摘要:

A stochastic analysis of panicle ignition has been given using a Fokker-Planck equation. Under critical conditions the steady slate probability density of particle temperature is found to be bimodal. A numerical solution to the time-dependent Fokker-Planck equation yields an estimate of the time scale over which a transition (’stochastic ignition“) from the kinetic to the diffusion regime takes place.

ISSN:0010-2202    

DOI:10.1080/00102209108951737

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The classical premixed isothermal flow reactor continues to be of interest in combustion and chemical engineering applications. For many of these problems, such as the combustion of toxics, the degree of conversion in a finite-length reactor is a critical consideration. Assuming global kinetics, we focus on the completeness of combustion for arbitrary reaction orders and stoichiometry, for both steady and nonsteady inflow conditions. For the case of steady inflow, it is shown that the deficient species may decay either algebraically or exponentially, or the reaction may go to completion, depending on the reaction order of the deficient component. Although this feature is preserved for nonsteady inflow conditions, sinusoidal (zero-mean) fluctuations in the inflow velocities and concentrations result in a nonzero time-average perturbation in the deficient component at any given downstream spatial location. Consequently, it is shown that when time-dependent perturbations in the inflow conditions are present, a longer reactor is generally required to achieve the same average degree of conversion as for the case of the steady-stale reactor.

ISSN:0010-2202    

DOI:10.1080/00102209108951738

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The soot emission problem of the diffusion flame around a fuel droplet was taken up and the droplet diameter at the sooting limit,DL, was determined using a high speed camera and an optical method of soot detection. Since no soot emission occurs from the flume if the droplet diameter is smaller thanDL. the magnitude ofDLmight be an index to the sooting tendency of liquid fuels, just like the fuel flow rate at the sooting limit which is a useful index to estimate the sooting tendency of gaseous fuels. The effects onDLof fuel type, fuel blending, and surrounding air velocity, as well as time variations of droplet diameter and flame height during combustion, were investigated. The main findings were: (1)D, was increased and the sooting tendency was lowered with the increase in the air velocity although it promoted fuel evaporation and soot formation; (2) When two types of fuels with different sooting tendencies were blended, the concentration dependence ofDLwas greatly affected by the relative volatility between the component fuels.

ISSN:0010-2202    

DOI:10.1080/00102209108951739

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Results are presented of direct numerical simulations of a two-dimensional temporally developing high speed mixing layer under the influence of a second-order non-equilibrium chemical reaction of the type A + B→Products+Heat. Simulations arc performed with different magnitudes of the convective Mach number and with different chemical kinetics parameters for the purpose of examining the isolated effects of the compressibility and the heat released by the chemical reaction on the structure or the layer, A full compressible code is developed and utilized, so that the coupling between mixing and chemical reaction is captured in a realistic manner.

ISSN:0010-2202    

DOI:10.1080/00102209108951740

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

An experimental study of the surface structure of high Reynolds number turbulent premixed hydrogen/air jet flames is reported. Test conditions involved various values of turbulence intensities relative to the laminar flame speed, and stable/neutral/unstable conditions for preferential diffusion, within the wrinkled and mixing-limited thin flamelet regimes. Measurements included laser light sheet imaging to characterize flame surface properties and condilional laser velocimetry to characterize the turbulence properties of the unburned gas. It was found that flame surface area (and thus the local turbulent burning velocity), flame brush thickness and the fractal dimension of the flame surface progressively increased with distance from the flameholder, with maximum values eventually limited by approach to the flame tip. Additionally, the rate of development of these properties with distance from the flameholder increased as turbulence intensities relative to the laminar flame speed increased. Finally, preferential diffusion reduced/increased the rate of increase of these properties for stable/unstable conditions. Taken together, these observations imply that models and correlations of turbulent premixed flame properties for turbulent jet flames must account for effects of flame development, e.g., distance from the flameholder or the point of ignition, and preferential diffusion, in order to adequately characterize flame properties.

ISSN:0010-2202    

DOI:10.1080/00102209108951741

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

This paper presents a numerical analysis of the thermal-expansion effects on the behavior of healing and vaporizing fuel droplet streams in the flow near a healed flat plate. The gas-phase Navier-Stokes equations are simplified using (he boundary-layer approximation, while the thermal-expansion effect appears as a simplified state equation relating the gas density to its temperature. The main driving force for the thermal expansion is a one-step chemical reaction of the Arrhenius type between the vaporized fuel and the air. In addition, the heating of the gas near Ihe wall contributes to the thermal expansion. The droplets are injected parallel to the wall at a specified streamwise location, and are assumed to remain spherical at all times. The droplet temperature is nonuniform and time dependent. Comparisons of the droplet behavior in terms of trajectories, heating and vaporization rates are made. In all cases, it is predicted that the droplets will be driven away from the hot wall into the cooler gas region, thus increasing their lifelimes. This behavior is more pronounced for smaller and slow-moving droplets.

ISSN:0010-2202    

DOI:10.1080/00102209108951742

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

During the early stages of combustion when bituminous coal particles are in a nearly single particle combustion environment, a surrounding mantle of volatile products is observed. The volatile cloud and the particle are emitting with bright luminosity, representing a single particle flame. These single particle flames were observed through a high speed video movie technique. The formation and the duration of the flame were found to be dependent on the gas environment such as temperature and oxygen concentration. This transient observation of visible flame formation was discussed along with modeling descriptions of a volatile flame based on the diffusion limited combustion.

ISSN:0010-2202    

DOI:10.1080/00102209108951743

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Reduced reaction mechanisms for methanol-air diffusion fiames are developed by systematic reduction of a skeletal mechanism. By examining the magnitude of the net production rates for the intermediate species, it is shown that for stagnation point diffusion flames the steady state assumption is reasonable for CH2OH, HCO, O, OH, H02, and H2O2, but not for CH2O. By eliminating these species from the skeletal mechanism, a five-step reduced mechanism is derived

ISSN:0010-2202    

DOI:10.1080/00102209108951744

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The intensity and frequency dependence of sound emitted by a premixed flame are related to the dynamics of the flame surface within the corrugated Hamlet approximation. For the turbulent flame brush, its velocity, thickness, and surface area are reviewed. If the turbulence has a Kolmogorov spectrum the acoustic power spectrum varies as ω−5/2.

ISSN:0010-2202    

DOI:10.1080/00102209108951745

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Steady, fully burning wall fires were studied using alkanes (n-pentane,n-hexane, andn-heptane) burning in air. Completeness of combustion was characterized and evaluated for the three alkanes studied and was compared with results previously obtained for alcohols (methanol, I-propanol, and I-butanol) burning in air. When the alkanes were burred, with CO and CO2the principal carbon-containing products, less than 15% of fuel carbon was converted to product CO as opposed to CO2. Accounting for incomplete combustion in a numerical model that employs a one-step overall reaction led to less than a 5% decrease in burning rates and a drop in peak temperatures of less than 50 K. By contrast, when alcohols are burned, more than 30% of fuel carbon is found in product CO. Slightly lower temperatures in the free-convection boundary layers of the alcohols might account for some of this when the reactions concerning the conversion of CO to CO2are considered. However, reaction mechanisms currently favored for the combustion of methanol under fuel-rich conditions would account for a large part of the differences in amounts of CO formed by the alcohols and the alkanes.

ISSN:0010-2202    

DOI:10.1080/00102209108951746

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor