Experimental study of the emissivity of flames resulting from the combustion of forest fuels

Heat transfer by radiation is taken into account in most models that predict the propagation of forest fires. This heat transfer mechanism is normally formulated according to the Stefan–Boltzmann law in terms of flame temperature and flame emissivity. This study focused on flame emissivity. Experimental studies carried out to compute the emissivity of the flames generated during the combustion of forest fuels were reviewed, thereby highlighting differences in methodologies and results. Since the results of these studies with regard to the exponential relationship between flame emissivity and flame thickness were not in agreement, two methods based on IR imagery were used in the present study to calculate flame emissivity values. Nine circular fuel beds with a diameter of 0.3–2.5 m were prepared with common Mediterranean species and burned as stationary fires. An exponential correlation between flame emissivity and flame thickness was observed for both methods. According to the results of this study, only flames thicker than 3.2 m would exhibit an emissivity close to that of a blackbody (0.9), and the associated extinction coefficient would be 0.72. A long-term retardant product was used to treat the fuel of two of the nine tests that were carried out and no effect on flame emissivity was observed.     

A model of particulate and species formation applied to laminar, nonpremixed flames for three aliphatic-hydrocarbon fuels

A detailed kinetic mechanism is developed that includes aromatic growth and particulate formation. The model includes reaction pathways leading to the formation of nanosized particles and their coagulation and growth to larger soot particles using a sectional approach for the particle phase. It is tested against literature data of species concentrations and particulate measurements in nonpremixed laminar flames of methane, ethylene, and butene. Reasonably good predictions of gas and particle-phase concentrations and particle sizes are obtained without any change to the kinetic scheme for the different fuels. The model predicts the low concentration of particulates in the methane flame (about 0.5 ppm) and the higher concentration of soot in the ethylene and butene flames (near 10 ppm). Model predictions show that in the methane flame small precursor particles dominate the particulate loading, whereas soot is the major component in ethylene and butene flames, in accordance with the experimental data. The driving factors in the model responsible for the quite different soot predictions in the ethylene and butene flames compared with the methane flame are benzene and acetylene concentrations, which are higher in the ethylene and butene flames. Soot loadings in the ethylene flame are sensitive to the acetylene soot growth reaction, whereas particle inception rates are linked to benzene in the model. A coagulation model is used to obtain collision efficiencies for some of the particle reactions, and tests show that the modeled results are not particularly sensitive to coagulation at the rates used in our model. Soot oxidation rates are not high enough to correctly predict burnout, and this aspect of the model needs further attention.     

对称与非对称碳氢火焰温度场重建实验研究

为了测量碳氢火焰的截面温度场分布,根据火焰烟黑辐射传递特性,提出了基于火焰发射辐射能图像的温度场重建模型,使用最小二乘QR矩阵分解法（LSQR法）求解该模型。数值模拟重建了轴对称和非对称分布的两类温度场,重建结果精度较高。进而对实验室的甲烷扩散火焰截面二维温度场进行了重建,结果与已有文献相比符合较好,特别是对非对称火焰,能准确还原其温度场分布特征,实验得到了较好的结果。     

Experimental and modeling study of the effects of adding oxygenated fuels to premixed n-heptane flames

The effects of methanol, dimethoxymethane (DMM), and dimethylcarbonate (DMC) on laminar premixed low pressure (30 Torr) n-heptane flames were investigated by using synchrotron photoionization and molecular-beam mass spectrometry (PI–MBMS) techniques. The overall C/O ratio was maintained constant (0.507) and the equivalence ratio was kept around 1.6 for all the tested flames. The composition of unburned mixtures was adjusted such that the post-flame temperatures were nearly equivalent for all the test conditions. Mole fraction profiles of major and intermediate species were derived and compared among the flames. Parallel computations were performed based on a modified model, and the predicted concentrations of flame species agree reasonably well with the measured results. Early production of CO₂ was observed in the DMC-doped flame. Reaction flux analysis suggested that it was caused by the decomposition of CH₃OCO radical, DMC molecule and CH₃OCOO radical. As oxygenated fuels were added, the concentrations of most C₁C₅ hydrocarbon intermediates were reduced while that of benzene (C₆H₆) also decreased apparently, and the extent of benzene reduction showed little difference among the oxygenate-doped flames. Reaction flux analysis indicated that, in all the tested flames, the primary pathway leading from small aliphatics to C₆H₆ was through C₃ + C₃ reactions, including the self-recombination reaction of propargyl radical (C₃H₃) and the cross reaction between C₃H₃ and allyl radical (a-C₃H₅). Considering that the temperatures of the tested flames were almost equivalent, the reduction of C₆H₆ concentration when doped with oxygenated fuels should be resulted from the reduced concentrations of its precursors. Furthermore, concentrations of certain oxygenated intermediates were also examined. The concentration of formaldehyde (CH₂O) was found to increase when flames were doped with oxygenated fuels, while those of acetaldehyde (CH₃CHO) and vinyl alcohol (C₂H₃OH) were nearly equivalent for all the flames. Methyl formate (CH₃OCHO) was detected only in the DMM-doped flame, which was attributed to the efficient CH₃OCHO formation pathway through the decomposition of CH₃OCHOCH₃ radical in the flame.     

Numerical and experimental study of soot formation in laminar diffusion flames burning simulated biogas fuels at elevated pressures

The effects of pressure and composition on the sooting characteristics and flame structure of laminar diffusion flames were investigated. Flames with pure methane and two different methane-based, biogas-like fuels were examined using both experimental and numerical techniques over pressures ranging from 1 to 20 atm. The two simulated biogases were mixtures of methane and carbon dioxide with either 20% or 40% carbon dioxide by volume. In all cases, the methane flow rate was held constant at 0.55 mg/s to enable a fair comparison of sooting characteristics. Measurements for the soot volume fraction and temperature within the flame envelope were obtained using the spectral soot emission technique. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows, which included complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Overall, the numerical simulations correctly predicted many of the observed trends with pressure and fuel composition. For all of the fuels, increasing pressure caused the flames to narrow and soot concentrations to increase while flame height remained unaltered. All fuels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. Adding carbon dioxide to the methane fuel stream did not significantly effect the shape of the flame at any pressure; although, dilution decreased the diameter slightly at 1 atm. Dilution suppressed soot formation at all pressures considered, and this suppression effect varied linearly with CO₂${\text{CO}}_2$ concentration. The suppression effect was also larger at lower pressures. This observed linear relationship between soot suppression and the amount of CO₂${\text{CO}}_2$ dilution was largely attributed to the effects of dilution on chemical reaction rates, since the predicted maximum magnitudes of soot production and oxidation also varied linearly with dilution.     

Experimental study of Markstein number effects on laminar flamelet velocity in turbulent premixed flames

Effects of turbulent flame stretch on mean local laminar burning velocity of flamelets, , were investigated experimentally in an explosion vessel at normal temperature and pressure. In this context, the wrinkling, At/Al, and the burning velocity, ut, of turbulent flames were measured simultaneously. With the flamelet assumption the mean local laminar burning velocity of flamelets, , was calculated for different turbulence intensities. The results were compared to the influence of stretch on spherically expanding laminar flames. For spherically expanding laminar flames the stretched laminar burning velocity, un, varied linearly with the Karlovitz stretch factor, yielding Markstein numbers that depend on the mixture composition. Six different mixtures with positive and negative Markstein numbers were investigated. The measurements of the mean local laminar burning velocity of turbulent flamelets were used to derive an efficiency parameter, I, which reflects the impact of the Markstein number and turbulent flame stretch—expressed by the turbulent Karlovitz stretch factor—on the local laminar burning velocity of flamelets. The results showed that the efficiency is reduced with increasing turbulence intensity and the reduction can be correlated to unsteady effects.     

Sooting tendencies of primary reference fuels in atmospheric laminar diffusion flames burning into vitiated air

In this study, the sooting tendencies of primary reference fuels (PRFs) are measured in term of yield sooting indices (YSIs) in methane diffusion flames doped with the vapors of PRFs. The present paper represents an incremental advance complementing the original methodology prescribed by McEnally and Pfefferle. The influence of both PRF formulation and CO₂ dilution of the coflowing air on the YSIs is also assessed. The diffusion flames burning in a coflowing oxidizer stream are established over the Santoro’s burner and vapor of the liquid fuel to be investigated is injected into the fuel stream. Laser extinction measurements are performed to map the two-dimensional field of soot volume fraction in the flame. For the pure liquid hydrocarbons investigated, i.e., n-hexane, n-heptane, isooctane, and benzene, the YSI reported in the original paper by McEnally and Pfefferle quantitatively predict the sooting propensities, measured here at much higher dopant concentrations. The present study therefore extends the consistency of the YSI methodology on the Santoro’s burner. For blends of n-heptane and isooctane, the sooting tendency of doped flames exhibits regular and monotonic trends and decreases with increasing n-heptane mole fraction or CO₂ dilution. Interestingly, the evolution of YSI with the isooctane mole fraction exhibits a strong similarity for varying CO₂ mole fraction. A quadratic least-squares fit is then derived, providing a phenomenological model of YSI as a function of both isooctane mole fraction in the fuel stream and CO₂ mole fraction in the oxidizer. A non-negligible cross effect of PRF formulation and CO₂ dilution on YSI is revealed. The method elaborated within the framework of the present paper could be extended to surrogate fuels. This would help develop a comprehensive database and empirical correlations that could predict the sooting propensities of different surrogate fuels, therefore their potentially mitigationed soot production through control of fuel composition and/or exhaust gas recirculation. This database would also be useful for the validation of CFD simulations incorporating sophisticated model of soot production.     

Analysis of the laminar flamelet concept for nonpremixed laminar flames

The goal of this paper is to investigate the application of the laminar flamelet concept to the multidimensional numerical simulation of nonpremixed laminar flames. The performance of steady and unsteady flamelets is analyzed. The deduction of the mathematical formulation of flamelet modeling is exposed and some commonly used simplifications are examined. Different models for the scalar dissipation rate dependence on the mixture fraction variable are analyzed. Moreover, different criteria to evaluate the Lagrangian-type flamelet lifetime for unsteady flamelets are investigated. Inclusion of phenomena such as differential diffusion with constant Lewis number for each species and radiation heat transfer are also studied. A confined co-flow axisymmetric nonpremixed methane/air laminar flame experimentally investigated by McEnally and Pfefferle (Combust. Sci. Technol. 116-117 (1996) 183-209) and numerically investigated by Bennett, McEnally, Pfefferle, and Smooke (Combust. Flame 123 (2000) 522-546), Cònsul, Pérez-Segarra, Claramunt, Cadafalch, and Oliva (Combust. Theory Modelling 7 (3) (2003) 525-544), and Claramunt, Cònsul, Pérez-Segarra, and Oliva (Combust. Flame 137 (2004) 444-457) has been used as a test case. Results obtained using the flamelet concept have been compared to data obtained from the full resolution of the complete transport equations using primitive variables. Finite-volume techniques over staggered grids are used to discretize the governing equations. A parallel multiblock algorithm based on domain decomposition techniques running with loosely coupled computers has been used. To assess the quality of the numerical solutions presented in this paper, a verification process based on the generalized Richardson extrapolation technique and on the grid convergence index (GCI) has been applied.     

Measurement of smoke point in velocity-matched coflow laminar diffusion flames with pure fuels at elevated pressures

Using a coflow burner, a quartz chimney, and a pressure vessel with good optical access, smoke points of pure fuels were measured in a laminar jet diffusion flame. The smoke point is a fundamental kinetic parameter, as this is the point where production of soot is exactly offset by its oxidation. Ethylene and methane, burning in a velocity-matched, overventilated coflow of air, were tested over a range of pressures from 1 to 16 atm. Fuel flow rate and air coflow rate were iteratively increased, keeping the exit velocity equal, until the smoke point was reached. The volumetric fuel flow and flame height were measured as a function of pressure to determine the functional relationship between these parameters and pressure. The volumetric fuel flow at the smoke point is observed to scale as a power law with pressure, while the smoke point height is best described by a log law with pressure. The residence time, based on flame height and exit velocity, was also calculated as a function of pressure and found to have a nonmonotonic behavior, with a peak at lower pressures.     

Experimental and computational infrared imaging of bluff body stabilized laminar diffusion flames

The concept of comparing measured and computed images is extended to the mid-infrared spectrum to provide a non-intrusive technique for studying flames. Narrowband radiation intensity measurements of steady and unsteady bluff body stabilized laminar ethylene diffusion flames are acquired using an infrared camera. Computational infrared images are rendered by solving the radiative transfer equation for parallel lines-of-sight through the flame and using a narrowband radiation model with computed scalar values. Qualitative and quantitative comparisons of the measured and computed infrared images provide insights into the flame stabilization region and beyond. The unique shapes and sizes of the flames observed in the measured and computed infrared images are similar with a few exceptions which are shown to be educational. The important differences occur in the flame stabilization region suggesting improvements in thermal control of the experiment and soot formation and heat loss models.     

Catalytic inhibition of laminar flames by transition metal compounds

Some of the most effective flame inhibitors ever found are metallic compounds. Their effectiveness, however, drops off rapidly with an increase of agent concentration, and varies widely with flame type. Iron pentacarbonyl, for example, can be up to two orders of magnitude more efficient than CF₃Br for reducing the burning velocity of premixed laminar flames when added at low volume fraction; nevertheless, it is nearly ineffective for extinction of co-flow diffusion flames. This article outlines previous research into flame inhibition by metal-containing compounds, and for more recent work, focuses on experimental and modeling studies of inhibited premixed, counterflow diffusion, and co-flow diffusion flames by the present authors. The strong flame inhibition by metal compounds when added at low volume fraction is found to occur through the gas-phase catalytic cycles leading to a highly effective radical recombination in the reaction zone. While the reactions of these cycles proceed in some cases at close to collisional rates, the agent effectiveness requires that the inhibiting species and the radicals in the flame overlap, and this can sometimes be limited by gas-phase transport rates. The metal species often lose their effectiveness above a certain volume fraction due to condensation processes. The influence of particle formation on inhibitor effectiveness depends upon the metal species concentration, particle size, residence time for particle formation, local flame temperature, and the drag and thermophoretic forces in the flame.     

Extinction of laminar partially premixed flames

Flame extinction represents one of the classical phenomena in combustion science. It is important to a variety of combustion systems in transportation and power generation applications. Flame extinguishment studies are also motivated from the consideration of fire safety and suppression. Such studies have generally considered non-premixed and premixed flames, although fires can often originate in a partially premixed mode, i.e., fuel and oxidizer are partially premixed as they are transported to the reaction zone. Several recent investigations have considered this scenario and focused on the extinction of partially premixed flames (PPFs). Such flames have been described as hybrid flames possessing characteristics of both premixed and non-premixed flames. This paper provides a review of studies dealing with the extinction of PPFs, which represent a broad family of flames, including double, triple (tribrachial), and edge flames. Theoretical, numerical and experimental studies dealing with the extinction of such flames in coflow and counterflow configurations are discussed. Since these flames contain both premixed and non-premixed burning zones, a brief review of the dilution-induced extinction of premixed and non-premixed flames is also provided. For the coflow configuration, processes associated with flame liftoff and blowout are described. Since lifted non-premixed jet flames often contain a partially premixed or an edge-flame structure prior to blowout, the review also considers such flames. While the perspective of this review is broad focusing on the fundamental aspects of flame extinction and blowout, results mostly consider flame extinction caused by the addition of a flame suppressant, with relevance to fire suppression on earth and in space environment. With respect to the latter, the effect of gravity on the extinction of PPFs is discussed. Future research needs are identified.     

Experimental and numerical study on laminar premixed NH3/H2/O2/air flames

Adding the product of water electrolysis (i.e. 2:1 volume of H₂ and O₂) is an effective strategy to enhance the combustion intensity of NH₃/air mixtures. In this work, the laminar burning velocity (LBV) of the obtained NH₃/H₂/O₂/air mixtures was measured at 303 K, 0.1 MPa and compared with the values predicted by seven mechanisms. To improve the prediction performance, a new mechanism is developed based on the existing mechanism and adopted for numerical simulation. The results of this study show that the LBV of NH₃ is significantly increased by additional H₂ and O₂. By comparison, it is found that H₂ shows a more significant promoting effect on LBV when the volume ratio of additional H₂ and O₂ is 2. The concentration of key radicals and the flame temperature increase remarkably due to the addition of H₂ and O₂, which promote the flame propagation. Furthermore, the experimental results also indicated that the additional H₂ and O₂ make the burned gas Markstein length decrease on the lean side and increase on the rich side.     

Structure of laminar sooting inverse diffusion flames

The flame structure of laminar inverse diffusion flames (IDFs) was studied to gain insight into soot formation and growth in underventilated combustion. Both ethylene-air and methane-air IDFs were examined, fuel flow rates were kept constant for all flames of each fuel type, and airflow rates were varied to observe the effect on flame structure and soot formation. Planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF), planar laser-induced incandescence of soot (soot PLII), and thermocouple-determined gas temperatures were used to draw conclusions about flame structure and soot formation. Flickering, caused by buoyancy-induced vortices, was evident above and outside the flames. The distances between the OH, PAH, and soot zones were similar in IDFs and normal diffusion flames (NDFs), but the locations of those zones were inverted in IDFs relative to NDFs. Peak OH PLIF coincided with peak temperature and marked the flame front. Soot appeared outside the flame front, corresponding to temperatures around the minimum soot formation temperature of 1300 K. PAHs appeared outside the soot layer, with characteristic temperature depending on the wavelength detection band. PAHs and soot began to appear at a constant axial position for each fuel, independent of the rate of air flow. PAH formation either preceded or coincided with soot formation, indicating that PAHs are important components in soot formation. Soot growth continued for some time downstream of the flame, at temperatures below the inception temperature, probably through reaction with PAHs.     

Computational investigation of non-premixed hydrogen-air laminar flames

Laminar diffusion hydrogen/air flames are numerically investigated. Detailed and global mechanisms are compared. NO formation is modelled by full nitrogen chemistry and the extended Zeldovich mechanism. A satisfactory agreement between the present predictions and the experiments of other authors is observed. Significance of different ingredients of mathematical modelling is analyzed. Minor roles of thermal diffusion and radiation, but a significant role of buoyancy is observed. It is observed that the full and quasi multi-component diffusion deliver the same results, whereas assuming Le = 1 to a remarkable difference. NO emissions logarithmically increase with increasing residence time. NO is the dominating nitrogen oxide. Its share increases with residence time, whereby NO₂ and N₂O show a reverse trend. It is observed that the NNH route plays a remarkable role in NO formation, where the share of the Zeldovich mechanism increases with residence time from about 20% to 85%, within the considered range.     

Numerical simulation of transcritical strained laminar flames

We investigate reactive and non-reactive strained flows associated with high pressure cryogenic rocket engines. A detailed high pressure fluid model based on thermodynamics of irreversible processes, statistical mechanics as well as kinetic theory of dense gases is used. This model insures the positivity of chemical entropy production and of molecular transport related entropy production. We first investigate a mixing layer between cold hydrogen and oxygen and the dramatic influence of nonideal transport near thermodynamic instabilities. Diffusion and partially premixed H₂–O₂ transcritical flame structures are then studied as well as strain extinction limits and dilution extinction limits.