摘要:

The micro-structure of a premixed, atmospheric-pressure, fuel-rich (equivalence ratio 2.6 ), flat flame of methane has been determined experimentally using a heated micro-probe coupled to on-line gas chromatography/mass spectrometry (GC/MS ). The identities and absolute concentrations of over 40 major and minor species have been established, including a large number of aromatics, substituted aromatics and polycyclic aromatic hydrocarbons (PAH ) by the direct analysis of samples withdrawn from the flame without pre-concentration. Mole fractions of the species quantified were in the range 0.45—2.0 × 10−9. The largest PAHs detected were the family of C20H12s (molecular weight 252 ), that include benzo (a )pyrene, perylene and benz (e )-acephenanthrylene, with peak concentrations reaching 1 ppmv. The results show the preferential formation of fused PAHs with even number of aromatic rings over PAHs with odd number of rings. The PAHs detected also suggest that they can be formed by at least 3 parallel routes; synthesis via the sequential addition of small building blocks, such as C2H2, by the recombination of smaller PAHs, or via isomerization. The measurements provide information on the identities and levels of hazardous air pollutants formed in combustion processes and represent useful new data for the development and validation of detailed reaction mechanisms describing their origin and fate.

ISSN:0010-2202    

DOI:10.1080/00102209508907792

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The regimes of Ni infiltration during combustion of a sample comprising layers of the Ti-C reactant mixture and Ni have been studied. Ni spreading during the layer-by-layer combustion of the mixture was found to proceed in two qualitatively different limiting modes. These modes differed in the direction of the metal flow upon melting: a thin Ni layer flowed toward the combustion front, while the thick one moved in the direction of the combustion front propagation. The characteristic times of the main stages of the SHS infiltration process, i.e., heating, combustion and infiltration, have been estimated. The type of infiltration has been determined from the ratio of the characteristic times of the Ni layer heating to the combustion of the respective mixture layer. A symmetrical Ni distribution across the sample thickness has been observed during the simultaneous combustion of the reactant layers. The Ni distribution in the final product was found to be connected with the infiltration inertia regime.

ISSN:0010-2202    

DOI:10.1080/00102209508907793

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Solutions of the steady, two-dimensional Navier-Stokes, thermal energy and species conservation equations have been computed for a low-speed dump combustor geometry with a downstream constriction. The equations are solved in the conservative finite-difference form on a nonuniform rectilinear grid of sufficient resolution to accurately capture the momentum/thermal boundary layers and, with somewhat lower accuracy, the flame structure. The chemistry is represented by a finite-rate reduced methane-air mechanism involving seven species. The computed flame shapes for different cavity lengths and mixture equivalence ratios are compared to those determined experimentally by OH (A) chemiluminescence. Except for the leanest mixture studied, the computed and experimental flame lengths agree to within a few percent. In general, the computed flame lengths depend more strongly on equivalence ratio than does the experimental. The dependence of computed flame shape on cavity length generally agrees with that determined from experiment. Computations indicate that the structure of the recirculation zone is similar to that of a nonreacting flow for short cavities, but is qualitatively different for longer cavities. These differences are a consequence of the heat release in the flame front. Simulations with additional fuel (representing a hydrocarbon waste)injected into the recirculation zone show two stages of heat release. The first is associated with premixed burning of the primary fuel-air mixture, while the second is associated with a diffusion flame at the interface between the oxygen-starved recirculation zone and the comparatively oxygen-rich combustion products from the primary flame. These simulations indicate that excess oxygen from a lean primary flame can be effectively utilized for waste destruction, even for very lean core flames resulting from oxygen enrichment. Oxygen utilization is seen to be somewhat better for longer cavities.

ISSN:0010-2202    

DOI:10.1080/00102209508907794

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A numerical model is developed to simulate flame spread over a thin solid fuel horizontally in a gravitational field. In this paper, the burning behaviors in normal and elevated gravity are analyzed. In normal gravity, the flame structures are investigated in detail in order to find the essential features of such spreading flame. The flame can be separated into an upper flame and a lower flame by the fuel plate. There are three high temperature regions: two of them are at the upper and lower flame fronts, respectively, and the other one is at the combustion plume, just behind the burnout point. The induced flow moves upward and forms a thermal plume above the fuel plate. An entrainment of air moves toward the flame, opposed to the upper spreading flame. Below the fuel plate, the induced flow moves concurrently with the lower spreading flame. From the reaction rate distribution, the characteristics of triple-reaction-zone is identified. The heat flux on the bottom side is more intensive and extends further upstream than that on top side, confirming that the controlling mechanism is determined by the spreading flame underneath the fuel plate. A qualitative comparison with experiments is given. A parametric study by elevating gravity is followed. A blow off limit occurs at about g = 5.5. During the process, the horizontal flame spread rate is found to be slower than the downward one behind g = 2.5, where the lower flame is broken due to the up-rising induced flow. The variation in flame structures as a function of gravity is discussed.

ISSN:0010-2202    

DOI:10.1080/00102209508907795

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In this paper, we present measurements of CO and NO emissions and the fraction of the heat of reaction emitted as radiation for a two stage burner. The fuel-air mixtures of different equivalence ratios were burned in two separate porous ceramic sections. The primary radiating surface in this burner is the outer cylindrical surface of the porous media. We compared the two stage results to those obtained by burning the entire mixture in a single stage. The emissions of NO and CO from the two stage burner were lower than those from the single stage burner. In addition, the emissions of the two stage burner could be minimized by proper choice of the equivalence ratios in the two stages without significantly affecting the radiant fraction. Typical values of NO and CO emissions from the two stage burner were l7-30 ppm and 10-75 ppm (corrected to 0% O2), respectively, for an overall equivalence ratio of 0.75-0.95. Typical radiant fractions were 35-45% for an energy input of 3.9-7.1 kW. For the two stage burner, the emissions and radiant fractions were relatively constant for a broad range of energy inputs.

ISSN:0010-2202    

DOI:10.1080/00102209508907796

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The interior of a squat rectangular solid, with isothermal noncatalytic impervious walls, is subdivided into two equal half volumes by a thin plastic film. One half-volume is filled with hydrogen, diluted with argon; the other, with oxygen, diluted with helium; the density, temperature, pressure, and (hence) “effective molecular weight” of the two half volumes are initially equal. At time zero, the film is broken to permit, briefly, interpenetration of the initially segregrated reactants, and ignition quickly follows. Except for quenching near cold walls, a diffusion flame results; it remains planar in microgravity. The subsequent position and temperature of the diffusion flame may be predicted, facilely, within a (Shvab-Zeldovich) formulation which adopts a direct one-step irreversible reaction. Specifically, we generalize the (Burke-Schumann) limit of diffusion-controlled, combustion to encompass: (1) a travelling flame (a moving boundary between fuel and oxidizer), and (2) differing diffusivities for fuel, oxidizer, and heat. Furthermore, we derive a sufficient condition for the extinction of burning by asymptotic analysis of a finite-rate, Arrhenius-type model of the one-step reaction. The conjunction of a numerical solution of the diffusion-controlled (Stefan-type) formulation, and an asymptotic solution of the finite-rate formulation, readily permits prediction of an upper bound on the time until extinction. An extended microgravity-testing interval, available during Shuttle flight, permits comparison of these predictions with experimental observations.

ISSN:0010-2202    

DOI:10.1080/00102209508907797

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The thermal effects of incident radiation on the burning characteristics of homogeneous solids are considered and the practical implications of these effects in solid rocket motors are assessed. The applicability of the equivalence principle (equivalence between radiation and initial temperature) is investigated experimentally and analytically. Experimental steady burning rates as a function of initial temperature and mean radiant flux are presented for a fine ammonium perchlorate (A P) composite propellant which indicate that the equivalence principle is accurate to within experimental error. The equivalence principle is also assessed analytically by considering the worst case conditions of condensed phase controlled burning. For deflagration and pyrolysis of solids controlled by condensed phase reactions it is shown that a modification of Ibiricu and Williams' high activation energy asymptotic burning rate expression allows consideration of the effect of incident radiation on steady burning rate over a wide range of propellant opacities. Numerical simulations are used to verify this modification. Thermal radiation from combustion gases in typical non-metalized solid rocket motors is examined using band model calculations. These calculations show that even pure molecular gas radiation (no particles) can reach near blackbody levels for realistic motor conditions. The effects of thermal radiation on burning rate and temperature sensitivity are also examined using the equivalence principle. It is shown that differences in burning rate and temperature sensitivity between motors and propellant strands which in the past have primarily been attributed to erosive burning, could in some instances be due to thermal gas radiation. The implications of gas radiation on combustion stability are also discussed. It is shown that radiation may have either a stabilizing or destabilizing effect, according to the initial temperature dependence of the temperature sensitivity σp(T0), although in most cases it is a stabilizing influence.

ISSN:0010-2202    

DOI:10.1080/00102209508907798

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Propagation of one-dimensional “laminar” flame in combustible mixtures with spatially-periodic longitudinal velocity oscillations is investigated within the thermo-diffusive flame model using the large activation energy asymptotic technique. The ratio of the flame propagation velocity in the periodically disturbed versus undisturbed fieldsXis examined as a function of the amplitude parameter Γ for the complete range of wavelength parameter y (0, ∞). It is found that for infinitely longwave lengths γ →0 the dimensionless flame velocityXdecreases monotonically with Γ from unity towards zero without any extinction. For intermediate wavelengthsXfirst increases to a maximum value above unity and then decreases with Γ and eventually at a critical value Γ > Γebecomes less than unity as it continues to decrease towards zero. Therefore, for a given wavelength a critical amplitude of velocity oscillation is identified that results in a maximum burning velocity for the combustible mixture. In the limit of exceedingly short wavelengths γ →∞ the dimensionless velocityXis double valued function of Γ resulting in fast and slow burning flames. Also, a maximum critical value of amplitude Γeis predicted above which the flame will extinguish. Since planar flames do not experience any stretch by either flow divergence or curvature, the extinction is attributed to voluminal stretch introduced by Buckmaster. The predictions are found to be consistent with the prior experimental observations.

ISSN:0010-2202    

DOI:10.1080/00102209508907799

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A mathematical technique is used to reduce several hydrogen/air reaction systems to one-and two-step schemes. The reduction technique is based on the use of intrinsic low-dimensional manifolds in composition space as introduced by Maas and Pope (1992). In this method it is assumed that the fastest reaction groups of the chemical source term are in steady-state

ISSN:0010-2202    

DOI:10.1080/00102209508907800

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Unsteady motions of 2-dimensional reactive flows have been simulated to investigate the hydrodynamic effect and the diffusive-thermal effect on the instability of freely expanding cylindrical flames. The numerical model includes compressibility, viscosity, heat conduction, molecular diffusion, one-step chemical reaction, and convection. We obtained the growth rates of disturbances superimposed on the cylindrical flames depending on their wave numbers. The results showed that the growth rates in the cylindrical flames are consistent with those in the plane flames for the case where the Lewis number is unity. Therefore, the hydrodynamic effect on the flame instability is independent of the mean curvature of the front. When the Lewis number is smaller/larger than unity, the growth rates in the cylindrical flames are lower/higher than those in the plane flames. It means that the instabilizing/stabilizing influence of the diffusive-thermal effect is less in the cylindrical flames than in the plane flames.

ISSN:0010-2202    

DOI:10.1080/00102209508907801

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor