摘要:

The present paper reports the results of an experimental study of flame stabilization by a jet of air, normal to the flow of homogeneous propane-air mixture. The objective of the present work was to study in detail the mechanism of flame stabilization by a transverse jet.

ISSN:0010-2202    

DOI:10.1080/00102209108951764

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Laminar counterflow combustion configurations have been studied extensively in the investigation of fundamental combustion phenomena, such as fuel dilution, strain rate, and pressure dependence on flame extinction limits. However very few studies have been performed on turbulent counterflow diffusion flames. The turbulent opposed flow configuration chosen here consists of a fuel jet pipe ofd= 6.4 mm in diameter directed downward toward an air jet 50 mm in diameter, shielded wilh a concentric air jet of 13 mm diameter. The vertical distance between the fuel jet exit and the air flow can be adjusted to investigate the influence of the overall strain rate along the centerline of this configuration. A hydrogen-helium mixture was chosen to investigate the effects of dilution on the structure of the diffusion flame. To investigate the combustion effects on the turbulent mixing, reacting and non reacting conditions were studied at the same Reynolds number. In order to study the reaction zone structure, high speed tomography based on Mie scattering was employed using a copper vapor laser and a Fastax high speed camera. Different seeding techniques were used to visualize both the air and fuel jets. The tomographic records were digitized and recorded in a digital computer for statistical treatment. Significant differences in the wrinkle scales between the reacting and the non reacting flows were found. A fractal statistical analysis of the tomography records was done to quantify these differences. Seeding of both fuel and air jets provided a means for the estimation of the reaction zone thickness. From the lime resolved tomographic records an evaluation of the stretch of the reaction zone boundaries was made.

ISSN:0010-2202    

DOI:10.1080/00102209108951765

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The influence of the presence and availability of ambient, “secondary” air on the oxidation of CO in partially premixed burner flames is considered, using laser-induced fluorescence. Composite two-dimensional images of the distributions of OH-radicals and CO-molecules in partially premixed bunsen flames, burning in the open air, arc compared with expectations based on the assumption of a closed system. The results indicate that many features of the structure of these flames arc intrinsically due to the interaction of the flame with the ambient environment. The imaging technique is further applied to elements from a burner system from a household appliance. In these experiments, the availability or secondary air is varied by changing the distance between two of the component burners. The images show a declining OH density between the burners as the distance decreases, with a concomitant increase in CO.

ISSN:0010-2202    

DOI:10.1080/00102209108951766

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A comparison with experimental dame-speed data is made for results from a simple formulation. conveniently reducible to quadrature, of laminar isobaric flame propagation in an initially homogeneous gaseous mixture. A direct one-step irreversible bimolecular second-order chemical reaction with large Arrhenius activation energy is adopted, but account is taken of the modified exothermicity owing to partial dissociation of the product species and to other causes of incomplete oxidation. The effects arising from differing diffusivities for heat transfer and reactant-species transport arc developed to within the limitations of the model. A tractable general expression is obtained for the steady-laminar-flame-propagation speed, by exploitation of the two-zone (convective-diffusive, diffusive-reactive) deflagration-wave structure. For simple-fuel/air mixtures, the predicted variation of flame speed with equivalence ratio φ agrees fairly well with experimental data, with the provision that (1) the cold-mixture transport properties are evaluated for the pertinent value of φ, and (2) the overall activation energy, taken lo be invariant with φ, is ascribed values in the 10–15 kcal/mole range. In particular. The experimentally observed off-stoichiometric (often fuel-rich) condition for peak flame speed in simple-fuel/air premixtures is recovered. However, for simple-fuel/oxygen mixtures, the predictions deviate from the data, although the sense of the stoichiometry for which the peak speed occurs is recovered. Finally, observations concerning the transient planar deflagration wave are made, as background useful for a study of spherical flame propagation, lo be reported separately.

ISSN:0010-2202    

DOI:10.1080/00102209108951767

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The propagation of a spherical flame radially outward (from an unspecified ignition with purely local and transient consequences) is examined by approximate analysis for an initially homogeneous gaseous premixture. A direct one-step irreversible bimolecular second-order chemical reaction with large Arrhenius activation energy is adopted, but account is taken of the modified exothermicity owing to the partial dissociation of product species and to other causes of incomplete oxidation. The effects arising from differing diffusivities for heat transfer and reactant-species mass transfer, and from varying the equivalence ratio of the premixture, are considered. Algebraic expressions and a simple quadrature are derived which suffice to describe the evolution of the spherical flame speed to the asymptotic planar-flame value, as the magnitude of the flame radius increases from values modestly in excess of the diffusive scale to values which are large multiples of the diffusive scale, for a flame with a two-zone (convective-diffusive, diffusive-reactive) structure. Limited published data on the variation of flamefront-speed-versus-time behavior with equivalence ratio for simple-hydrocarbon/air mixtures are examined in view of the results.

ISSN:0010-2202    

DOI:10.1080/00102209108951768

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The combustion of premixed hydrogen/air mixtures flowing over a hot catalytic and non-catalytic surface has been modelled using a modified Newton's method and an adaptive gridding algorithm for the solution of the governing boundary layer equations. Detailed transport, including thermophoretic effects, and gas phase chemical kinetics were included in the model. The surface boundary conditions in the catalytic case include a mass transport limited surface reaction of H2and O2to form H2Oand OH. The relative amounts of H,0 and OH were taken from available experimental data and were varied to test the effect of the amount of desorbed OH on radical profile shapes. Heat release and O-atom profiles exhibit a double peak as a result of OH desorption. The heat release in the catalytic case is significantly lower near the leading edge of the plate resulting in slower flame propagation into the boundary layer. Recombination reactions of the unstable species (O, OH, H, HO2, H2O2) on the wall were included in the non-catalytic case. The sticking coefficients of the recombination reactions were varied to test the effect of the recombination rate on radical profile shapes.

ISSN:0010-2202    

DOI:10.1080/00102209108951769

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The extinction of counterflow diffusion flames in counter and co-rotating finite-jets is experimentally and theoretically investigated. In the experimental study of methane and butane diffusion dames, the critical minimum values of oxidizer concentration as a function of the jet angular velocity ω is determined for various values of the jet linear velocity. It is found that the oxidizer concentration at extinction decreases to a minimum value at the critical angular velocity ω = ωr, and thereafter increases with the jet angular velocity. This tendency is similar to that observed earlier for the limiting fuel concentration at extinction. Also, the tests for the extinction of methane flames in counter versus co-rotating jets show that the difference between the results for the two flows appears for ω > ωr,i.e. after the onset of the secondary recirculating flow. Under the latter condition, the limit-oxygen and fuel concentrations are found to be higher in counter versus co-rotating jets. In the theoretical part, the large activation energy asymptotic technique is applied, assuming viscous and incompressible hydrodynamics, for the thermo-diffusive analysis of the diffusion flame structure and extinction with general Lewis numbers of the reactants. Qualitative comparisons are made between the calculations and the laboratory observations. Among possible interactions between stretch rates, vorticity rates and the flamelets, the influence of helicily H = to ω v on the strained diffusion flamelets and its role in the statistical description of turbulent diffusion flames is discussed.

ISSN:0010-2202    

DOI:10.1080/00102209108951770

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Detailed numerical calculations are performed to determine the structure of heptane-air diffusion flames, and the results are compared with experimental measurements. The configuration used is the diffusion flame stabilized in the vicinity of a stagnation plane, which is formed by directing an oxidi2ing gas flow onto the vaporizing surface of a pool of heptane. Profiles of the concentration of various stable species and of the temperature have been measured by gas chromatography and by thermocouples. respectively. To evaluate the influence of strain on the structure or the flame, the measurements taken at a fixed composition of the oxidizer stream and at two values of the strain rate were chosen for comparison with the calculated results. The computations were performed using a chemical kinetic mechanism consisting of forty-two elementary reactions involving eighteen species. To simplify the chemical kinetic mechanism, it was assumed that heptane is attacked by radicals to form the heptyl radical whose decomposition to CH3and C3H6is represented by a one-step overall reaction. Good agreement was obtained between the results of the detailed numerical calculations and the experimental measurements. It was found that aspects of the structure of heptane-air diffusion flames relevant to asymptotic descriptions are similar to those of methane-air diffusion flames. Hence, the previously developed techniques for asymptotic analysis of the structure of methane-air flames can be used for analyzing the structure of heptane-air flames.

ISSN:0010-2202    

DOI:10.1080/00102209108951771

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Joint Reactive-Mie-Scattering (RMS) and Planar-Laser-Induced-Fluorescence (PLIF) techniques have been employed to study the impact of jet-shear-layer development upon flame behavior. Images of the complete penetration of the shear-layer vortex through the flame surface have been successfully captured utilizing these planar imaging techniques. Studies of these images have indicated that the degree of flame/flow interaction varies with respect to a wide range of flame conditions and results in the observation of several different types of flame zones—flamelets, multiple flamelets, thick flame zones, and local extinction. The probability of observing the different types of flame behavior was measured, and the results are presented. Analysis of the probability data indicates that the root-mean-square (rms) flame-surface fluctuation.L, provides a proper length for correlating the statistical behavior of the flame. Significant changes in the flame behavior were observed to occur at two critical values ofL—the minimal thermal thickness (∼ 1 mm) and the nozzle diameter (5 mm).

ISSN:0010-2202    

DOI:10.1080/00102209108951772

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Recent experimental work in the area of spark ignition of hydrocarbon fuel sprays has demonstrated that it is impossible to define a unique minimum ignition energy. Instead there exists a range of energies over which ignition becomes more or less probable. In connection with recent experimental and modelling work studying the spark ignition of a monodisperse fuel spray, a model has been developed to predict the ignition frequency based on the variation of the spark energy and the instantaneous spark gap equivalence ratio. Random normal distributions of these two parameters were generated with a Monte Carlo routine and used in conjunction with a Characteristic Time Model for ignition. The resulting ignition frequency simulations were nearly identical to the experimentally observed values, supporting the hypothesis that the probabilistic nature of spray ignition is the result of variations in the energy levels of individual sparks and the random distribution of droplets in the fuel spray.

ISSN:0010-2202    

DOI:10.1080/00102209108951773

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor