CH4 Flames Research Articles

Heat Transfer in Industrial Combustion

7R51. Heat Transfer in Industrial Combustion. - CE Baukal Jr (R&D Test Center, John Zink Co LLC, Tulsa OK). CRC Press, Boca Raton FL. 2000. 545 pp. ISBN 0-8493-1699-5. $89.95Reviewed by Lea-Der Chen (Dept of Mech Eng, Univ of Iowa, 2129EB, Iowa City IA 52242).This book, as stated in Preface, is intended to “fill a gap in the literature for books on heat transfer in industrial combustion, written for the practicing engineers.” This reviewer would like to congratulate the author for a well-written and a comprehensive book on industrial combustion. This book covers the three modes of heat transfer and their relevance to industrial combustion. It also covers the modeling strategies, including the thermodynamic equilibrium approximation, for example, using the robust NASA equilibrium code; correlations for heat calculation; and CFD-base multidimensional modeling. In addition, this book gives a comprehensive introduction of the experimental techniques that are important to combustion measurements. With the comprehensive review of the literature and the clear presentation of fundamentals of mass, momentum, and heat transport in combustion flames, this book should find its place as a valuable reference to practitioners in the field of combustion. As a university professor, this reviewer also finds that this book would be very useful when teaching my combustion and propulsion class (a senior-level technical elective or first-year graduate-level course). Examples from the book such as submerged burners and flame impingements will be used to illustrate the breadth and complexities of the field. There are 12 chapters, five appendices, and two indices in the book. These chapters are, in numerical order: introduction, some fundamentals of combustion, heat transfer modes, heat sources and sinks, computer modeling, experimental techniques, flame impingement, heat transfer from burners, heat transfer in furnaces, lower temperature applications, higher temperature applications, and advanced combustion systems. The Appendices are reference sources for further information on: common conversions; methods of expressing mixture ratios for CH4, C3H8, and H2; properties for CH4, C3H8, and H2 flames; fluid dynamics equations; and material properties. The book concludes with an author index and a detailed subject index. Each chapter begins with a summary of a literature review and is followed by in-depth discussion of the chapter’s subject matter. The book gives a clear link of fundamental principles to industrial combustion. At the same time, it also gives readers a working level of calculation that can be used to estimate heat transfer performance. Should the author plan a second edition of the book, perhaps it could be expanded into a handbook for heat transfer calculation for industrial combustion. In doing so, this reviewer also would like to suggest that Chapter 5 (Computer Modeling) be expanded to give more examples that can further illustrate the utilities and limitations of commercial CFD codes for heat transfer calculation of the industrial combustion examples. This reviewer recommends Heat Transfer in Industrial Combustion to his peers as well as to the libraries of institutions for higher learning.

Flame Synthesis of Aluminum Nitride Filler-Powder.

This short communication offered one of flame aerosol synthesis of aluminum nitride, AlN, as a new filler-powder supply-route for resin polymer composite system filled with ceramic particles, which would have a promising advantage in thermal conductivity. Flame AlN synthesis was based on the aerosol preparation of conventional LPG-O2-N2-NH3 system flame, and the particulate raw material. Effects of flame temperature and heat amount were studied on the direct-nitridation of Al through CH4, C3H8, C2H2 flame of LPG-O2-N2-NH3 system. As-prepared resultants were consisted of an Al-O-N system and a remained Al raw material, and their annealed powder was comprised of an AlN majority and a little α-Al2O3. Heating amount increments resulted in the yield-increase of polyphasic intermediate system of Al-O-N and that of AlN. Flame synthesized powder had a fairly large aerosol particle size, roughly 10 μm in average size, and a bimodal size distribution. Especially, the new AlN powder had a non-squarish shape rather than one of marketed products with angular morphology, which would be good for high-density packing. Feasible potential of this synthesis method was confirmed to provide the AlN powder having suitable properties as the filler-powder.

Extinction of Laminar Counterflow Diffusion Flames of CH4 and C3H8 Fuels with Inert Jet Impingement

Experimental results of N2-diluted counterflow diffusion flames of CH4 and C3H18 vs, air with local extinction are reponed. The local extinction was caused by inert jet impingement on flames at selected locations either from the fuel or oxidizer side of the reaction zone. This was done to simulate how local extinction affects flame extinction over a larger flame area. The results are: (1) Local extinction of both CH4 and C3H8 flames occurs at a lower strain rate when the inert jet impingement originates from the airside. (2) The global extinction strain rate for CH4 flames is insensitive to the location and number (one vs. three) of local extinction sites. (3) For C3H8 flames, one single inert jet impinging from the airside along the centerline is more effective in causing global extinction than three inert jets impinging at regions away from the centerline. This suggests that flame extinction over a larger area may depend on strategically selected smaller local extinction sites. Furthermore, since similar results of (3) are not observed in this study for CH4 flames, fuel chemistry may also play a role in the effectiveness of local extinction transitioning to global extinction. Differences between flame stabilization mechanisms with and without local extinction are discussed and the implications for turbulent diffusion flames are outlined.

UV to Near-IR CO Emissions from O + C2H2 and O + C3O2 Flames at Low Pressure and High Temperature

Emission spectra from C2H2 + O flames have been studied in a low-pressure, fast flow discharge system under highly dilute, fuel-lean conditions from 295 to 873 K. Our previous work with room-temperature flames has been extended to look for temperature-induced changes in the relative populations of triplet CO(a), (a‘), (d), and (e) reaction products by observing chemiluminescent emission in the 185−900 nm wavelength region. The effect of temperature on the relative yields of the CO triplet states was small, and the production rates increased linearly with the temperature-dependent rate of the initial O + C2H2 reaction step. To test the importance of C2O as a major pathway to triplet state CO, similar experiments were performed on C3O2 + O flames. The relative yields of the CO triplet states showed little temperature dependence; however, the overall triplet production rate increased much faster with temperature than the initial O + C3O2 reaction and may be related to temperature variation of the branching ratio yielding C2O in the initial reaction. Relative vibrational level populations were determined in the CO triplet states and were slightly hotter in C3O2 flames than in C2H2 flames throughout the temperature range.

A Study of Partial Extinction and Reignition Effects in Turbulent Non-Premixed Jet Flames of CH4 and CO/H2/N2 with a Two-Scalar Reactedness-Mixture Fraction Presumed PDF Model

A method for modeling the effects of finite-rate chemistry such as partial extinctions and reignitions is developed and used to compute turbulent CO/H2/N2 or CH4-air-piloted jet diffusion flames close to extinction. The method is combined with a two-dimensional Large Eddy Simulation procedure employing a partial equilibrium/two-scalar exponential Probability Density Function (PDF) combustion submodel applied at the subgrid scale (SGS) level. Subgrid motions are modeled with a first-order closure utilizing an anisotropic subgrid eddy-viscosity and two equations for the subgrid scale turbulent kinetic and scalar energies. Statistical independence of the joint PDF scalars is here avoided and the required moments are obtained from an extended scale-similarity assumption. Extinction is determined by comparing the local Damkohler number against a ‘critical’ local limit related to the Gibson scalar scale and the reaction zone thickness. The post-extinction regime is treated with a reactedness progress variable computed from a Lagrangian transport equation and a two-scalar reacledness-mixture fraction presumed PDF. Comparisons with well documented turbulent flame data suggested the ability of the presented methodology to describe many experimental trends and variations of the momentum and reactive scalar mixing fields at flame conditions close to extinction.

99/00811 Scientific and research activities of Oil and Gas Institute in Krakow

A digital particle image velocimetry technique that is appropriate for the experimental derivation offundamental flame properties was implemented. The technique allows for the determination of the instantaneous flowfield and is essential for fluid mechanics measurements in reduced gravity environments. Measurements of laminar flame speeds were conducted in the stagnation flow configuration just before a flame undergoes a transition from planar to Bunsen flame. Results obtained for lean CH4/air and C2H2/air flames were found to be in close agreement with prefious laser Doppler velocimetry data. Subsequently, measurements were conducted for the CH4 and C2H2 flames by independently varying the equivalence ratio and flame temperature to distinguish between temperature and concentration effects. The laminar flame speeds were also calculated using the GRI-Mech 3.0 mechanism. It was convincingly shown that under high-O2 and low-temperature conditions, the experimental laminar speeds are over predicted by the simulations especially for C2H6 flames. Additional experiments were conducted by adding H2 to lean C2H6/air flames and by diluting those mixtures by either He or N2 to vary the flame temperature. While for the He dilution case, the predictions noticeably overpredict the experiments, for N2 dilution, close aggrement was observed. Analyses of the flame structures revealed that for those fuel-lean flames, the burning rate largely depends on the competition of the two-body branching and three-body termination reaction between H and O2. It was not possible to point to possible kinetic deficiencies other than referring to uncertainties associated with the rates and collision efficiencies of three-body reactions. The high-O2 low-temperature region is of interrest not ouly to lean-premixed combustion, but also to flame ignition, and requires further exploration.

99/00812 Structural dependence of heterolytic bond dissociation energy of carbon-carbon g bonds in hydrocarbons

A digital particle image velocimetry technique that is appropriate for the experimental derivation offundamental flame properties was implemented. The technique allows for the determination of the instantaneous flowfield and is essential for fluid mechanics measurements in reduced gravity environments. Measurements of laminar flame speeds were conducted in the stagnation flow configuration just before a flame undergoes a transition from planar to Bunsen flame. Results obtained for lean CH4/air and C2H2/air flames were found to be in close agreement with prefious laser Doppler velocimetry data. Subsequently, measurements were conducted for the CH4 and C2H2 flames by independently varying the equivalence ratio and flame temperature to distinguish between temperature and concentration effects. The laminar flame speeds were also calculated using the GRI-Mech 3.0 mechanism. It was convincingly shown that under high-O2 and low-temperature conditions, the experimental laminar speeds are over predicted by the simulations especially for C2H6 flames. Additional experiments were conducted by adding H2 to lean C2H6/air flames and by diluting those mixtures by either He or N2 to vary the flame temperature. While for the He dilution case, the predictions noticeably overpredict the experiments, for N2 dilution, close aggrement was observed. Analyses of the flame structures revealed that for those fuel-lean flames, the burning rate largely depends on the competition of the two-body branching and three-body termination reaction between H and O2. It was not possible to point to possible kinetic deficiencies other than referring to uncertainties associated with the rates and collision efficiencies of three-body reactions. The high-O2 low-temperature region is of interrest not ouly to lean-premixed combustion, but also to flame ignition, and requires further exploration.

Laser-Induced Fluorescence of CF2 from a CH4 Flame and an H2 Flame with Addition of HCFC-22 and HFC-134a

Laser-induced fluorescence (LIF) of CF2 radicals in atmospheric diffusion flames is reported for the first time. When a CH4 flame is premixed with HCFC-22 (CCIF2H) or HFC-134a (CF3CFH2), fluorescence spectra between 250 and 314 nm of the Ã1B1–X̃1A1 transition from CF2 are observed when the flame is probed by a KrF laser. When these two halocarbons are added to an atmospheric H2 flame, similar LIF spectra of CF2 are also observed. Two-dimensional LIF images from the flames show that the CF2 radicals are localized between the flame center and the flame front.

CCl, CH, and NO LIF measurements in methane-air flames seeded with chlorinated species: Influence of CH 3Cl and CH 2Cl 2 on CCl and NO formation

In this work, the laser-induced fluorescence (LIF) technique is used to detect minor species (CCl, NO, and CH) in premixed stoichiometric methane-air flames seeded with monochloromethane or dichloromethane. Quenching data are extracted from time-resolved fluorescence lifetime measurements for all the excited species. First quenching measurements of CCl under flame conditions are reported. It is shown that LIF measurements are strongly perturbed by the presence of background emissions issued from the radiative relaxation of photolytic fragments (HCl*, CCl*, CH*, and C2*) formed upon laser excitation. The parent molecules that are partly responsible for these emissions are C2H3Cl (for HCl*, CH*) and CHCl2 (for CCl*). Profiles of both photolytic fragments and species directly measured by LIF are used to study the influence of CH3Cl and CH2Cl2 addition on CCl and NO formation in methane-air flames. CCl radical is found to be formed in the reaction zone of the flames. The reaction path leading to CCl appears to be dependent on the nature of the chlorinated hydrocarbon (CHC) seeded in the flame. The suggested reaction paths may preferentially involve the contribution of CHCl2 in case of CH2Cl2 degradation and CH2Cl in case of CH3Cl degradation. An important increase of NO in presence of CHC is pointed out for the first time. The NO formation in flames containing CHC appears to occur in the reaction zone of the flames, and [NO] is found to be constant in the burned gases: This suggests a predominance of the prompt-NO mechanism in this kind of flame as confirmed experimentally by the observed [CH] increase. Reaction paths involving the degradation of CHCs, particularly CHCl2, should largely contribute to the formation of CH in flames seeded with CHCs.

Formation of NO x, CH 4, and C 2 species in laminar methanol flames

Computational and experimental results at normal atmospheric pressure are reported for methanol-air premixed and diffusion flames, the latter including fuel streams partially premixed to equivalence ratios down to 2, yielding two-stage flames. Measurements of the concentrations of NOx, of CH4, and of the sum of the concentrations of C2H2, C2H4, and C2H6 (collectively termed C2 species) are made for these flames. By employing 184 elementary steps, 52 of which are associated with the nitrogen chemistry, computations are performed for freely propagating premixed flames with equivalence ratios from 0.5 to 1.9 and for counterflowing two-stage flames with equivalence ratios from 2 to infinity at different initial conditions. Most of the elementary rate constants have been validated in previous experimental and computational studies of H2, CO, CH4, and CH3OH flames. Those that have not mainly concern consumption of CH3O and CH2OH, especially notably CH2OH+H→CH3+OH, which is the dominant reaction leading to the formation of both CH4 and C2 species in these experiments and for which the rate parameters are determined herein. Excellent agreements between experiments and predictions are obtained. This completes the determination of elementary rate parameters for methanol-air flames, to the extent possible on the basis of currently available experimental information, and it thereby provides a reaction mechanism for these flames.

NO Destruction and Regeneration in CO2 Enriched CH4 Flame. (A Fundamental Study on CO2 Recycled Coal Combustion)

The effect of initial CO 2 concentration on NO destruction and regeneration was investigated as part of our ongoing research on NO emission from CO 2 recycled coal combustion, which could realize easy CO 2 recovery and drastic N() reduction simultaneously. Experiments were carried out in a flat flame burner for CH 4 /O 2 /Ar/NO/CO 2 flame at various excess oxygen ratios (from 0.6 to 1.2) and CO 2 concentrations (from 0% to 48%). It was found that CO 2 has a pronounced negative effect on the destruction of NO under fuel-rich condition. In contrast, under fuel- lean condition, the effect of CO 2 becomes smaller even at high CO 2 concentrations. Kinetic modeling showed that a large part of NO is regenerated through the reaction N+CO 2 = NO+CO at high CO 2 concentrations. The effect of CO 2 is not so significant under fuel-lean condition because this reaction is no longer competitive with other NO formation reactions in this case.

97/04901 A review of NO x formation and reduction mechanisms in combustion systems, with particular reference to coal : Williams, A. et al. J. Institute of Energy, September 1997, 70, 102–113

In this paper a detailed sub-mechanism NOMecha2.0 for NOx chemistry at high temperature is validated on a large experimental database obtained in flames, a jet-stirred reactor (JSR) and a plug-flow reactor under sub-atmospheric and atmospheric conditions. This sub-mechanism mainly based on the one implemented in the GDFkin®3.0_NCN model for prompt-NO formation has been thoroughly updated. Prompt-NO formation results from the reaction between CH and N2 yielding NCN and H. Then, NCN radicals react with H and O-atoms yielding HCN + N and CN + NO, respectively. Care was taken in the choice of the enthalpy of formation of the N-species, especially for NCN radicals for which the enthalpy values reported in the literature differ by more than 10 kJ mol−1. Critical rate constant values for the two main reactions (CH + N2 → NCN + H and NCN + H → HCN + N) and the branching ratio of the reactions NCN + H → products are discussed. Validation focuses mainly on prompt-NO formation in low pressure flames stabilized with different fuels (CH4, C2H6, C3H8 and C2H2) using data from the literature. It draws in particularly on the extended experimental database, including species profiles (CH, NCN, CN, HCN, NCO and NO) previously obtained in Lille in CH4 and C2H2 flames, which was extended in this work. Validation also considers experiments dedicated to HCN oxidation in a JSR or in flames, and to HNCO oxidation performed in a plug-flow reactor. In order to improve the prediction of HNCO, HCN/HNC isomerization has been implemented in NOMecha2.0. Finally the NOMecha2.0 sub-mechanism was satisfactorily coupled to a detailed chemical mechanism from the literature dedicated to the oxidation of acetylene at high temperature to predict NO formation.

Scalar Dissipation Measurements in Turbulent Jet Diffusion Flames of Air Diluted Methane and Hydrogen

Simultaneous two-dimensional Rayleigh and fuel Raman images have been collected in air-diluted methane and hydrogen jet diffusion flames. Temperature, fuel mass fraction and mixture fraction images are derived by a two-scalar approach based on one-step chemistry and equal species diffusivities. This enables calculation of two components of the scalar dissipation rate x-The inherently weak Raman signal has been maximised by intra-cavity measurements, using a flashlamp-pumped dye laser. In addition, the Raman signal-to-noise ratio is drastically improved by a novel contour-aligned smoothing technique which exploits the high correlation between the Rayleigh and Raman signals. Quantitative measurements of scalar dissipation are presented, including probability density functions for components of x- Profiles of mean and rms mixture fraction show the usual features already documented in other published results for this type of flame. Probability density functions of ξ are close to Gaussian on the axis, and tend to bimodal at the edge of the flame. Results for the CH4 flames indicate that the mean of x shows little change with Reynolds number. In the H2 flame, mean values for the axial and radial components of the scalar dissipation rate, x are nearly the same, indicating a more isotropic structure than in the CH4 flames. For both fuels, the pdf of ln(x) on the axis is more peaky than a lognormal distribution and somewhat skewed. The profiles of ( x\\ 𝛈 show a nonlinear dependence on mixture fraction and have no clear resemblance to the skewed, monomodal shapes seen in cold flows. In the H2 flame there is a strong correlation between instantaneous, local values of scalar dissipation and the departure from equilibrium, as measured by temperature depression.

NOxand NO2Emission of Swirl-Stabilized Nonpremixed Flames of a H2—CH4Mixture

NOx emission levels and the NO2/NOx ratios were studied in swirl-stabilized H2—CH4 flames and comparison was made with H2 flames under similar conditions (i.e. system parameters such as Reynolds and swirl numbers, and the recirculation zone residence time). Changing the fuel composition from 100% H2 to 50% H2—CH4 was found to have moderate effects on the NOx emission level. The NO2/NOx ratios with CH4. substitution can be significantly higher than pure H2 flames (about 80–95% for H2—CH4 flames, compared to 35–60% for H2 flames), partly explaining its high values in practical gas turbine combustors.

Degenerate four-wave mixing measurements of methyl radical distributions in hydrocarbon flames: Comparison with model predictions

We report an investigation of the use of degenerate four-wave mixing (DFWM) to measure CH3 radical distributions in premixed hydrocarbon flames. Experimental results are compared with flame model calculations. By exciting the Herzberg β1 band near 217 nm, we monitored DFWM signals from CH3 as a function of height above a laboratory flat-flame burner at atmospheric pressure. Coherent anti-Stokes-Raman spectroscopy (CARS) of N2 was performed to obtain temperature profiles. The dependence of the DFWM signal on temperature was shown theoretically to be reduced by the short predissociative lifetimes of the CH3 upper states. A two-color FWM mixing experiment indicated that DFWM from CH3 in CH4 flames was predominately the result of population gratings. Using population-grating theory, the DFWM signal profile was corrected for varying temperature, beam absorption, and lower-state lifetimes. The results were in reasonable agreement with flame calculations and demoustrate that DFWM can provide useful relative concentration profiles of a polyatomic radical in rich hydrocarbon flames. However, we found that a weak DFWM background signal limited detection sensitivity ≈70 ppm of CH3 in CH4 flames. The background was comparable to the CH3 signal in C3H8 flames, and it obscured the CH3 signal in rich C2H2 and C2H4 flames. Strategies to alleviate the background are discussed.

Quantitative CH determinations in low-pressure flames

Quantitative linear laser-induced fluorescence (LIF) has been applied to measure CH concentration in low-pressure propane/air and methane/air flames. Temperature measurements at the peak of the CH distribution give 1600 and 1700 K for those flames, respectively, using CH B-X, A-X, and OH A-X excitation LIF scans. Quenching was taken into account by measuring the temporally resolved decay of the signal. The CH absolute number density determination was carried out in two different electronic bands and the LIF signal was calibrated with two independent methods: Rayleigh scattering of N2 and Raman scattering of H2. The results show agreement to better than 10% between electronic bands and 30% between calibration methods. Overall, the technique is well suited to measure concentrations in the ppm and sub-ppm range with an average error of 25%. This degree of accuracy is adequate to test flame kinetic models. CH profiles and peak amounts of 4–11 ppm for the propane and methane flames agree well with model predictions based on the GRI-Mech 2.11 mechanism. Flame CH, temperature, and NO measurements were used to deduce rate constants for the CH+N2 reaction near 1600 K.

Advances in the chemistry of insect control, III

A series of experiments were performed to investigate the relationship between syngas composition (H2/CO/CO2/N2/O2) and NO formation. Laser-saturated fluorescence measurements of NO in a premixed syngas flat flame from a heat flux burner were recorded for various fuel compositions, dilution ratios and equivalence ratios. Quantitative measurements were compared with the predictions from CHEMKIN software using four different chemical mechanisms, i.e., GRI mech 3.0, Chemical Reaction Engineering and Chemical Kinetics 1407, GDF-Kin 3.0, and the Mendiara and Glarborg mechanism; each mechanism included reaction subsets for NOx formation. These models were validated against a CH4 flame; the GDF-Kin 3.0 mechanism produced the best agreement with measurements. NO measurements for CO/H2 syngas fuel with 50% CO2 dilution at various equivalence ratios found the maximum NO production near stoichiometry. The majority of NO produced for these conditions is predicted to come from the combination of the NNH and N2O pathways. Investigation of NO production during combustion of different CO/H2 ratio syngas diluted with 60% N2 showed that increasing the H2 fraction decreased the total NO concentration. However, normalizing the NO production rate by the fuel mass consumption rate showed H2 produces more NO per gram of fuel consumed. Predictions of temperature and NO rate of production along the flame axis showed that the syngas with high H2 content had a flame front closer to the burner exit and NO production rate greater than the high CO case. Dilution of CO/H2 syngas fuel with N2 and CO2 showed that CO2 has a stronger reduction in NO emission than N2. GDF-Kin 3.0 predicted that dilution with CO2 caused a greater reduction in flame temperature than the same volume of N2. Predicted NO rate of production and reaction sensitivity analysis predicted that the Zel’dovich pathway was dominant for undiluted syngas. However, as the flame temperature reduced, the Zel’dovich pathway was inhibited to a greater extent than the N2O and NNH pathways so that, at high dilution, NO production was driven by the N2O and NNH pathways.

Chemical Structures of Fuel-Rich, Premixed, Laminar Flames of C6H5Cl/CH4/O2/Ar Mixtures

Abstract Chemical structures of atmospheric-pressure, fuel rich (sooting) premixed, laminar, flat flames of C6H5Cl and CH4 have been investigated using a flat flame burner. Species mole fraction profiles were determined by using micro-probe sampling coupled to on-line gas chromatography/mass spectrometry. In addition to those of reactants, mole fraction profiles have been determined for CO, CO2, HCl, H2O, H2, C2H6, C2H4, C2H2, C4H2, C4H4, C5H6, C6H6, C6H5CH3, C6H5C2H3, C6H5C2H, C6H5OH, ClC6H4OH, ClC6H4CH3, C6H4Cl2, and C10H8. In a fuel lean flame of pure C6H5Cl highly chlorinated hydrocarbons such as CCl4, Cl2C6H3OH, C5HCl4, C2HCl3, C6H3Cl3 and C2Cl4 were also observed. The roles of these species, in view of the current ideas of the mechanism of combustion of C6H5Cl are discussed.

Generalized State-Property Relations for Nonluminous Flame Absorption Coefficients

The work reported here simplifies computing the local Planck-mean absorption coefficient in nonluminous flames as a function of the mixture fraction and fuel composition. Equilibrium is assumed for fuel/air mixtures up to the point where carbon is predicted to condense, beyond which the gaseous products are assumed to be frozen and to be diluted with cold fuel. The resulting algebraic expressions are suitable for inclusion in any turbulent or laminar diffusion flame model predicated on single-step chemistry. The method accounts for the nongray nature of the gaseous combustion products and their variation in concentration and temperature with mixture fraction, at a computational penalty little more than that for estimating variable fluid properties. Nonluminous flames (in air) of H2, CO, CH3OH, CH4 and lean regions of fuels with the general form CxHyOz can be modeled satisfactorily. The effects of pressure-pathlength and heat loss on the absorption coefficient are addressed.

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.