摘要:

The process of spark ignition and the subsequent flame propagation in an internal combustion engine have been investigated. A unique theoretical model which considers the various physical and chemical phenomena associated with the ignition process has been developed. It employs a two-dimensional cylindrical coordinate system and assumes axial and radial symmetry. The model employs also a detailed chemical reaction scheme for a methane-air mixture which contains 29 chemical species and 97 reactions. The thermodynamic and transport properties are evaluated by using statistical thermodynamics and molecular theory approach while including the various energy modes stored in the mixture particles. The appropriate conservation equations are solved numerically by using an integration of the PHOENICS and the CHEMKIN codes. It was concluded from the numerical results that the spark kernel growth can be described as a two-step process. The early short stage (1–5 μs), which involves a pressure wave emission, is followed by a much longer (1–10 ms) diffusive period. In the early stage the mass and energy transfer processes are very much dominated by the pressure wave and the violently expanding plasma kernel, with only negligible contribution of the chemical reactions to the kernel development During the diffusive stage, when the contribution of the chemical reactions to the kernel expansion is sensible, an inflammation zone is created. Based on a parametric study it was concluded that the spark kernel expansion can be enhanced by increasing the spark power during the early stage of its development, decreasing the electrodes' diameter and increasing their gap distance as these reduce substantially the energy losses to the electrodes.

ISSN:0010-2202    

DOI:10.1080/00102209508960387

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Flame curvature in axisymmetric turbulent jet flames are obtained experimentally foru'/SLfrom 0.8–1.5. The measurements indicate that the flame curvature statistics exhibit a substantial change as a function of the axial location in turbulent jet flames primarily due to the evolution of flame wrinkle structures. In particular, it is found that the flame curvature tends to be very small near the burner exit, while development of the flame wrinkle structures further from the burner exit results in an increase in magnitude and asymmetry in flame curvature probability density functions (pdΓs). The mean flame curvature in the vertical plane is close to zero in spite of the asymmetry in the flame curvature pdΓs except near the flame tip where the mean flame curvature becomes negative due to the frequent occurrences of cusps. In contrast, the flame curvature in the horizontal plane are strongly affected by the burner dimension and geometry in that the magnitude of both positive and negative flame curvatures are increased by the finite curvature of the conical jet flame shape in the mean. The mean flame curvature in the horizontal plane tends to be of the order of the burner radius. The flame orientation statistics of turbulent jet flames have also been obtained, which show a similar evolution as a function of the distance from the burner exit

ISSN:0010-2202    

DOI:10.1080/00102209508960388

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Ignition of a cold fuel injection into a supersonic hot airstream is analyzed both asymptotically and numerically. The present study extends the Liñán and Crespo's analysis of ignition in a mixing layer of parallel streams to the analysis of ignition in a jet flow. The ignition distance is related explicitly to the thermodynamic parameters and the width of jet The asymptotic analysis not only correlates the ignition distance in a fuel jet with that in a mixing layer of parallel streams, but also reveals the difference between them. It is shown that ignition mechanism of a fuel jet in an infinite air stream can be divided into three regimes, ignition regime in a mixing layer of parallel streams, ignition regime determined by jet scale and chemical scale, and ignition regime controlled by chemical scale only, according to the width of jet The present analysis theoretically exhibits the linear dependence of ignition distance on the logarithmic function of jet width. A good agreement is established between the results obtained from the asymptotic analysis and those obtained by numerical simulations.

ISSN:0010-2202    

DOI:10.1080/00102209508960389

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Interaction between a premixed flame and a fine vortex tube perpendicular to the flame is simulated numerically in order to study the effects of the rotating velocity and tube diameter on flame propagation along the vortex tube. The vortex tube has an initial circumferential velocity ranging from 1.8 to 36 times the laminar burning velocity and an initial core diameter ranging from 0.18 to 1.71 times the flame thickness. It is shown that a premixed flame can be accelerated along the vortex tube axis, and that the propagation velocity is proportional to the maximum circumferential velocity of the vortex tube. It is also shown that the influence of the vortex on the flame propagation can be neglected when the Reynolds number based on the vortex tube diameter and maximum circumferential velocity is below 10. The proportionality factor between the propagation velocity and circumferential velocity increases above the critical number with azimuthal speed and reaches about one at the Reynolds number of about 60.

ISSN:0010-2202    

DOI:10.1080/00102209508960390

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A kinematic equation of motion for an iso-surface describing premixed turbulent combustion in the flamelet regime is used to construct a conceptually transient normalised scalar field defined at each point as the shortest distance to the flame sheet The probability density function of this normalised scalar field is related to the surface density of the flame. Closure of the mean reaction rate term depends on the surface density of the flame sheet, and hence, determination or the probability density function of the normalised scalar field closes the mean reaction rate term.

ISSN:0010-2202    

DOI:10.1080/00102209508960391

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

During the study on the ignition process of large carbon/char particles, we have made the discovery throught experiments and numerical simulation, that the ignition of a large carbon/char particle, different from that of fine particle, has no sudden rising in temperature, thus there has no existence of the d2T/dt2= 0. Based on the characteristics of the large carbon/char particle ignition process, this paper puts forward a new ignition criterion, according to which we set up ignition model of a large carbon/char particle and deduce its ignition expression identical with that of a fine carbon/char particle. It is proved throught experiments that the generalized relationship between the ignition temperature and coal properties deduced in the line particle carbon/char model is still suitable to use in the ignition of large carbon/char particles.

ISSN:0010-2202    

DOI:10.1080/00102209508960392

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Experimental and theoretical studies on the spontaneous ignition process of isolated fuel droplets were carried out. Time dependent temperature fields around the igniting droplets were observed by interferometry so that two step temperature rise can be detected. Some experiments are performed under microgravity to obtain reference data. Induction times are examined as a function of ambient temperature. As a result, a zero temperature coefficient region is found, which is equivalent to the NTC (negative temperature coefficient) region for the ignition of premixed gas. A numerical model is developed applying a simplified chemical reaction model that includes the low and the high temperature reactions. The model is able to reproduce the two step temperature rise and the roles of the two kinds of reactions on the ignition process up to the establishment of a diffusion flame around the droplet are examined.

ISSN:0010-2202    

DOI:10.1080/00102209508960393

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A numerical study of a premixed methane/air flame on a 4 mm slit burner is presented. A local grid refinement technique is used to deal with large gradients and curvature of all variables encountered in the flame, keeping the number of grid points within reasonable bounds. The method used here leads to a large reduction in the number of mesh points, compared to global or line-by-line refinement techniques.

ISSN:0010-2202    

DOI:10.1080/00102209508960394

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A method to predict burning rate of Mg-NaNO3, propellant as a function of concentration and particle size of the ingredients with consideration of metal particle agglomeration during combustion has been developed. Experimental results indicate that compositions containing NaNO3, of finer particle size (50μ) give higher burning rate at high fuel content than at the stoichiometric ratio, whereas compositions containing NaNO3, of coarser particle size (250 μ) show higher burning rate at higher oxidizer content maximum at the stoichiometric ratio.

ISSN:0010-2202    

DOI:10.1080/00102209508960395

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Experimental measures are developed for the description of anisotropic growth in the early stages, of the flame kernel development in an operating spark ignition internal combustion engine. These measures are developments of the support area concept introduced by Bradley and Abdel-Gayed. By applying the technique of multi-scale analysis derived from texture analysis to such anisotropy measures it has shown to be possible to obtain time dependent information about the effect of the various local length scales in the processes that influence early flame growth. The analysis techniques employed are demonstrated to offer very wide scope for characterising anisotropic time varying combustion and the basis is laid for a formal development of this topic as presented in the companion paper. An interesting observation is that for cycles other than partial combustions the largest length scale of process effecting the flame kernel on tends to increase with time during the early flame growth.

ISSN:0010-2202    

DOI:10.1080/00102209508960396

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor