressure. Before rupture of thin film, the explosion overpressure evolution at various monitoring points could be reproduced using the theoretical model considering self-accelerating flame propagation.

The effect of oxygen content on the turbulent flame speed of ammonia/oxygen/nitrogen expanding flames under elevated pressures

Ammonia is a promising, but weakly reactive, carbon-free fuel and a promising hydrogen carrier for renewable energy. In this study, the turbulent flame speed (ST) of stoichiometric ammonia/oxygen/nitrogen mixtures under oxygen enrichment conditions was investigated at elevated pressures using a fan-stirred constant volume combustion chamber. Turbulent flame speed is found to increase with oxygen content at all pressures and turbulent intensities (u′) studied. In contrast, the turbulent-to-laminar flame speed ratio (ST/SL) was found to decrease with the oxygen content, mainly due to SL increasing faster than ST. The self-similar propagation characteristics of ammonia flame as flame radius develops are similar to other fuels and scale as a one-half power law, but it bends down as oxygen content increases. Several correlations of turbulent flame speed are validated against the present data, and it is found that a fitting relationship including pressures and turbulent intensities is independent of fuel type and performs best among literature proposed correlations. Moreover, the correlations of ammonia get similar results with other fuels when turbulent length scale adopted same definition of laminar flame thickness, in contrast, the ST/SL is larger for ammonia compared to methane at the same Reynolds number. Two new correlations of turbulent flame speed based on Karlovitz number (Ka) and Damköhler number (Da) are presented and narrowed the data scatter. Ammonia/oxygen/nitrogen mixture with ηO2 = 0.4 get similar turbulent flame speed as methane/air. A flame surface wrinkling analysis and stretch sensitivity analysis showed that the sensitivity of the higher stretch rate combined with the wrinkling acceleration was responsible for the observed increase in ST/SL ratio.

Assessing ignitions of explosive gas mixtures by low-energetic electrical discharges using OH-LIF and 1D-simulations

ABSTRACT Spark ignition by low-energetic electrical discharges in combustible fuel/air mixtures is a significant safety risk in various industries. In the present paper, OH-LIF measurements and numerical simulations of the ignition and early flame propagation of hydrogen/air, propane/air and ethene/air mixtures are reported. The experiments, carried out at four energy levels close to the respective minimum ignition energy (MIE), show that higher energy input leads to a wider flame radius, which is in agreement with one-dimensional numerical simulations. Further, the evaluation of the radial OH distribution reveals the highly stochastic nature of this process if the introduced energy is near the respective MIE. By utilizing the combined methodical approach of experiments and simulations, the key factors impairing the experimental repeatability are identified. The numerical results indicate at lower energies a longer time delay between source time and ignition, during which loss processes and perturbations may lead to extinction. One important influencing factor is the three-dimensional flow induced by the discharge, effectively increasing the experimental scatter. The presented work facilitates a detailed view on the complex physiochemical mechanisms dominating ignitions of explosive gas mixtures by low-energetic electrical discharges.

Tailored mixture properties for accurate laminar flame speed measurement from spherically expanding flames: Application to H[formula omitted]/O[formula omitted]/N[formula omitted]/He mixtures

The uncertainty on the laminar flame speed extracted from spherically expanding flames can be minimized by using large flame radius data for the extrapolation to zero stretch-rate. However, at large radii, the hydrodynamic and thermo-diffusive instabilities would induce the wrinkling of the flame surface and limit the range of usable data. In the present study, we have employed the flame stability theory of Matalon to tailor the properties of the initial mixture so that onset of cellular flame would occur at a pre-determined, large radius. This approach was employed to measure the laminar flame speeds of H2/O2/N2/He mixtures with equivalence ratios from 0.6 to 2.0, at pressures of 50/80/100 kPa and initial temperature of 300 K. For most experiments we performed, the uncertainty related to the extrapolation to zero stretch-rate (performed with the linear curvature model) was below 2% as shown by the position of the data points in the (Lb/Rf,U, Lb/Rf,L) plan, where Lb is the burned Markstein length; and Rf,L and Rf,U are the flame radii at the lower and upper bounds of the extrapolation range. Unsteady 1-D simulations using four chemical mechanisms were performed to show that unstretched laminar flame speed can be well-characterized with relative error below 10% for most conditions. The flame dynamical response to stretch rate could be captured by the mechanisms only under some conditions. Further analyses on critical flame radius were carried out: (i) the dominant parameters were identified based on a local sensitivity analysis; and (ii) the uncertainty on the predicted critical radius was determined through an uncertainty propagation approach. Uncertainty in critical flame radius calculation comes mostly from the uncertainty on fundamental transport and kinetic data. The present work indicates that although the stability theory of Matalon provides a well defined framework to tailor the mixture properties for improved flame speed measurement, the inaccuracy of some of the required parameters can result in significantly over-estimated critical radius for cellular flame onset which compromises the accuracy of the tailoring procedure.

Cellular flame structures and thermal characteristics of axi-symmetric ceiling fires: An experimental study and scaling analysis

The present study provides an experimental characterization of cellular flame structures and thermal characteristics of axi-symmetric ceiling fires. Experiments were carried out employing circular gaseous burners of various diameters at the ceiling facing downward using methane and propane as fuel. For this configuration, a hot buoyant diffusion flame burning beneath the cold fuel gas issuing from the ceiling above was naturally resulted in. A cellular flame structure, due to the inherent “Rayleigh-Taylor’’ instability of this buoyant flow configuration, was formed beneath the ceiling. The flame length (radius), temperature and heat flux distribution along the ceiling as well as characteristic inner cell size evolution of this special cellular flame were quantified experimentally for such ceiling fire of various conditions. Scaling analysis was performed to analyze the obtained parameters of the cellular flame structure and thermal characteristics of the ceiling fire. Results showed that the overall length (radius) of the cellular flame has a 2/5 power dependence on heat release rate and is independent of burner sizes and fuel types. Temperature profile and heat flux distribution for various conditions can be universally expressed as a function of L/Rf, the distance from the burner center normalized by the flame length. The characteristic cell size of the cellular flame was related to temperature, which can be well represented by a 2.33 power function based on the “Rayleigh-Taylor’’ instability scaling analysis. The cell size spatial evolution inside the cellular flame was self-similar corresponding to the dependence of temperature on relative position, and a non-dimensional function was obtained. This study provides fundamental data and a scaling analysis into the basic physics of the cellular flame behavior and thermal characteristics of ceiling fires.

Prediction of Deflagrative Explosions in Variety of Closed Vessels

In this paper the multi-phenomena deflagration model is used to simulate deflagrative combustion of several fuel–air mixtures in various scale closed vessels. The experimental transient pressure of methane–air, ethane–air, and propane–air deflagrations in vessels of volume 0.02 m3, 1 m3, and 6 m3 were simulated. The model includes key mechanisms affecting propagation of premixed flame front: the dependence of laminar burning velocity of concentration, pressure, and temperature; the effect of preferential diffusion in the corrugated flame front or leading point concept; turbulence generated by flame front itself or Karlovitz turbulence; increase of the flame front area with flame radius by fractals; and turbulence in the unburned mixture. Laminar velocity dependence on concentration, pressure, and temperature were calculated using CANTERA software. Various scale and geometry of used vessels induces various combustion mechanism. Simulations allow insight into the dominating mechanism. The model demonstrated an acceptable predictive capability for a variety of fuels and vessel sizes.

Validation of a LES Spark-Ignition Model (GLIM) for Highly-Diluted Mixtures in a Closed Volume Combustion Vessel

<div class="section abstract"><div class="htmlview paragraph">The establishment of highly-diluted combustion strategies is one of the major challenges that the next generation of sustainable internal combustion engines must face. The desirable use of high EGR rates and of lean mixtures clashes with the tolerable combustion stability. To this aim, the development of numerical models able to reproduce the degree of combustion variability is crucial to allow the virtual exploration and optimization of a wide number of innovative combustion strategies.</div><div class="htmlview paragraph">In this study ignition experiments using a conventional coil system are carried out in a closed volume combustion vessel with side-oriented flow generated by a speed-controlled fan. Acquisitions for four combinations of premixed propane/air mixture quality (Φ=0.9,1.2), dilution rate (20%-30%) and lateral flow velocity (1-5 m/s) are used to assess the modelling capabilities of a newly developed spark-ignition model for large-eddy simulation (GLIM, GruMo-UniMORE LES Ignition Model). The model accounts for all the main physical phenomena governing flame kernel growth, including electric circuit and over-adiabatic thermal expansion, which are included in the LES formalism of ECFM-LES combustion model. In the first part of the study the agreement of the simulation results against measurements is carried out to assess a validated non-reacting condition and converged flow statistics. Then, combustion simulations are carried out, and ignition events are repeated at random timings to replicate flow variability at ignition. Finally, optical comparison is carried out for simulated enflamed volume against high-frequency Schlieren images for all the cases, and measurements of flame radius growth are presented. The agreement of the quantitative flame measurements, as well as the qualitative resemblance of flame development, indicate the use of the presented GLIM ignition model as a valuable model for multi-cycle engine simulations, with particular relevance on unstable and CCV-affected conditions</div></div>

Experimental and Numerical Analysis of Gas Premix Turbulent Flames Stabilized in a Swirl Burner With Central Bluff Body

Abstract Lean premixed gas turbulent flames stabilized in the flow generated by an industrial swirl burner with a central bluff body are experimentally found to behave bistable. This bistable behavior, which can be triggered via a small change in some of the controlling parameters, for example, the bulk equivalence ratio, consists in a rather sudden transition of the flame from completely lifted to well attached to the bluff body. This has impact on combustion dynamics, emissions, and pressure losses. While several experimental investigations exist on this topic, numerical analysis is limited. This work is therefore also of numerical nature, with a twofold scope: (a) simulation and validation with experiments of the bistable flame behavior via computational fluid dynamics (CFD) in the form of large eddy simulation (LES) and (b) analysis of CFD results to shed light on the flame stabilization properties. LES results, in case of the lifted flame, show that the vortex core is sharply precessing at a given frequency. Phase averaging these results at the frequency of precession clearly indicates a counterintuitive and unexpected presence of reverse flow going all the way through the flame apex and the bluff body tip. The counterintuitive presence of a lifted flame is explained here in terms of the phase averaged data, which show that the flame apex is not placed at the center of the spinning reverse flow region. It is instead slightly shifted radially outward where the axial velocity recovers to low positive values of the order of the turbulent burning rate. A simple one-dimensional flame stabilization model is applied to explain this peculiar flame behavior. This model provides first an estimation of the flame radius of curvature in terms of axial velocity and turbulence quantities. This radius is therefore used to determine the total flux of reactants into the flame, given by an axial convection and radial diffusion contributions. Subsequently, the possibility of the flame positioned at the center of the vortex is excluded based on the balance between this flux and the turbulent burning rate. A clear explanation of the mechanism leading to the sudden flame jump has instead not been identified and only some hypotheses are provided.

Combustion-induced turbulent flow fields in premixed flames

Combustion-induced turbulence is studied in explosions of methane/air in a steel sphere with optical access. Rms turbulent velocities, u', created by four peripheral variable speed fans, were measured by Particle Image Velocimetry, PIV, which indicated near-uniform, isotropic turbulence, with small mean velocities. The gaseous expansion of a near-spherical propagating flame generated a radially outwards velocity of unburned mixture, the intensity of which depended upon the burning velocity and volume increase due to combustion. The study is to measure any extra turbulence created by this outwards velocity pulse. Only wave lengths less than the flame circumference can wrinkle the flame and increase the burning velocity, ut. Consequently the effective rms velocity at the flame front affecting its propagation, uk', is less than u'. Analysis shows, uk', to be a function of flame radius. The velocity pulse generated additional turbulence, enhancing uk'. This enhanced ut, giving further positive feedback. This higher rms value is designated as a spatial rms turbulent velocity, us'. PIV-measured gas velocities are resolved into a mean radial velocity, U-r and, us', with their radial profiles. These are derived from explosions at different equivalence ratios, pressures, and temperatures. At the radii at which ut are measured, us'/uk' is enhanced by the high rates of burning and volumetric expansion. This ratio attained a high value of 2.5, with stoichiometric CH4/air. Values of ut measured in explosions are higher than those measured in steady-state burners, attributable to the induced us', enhancing ut, in explosions, with no such effect in burners.

The effects of addition of carbon dioxide and water vapor on the dynamic behavior of spherically expanding hydrogen/air premixed flames

Utilizing efficiently and securely hydrogen as clean energy source, it is required not only to analyze the experimental data under a certain condition but also to create the mathematical model for the prediction of flame propagation velocity under various conditions. Thus, it is significant to understand the characteristics of dynamic behavior of hydrogen/air premixed flames and to elucidate the effects of addition of inert gas, i.e. carbon dioxide CO2 and water vapor H2O. We performed the experiments of hydrogen explosion in two types of closed chambers to observe spherically expanding flames using Schlieren photography. Wrinkles on the flame surface were clearly observed in low equivalence ratios. Analyzing the Schlieren images, the flame propagation velocity depending on the flame radius was obtained. Increasing the addition of inert gas, the propagation velocity decreased, especially in the case of CO2 addition. The propagation velocity increased monotonically as the flame radius became larger. The appearance of flame acceleration was found, which was caused by the evolution of wrinkles on the flame surface. Moreover, the Markstein length decreased as the concentration of inert gas became higher, indicating that the addition of inert gas promoted the instability of hydrogen flames. Furthermore, the wrinkling factor, closely related with the increment in propagation velocity, decreased as the inert-gas concentration became higher. The wrinkling factor normalized by the propagation velocity of flat flame increased, on the other hand, under the conditions of high inert-gas concentration, except for near the quenching conditions. This indicated that the addition of CO2 or H2O promoted the unstable motion of hydrogen flames, which could be due to the enhancement of the diffusive-thermal effect. Based on the characteristics of dynamic behavior of hydrogen flames, the parameters used in the mathematical model on propagation velocity including flame acceleration was obtained, and then the flame propagation velocity under various conditions was predicted.

Improvement of Calculations for Turbulent Premixed Flame Characteristics Determination using PDF Monte Carlo Simulation

This study aims at simulating turbulent premixed flame in a constant-pressure vessel (P = 1 atm) where the turbulence is supposed to be homogeneous and isotropic. The mixture of gas is composed by iso-octane-air. The realized CFD were based on Lagrange approach in Monte Carlo simulations. We focused on calculations of; flame radii R F , the flame propagation velocity S t , flame-brush thick ness  t an d flammability limit. During the study, influencing crucial parameters such as, the equivalence ratio  and the turbulence intensity u’ were considered. Results show that the equivalence ratio enhances the flame propagation when passing from lean to stoichiometric flames. Also, the turbulence intensity yields a notable growth for the flame characteristics mentioned above. Moreover, we noticed that the flammability limit is strongly depending of the turbulence intensity and the equivalence ratio. More precisely, we remarked that the minimum ignition energy (MIE) was situated quite smaller than the stoichiometric condition. But, it increased with the turbulence intensity.

The Influence of Hydrogen on Vaporization, Mixture Formation and Combustion of Diesel Fuel at an Automotive Diesel Engine

Hydrogen can be a viable alternative fuel for modern diesel engines, offering benefits on efficiency and performance improvement. The paper analyses the results of a thermodynamic model developed by authors in order to study the influence of Hydrogen addition on a process like vaporization, mixture forming, and combustion at the level of diesel fuel droplets. The bi-zonal model is applied for a dual-fueled diesel engine K9K type designed by Renault for automotives. For the engine operating regime of 2000 rpm speed and 55% engine load, the diesel fuel is partially substituted by Hydrogen in energetic percents of 6.76%, 13.39%, and 20.97%, the engine power being maintained at the same level comparative to classic fueling. At Hydrogen addition, the diesel fuel jets atomization and diesel fuel droplets vaporization are accelerated, the speed of formation of the mixture being increased. Comparative to classic fueling, the use of Hydrogen leads to diesel droplets combustion intensification, with a shortened autoignition delay, reduction of combustion duration, and increase of flame radius.

Analysis of combustion peculiarities in flame zone of aluminium particle

Metallic particles being added to rocket fuel contribute to increasing its energetic efficiency. In the present paper, the physical model of aluminum particle combustion is developed. The model takes into account accumulation and distribution of condensed oxide in the flame, which is divided into characteristic zones at that. The thermodynamic analysis of flame parameters for a single particle burning in medium «79% Ar + 21% O2» was carried out. The mathematical description of the model is presented, which lets to calculate dependencies of temperature and oxidizing components concentration on relative flame radius R‾ which is ratio of inner flame points coordinate R to current particle radius R0. The calculation method is based on combined use of heat and oxidizer flows balance and on data of equilibrium thermodynamic analysis of flame zone. The calculation of the mentioned dependencies were carried out for aluminium particles 220 μm (220 μm) in diameter at ambient environment pressure equal 0,1 MPa. The following radii were determined: radius of stoichiometry of metal and oxygen flows, radius of maximum temperature value reach and that of flame boundary. New parameter η is introduced which determines degree of metal transformation into condensed oxide and depends on thermodynamics parameters of flames (on temperature and oxidizing components concentration). The maximum of oxide particles distribution is established to be in zone of 5,5 ≤ R/R0 < 6,6, at that η value reaches 90%. The results of present work are compared with data of theoretical and experimental investigations of other authors. The accountability of equilibrium thermodynamics is shown to be mandatory part of models of aluminum particles combustion.

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

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

Propagation of weakly stretched premixed spherical spray flames in localized homogeneous and heterogeneous reactants

Propagation of weakly stretched spherical flames in partially pre-vaporized fuel sprays is theoretically investigated in this work. A general theory is developed to describe flame propagation speed, flame temperature, droplet evaporation onset, and completion locations. The influences of liquid fuel and gas mixture properties on spherical spray flame propagation are studied. The results indicate that the spray flame propagation speed is enhanced with increased droplet mass loading and/or evaporation heat exchange coefficient (or evaporation rate). Opposite trends are found when the latent heat is high due to strong evaporation heat absorption. Fuel vapor and temperature gradients are observed in the post-flame evaporation zone of heterogeneous flames. The evaporation completion front location considerably changes with flame radius. For larger droplet loading and a smaller evaporation rate, the fuel droplet tends to complete evaporation behind the flame front. Flame bifurcation occurs with high droplet mass loading under large latent heat, leading to multiplicity of flame propagation speed, droplet evaporation onset, and completion fronts. The flame enhancement or weakening effects by the fuel droplet sprays are revealed by the enhanced or suppressed heat and mass diffusion process in the pre-flame zone. Besides, for heterogeneous flames, heat and mass diffusion in the post-flame zone also exists. The mass diffusion for both homogeneous and heterogeneous flames is enhanced with a decreased Lewis number. The magnitude of the Markstein length is considerably reduced with increased droplet loading. Moreover, post-flame droplet burning behind the heterogeneous flame influences the flame propagation speed and Markstein length when the liquid fuel loading is relatively low.

Reynolds number scaling of burning rates in spherical turbulent premixed flames

Abstract

Effect of CO2 on self-acceleration properties of syngas/air turbulent premixed flame at different equivalence ratios

For studying the effect of carbon dioxide (CO2) on the self-accelerating properties of syngas/air mixture's turbulent premixed flame, an experimental study was carried out on 90%H2/10%CO/air mixture at the turbulence intensity of 1.31 m/s. The effects of various equivalence ratios (0.6, 0.8 and 1.0) and carbon dioxide contents (10%, 20% and 30%) on the flame evolution and propagation speed were studied. The effects of the CO2 contents and equivalence ratios on the self-acceleration properties in the flame propagation transition stage were evaluated. The effects of effective Lewis number and Zel'dovich number on the fractal excess and acceleration exponent were analyzed. The results showed that, as the CO2 contents increased, the development process of the flame slowed down, and the rate of growth of the flame's radius and the turbulent premixed flame propagation speed decreased at the same radius. The fractal excess and acceleration exponent increased with the increasing CO2 contents and decreased with the increasing equivalence ratio. Under different equivalence ratios and CO2 contents, the syngas/air premixed flame acceleration exponent was less than 1.5 in the transition stage.

Detailed simulation of laser-induced ignition, spherical-flame acceleration, and the origins of hydrodynamic instability

Ignition of a lean hydrogen–oxygen premixture by focused-laser-induced breakdown and subsequent three-dimensional expanding-flame instabilities are simulated in high detail. Both diffusive–thermal and hydrodynamic (Darrieus–Landau) instabilities are active and accelerate the flame expansion. The fluid is a partially-ionized gas in local thermodynamic equilibrium with detailed kinetics and transport models, starting from initial conditions from an auxiliary simulation based on a two-temperature local thermodynamic non-equilibrium model. After the decay of the initial laser-induced plasma, the r ∼ t1.5 growth in time of the flame radius matches theory and experimental observations. Based on hydrodynamic theory for spherical-flame propagation, a global Karlovitz number is defined as the ratio of the hydrodynamic to flame-distortion time scales. It initially increases during the diffusive–thermal instability stage, then with the onset of significant baroclinic torque, this trend reverses, with vorticity production becoming the dominant mechanism of instability.

Resolving flame thickness using high-speed chemiluminescence imaging of OH* and CH* in spherically expanding methane–air flames

The coupling of CFD simulations with detailed chemical kinetics presents great progress in predicting the complex behavior of reacting flows, but also requires validated input parameters in the form of experimental data. The spatial profile of a combustion wave represents one such parameter, which can be directly measured using chemiluminescence imaging of a spherically expanding flame. In this work, emission signals from electronically excited methylidyne (CH*) and hydroxyl (OH*) radicals near 434 nm and 315 nm, respectively, from spherically expanding methane–air flames at 1 atm and 298 K were recorded for equivalence ratios of 0.8, 1.0, and 1.2. Spatial profiles of normalized intensity were compared to predicted profiles from AramcoMech2.0. The effect of image resolution was investigated by repeating experiments for three levels of image pixel density. An Abel inversion was employed to extract intensity profiles of CH* and OH* at flame radii up to 6.5 cm. Measured flame thickness increased as flames grew in size, but this behavior diminished as image resolution increased. A linear stretch correlation was used to extrapolate measured thicknesses to an unstretched thickness for each experimental condition. Radical-based flame thicknesses and corresponding spatial profiles were found to be highly dependent on image resolution, and at high resolution, measured flame thickness appeared to approach the kinetically predicted radical-based thicknesses. This paper lays the foundation for future, comprehensive measurements of spherical, laminar flames that can resolve the flame zone details to a level of precision not typically seen in the literature, providing benchmark data for both kinetics model validation and CFD model inputs. As a result, the measurements thus far indicate that the measured flame zone thickness based on electronically excited species is much closer to the length scale typically predicted by kinetics models than what has been seen in most experiments to date.

Experimental and kinetic study of laminar flame characteristics of H2/O2/diluent flame under elevated pressure

Laminar burning velocity, Markstein length, and critical flame radius of an H2/O2 flame with different diluents, He, Ar, N2 and CO2, were measured under elevated pressure with different diluent concentrations. The effects of pressures, diluents, and dilution and equivalence ratios were studied by comparing calculated and experimental results. The laminar burning velocity showed non-monotonic behavior with pressure when the dilution ratio was low. The reason is the radical pool reduced with increasing pressure and leads to the decrease of overall reaction order from larger than 2 to smaller than 2, and further leads to this non-monotonic phenomenon. A modified empirical equation was presented to capture the relationship between active radicals and laminar burning velocity. Critical radii and Markstein lengths both decrease with initial pressure and increase with equivalence ratio and dilution ratio. The calculated critical radii indicate that the Peclet number and flame thickness control the change of Rcr. It can be found that Leeff has a significant influence on Peclet number and leads to the decrease of critical flame radii of Ar, N2, and CO2 diluted mixture. Interestingly, the CO2 diluted mixture has the lowest Markstein length under stoichiometric conditions and a high value under fuel-rich conditions, consistent as the flame instability observed on the flame images. The reason is that the Leeff of CO2 diluted mixture increased rapidly with the equivalence ratio.