Flame Speed Data Research Articles

A detailed chemical kinetic model for high temperature ethanol oxidation

A detailed chemical kinetic model for ethanol oxidation has been developed and validated against a variety of experimental data sets. Laminar flame speed data (obtained from a constant volume bomb and counterflow twin-flame), ignition delay data behind a reflected shock wave, and ethanol oxidation product profiles from a jet-stirred and turbulent flow reactor were used in this computational study. Good agreement was found in modeling of the data sets obtained from the five different experimental systems. The computational results show that high temperature ethanol oxidation exhibits strong sensitivity to the fall-off kinetics of ethanol decomposition, branching ratio selection for C2H5OH + OH Products, and reactions involving the hydroperoxyl (HO2) radical. The multichanneled ethanol decomposition process is analyzed by RRKM/Master Equation theory, and the results are compared with those obtained from earlier studies. The ten-parameter Troe form is used to define the C2H5OH(+M) CH3 + CH2OH(+M) rate expression as k∞ = 5.94E23 T−1.68 exp(−45880 K/T) (s−1) ko = 2.88E85 T−18.9 exp(−55317 K/T) (cm3/mol/sec) Fcent = 0.5 exp(−T/200 K) + 0.5 exp(−T/890 K) + exp(−4600 K/T) and the C2H5OH(+M) C2H4 + H2O(+M) rate expression as k∞ = 2.79E13 T0.09 exp(−33284 K/T) (s−1) ko = 2.57E83 T−18.85 exp(−43509 K/T) (cm3/mol/sec) F cent = 0.3 exp(−T/350 K) + 0.7 exp(−T/800 K) + exp(−3800 K/T) with an applied energy transfer per collision value of = 500 cm−1. An empirical branching ratio estimation procedure is presented which determines the temperature dependent branching ratios of the three distinct sites of hydrogen abstraction from ethanol. The calculated branching ratios for C2H5OH + OH, C2H5OH + O, C2H5OH + H, and C2H5OH + CH3 are compared to experimental data. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 183–220, 1999

An experimental and kinetic modeling study of propyne oxidation

The oxidation of propyne was studied experimentally in an atmospheric-pressure flow reactor and in laminar premixed flames. Species profiles were obtained for propyne oxidation experiments conducted in the Princeton turbulent flow reactor (PTFR) in the intermediate- to high-temperature range (∼1170 K) for lean, stoichiometric, and rich conditions. Laminar flame speeds of propyne/(18% O 2 in N 2 ) mixtures were determined, over an extensive range of equivalence ratios, at room temperature and atmospheric pressure, using the counterflow twin flame configuration. A detailed chemical kinetic model of high-temperature propyne oxidation, consisting of 437 reactions and 69 species, was developed. It is shown that this kinetic model predicts reasonably well the flow-reactor and flame-speed data determined in this study and the shock tube ignition data available in the literature. The remaining uncertainties in the reaction kinetics of propyne oxidation are discussed.

Ethane oxidation at elevated pressures in the intermediate temperature regime: Experiments and modeling

Ethane oxidation has been experimentally studied in the intermediate temperature regime under lean conditions using a flow reactor. Species profiles have been obtained for H 2, CO, CO 2, CH 2O, CH 4, C 2H 4, C 2H 6, C 2H 4O, and CH 3CHO at pressures of 3, 6, and 10 atm for temperatures ranging from 915 to 966 K using a constant equivalence ratio of ∼ 0.2 (in air). To model this data a detailed chemical kinetic model for ethane oxidation was developed. An optimized reaction mechanism, originally developed to model natural gas combustion, was expanded to include reactions pertinent to the lower temperature, elevated pressure conditions encountered in the flow reactor. The expanded mechanism consists of 277 elementary reactions and contains 47 species. By adjusting the rate coefficients of two key reactions the model was brought into agreement with experiment at 6 atm; however, the model indicates a larger pressure sensitivity than was measured experimentally. Results indicate that HȮ 2 is of primary importance in the regime studied; controlling the formation of many of the observed intermediates including the aldehydes and ethylene oxide. The results also point to the importance of continued investigation of the reactions of HȮ 2 with C 2H 6, Ċ 2H 5, and C 2H 4 to further the understanding of ethane oxidation in the intermediate temperature regime. The expanded mechanism has also been tested against shock-tube ignition delay and laminar flame speed data and was found to be in good agreement with both the original GRI-Mech and the experimental data for both methane and ethane.

A Generalized Intermediate Approach to Combustion Chemistry: Application to Methane Ignition and Flame Propagation

A chemical reaction mechanism for methane combustion was developed by a method different in principle from previous efforts that start from detailed chemistry and use steady-state and other approximations. It describes, accurately and with the same parameter set, such different phenomena as flames and high and intermediate temperature ignition. The approach used for chemistry simplification resolves the main features of hydrocarbon oxidation into steps, each with simple kinetic sense, describing thermal decomposition of fuel, true and degenerate chain branching, propagation, termination, and the formation of final products. Rate coefficient expressions for elementary reactions were taken from the literature, while those of generalized reactions were optimized using an automated regression procedure against ignition and flame speed data. The predicted composition and pressure dependences for ignition and combustion of methane and methane-ethane mixtures are in good agreement with experiment.

The oxidation of methane at elevated pressures: Experiments and modeling

A detailed chemical kinetic model has been developed for methane oxidation which is applicable over a wide range of operating conditions. A reaction mechanism, originally developed for high-temperature methane oxidation, was expanded and extended to include reactions pertinent to the lower temperature, elevated pressure conditions encountered in the flow reactor experiments performed in the study. The resulting 207-reaction, 40-species mechanism is capable of reproducing the experimental species concentrations for each of the cases studied. The concentration profiles of reactant, intermediate, and product species, including CH 4, CH 2O, CH 3OH, H 2, C 2H 6, C 2H 4, CO, and CO 2, were obtained in the High Pressure Optically Accessible Flow Reactor (HiPOAcFR) facility for temperatures ranging from 930 to 1000 K and pressures of 6 and 10 atm. Based on the model, no appreciable change in reaction pathway was observed over the pressure and temperature range studied, with HO 2 providing the major route for CH 3 oxidation. CH 2O was found to be a vital intermediate for all of the CH 4 oxidation paths. In addition, inclusion of trace amounts of CH 2O measured at the initial sampling location into the model initial conditions greatly reduced the predicted time to onset of fuel disappearance and enhanced the model agreement. This result is consistent with past engine studies which have found that CH 2O is a significant pro-knock additive when added to a methane base fuel. The expanded reaction mechanism was also tested against shock-tube ignition delay and laminar flame speed data and was found to be in good agreement with the relevant experimental data.

US 4,966,931 Fire retardant polypropylene

After the restriction on several ozone-depleting compounds, including the high efficiency fire suppressant Halon 1301(CF3Br), several alternatives have been proposed. Among them, HFC-227-(C3HF7) and HFC-125 (C2HF5) represent two of the most-used fire suppressants in the industry because of their environmentally favorable properties. Due to their increasing demand, it is very important to understand their combustion properties to optimize their applications and to prevent undesirable events. To this end, the present work examined the effect of C2HF5 and C3HF7 on CH4 and C3H8 laminar flame speeds and ignition delay times. The experimental techniques included freely propagating flames to obtain un-stretched, laminar flame speed and a shock tube for the ignition delay times in fuel-O2-suppressant mixtures highly diluted in Ar (∼98%) using OH* emission near 307 nm. The laminar flame speed experiments were performed at 1 atm over a range of equivalence ratios from 0.7 to 1.3, and the shock-tube tests were done near 1.5 atm over a 1350–2200 K temperature range. A chemical kinetics mechanism was assembled using a HFC set of reactions together with a recently updated C0-C5 hydrocarbon mechanism and OH* chemistry. The results suggest that the tested agents may not be good alternatives as ignition preventers, although they can reduce the laminar flame speed, as a proof that they can be used as fire extinguishers. Comparisons between modeled and experimental data show that the HFC sub-mechanism behaves well, however it can be improved. Surprisingly, a sensitivity analysis shows that many of the top reactions containing fluorinated compounds are classified as ignition-promoters, especially for the experiments with CH4. This work presents some of the first fundamental ignition delay time and flame speed data for HFC-227 and -125, and the results can be used as the basis for future HFC-based chemical kinetics mechanism improvements and to further understand their impact on the combustion process.

An experimental and theoretical study of porous radiant burner performance

We have studied experimentally the stability and heat transfer characteristics of lean premixed, methane-air flames embedded in a porous layer. The work is directly relevant to understanding the performance and operating behavior of porous radiant burners (PRB). Flame speed and radiant output data were obtained for different stoichiometries and flame locations in porous ceramic foam. The results indicate that stable combustion at elevated flame speeds can be maintained in two different spatial domains: one spanning the upstream half of the porous region and the other in a narrow region near the exit plane. The heat release and radiant output are also found to increase as the flame is shifted toward the middle of the porous layer. A one-dimensional laminar premixed flame model incorporating a radiatively participating inert porous medium was used to describe the test conditions. Calculations using a one-step reaction confirmed the observation of two stable flame regions. The predicted flame speeds and radiant output agree favorably with the experimental trends.

A new spectral method for calculation of the time-varying area of a laminar flame in homogeneous turbulence

A novel approach to flame area simulation is presented, based on the physical processes of chaotic mixing. The “wrinkled” flame front is represented by a length-scale distribution of distortions of the surface, bounded by the chamber or flame kernel size, and the laminar flame thickness. The turbulent fluid in which the flame propagates is modelled as a length scale spectrum of vortices, with the contribution of each scale to the total turbulent kinetic energy being obtained from a prescribed energy spectrum function. Vortex-flame interaction is modelled by: (a) flame area generation by the vortices at their length scale, (b) stretching and folding by generation of flame area at larger scales. The effect of flame propagation on the radius of curvature of the flame constitutes the third mechanism for altering the “wrinkle” spectrum. The resulting integro-differential equation is solved numerically by a time-marching procedure to yield the evolution of surface area and asymptotic (equilibrium) limit for given conditions of turbulence and prescribed laminar flame speed. Good agreement with experimental results is obtained for the fractal dimension of passive scalar and flame surfaces, and also with turbulent flame speed data over a wide range of conditions. The model also makes predictions of flame-speed trends in the near-wall region in agreement with observed behaviour, and also shows that there is a weak dependence of turbulent flame speed on S l additional to that on u′ /S l used in the Bradley correlations. The effects of turbulent flame stretch and vortices smaller than the laminar flame thickness are not modelled, leading to reduced agreement in the high u′ /S l limit. The physical nature of the modelling means that these, and other phenomena such as holes in the flame front caused by rapid local stretching, can be easily included in future work.

What Causes Slower Flame Propagation in the Lean-Combustion Engine?

Previous engine data suggest that slower flame propagation in lean-burn engines could be due to slower flame expansion velocity at lean conditions than at stoichiometric combustion. Two classes of model, a quasi-dimensional engine-simulation program and a multidimensional engine-flow and combustion code, were used to study this effect in detail and to assess the capabilities of the models to resolve combustion details. The computed flame-speed data from each program differed somewhat in magnitude, but the predicted trends at various equivalence ratios were quite similar. The trends include: (1) The peak in-cylinder burned-gas temperature decreases by about 300 K as the equivalence ratio is decreased from 0.98 to 0.70. (2) Both the laminar flame speed and the flame-propagation speed, the latter computed from the time derivative of flame radius, decrease with decreasing equivalence ratio. (3) The turbulent burning speed, defined as the ratio of specific fuel-burning rate to the product of the flame frontal area and unburned-mixture density, is relatively insensitive to equivalence-ratio variations at the same flame-radius position. The previous experimental finding that the reduction in flame-propagation speed with decreasing equivalence ratio is caused mainly by the lower thermal-expansion speed, calculated by subtracting the turbulent burning speed from the flame-propagation speed, was confirmed. This is a consequence of lower burned-gas temperature for the lean case. Regarding the reliability of the models to resolve the combustion details, limitations of the models are identified and discussed in detail.

Concentration Effects on Flame Acceleration by Obstacles in Large-Scale Methane-Air and Propane-Air Vented Explosions

Abstract An experimental study of flame and pressure development have been performed in a large scale 50 m3 obstructed tube of diameter 2.5 m and length 10 m and with one end closed and one open to the atmosphere. Homogeneous clouds of variable concentrations of cither methane/air or propane/air mixtures have been accelerated in the tube by five regularly spaced obstacles which blocked off 30% of the tube cross-sectional area. Two types of ignition sources were used to initiate the explosions. The planar ignition mode had 19 weak match heads that covered the whole of the tube cross-section at the closed end of the tube. The point source ignition mode used two weak match heads located at the centre-line, also at the closed end of the tube. In addition to the conventional pressure and flame-arrival probes, some of the tests also included hot-wire probes for flow and turbulence measurements. Both the pressure and flame speed data showed a strong dependence with fuel/air concentrations. Maximum pressures and ...

Temperature and Velocity Measurements in Premixed Turbulent Flames

Turbulent flame speed data for premixed flames of methane-air, propane-air and ethylene-air mixtures stabilized in grid turbulence are reported and discussed. It is shown that turbulence effects on flame speed cannot be fully correlated by the turbulence length scale and r.m.s. velocity in the cold flow. Rather there appear to be significant flame-flow-turbulence interactions affecting both turbulence level in the reaction zone and measured flame speeds. Results of detailed velocity measurements, including autocorrelations, by laser velocimetry are used to elucidate the nature of these interactions. It is concluded that flame speed experiments must be designed and conducted to provide sufficient information (e.g., boundary conditions) to allow for reconstruction of the flow field and these interactions by modelers if the data are to be of value in turbulent combustion model development and evaluation.

Turbulence Effects on Flame Speed and Flame Structure

Turbulence effects on methane-air flames stabilized in grid turbulence were investigated through measurements of flame speed and mean and fluctuating flame temperature profiles. Published turbulent flame speed correlations were able to correlate the experimental flame speed data but were contradictory in indicating flame structure and combustion mechanisms. A simple, one-dimensional, wrinkled laminar flame model was used to predict characteristic flame temperature fluctuation levels. Comparison of these predictions with measured temperature fluctuations indicated that the majority of the flames studied were wrinkled laminar flames. However, a wrinkled laminar flame structure was inappropriate for the most intensely turbulent flames examined.

Methane flame extinguishment with layered halon or carbon dioxide

In quenching tests of downward propagation in stoichiometric methane-air mixtures based on flammability limits and flame speed data, Halon 1301 was about five times more effective, on a volumetric basis, than carbon dioxide as an extinguishant.

The Influence of Turbulence on Flame Propagation Rates

A technique for the experimental study of laminar and turbulent flame propagation is presented. The method consists of igniting, by a single spark discharge, a smal l zone in a free jet of fuel-air mixture. This approximately spherically shaped burning zone, called a flame globule , was shadowgraphed, us ing a high-speed mot ion picture camera, as it passed downstream with the mixture. Measurements taken from the photographs resulted in the flame speed data. 1 The experimental technique appears to have considerable value for the study of the propagation of laminar and turbulent flames. The data are quite consistent and reproducible, and are adaptable to the study of the transient phase of the turbulence effect. 2 The measured laminar flame speeds are in good agreem e n t with those obtained by other investigators. 3 An encouraging quantitat ive correlation between an empirical equation and the experimental data is shown, but the relation to the scale of the turbulence was satisfactory for only one of the two screens used. The correlation was unsuccessful for very lean and rich mixtures . This was due to the fact tha t the rate of turbulent flame growth, for these cases, was lower t h a n the laminar values.