Lean and ultralean stretched propane-air counterflow flames

Stretched laminar flame structures for a wide range of C₃H₈-air mixtures vs hot products are investigated by laser-based diagnostics and numerical simulation. The hot products are produced by a lean H₂-air premixed flame. The effect of stretch rate and equivalence ratio on four groups of C₃H₈-air flame structures is studied in detail by Raman scattering measurements and by numerical calculations of the major species concentration and temperature profiles. The equivalence ratio, ?, is varied from a near-stoichiometric condition (?=0.86) to the sublean limit (?=0.44) and the stretch rate varies from 90 s⁻¹ to near extinction. For most of these C₃H₈-air lean mixtures, hot products are needed to maintain the flame. The significant feature of these flames is the relatively low flame temperatures (1200-1800 K). For this temperature range, the predicted C₃H₈-air flame structure is sensitive to the specific chemical kinetic mechanism. Two types of flame structures (a lean self-propagating flame and a lean diffusion-controlled flame) are obtained based on the combined effect of stretch and equivalence ratio. Three different mechanisms, the M5 mechanism, the Optimized mechanism, and the San Diego mechanism, are chosen for the numerical simulations. None of the propane chemical mechanisms give good agreement with the data over the entire range of flame conditions.     

Important role of chemical interaction on flame extinction in downstream interaction between stretched premixed H2-air and CO-air flames

Important role of chemical interaction in flame extinction is numerically investigated in downstream interaction among lean (rich) and lean (rich) premixed as well as partially premixed H₂- and CO-air flames. The strain rate varies from 30 to 5917 s⁻¹ until interacting flames cannot be sustained anymore. Flame stability diagrams mapping lower and upper limit fuel concentrations for flame extinction as a function of strain rate are presented. Highly stretched interacting flames are survived only within two islands in the flame stability map where partially premixed mixture consists of rich H₂-air flame, extremely lean CO-air flame, and a diffusion flame. Further increase in strain rate finally converges to two points. It is found that hydrogen penetrated from H₂-air flame (even at lean flame condition) participates in CO oxidation vigorously due to the high diffusivity such that it modifies the slow main reaction route CO + O₂ → CO₂ + O into the fast cyclic reaction route involving CO + OH → CO₂ + H. These chemical interactions force even rich extinction boundaries with deficient reactant Lewis numbers larger than unity to be slanted at high strain rate. Appreciable amount of hydrogen in the side of lean H₂-air flame also oxidizes the CO penetrated from CO-air flame, and this reduces flame speed of the H₂-air flame, leading to flame extinction. At extremely high strain rates, interacting flames are survived only by a partially premixed flame such that it consists of a very rich H₂-air flame, an extremely lean CO-air flame, and a diffusion flame. In such a situation, both the weaker H₂- and CO-air flames are parasite on the stronger diffusion flame such that it can lead to flame extinction in the situation of weakening the stronger diffusion flame. Important role of chemical interaction in flame extinction is discussed in detail.     

Effect of hydrocarbon addition on tip opening of hydrogen-air bunsen flames

The effect of hydrocarbon addition on tip opening of lean and stoichiometric hydrogen-air flames is studied computationally by performing two-dimensional numerical simulations. The numerical study reveals that the flame tip of the H₂-air burner stabilized flame is open at lean and stoichiometric mixture conditions. The flame tip closes upon hydrocarbon addition. The tip closing is mainly affected by preferential diffusion of the multi-component mixture and the stretch effects. In the addition of light hydrocarbon (CH₄), the tip closing starts at a higher percentage of hydrocarbon addition in H₂-air flames. Whereas, upon the addition of heavy hydrocarbons such as propane and butane in H₂-air flames, tip closing starts with a lesser amount of hydrocarbon addition. Temperature, OH mole fraction and heat release rate have been investigated, focusing on the flame structure at the tip. The tip opening regime diagram for H₂–CH₄-air, H₂–C₃H₈-air and H₂–C₄H₁₀-air mixtures are presented.     

A study on flame structure and extinction in downstream interaction between lean premixed CH4-air and (50% H2 + 50% CO) syngas-air flames

Downstream interactions between lean premixed flames with mutually different fuels of (50% H₂ + 50% CO) and CH₄ are numerically investigated particularly on and near lean extinction limits in order to provide fundamental database for the design of cofiring burners with hydrocarbon and syngas under a retrofit concept. In the current study the anomalous combination of lean premixed flames is provided such that even a weaker CH₄-air flame temperature is higher than a stronger syngas-air flame temperature, and, based on a deficient reactant concept, the effective Lewis numbers Le_eff ≈ 1 for lean premixed (50% H₂ + 50% CO)-air mixture and Le_D < 1 for CH₄-air mixture. It is found that the interaction characteristics between lean premixed (50% H₂ + 50% CO)-air and CH₄-air flames are quite different from those between the same hydrocarbon flames. The lean extinction boundaries are of slanted shape, thereby indicating strong interactions. The upper extinction boundaries have negative flame speeds while the lower extinction boundaries have both negative and positive flame speeds. The results also show that the flame interaction characteristics do not follow the general tendency of Lewis number, which has been well described in interactions between the same hydrocarbon flames, but have the strong dependency of direct interaction factors such as flame temperature, the distance between two flames, and radical-sharing. Importance of chain carrier radicals such as H is also addressed in the downstream interactions between lean premixed (50% H₂ + 50% CO)-air and CH₄-air flames.     

Prediction of the liftoff,blowout and blowoff stability limits of pure hydrogen and hydrogen/hydrocarbon mixture jet flames

The paper presents experimental studies of the liftoff and blowout stability parameters of pure hydrogen, hydrogen/propane and hydrogen/methane jet flames using a 2 mm burner. Carbon dioxide and Argon gas were also used in the study for the comparison with hydrocarbon fuel. Comparisons of the stability of H₂/C₃H₈, H₂/CH₄ and H₂/CO₂ flames showed that H₂/C₃H₈ produced the highest liftoff height and H₂/CH₄ required highest liftoff, blowoff and blowout velocities. The non-dimensional analysis of liftoff height was used to correlate liftoff data of H₂, H₂/C₃H₈, H₂/CO₂, C₃H₈ and H₂/Ar jet flames tested in the 2 mm burner. The suitability of extending the empirical correlations based on hydrocarbon flames to both hydrogen and hydrogen/hydrocarbon flames was examined.     

Numerical study of experimental feasible heat release rate markers for NH3–H2-air premixed flames

Ammonia (NH₃) and hydrogen (H₂) as carbon-free fuels have attracted much attention for combustion applications in recent years. Co-firing ammonia with hydrogen provides a solution to overcome the extremes in the reactivities of both pure ammonia and hydrogen fuels. Heat release rate (HRR) is one of the most important quantities in the study of turbulent combustion, but direct measurement of local HRR is not experimentally feasible. In this study, we explored several quantities, [NH], [O], and the gradient of [OH] (Grad [OH]) as potential experimentally feasible HRR markers for NH₃–H₂-air premixed flames using numerical simulations. The performance of these quantities over a wide range of equivalence ratios and H₂ blending ratios have been examined, and some key reactions have been identified to explain the corresponding variations of the correlation for [NH] and [O]. It is concluded that the [NH] and Grad [OH] can be used in general as a suitable HRR marker for NH₃–H₂-air premixed flames, and the use of [NH] is especially recommended for lean flame conditions. A strategy that slightly shifts the [NH] and Grad [OH] profiles to overlap the corresponding HRR shows a further improvement on the performance of [NH] and Grad [OH]. The use of [O] can be considered for rich flame conditions while cautions are needed for conditions with high H₂ blending ratios.     

The controlling chemistry of surface deposition from sodium and potassium seeded flames free of sulfur or chlorine impurities

Sodium and potassium salt deposition have been studied in a series of propane and hydrogen flames free of sulfur or halogen impurities. With the collection probe in the 400 to 800 K range, samples of pure carbonate are observed and more importantly the rates of, for example, sodium carbonate deposition measured in terms of alkali metal are identical to those previously reported for sodium sulfate formation and also those observed for dominant NaCl deposition. Moreover, the behavior of Na₂CO₃ deposition mirrors exactly that of Na₂SO₄ in this temperature range. It shows a corresponding first order dependence on flame total sodium concentration, a zero order dependence on flame carbon, an insensitivity to fuel type, equivalence ratio, flame temperature, flow rate, probe material, or the nature of the sodium speciation in the flame, be it atomic or the hydroxide, or the state of the flame equilibration. A constant rate of deposition between 330 and 800 K conveys formation kinetics with a zero activation energy and that the surface accommodates atomic sodium equally well, be it below or above its dew point temperature and also at a seemingly approximately equal rate to that of flame NaOH. The fact that Na₂CO₃ cannot exist in the gaseous state in a flame finally proves irrefutably that these alkali deposition processes producing sulfate, carbonate or halide salts are heterogeneous in nature. The high collection efficiencies of the surface for alkalis have been confirmed by a further independent new calibration method for flame total alkali content. Also deposition rates are seen to be extremely similar in C₃H₈ /O₂ flames heavily diluted with either He, Ne, or Ar and also in a very fuel rich H₂ or D₂ flame. As with sulfate deposition, the rate of deposition is predominantly controlled by the actual flux of alkali in the flame gases that are intercepted by the collection probe. Moreover, there is an insensitivity to probe geometry and the nature of the flame flowfield, be it laminar or turbulent. The theoretical understanding of the complex boundary layer penetration and deposition mechanism is still inadequate in explaining these observations. The most intriguing results and differences from sulfate deposition have been observed on probes at lower temperatures (330-370 K). Although the formation of NaHCO₃, and more so KHCO₃, was expected to compete with that of their carbonates, in the case of sodium under fuel lean conditions only a small competing contribution of NaNO₃ formation was noted. This was very marginal for fuel rich conditions. However, with potassium the effects were enhanced and KNO₃ competes significantly with K₂CO₃ under fuel lean conditions. However, in fuel rich flames an unexpected dominant formation of potassium oxalate, K₂C₂O₄, was observed, along with some K₂CO₃ and a small amount of KHCO₃. Thermodynamic expectations in this lower temperature regime tend to suggest nitrate>bicarbonate>carbonate>oxalate. This is our first clearly observed non-equilibrium deposition behavior where the flame begins to display a pivotal role in controlling the surface molecular distribution. It also raises the possibility that low temperature surfaces in flames may be a new route for synthesizing certain thermodynamically metastable materials.     

Effects of dimethyl methylphosphonate on premixed methane flames

The impact of dimethyl methylphosphonate (DMMP) was studied in a premixed methane/oxygen/N₂-Ar flame in a flat flame burner slightly under atmospheric pressure at two different equivalence ratios: rich and slightly lean. CH₄, CO, CO₂, CH₂O, CH₃OH, C₂H₆, C₂H₄, and C₂H₂ profiles were obtained with a Fourier Transform Infrared (FTIR) spectrometer. Gas samples, analyzed in the FTIR, were extracted from the reaction zone using a quartz microprobe with choked flow at its orifice. Temperature profiles were obtained by measuring the probe flow rate through the choked orifice. Flame calculations were performed with two existing detailed chemical kinetic mechanisms for organophosphorus combustion. DMMP addition caused all profiles except that of CH₃OH to move further away from the burner surface, which can be interpreted as a consequence of a reduction in the adiabatic flame speed. Experimentally, the magnitude of the shift was 50% greater for the near-stoichiometric flame than for the rich flame. Experimental CH₃OH profiles were four to seven times higher in the doped flames than in the undoped ones. The magnitude of this effect is not predicted in the calculations, suggesting a need for further mechanism development. Otherwise, the two mechanisms are reasonably successful in predicting the effects of DMMP on the flame.     

Adiabatic burning velocity and cellular flame characteristics of H2–CO–CO2–air mixtures

The objective of this work was to study the effect of dilution with carbon dioxide on the adiabatic burning velocity of syngas fuel (with various H₂/CO ratios)-air(21% O₂–79% N₂ by volume) mixtures along with detailed understanding of cellular flame structures. Heat flux method with a setup similar to that of de Goey and co-workers [1] was used for measurement of burning velocities. Validation experiments were done for H₂ (5%)–CO (95%)–air and H₂ (5%)–CO (45%)–CO₂ (50%)–air mixtures at various equivalence ratios and the results were in good agreement with published data in the literature. The mixtures considered in this work had 1:4, 1:1 and 4:1 H₂/CO ratio in the fuel and 40%, 50% and 60% CO₂ dilution. The burning velocity increased significantly with the increase in H₂ content in mixture of H₂–CO with fixed CO₂ dilution. The burning velocity reduced remarkably with carbon dioxide dilution in H₂–CO mixture due to reduction in heat release, flame temperature and thermal diffusivity of the mixture. The location of peak adiabatic burning velocity shifted from ? = 1.6 for 40% CO₂ to ? = 1.2 for 60% CO₂, whereas it remained unchanged with variation of H₂:CO ratio (4:1, 1:1 and 1:4) at a given CO₂ dilution. A comparison of experiments and simulations indicated that the Davis et al. [2] mechanism predicted burning velocities well for the most of experimental operating conditions except for rich conditions. For some lean mixtures, flames exhibited cellular structures. In order to explain the structures and generate profiles of various field variables of interest, computations of three dimensional porous burner stabilized cellular flames were performed using commercial CFD software FLUENT. Simulations for lean H₂ (25%)–CO (25%)–CO₂ (50%)–air mixtures (? = 0.6 and 0.8) produced cellular flame structures very similar to those observed in the experiments. It was found that the in the core region of a typical cell, stretch rate was positive, the volumetric heat release rate was high and the net reaction rate for the reaction O + H₂ ? H + OH and the net consumption rate of H₂ were both high.     

Extinction and near-extinction instability of non-premixed tubular flames

Tubular non-premixed flames are formed by an opposed tubular burner, a new tool to study the effects of curvature on extinction and flame instability of non-premixed flames. Extinction of the opposed tubular flames generated by burning diluted H₂, CH₄ or C₃H₈ with air is investigated for both concave and convex curvature. To examine the effects of curvature on extinction, the critical fuel dilution ratios at extinction are measured at various stretch rates, initial mixture strengths and flame curvature for fuels diluted in N₂, He, Ar or CO₂. In addition, the onset conditions of the cellular instability are mapped as a function of stretch rates, initial mixture strengths, and flame curvature. For fuel mixtures with Lewis numbers much less than unity, such as H₂/N₂, concave flame curvature towards the fuel suppresses cellular instabilities.     

Chemical effects on molecular species concentrations in turbulent fires

The measurement and modeling of molecular species concentrations in turbulent pool and buoyant jet flames is described. The experimental parameters included burner diameter (2.8 mm jet nozzle, 190, 381, and 762 mm pools), theoretical combustion heat release rate (10–283 kW), lip size (0–25 mm), and fuel (CH₄, C₃H₈). Time-averaged species concentrations were obtained through axial and radial sampling probe traverses. A novel sampling probe was developed which provides a constant mass flow of flame gases that is not biased toward either hot or cold gas eddies.Local concentrations of major gas species (fuel, O₂, CO, CO₂, H₂O, N₂) in the fire are correlated by the mixture fraction, which is the fraction of atomic species present which originated in the supplied fuel. The correlation appears to be independent of pool diameter, lip size, and heat release rate. These turbulent correlations differ from the corresponding curves for laminar flames primarily due to composition broadening resulting from time average measurements of widely fluctuating components. We obtained higher than expected concentrations of CO and CO₂ in centerline measurements near the fuel source. An attempt is made ot explain these findings based on non-equal species diffusivity and local radiative extinction.The correlations obtained in this work form the basis for two closely related models: (1) for predicting mean species concentrations in turbulent flames by weighting laminar data with an assumed pdf of the mixture fraction, and (2) the chemical scaling of turbulent pool fires using Froude modeling principles. These applications are briefly discussed.     

Effect of flame stretch in downstream Interaction between premixed syngas-air flames

The effect of strain rate in downstream interactions between lean (rich) and lean (rich) premixed syngas flames with the fuel composition of 50% H₂ and 50% CO is numerically investigated by varying the strain rate in the range of 5∼500 s⁻¹. The flame stability maps for several strain rates are presented and main concerns are focused on the downstream interactions on the lean and rich extinction boundaries. The fuel composition of 50% H₂ and 50% CO with effective Lewis numbers larger than unity for both lean and rich extinction boundaries is chosen for grasping the important role of hydrogen with the deficient reactant Lewis numbers much smaller than unity. The results show that the lean extinction boundaries have the slanted shape, thereby leading to strong interactions; meanwhile the rich extinction boundaries at appropriately low strain rates are of square, indicating weak interactions. However, at highly strained interacting rich flames, the rich extinction boundaries show a slanted shape, thereby leading to strong interactions even for Lewis numbers much larger than unity. In such situations, thermal and chemical interactions are explained in detail. It is found that, in interacting flames, the excessive heat loss of the stronger flame partly to the weaker flame and mostly to the ambience is the mechanism of flame extinction.     

The ignition characteristics of the pre-chamber turbulent jet ignition of the hydrogen and methane based on different orifices

In this paper, the combustion characteristics of premixed CH₄-air and H₂-air mixtures with different excess air coefficients ignited by hot jet or jet flame are investigated experimentally in a constant volume combustion chamber (CVCC). The small volume pre-chambers with different orifices (2 or 3 mm in diameter) in the passive or active pre-chamber were selected. Both the high-speed Schlieren and OH1 chemiluminescence imaging are applied to visualize the turbulent jet ignition (TJI) process in the main chamber. Results show that the variation of orifice has diverse influences on the turbulent jet ignitions of methane and hydrogen. Smaller orifices will reduce the temperature of the jet due to the stronger stretch and throttling effect, including change of lean flammability limit, ignition delay, and re-ignition location. Furthermore, shock waves and pressure oscillations were captured in the experiments with hydrogen jets. The former is related to the jet velocity, while the latter is mainly affected by the mixture thermodynamic states in the main chamber. Furthermore, the re-ignition location is discussed. If the mixture reactivity and the jet energy are sufficiently high, the reaction will be initiated at the tip of the jet in a short time. On the contrary, a relatively long time is required to prepare the mixture during the entrainment when the reactivity is not high enough, and the corresponding re-ignition location will move towards the orifice exit owing to the temperature decline at the tip. Finally, the ignition mode transition of hydrogen jet in lean cases with a 2 mm orifice is explained.     

Combustion enhancement and inhibition of hydrogen-doped methane flame by HFC-227ea

Hydrogen enriched with compressed natural gas is an efficient and environment-friendly gaseous fuel. However, the safety issues of mixture and the method to control or weaken their combustion are highly concerned. To explore the inhibition effect of halogenated fire suppressants on the mixture, the effect of HFC-227ea on the laminar premixed methane/air flames, with different fractions of H₂, have been studied. Burning velocities have been measured with constant-volume combustion chamber and kinetically modelled a recently assembled kinetic mechanism. The fractions of H₂ influence the enhancement and inhibition effect of HFC-227ea, and it is less effective with the lean mixture. In stoichiometric condition, HFC-227ea showed good inhibition effect on the mixture flames. The HFC-227ea increased the burning velocities of CH₄-0% H₂-air and CH₄-10% H₂-air flames at leanest condition, whereas the increased burning velocity arising from HFC-227ea not occurred as the addition of H₂ above 20%. Experimental results coincided well with numerical results, however the agreement was poor for the leanest flames at low agent loading. Lastly, kinetic mechanism analysis was used to interpret the combustion enhancement and inhibition effect of hydrogen-doped methane flame by HFC-227ea.     

On the structure of the near field of oxy-fuel jet flames using Raman/Rayleigh laser diagnostics

An experimental study on turbulent non-premixed jet flames is presented with focus on CO₂-diluted oxy-fuel combustion using a coflow burner. Measurements of local temperatures and concentrations of the main species CO₂, O₂, CO, N₂, CH₄, H₂O and H₂ were achieved using the simultaneous line-imaged Raman/Rayleigh laser diagnostics setup at Sandia National Laboratories. Two series of flames burning mixtures of methane and hydrogen were investigated. In the first series, the hydrogen molar fraction in the fuel was varied from 37% to 55%, with a constant jet exit Reynolds number Re_Fuel of 15,000. In the second series the jet exit Reynolds number was varied from 12,000 to 18,000, while keeping 55% H₂ molar fraction in the fuel. Besides local temperatures and concentrations, the results revealed insights on the behaviour of localized extinction in the near-field. It was observed that the degree of extinction increased as the hydrogen content in fuel was decreased and as the jet Reynolds number was increased. Based on the distribution of the temperature, a fully burning probability index able to quantify the degree of extinction along the streamwise coordinate was defined and applied to the present flame measurements. A comparison of measured conditional mean of mass fractions and laminar flame calculations underlined the significant level of differential diffusion in the near-field that tended to decrease farther downstream. The results also showed high local CO levels induced by the high content of CO₂ in the oxidizer and flame products. A shift of maximum flame temperature was observed toward the rich side of the mixture fraction space, most likely as a consequence of reduced heat release in the presence of product dissociation. Main characteristics of laser Raman scattering measurements in CO₂-diluted oxy-fuel conditions compared to air-diluted conditions are also highlighted. Most data, including scalar fluctuations and conditional statistics are available upon request.     

Flame structure and kinetic studies of carbon dioxide-diluted dimethyl ether flames at reduced and elevated pressures

The flame structure and kinetics of dimethyl ether (DME) flames with and without CO₂ dilution at reduced and elevated pressures were studied experimentally and computationally. The species distributions of DME oxidation in low-pressure premixed flat flames were measured by using electron-ionization molecular-beam mass spectrometry (EI-MBMS) at an equivalence ratio of 1.63 and 50 mbar. High-pressure flame speeds of lean and rich DME flames with and without CO₂ dilution were measured in a nearly-constant-pressure vessel between about 1 and 20 bar. The experimental results were compared with predictions from four kinetic models: the first was published by Zhao et al. (2008) [9], the second developed by the Lawrence Livermore National Laboratory (LLNL) (Kaiser et al., 2000) [13], and the third has been made available to us as the Aramco mechanism (Metcalfe et al., 2013) [14]; as the fourth, we have used an updated model developed in this study. Good agreement was found between measurements and predictions from all four models for all major and most typical intermediate species with and without CO₂ addition in low-pressure flat flame experiments. However, none of the models was able to reliably predict high-pressure flame speeds. Although the updated model improved the prediction of flame speeds for lean mixtures, errors remained for rich conditions at elevated pressure, likely due to uncertainty in the rates of CH₃ + H(+M) = CH₄(+M) and the branching and termination reaction pair of CH₃ + HO₂ = CH₃O + OH and CH₃ + HO₂ = CH₄ + O₂. CO₂ addition considerably decreased the flame speed. Kinetic comparisons between inert and chemically active CO₂ in DME flames showed that CO₂ addition affects rich and lean DME flame kinetics differently. For lean flames, both the inert third-body effect and the kinetic effect of CO₂ reduce H-atom production. However, for rich flames, the inert third-body effect increases H-atom production via HCO(+M) = H + CO(+M) and suppression of the kinetic effect of CO₂ by shifting the equilibrium of CO + OH = CO₂ + H.     

Effects of multi-component diffusion and heat release on laminar diffusion flame liftoff

Numerical simulations were conducted of the liftoff and stabilization phenomena of laminar jet diffusion flames of inert-diluted C₃H₈ and CH₄ fuels. Both non-reacting and reacting jets were investigated, including multi-component diffusivities and heat release effects (buoyancy and gas expansion). The role of Schmidt number for non-reacting jets was investigated, with no conclusive Schmidt number criterion for liftoff previously arrived at in similarity solutions. The cold-flow simulation for He-diluted CH₄ fuel does not predict flame liftoff; however, adding heat release reaction lead to the prediction of liftoff, which is consistent with experimental observations. Including reaction was also found to improve liftoff height prediction for C₃H₈ flames, with the flame base location differing from that in the similarity solution - the intersection of the stoichiometric and iso-velocity (equal to 1-D flame speed) is not necessary for flame stabilization (and thus liftoff). Possible mechanisms other than that proposed for similarity solution may better help to explain the stabilization and liftoff phenomena.     

Experimental and modeling studies of C2H4/O2/Ar, C2H4/methylal/O2/Ar and C2H4/ethylal/O2/Ar rich flames and the effect of oxygenated additives

The addition of dimethoxymethane (DMM or methylal) and diethoxymethane (DEM or ethylal) to a rich ethylene/oxygen/argon flame has been investigated by measuring the depletion of soot precursors. Three rich premixed ethylene/oxygen/argon (with and without added methylal or ethylal) flat flames have been stabilized at low-pressure (50 mbar) on a Spalding–Botha type burner with the same equivalence ratio of 2.50. Identification and monitoring of signal intensity profiles of species within the flames have been carried out by using molecular beam mass spectrometry (M.B.M.S.). The replacement of some C₂H₄ by C₃H₈O₂ or C₅H₁₂O₂ is responsible for a decrease of the maximum mole fractions of the detected intermediate species. This phenomenon is noticeable for C₂–C₄ intermediates and becomes more effective for C₅–C₁₀ species, mainly when C₃H₈O₂ added.A new kinetic model has been elaborated and contains 546 reactions and 107 chemical species in order to simulate the three investigated flames: C₂H₄/O₂/Ar, C₂H₄/DMM/O₂/Ar and C₂H₄/DEM/O₂/Ar. The reaction mechanism well reproduces experimental mole fraction profiles of major and intermediate species, and underlines the effect of methylal and ethylal addition on species concentration profiles for these flames.     

Radiation intensity imaging measurements of methane and dimethyl ether turbulent nonpremixed and partially premixed jet flames

Quantitative time-dependent images of the infrared radiation intensity from methane and dimethyl ether (DME) turbulent nonpremixed and partially premixed jet flames are measured and discussed in this work. The fuel compositions (CH₄/H₂/N₂, C₂H₆O/H₂/N₂, CH₄/air, and C₂H₆O/air) and Reynolds numbers (15,200–46,250) for the flames were selected following the guidelines of the International Workshop on Measurement and Computation of Turbulent Nonpremixed Flames (TNF Workshop). The images of the radiation intensity are acquired using a calibrated high speed infrared camera and three band-pass filters. The band-pass filters enable measurements of radiation from water vapor and carbon dioxide over the entire flame length and beyond. The images reveal localized regions of high and low intensity characteristic of turbulent flames. The peak mean radiation intensity is approximately 15% larger for the DME nonpremixed flames and 30% larger for the DME partially premixed flames in comparison to the corresponding methane flames. The trends are explained by a combination of higher temperatures and longer stoichiometric flame lengths for the DME flames. The longer flame lengths are attributed to the higher density of the DME fuel mixtures based on existing flame length scaling relationships. The longer flame lengths result in larger volumes of high temperature gas and correspondingly higher path-integrated radiation intensities near and downstream of the stoichiometric flame length. The radiation intensity measurements acquired with the infrared camera agree with existing spectroscopy measurements demonstrating the quantitative nature of the present imaging technique. The images provide new benchmark data of turbulent nonpremixed and partially premixed jet flames. The images can be compared with results of large eddy simulations rendered in the form of quantitative images of the infrared radiation intensity. Such comparisons are expected to support the evaluation of models used in turbulent combustion and radiation simulations.