Laminar hydrocarbon flame structure

From the very first experimental studies, based essentially on flame propagation velocity and flame emission measurements, successive improvements in analytical and numerical techniques have contributed to make the analysis of laminar flame structure a powerful tool for extending the knowledge on combustion chemistry, thermodynamics, and transport properties. This better knowledge is very beneficial to design efficient combustion devices with reduced pollutant emission. Overall net species production rates are derived from the experimental determination of the evolution of the gas stream velocity, temperature, and species concentrations in the direction normal to the flame front. For species involved in a limited number of reactions, rate constants can be calculated at the next step. The development, in the early 1980s, of numerical codes for simulating the structure of one-dimensional laminar premixed flames the flame structure data to be directly used for validating detailed reaction mechanisms. Species analyses are still performed with techniques based on local gas sampling by probes, despite flame perturbations, but flame structure analyses have been markedly enriched by the use of non-intrusive spectroscopic techniques. The former allow the analysis of a large variety of species, and they have proven to be very well adapted to the large number of intermediate species formed in rich flames or in flames fed by heavy fuel molecules. The molecular beam mass spectrometry technique has been recently improved by the use of new photoionization sources that allow identification of isomers and extend the knowledge on intermediate species involved in formation of benzene, polycyclic aromatic hydrocarbons, and soot in flames. Amongst various spectroscopic techniques applied to flame structure analyses, laser-induced fluorescence has been largely used to perform accurate quantitative measurements of intermediate radicals that play a key role in the prompt-NO mechanism. In this study, the contribution of flame structure studies to a better knowledge of formation mechanisms of benzene and NO _x is briefly reviewed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 22–42, July–August, 2009.     

Laminar flame theory. Part II

The laminar flame theory under the condition of minimum entropy production is presented. The equation of the temperature boundary layer is obtained for Le=0, and its general solution is found. It is shown that the mechanism of laminar flame propagation can be treated from the viewpoint of stationary thermal explosion in a cylindrical vessel of radius 2 as the critical regime in which the burnup is taken into account in an unusual form. Tomsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 5, pp. 22–30, September–October, 1993     

Laminar flame theory. Part III

We present the final part of the laminar flame theory based on the notion of the minimum entropy production in weakly nonequilibrium systems. A correction has been made to the Zel'dovich-Frank Kamenetskii-Landau formula for the laminar flame velocity at large Lewis numbers. If the flame velocity calculated in this work corresponds to the stable regime of front propagation, the problem under consideration is analogous to the Kolmogorov-Petrovskii-Piskunov problem. With sufficiently high reaction orders, the width of a thermally diffusive boundary layer increases by a logarithmic law. V. V. Kuibyshev Tomsk State University, Tomsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 6, pp. 25–33, November–December, 1993.     

Laminar propane-air flame in a weak electric field

Three mechanisms of the electric field influence on combustion of gases are considered: ohmic heating, effect of chemical reactions on the kinetics, and ionic wind, which is the most important factor. Experimental data presented in the paper testify to a local action of the electric field directly on the zone of chemical reactions, which leads to deformation of the flame front.     

Laminar flame in the approximation of weak compressibility of reacting flows

The complete system of hydrodynamic equations describing the development of instability of the reaction front in the hydrodynamic approximation is reduced to a closed system of surface equations with the use of Lagrange variables, integrals of motion, and their analogs. In the adiabatic approximation, it is shown how to take into account the acoustic vibrations of the gas density caused by this motion.     

Steady combustion of solid fuel gas-suspensions. Laminar diffusion two-phase flame

Odessa. Translated from Fizika Goreniya Vzryva, Vol. 26, No. 6, 54–62, November–December, 1990.     

Laminar flame speeds and extinction limits of conventional and alternative jet fuels

Experimental results of laminar flame speeds and extinction stretch rates for the conventional (Jet-A) and alternative (S-8) jet fuels are acquired and compared to the results from our earlier studies for neat hydrocarbon surrogate components, including n-decane and n-dodecane. Specifically, atmospheric pressure laminar flame speeds are measured using a counterflow twin-flame configuration for Jet-A/O₂/N₂ and S-8/O₂/N₂ mixtures at preheat temperatures of 400, 450, and 470 K and equivalence ratios ranging from 0.7 to 1.4. The flow field is recorded using digital particle image velocimetry. Linear extrapolation is then applied to determine the unstretched laminar flame speed. Experimental data for the extinction stretch rates of the nitrogen diluted jet fuel/oxidizer mixtures as a function of equivalence ratio are also obtained. In addition, the experimental data of Jet-A are compared to the computed values using a chemical kinetic mechanism for a kerosene surrogate reported in literature. A sensitivity analysis is further performed to identify the key reactions affecting the laminar flame speed and extinction stretch rate for this kerosene surrogate.     

Laminar flame structure in a low-pressure premixed H2/O2/Ar mixture

The chemical structure of a stoichiometric hydrogen-oxygen flame stabilized on a flat burner at a pressure of 47 torr is studied by molecular-beam mass spectrometry and computer simulation. Concentration profiles are measured for the stable flame components H₂, O₂, and H₂O, as well as for the atoms and radicals H, O, and OH. The concentration profiles calculated using two different kinetic mechanisms are in acceptable agreement with experimental data. It is also shown that for sampling probes with thin walls (<-0.1 mm) at their tips, there is no need to perform the calculations using a specified temperature profile measured by thermocouples located near the aperture of the sampler, as recommended by a number of authors. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 3, pp. 29–34, May–June 1999.     

含氨燃料预混火焰的层流火焰速度及NO排放特性

将甲烷或氢气与氨气共燃可以克服NH₃火焰的点火能量高、燃烧速度慢的缺点。为了解NH₃作为燃料的燃烧特性,对含NH₃燃料进行一维层流预混火焰数值模拟,研究其层流火焰速度及NO排放特性。采用文献中5个简化反应机理进行数值计算,发现Okafor机理模拟NH₃/CH₄/air火焰精度更高;Xiao机理模拟NH₃/H₂/air、NH₃/air精度适中,计算时间较短。此外,开展了当量比、燃料混合物组分比例、压力等参数对含NH₃燃料燃烧时烟气中NO浓度影响的研究。研究发现：含NH₃燃料燃烧时NO主要通过OH、H、O自由基和O₂分子的消耗而生成,主要通过与NH_i（i=0, 1, 2）自由基反应消耗;含NH₃燃料在富燃状态下燃烧可有效减少NO排放,但富燃燃烧效率低,可采用富燃-贫燃分级燃烧技术来提高燃烧效率,同时保持NO的低排放;掺有较多NH₃的含NH₃燃料在中高压下燃烧时可有效减少NO排放。     

Laminar flame speed measurements of dimethyl ether in air at pressures up to 10 atm

Laminar flame speed measurements of dimethyl ether/air mixtures were made at 1, 5, and 10 atm with equivalence ratios ranging from 0.7 to 1.6. All experiments were performed in a large cylindrical constant-volume bomb with optical access. A new method for converting flame images into flame radii was used. Results reported in other studies were investigated, and some explanations on the disparities found are presented. A full uncertainty analysis was performed combining precision errors from data scatter with predicted systematic errors. Uncertainties ranging between 4.2% and 8.6% were found depending on the equivalence ratio and initial pressure. Experimental results agreed well with some other spherical flame experiments and counterflow flame measurements, but were found to be much lower than PIV-based stagnation flame results. Also, two spherical flame studies deviated significantly both in magnitude and trend. Critical radii and Peclet numbers, defined by the onset of rapid flame acceleration, were recorded for all high-pressure experiments. Markstein lengths were measured and showed a decreasing trend with increasing equivalence ratio. Three different methods were used to define the laminar flame thickness, and large disparities were found between them. In this study, the modeled temperature gradient method for the definition of flame thickness is preferred over other methods. Modeling was performed with the latest version of a C3 chemical kinetics mechanism. Good agreement is seen between the experimental results and the model at all pressures. Emphasis is placed in this paper on reporting experimental uncertainties, calculated density ratios, flame temperatures, and flame radii ranges used for data analysis, and the results resolve some discrepancies seen in the literature for dimethyl ether flame speeds.     

Defect-free drying of large fine-particle zirconia compacts prepared by gelcasting method

In this work, drying of bodies prepared by gelcasting fine submicrometre-sized zirconia particles was studied and a drying process for defect-free bodies with large cross-sections was proposed. It was found that the cracking of large bodies could be prevented by reducing the monomer content and using appropriate non-volatile cosolvents. Glycerol and polyethylene glycols with different molecular weights were used as non-volatile cosolvents in aqueous ceramic suspensions. The complex effects of the individual cosolvents on the gelcasting process and, in particular, on the drying step were investigated and explained. The applicability of individual cosolvents for the gelcasting process were discussed and their optimal use was indicated.     

Laminar transient flow in pipes

The effects of fluid viscosity, pipe length, radius and pressure wave celerity are examined by solving the equations of fluid motion numerically for impulsively started flows. Velocities are obtained in terms of axial and radial positions and time. Pressures are obtained as a function of axial position and time using an averaging process. The dimensionless group \documentclass{article}\pagestyle{empty}\begin{document}$\frac{{\rho a{\rm R}^2}}{{\mu {\rm L}}}$\end{document} is shown to determine the development of these profiles. At low values of this group, a significant axial variation in the velocity profiles occurs. The pressure fluctuations accompanying startup however are progressiveely more damped as this group decreases. Such fluctuations are shown theoretically and experimentally to disappear when the aforementioned group falls to about 1.6. The numerical method used is capable of being applied to other transient pipe flows.     

Crack Bifurcation in Laminar Ceramic Composites

Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al₂O₃ layers sandwiched between thicker layers of Zr(12Ce)O₂. The Al₂O₃ layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O₂ layers were nearly free of residual, tensile stresses because they were much thicker than the Al₂O₃ layers. The ceramic composites were fabricated by a green tape and codensification method. Different specimens were fabricated to examine the effect of the thickness of the Al₂O₃ layer on the bifurcation phenomena. Bar specimens were fractured in four-point bending. When the propagating crack encountered the Al₂O₃ layer, it bifurcated as it approached the Zr(12Ce)O₂/ Al₂O₃ interface. After the crack bifurcated, it continued to propagate close to the center line of the Al₂O₃ layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O₂ layer, where it bifurcated again as it entered the next Al₂O₃ layer. If the loading was stopped during bifurcation, the specimen could be unloaded prior to complete fracture. Although the residual stresses were nearly identical in all Al₂O₃ layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm.     

Laminar Drag Reduction in Hydrophobic Microchannels

The apparent slip effects of laminar water flow in smooth hydrophobic microchannels and patterned hydrophobic microchannels were investigated. A series of experiments were performed to demonstrate the drag reductions for laminar water flow in hydrophobic microchannels. These microchannels were fabricated from silicon wafers using photolithography and were coated with hydrophobic octadecyltrichlorosilane (OTS). To generate a larger drag reduction, the patterned hydrophobic microchannels were fabricated to allow the liquid to flow over a region of trapped air in the cavity between the microridges. With the geometrical dimensions used, pressure drop reductions ranging from 10 to 30 % were found in the smooth microchannels and patterned microchannels. The pressure drop reduction was shown to increase with increasing microridge spacing and decreasing microchannel width. Using micro‐particle image velocimetry (PIV), we measured an apparent slip velocity at the wall of approximately 8 % of the centerline velocity, yielding a slip length of approximately 2 μm in the smooth hydrophobic microchannel. Theoretically, the analytical solution derived for three‐dimensional flow in a rectangular duct is presented to predict the slip velocity and slip length at the wall based on the pressure drop measurement. These results are in agreement with the experimental data obtained using micro‐PIV.     

The role of flame flux in opposed-flow flame spread

The flame spread process is driven by the net heat flux to the specimen surface, including the flux from the flame itself. This flame flux is important since it comprises a major part of the driving force causing flame fluxes were obtained. The values which are reported do not appear consistent and show more deviation among materials than would be anticipated. The most common fire test used for obtaining engineering data on flame spread (ASTM E 1321) also is not formulated in terms of flame flux as a driving force. This motivated an experimental programme, whereby six materials have been studied using the flame spread geometry of the ASTM E 1321 test, but with additional instrumentation for recording heat fluxes. The flame fluxes obtained experimentally in this study show much less variation among materials than the comparable data from the literature survey.     

Laminar shear mixing in reaction injection molding

Turbulent flow in the impingement mixing head of a reactions injection molding machine produces a laminated mixture, whose striations are mostly so thick that any subsequent copolymerization would be hindered by diffusion. However, further reduction in striation thickness occurs during laminar flow through the runner. This is calculated here and is a function of the radial position in the runner and of its length/diameter ratio. So, for example, when L/D = 10, it is predicted that at least 79 percent of, the reagent striations would be thin enough to permit reaction under kinetically controlled conditions.