Extinction and flame bifurcations of stretched dimethyl ether premixed flames

Extinction limits and flame bifurcation of lean premixed dimethyl ether–air flames are numerically investigated using the counterflow flame with a reduced chemistry. Emphasis is paid to the combined effect of radiation and flame stretch on the extinction and flammability limits. A method based on the reaction front is presented to predict the Markstein length. The predicted positive Markstein length agrees well with the experimental data. The results show that flow stretch significantly reduces the flame speed and narrows the flammability limit of the stretched dimethyl ether–air flame. It is found that the combined effect of radiation and flow stretch results in a new flame bifurcation and multiple flame regimes. At an equivalence ratio slightly higher than the flammability limit of the planar flame, the distant flame regime appears at low stretch rates. With an increase in the equivalence ratio, in addition to the distant flame, a weak flame isola emerges at moderate stretch rates. With a further increase in the equivalence ratio, the distant flame and the weak flame branches merge together, resulting in the splitting of the weak flame branch into two weak flame branches, one at low stretch and the other at high stretch. Flame stability analysis demonstrates that the high stretch weak flame is also stable. Furthermore, a K-shaped flammability limit diagram showing various flame regimes and their extinction limits is obtained.     

Burning velocity correlation of methane/air turbulent premixed flames at high pressure and high temperature

Turbulent burning velocities for methane/air mixtures at pressures ranging from atmospheric pressure up to 1.0 MPa and mixture temperatures of 300 and 573 K were measured, which covers the typical operating conditions of premixed-type gas-turbine combustors. A bunsen-type flame stabilized in a high-pressure chamber was used, and OH-PLIF visualization was performed with the pressure and mixture temperature being kept constant. In addition to a burner with an outlet diameter of 20 mm for the high-pressure experiments, a large-scale burner with an outlet diameter of 60 mm was used at atmospheric pressure to extend the turbulence Reynolds number based on the Taylor microscale, R_λ, as a common parameter to compare the pressure and temperature effects. It was confirmed that R_λ over 100 could be attained and that u′/S_L could be extended even at atmospheric pressure. Based on the contours of the mean progress variable c = 0.1 determined using OH-PLIF images, turbulent burning velocity was measured. S_T/S_L was also found to be greatly affected by pressure for preheated mixtures at 573 K. The bending tendency of the S_T/S_L curves with u′/S_L was seen regardless of pressure and mixture temperature and the R_λ region where the bending occurs corresponded well to the region where the smallest scale of flame wrinkling measured as a fractal inner-cutoff approaches the characteristic flame instability scale and becomes almost constant. A power law of S_T/S_L with (P/P₀)(u′/S_L) was clearly seen when S_T was determined using c = 0.1 contours, and the exponent was close to 0.4, indicating agreement with the previous results using the mean flame cone method and the significant pressure effects on turbulent burning velocity.     

Measurement of unstable burning velocities of iso-octane–air mixtures at high pressure and the derivation of laminar burning velocities

A new technique is reported for measuring burning velocities at high pressures in the final stages of two inwardly propagating flame kernels in an explosion bomb. The flames were initiated at diametrically opposite spark electrodes, close to the wall, in quiescent mixtures. Measurements of pressure and flame kernel propagation speeds by high-speed photography showed the burning velocities to be elevated above the corresponding laminar burning velocities as a result of the developing flame instabilities. The enhancement increased with increase in pressure and decreased with increase in Markstein number. When the Markstein number was negative, instabilities could be appreciable, as could the enhancement. For the iso-octane–air mixtures investigated, where the mixtures had well-characterised Markstein numbers or critical Peclet numbers at the relevant pressures and temperatures, it was possible to explain the enhancement quantitatively by the spherical explosion flame instability theory of Bechtold and Matalon, provided the critical Peclet number was that observed experimentally, and allowance was made for the changing pressure. With this theoretical procedure, it was possible to derive values of laminar burning velocity from the measured values of burning velocity over a wide range of equivalence ratios, pressures, and temperatures. The values became less reliable at the higher temperatures and pressures as the data on Markstein and critical Peclet numbers became less certain. It was found that with iso-octane as the fuel the laminar burning velocity decreased during isentropic compression.     

Laminar and unstable burning velocities and Markstein lengths of hydrogen–air mixtures at engine-like conditions

Hydrogen offers an attractive alternative to conventional fuels for use in spark ignition engines. It can be burned over a very wide range of equivalence ratios and with considerable exhaust gas recirculation. These help to minimise pumping losses through throttleless operation and oxides of nitrogen (NO_x) production through reduced temperature. Full understanding of hydrogen-fuelled engine operation requires data on the laminar burning rate of hydrogen–air residuals under a wide range of conditions. However, such data are sparse. The present work addresses this need for experimental data. Spherically expanding H₂–air flames were measured at a range of temperatures, pressures, and equivalence ratios and with varying concentrations of residuals of combustion. Unstretched burning velocities, u_l, and Markstein lengths, L_b, were determined from stable flames. At the higher pressures, hydrodynamic and diffusional-thermal instabilities caused the flames to be cellular from inception and prohibited the derivation of values of u_l and L_b. The effect of pressure on the burning rate was demonstrated to have opposing trends when comparing stoichiometric and lean mixtures. The present measurements were compared with those available in the literature, and discrepancies were attributed to neglect, in some works, the effects of stretch and instabilities. From the present measurements, the effects of pressure, temperature, and residual gas concentration on burning velocity are quantified for use in a first step towards a general correlation.     

Dynamics and burning limits of near-limit hot,warm, and cool diffusion flames of dimethyl ether at elevated pressures

The near-limit diffusion flame regimes and extinction limits of dimethyl ether at elevated pressures and temperatures are examined numerically in the counterflow geometry with and without radiation at different oxygen concentrations. It is found that there are three different flame regimes—hot flame, warm flame, and cool flame—which exist, respectively, at high, intermediate, and low temperatures. Furthermore, they are governed by three distinct chain-branching reaction pathways. The results demonstrate that the warm flame has a double reaction zone structure and plays a critical role in the transition between cool and hot flames. It is also shown that the cool flame can be formed in several different ways: by either radiative extinction or stretch extinction of a hot flame or by stretch extinction of a warm flame. A warm flame can also be formed by radiative extinction of a hot flame or ignition of a cool flame. A general €-shaped flammability diagram showing the burning limits of all three flame regimes at different oxygen mole fractions is obtained. The results show that thermal radiation, reactant concentration, temperature, and pressure all have significant impacts on the flammable regions of the three flame regimes. Increases in oxidizer temperature, oxygen concentration, and pressure shift the cool flame regime to higher stretch rates and cause the warm flame to have two extinction limits. At elevated temperatures, it is found that there is a direct transition between the hot flame and warm flame at low stretch rates. The results also show that, unlike the hot flame, the cool flame structure cannot be scaled by using pressure-weighted stretch rates due to the its significant reactant leakage and strong dependence of reactivity on pressure. The present results advance the understanding of near-limit flame dynamics and provide guidance for experimental observation of different flame regimes.     

Photoionization mass spectrometry and modeling studies of the chemistry of fuel-rich dimethyl ether flames

Reaction paths are identified for dimethyl ether (DME) combustion using modeling of new data from fuel-rich DME flat flames. A molecular-beam flame-sampling photoionization mass spectrometer, employing VUV synchrotron radiation, is applied to the measurement of mole fractions for 21 flame species in low-pressure premixed fuel-rich (Φ = 1.2, 1.68) DME/oxygen/argon flat flames. This approach is capable of resolving and identifying isomers and other flame species of near equal masses with ionization thresholds that differ by as little as 0.1 eV. The measurements agree well with flame modeling predictions, using a recently revised high-temperature DME kinetic mechanism, which identify reaction paths quite analogous to alkane combustion. They further reveal the presence of ethyl methyl ether, a molecule previously unobserved in flames and not included in present flame models.     

Laminar burning velocities of 2-methyltetrahydrofuran at elevated pressures

The laminar burning velocities (LBVs) and cellular instability of 2-methyltetrahydrofuran (2-MTHF) were investigated at the unburned temperature of 423 K and pressures from 1 to 10 atm in a cylindrical constant-volume vessel. The LBVs of 2-MTHF/air flame exhibit a notably dropping with increasing pressure. The cellular instability analysis indicates that the critical flame radius of 2-MTHF/air mixture monotonically increases with increasing pressure and the flame surface suffers more badly cellularity under higher pressures. The critical flame radius exhibits non-monotonic variation versus ? and the most unstable flames appear at ? ≈ 1.3. It is observed that the measured Markstein length of 2-MTHF/air mixture decreases with increasing ? and P_u, leading to an earlier formation of wrinkling and cracks with respect to preferential-diffusional instability. Further investigation found that by using a mixture of 14.2% oxygen with 85.8% helium in place of air as bath gas at 10 atm can effectively suppress the cellular instability. Two recently developed models were used to simulate the experimental results and explore the chemical kinetic effects on LBV. Reaction path analysis reveals that the most consumption of 2-MTHF/air at stoichiometric conditions is through the abstraction of H-atom to form radical C₅H₉O-5. While the competitiveness of decomposition by CC scission yielding CH₃ and tetrahydrofuran radical is relatively weak. Sensitivity analysis illustrates that small-species reactions show a controlling effect on LBV. The increasing pressure leads to an evident increase in the sensitivity coefficient of the recombination reaction H + O₂ (+M)=HO₂ (+M). The reduction of H atom concentration will cause competition to the initiation reaction H + O₂ = O+OH. This could lower the overall oxidation rate and reduce the burning velocity.     

Measurements of three-dimensional mean flame surface area ratio in turbulent premixed Bunsen flames

A data processing scheme with particular emphasis on proper flame contour smoothing is developed and applied to measure the three-dimensional mean flame surface area ratio in turbulent premixed flames. The scheme is based on the two-sheet imaging technique such that the mean flame surface area ratio is an average within a window covering a finite section of the turbulent flame brush. This is in contrast to the crossed-plane tomograph technique which applies only to a line. Two sets of Bunsen flames have been investigated in this work with the turbulent Reynolds number up to 4000 and the Damköhler number ranging from less than unity to close to 10. The results show that three-dimensional effects are substantial. The measured three-dimensional mean flame surface area ratio correlates well with a formula similar to the Zimont model for turbulent burning velocity but with different model constants. Also, the mean flame surface area ratio displays a weak dependency on turbulence intensity but a strong positive dependency on the turbulence integral length scale.     

Experimental investigation of partially premixed,highly-diluted dimethyl ether flames at low temperatures

Atmospheric-pressure highly-diluted laminar dimethyl ether (DME)–oxygen flames with temperatures below 1500 K were stabilized for the first time on a specially designed burner which allows preheating of the gas streams of fuel and oxidizer. With regard to the partially premixed structure of these flames which contain up to 90% argon in the unburnt gases, molecular-beam mass spectrometry (MBMS) with electron ionization (EI) was used to investigate chemical species profiles of reactants, intermediates, and products at a series of lateral positions and as function of distance from the burner. The flame structure reveals a near one-dimensional behavior at the flame front and beyond, towards the burnt gas. In a systematic approach, combustion parameters including stoichiometry, dilution, and gas preheating temperature were varied. The partial premixing effects upon the flame were revealed by comparing the distribution of flame species in a full two-dimensional concentration field above the burner, which is a starting point to model such flames in further studies. Formaldehyde and the methyl radical as two prominent species in the combustion process of DME were used to discuss characteristics of both high- and low-temperature kinetics.     

Role of hydroxyl production and heat release in the two-zone fuel-rich adiabatic dimethyl ether/air flames at atmospheric pressure

Fuel-rich laminar adiabatic flames of premixed dimethyl ether/air mixtures at a high initial temperature and atmospheric pressure have been studied by numerical simulation and sensitivity analysis. These flames, having two heat release zones, are of great interest as an unusual and little-studied subject. We have investigated the chemical processes occurring in the two zones and analysed the mechanism of heat release in the flame. It has been found that the key reactions that have a significant influence on the flame speed are those involving dimethyl ether and the products of its incomplete oxidation. Calculation of the heat release rate confirms the presence of two heat release zones in the flame. A comparison of the reactions making a major contribution to the heat release with those significantly affecting the flame speed indicates that the main factor determining the flame speed is the formation of hydroxyls, rather than heat release. Analysis of the flame speed sensitivity shows that in the case of a two-zone structure of the flame, its speed is mainly determined by the reactions taking place in the low-temperature zone. That is, the cool zone with a higher temperature gradient is the leading one.     

An investigation of flame zones and burning velocities of laminar unconfined methane-oxygen premixed flames

Numerical and experimental investigations of unconfined methane-oxygen laminar premixed flames are presented. In a lab-scale burner, premixed flame experiments have been conducted using pure methane and pure oxygen mixtures having different equivalence ratios. Digital photographs of the flames have been captured and the radial temperature profiles at different axial locations have been measured using a thermocouple. Numerical simulations have been carried out with a C2 chemical mechanism having 25 species and 121 reactions and with an optically thin radiation sub-model. The numerical results are validated against the experimental and numerical results for methane-air premixed flames reported in literature. Further, the numerical results are validated against the results from the present methane-oxygen flame experiments. Visible regions in digital flame photographs have been compared with OH isopleths predicted by the numerical model. Parametric studies have been carried out for a range of equivalence ratios, varying from 0.24 to 1.55. The contours of OH, temperature and mass fractions of product species such as CO, CO₂ and H₂O, are presented and discussed for various cases. By using the net methane consumption rate, an estimate of the laminar flame speed has been obtained as a function of equivalence ratio.     

Assessment of counterflow to measure laminar burning velocities using direct numerical simulations

The laminar burning velocity is a fundamental property that is extensively used in the study and modelling of premixed combustion processes. A counterflow flame configuration is commonly used to measure this quantity for different combustion systems. In this procedure, the burning velocities are typically measured at various low stretch conditions and the unstretched burning velocity is extrapolated from these measurements. This extrapolation is done assuming a theoretically one-dimensional system along the centre-line. We analyse the validity of this assumption by performing DNS studies with finite rate chemistry of the experimental counterflow configuration. The extrapolation process using one-dimensional computations is performed on the DNS data and the extrapolated value is compared to the computed laminar burning velocity for the chemical mechanism used. We show that the assumption works well if the nozzle exit velocity has a nearly top-hat profile. For non-uniform velocity profiles, it is shown that the temperature curvature at the centre-line becomes important. This effect cannot be captured by the one-dimensional formulation. Thus, experimental studies measuring laminar burning velocity need to ensure that the nozzle velocity profile is very close to uniform. The extrapolation to zero stretch using 1D counterflow simulations can be performed in different ways. Based on the results obtained in this paper, a simple and accurate extrapolation method is proposed.     

The effects of dimethyl ether and ethanol on benzene and soot formation in ethylene nonpremixed flames

Soot volume fractions, C1-C12 hydrocarbon concentrations, and gas temperature were measured in ethylene/air nonpremixed flames with up to 10% dimethyl ether (CH₃OCH₃) or ethanol (CH₃CH₂OH) added to the fuel. The measurement techniques were laser-induced incandescence, photoionization mass spectroscopy, and thermocouples. Oxygenated hydrocarbons have been proposed as soot-reducing fuel additives, and nonpremixed flames are good laboratory-scale models of the fuel-rich reaction zones where soot forms in many full-scale combustion devices. However, addition of both dimethyl ether and ethanol increased the maximum soot volume fractions in the ethylene flames studied here, even though ethylene is a much sootier fuel than either oxygenate. Furthermore, dimethyl ether produced a larger increase in soot even though neat dimethyl ether flames produce less soot than neat ethanol flames. The detailed species measurements suggest that the oxygenates increase soot concentrations because they decompose to methyl radical, which promotes the formation of propargyl radical (C₃H₃) through C1 + C2 addition reactions and consequently the formation of benzene through propargyl self-reaction. Dimethyl ether has a stronger effect than ethanol because it decomposes more completely to methyl radical. Ethylene does not decompose to methyl, so its flames are particularly sensitive to this mechanism; the alkane-based fuels used in most practical fuels do decompose to methyl radical, so the mechanism will be much less important for practical devices.     

Effects of Lewis number,density ratio and gravity on burning velocity and conditional statistics in stagnating turbulent premixed flames

DNS is performed to analyse the effects of Lewis number (Le), density ratio and gravity in stagnating turbulent premixed flames. The results show good agreement with those of Lee and Huh (Combustion and Flame, Vol. 159, 2012, pp. 1576–1591) with respect to the turbulent burning velocity, S_T, in terms of turbulent diffusivity, flamelet thickness, mean curvature and displacement speed at the leading edge. In all four stagnating flames studied, a mean tangential strain rate resulting in a mean flamelet thickness smaller than the unstretched laminar flame thickness leads to an increase in S_T. A flame cusp of positive curvature involves a superadiabatic burned gas temperature due to diffusive–thermal instability for an Le less than unity. Wrinkling tends to be suppressed at a larger density ratio, not enhanced by hydrodynamic instability, in the stagnating flow configuration. Turbulence is produced, resulting in highly anisotropic turbulence with heavier unburned gas accelerating through a flame brush by Rayleigh–Taylor instability. Results are also provided on brush thickness, flame surface density and conditional velocities in burned and unburned gas and on flame surfaces to represent the internal brush structures for all four test flames.     

A liquid film or droplet of miscible binary fuel burning on a heated surface at elevated pressures

An experimental study has been made of the combustion and evaporation of miscible binary fuels on a heated surface at elevated pressures. Photographic observation was made for elucidating how the binary fuel droplets or films behaved on the heated surface and for obtaining the characteristic lifetime as a function of the surface temperature, the ambient pressure and the initial concentration of highly volatile component of the fuels. The binary fuels tested were mixtures of n-hexane and n-decane. The results showed that the lifetime curve both for the combustion and for the evaporation of n-hexane/n-decane on the heated surface had two peaks at lower pressures, the first peak in the film-type evaporation region and the second peak in the spheroid-type evaporation region. The preferential evaporation of the highly volatile component was responsible for the appearance of the first peak in the film-type evaporation region. The second peak corresponded to Leidenfrost point. The increase in the ambient pressure caused a shift of the lifetime curve toward higher surface temperature and a decrease in the lifetime at two peaks. The lifetime curve decreased monotonically with increasing the surface temperature and the spheroid-type evaporation was not observed at higher pressures. The increase in the initial concentration of highly volatile component caused a shift of the first peak toward higher surface temperature, a decrease in its lifetime and a slight shift of the second peak toward lower surface temperature. The combustion caused a remarkable decrease in the lifetimes around the first peak and in the spheroid-type evaporation region and a slight increase in the surface temperature at the first peak, while the surface temperature at the second peak remained almost unchanged.     

A flamelet analysis of the burning velocity of premixed turbulent expanding flames

Direct numerical simulation is a very powerful tool to evaluate the validity of new models and theories for turbulent combustion. In this paper, direct numerical simulations of spherically expanding premixed turbulent flames in the corrugated flamelet regime are performed. The flamelet-generated manifold method is used to deal with detailed reaction kinetics. The numerical method is validated for both laminar and turbulent expanding flames. The computational results are analyzed by using an extended flame stretch theory. It is investigated whether this theory is able to describe the influence of flame stretch and curvature on the local burning velocity of the flame. If the full profiles of flame stretch and curvature through the flame front are included in the theory, the local mass burning rate is predicted accurately. The influence of several approximations, which are used in other existing theories, is studied. When flame stretch is assumed to be constant through the flame front or when curvature of the flame front is neglected, the theory fails to predict the local mass burning rate.     

Ultrasonic velocity of binary systems at elevated pressures

Various empirical theories of ultrasonic velocity have been applied to three binary liquid mixtures, under pressures up to 200 MPa and their validity have been tested. A pressure dependent study of ultrasonic velocities has been made at 303.15 K. The agreement between theory and experiment is found to be quite satisfactory.     

Effect of Soret diffusion on lean hydrogen/air flames at normal and elevated pressure and temperature

The influence of Soret diffusion on lean premixed flames propagating in hydrogen/air mixtures is numerically investigated with a detailed chemical and transport models at normal and elevated pressure and temperature. The Soret diffusion influence on the one-dimensional (1D) flame mass burning rate and two-dimensional (2D) flame propagating characteristics is analysed, revealing a strong dependency on flame stretch rate, pressure and temperature. For 1D flames, at normal pressure and temperature, with an increase of Karlovitz number from 0 to 0.4, the mass burning rate is first reduced and then enhanced by Soret diffusion of H₂ while it is reduced by Soret diffusion of H. The influence of Soret diffusion of H₂ is enhanced by pressure and reduced by temperature. On the contrary, the influence of Soret diffusion of H is reduced by pressure and enhanced by temperature. For 2D flames, at normal pressure and temperature, during the early phase of flame evolution, flames with Soret diffusion display more curved flame cells. Pressure enhances this effect, while temperature reduces it. The influence of Soret diffusion of H₂ on the global consumption speed is enhanced at elevated pressure. The influence of Soret diffusion of H on the global consumption speed is enhanced at elevated temperature. The flame evolution is more affected by Soret diffusion in the early phase of propagation than in the long run due to the local enrichment of H₂ caused by flame curvature effects. The present study provides new insights into the Soret diffusion effect on the characteristics of lean hydrogen/air flames at conditions that are relevant to practical applications, e.g. gas engines and turbines.     

Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation

Ignition delay times and OH concentration time-histories were measured in DME/O₂/Ar mixtures behind reflected shock waves. Initial reflected shock conditions covered temperatures (T₅) from 1175 to 1900 K, pressures (P₅) from 1.6 to 6.6 bar, and equivalence ratios (?) from 0.5 to 3.0. Ignition delay times were measured by collecting OH^∗ emission near 307 nm, while OH time-histories were measured using laser absorption of the R₁(5) line of the A-X(0,0) transition at 306.7 nm. The ignition delay times extended the available experimental database of DME to a greater range of equivalence ratios and pressures. Measured ignition delay times were compared to simulations based on DME oxidation mechanisms by Fischer et al. [7] and Zhao et al. [9]. Both mechanisms predict the magnitude of ignition delay times well. OH time-histories were also compared to simulations based on both mechanisms. Despite predicting ignition delay times well, neither mechanism agrees with the measured OH time-histories. OH Sensitivity analysis was applied and the reactions DME ↔ CH₃O + CH₃ and H + O₂ ↔ OH + O were found to be most important. Previous measurements of DME ↔ CH₃O + CH₃ are not available above 1220 K, so the rate was directly measured in this work using the OH diagnostic. The rate expression k[1/s] =  1.61 × 10⁷⁹T^−18.4 exp(−58600/T), valid at pressures near 1.5 bar, was inferred based on previous pyrolysis measurements and the current study. This rate accurately describes a broad range of experimental work at temperatures from 680 to 1750 K, but is most accurate near the temperature range of the study, 1350-1750 K. When this rate is used in both the Fischer et al. and Zhao et al. mechanisms, agreement between measured OH and the model predictions is significantly improved at all temperatures.