摘要:

Using laser imaging data for scalar mixing collected in turbulent jets of Frcon-12, synthesised images are generated for reaction zones in turbulent nonpremixed flames of hydrogen at high Damkholer numbers. It is found that Quasi-Equilibrium Distributed Reaction (QEDR) zones govern the flame structure except in the first fifteen jet diameters near the nozzle. This structure which otherwise follows the stoichiometric mixture fraction ξ, tends towards that of scalar dissipation, χas the distance from the nozzle increases.

ISSN:0010-2202    

DOI:10.1080/00102209008951645

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Molecular mixing and finite rate chemical reactions in a two dimensional viscous vortex are examined analytically. Two species initially separated across a plane are allowed to diffuse and react in the presence of a line vortex situated at this separation plane. Solution of the species diffusion and reaction equations are obtained locally. From these solutions, the concentration field of the species is composed. The probability density distributions are calculated using Taylor's frozen flow approximation. They are determined for a range of vortex strengths and for several values of Schmidt number at different times during the growth of the vortex. An asymptotic analysis is presented with a favorable comparison of results in the high vortex strength limit. The reacting vortex is computed by use of Green's function solution of the species equation. The results for the reacting vortex are compared with those for the non-reacting vortex and some insight is gained concerning the form of the probability density function in such configurations. The mixedness parameter is found to be approximately linear with Reynolds number (based upon vortex strength), linear with time, and weakly dependent upon Schmidt number.

ISSN:0010-2202    

DOI:10.1080/00102209008951646

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Based on spatial and temporal averaging of the conservation equations, an approximate analysis provides a convenient vehicle for analyzing the nonlinear behavior of unsteady motions in combustion chambers. Recent work has been concerned with the conditions for existence and stability of limit cycles, and in particular their dependence on the order of nonlinear terms contained in the equations of motion. It seems to be generally true that if only nonlinear gasdynamic terms to second order are accounted for, a stable nontrivial limit cycle is unique. This implies that no initial disturbance will cause a linearly stable system to execute a limit cycle, a result contrary to prior experience with, for example, solid propellant rocket motors. In this paper, we study some conditions under which true nonlinear instabilities may be found, with special attention focused on the effect of nonlinear gasdynamics. Our results indicate that third-order terms in the acoustic motions do not lead to triggering to either a stable or an unstable limit cycle; they only modify the stability domain of the system. However, the interactions between mean flow and nonlinear acoustic waves may trigger a linearly stable system to an unstable limit cycle. The influence of nonlinear combustion response may have quite different consequences and will be discussed in Part II of this work.

ISSN:0010-2202    

DOI:10.1080/00102209008951647

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Two-dimensional flame structure measurements of a freely propagating premixed turbulent flame were examined for fractal character over a range of turbulent Reynolds numbers from 52 to 1431 and Damkohler numbers from 10 to 889. The fractal dimension was found to increase with u'/SL, from 2.13 at u'/SLof 0.25 to 2.32 at u'/SLof 11.9. An inner cutoff was not identified due to limited spatial resolution, however, it was shown that the inner cutoff was smaller than the Gibson scale for the conditions studied. The outer cutoff was found to occur at scales comparable to the integral scale, however, the cutoff was a gradual, rather than sharp, transition from fractal behavior. A heuristic relationship between the turbulent flame surface fractal dimension and u'/SLwas developed which agrees well with the fractal dimension measurements made in this study, as well as those made in a number of different flame configurations.

ISSN:0010-2202    

DOI:10.1080/00102209008951648

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A model for the radiation affected liquid fuel burning on water is developed which improves substantially on an existing model by including the turbulent fire plume and the explicit effects (hotness and optical thickness) of fire radiation. In achieving these objectives, the experimental literature on flame height and velocity is utilized. The fire plume above the slick is divided into three regions: the continuous flame; the intermittent flame; and the thermal plume. To describe the fire plume, Taylor's entrainment model is used which assumes top-hat profiles for the radial velocity and temperature and relates the entrainment and vertical velocities at a given height. The contributions from gaseous combustion products, mainly CO2and H2O, as well as from particulate matter to radiative heat transfer are also accounted for. Both linear and nonlinear solutions are obtained and compared to examine the accuracy of the usual approach of linearization in the radiative transport equations. The effect of radiation on fuel burning is demonstrated as a function of the flame hotness and optical thickness. Radiation is shown to lower the temperatures in the high temperature region of the flame and raise the temperatures in the low temperature region near the fuel surface. It is further shown that fuel burning increases monotonically with increase in radiation effects. It is concluded that the contribution of fire radiation to fuel burning is substantial and may exceed that of conduction heat transfer by an order of magnitude. The model is also compared with available data for crude oil burning on water, yielding reasonable trends within the uncertainty of the experimental literature.

ISSN:0010-2202    

DOI:10.1080/00102209008951649

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Simultaneous laser Raman, Rayleigh and fluorescence measurements have been made in diluted turbulent methane diffusion flames. Time and space resolved results for CH4, O2, N2, H2O, CO, CO2, H2, OH, temperature and mixture fraction arc presented as scatter plots and conditional probability density functions for conditions close to extinction. Dilution, whether by air or nitrogen, results in a flame structure with wider reaction zones, and it is shown that the reaction zone width markedly affects the development of bimodality of species and temperaturepdfsaround stoichiometric composition. The position of the mean stoichiometric contour, relative to the shear layer, is also found to alter the shapes ofpdfsof temperature and species. Hydrogen and hydroxyl radical mass fractions are much in excess of predictions for stretched laminar flames. Dilution with air does not seem to alter the chemical composition near extinction, either at stoichiometric mixture or in the richer reactive region of the flame. and there are strong indications that a two-variable approach to the modelling of the chemical kinetics is feasible.

ISSN:0010-2202    

DOI:10.1080/00102209008951650

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A new optical set up for the measurement of the angular distribution of the polarized components of light scattered by single droplets has been realized. It allows the simultaneous determination of the size, velocity and complex refractive index of the droplets. The vaporization and chemical transformation in the liquid phase have been studied with this technique in the temperature range between 20°C to 700°C for arrays of droplets of pure hydrocarbons and commercial light oils. It was found that droplets composed by commercial blending became opaque in the visible (λ = 514.5 nm) while those composed by pure compounds exhibit only a vaporization without reaction in the liquid phase. This effect is attributed to a class of aromatic compounds with polar characteristics and it is qualitatively similar, although less relevant, to that which takes place inside droplets of heavy oils at high temperature. The increase of absorbivity of heated up droplets has been detected also in spray in burning conditions and the effect may have relevance in the formation of particulate phase during combustion.

ISSN:0010-2202    

DOI:10.1080/00102209008951651

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We consider small two-dimensional spatial perturbations to a one-dimensional oblique detonation wave, and examine how they behave as the distance along the wave, in the downstream direction, is increased. With the kinetics modeled by one-step Arrhenius kinetics it is shown that, for large activation energy, the perturbations grow. This result is interpreted as astructural instabilitywhich will prevent the existence of one-dimensional oblique detonation waves. This result is compared with numerical results of Fujiwara,

ISSN:0010-2202    

DOI:10.1080/00102209008951652

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Percolation theory deals with the statistics of random arrays. Two-dimensional percolation theory has been used as a model of forest fires. The important result is that if burnable elements are randomly placed on a two-dimensional array with a packing ratio p, then there is a value pcbelow which fires will not propagate, and above which a fire will propagate from one end of the array to the other.

ISSN:0010-2202    

DOI:10.1080/00102209008951653

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor