摘要:

Classical studies of boundary layer diffusion flames have neglected radiation. As a first step toward a better understanding of radiation heat transfer in fires, a numerical solution is obtained for a steady, laminar, radiating, combusting, boundary layer over a pyrolyzing fuel slab. Two Shvab-Zeldovich coupling functions are needed in the analyses, which treat forced and free flows separately. For these laminar systems, an optically thin approximation for radiation is valid, based on previous measurements of soot volume fraction in combusting boundary layers. As in non-radiating combustion, the mass transfer number, B, and the mass consumption number, rp, are the dominant parameters controlling the local pyrolysis rate and the excess pyrolyzate, respec- tively. The dominant role of the dimensionless heat of combustion, Dc, in determining the flame temperature makes it a significant parameter in radiating systems. The surface temperature and emissivity characterize the surface emission, which can dominate flame radiation for solid fuels of small dimension. The flame emission is controlled by the optical thickness and a radiation para- meter, which indicates the relative strength of conduction to radiation. A comparison between numerical and experimental pyrolysis rates shows good agreement for a case where surface emission dominates flame radiation [polymethylmethacrylate (PMMA) burning in air]. Values of a mean absorption coefficient and soot generation rates are also obtained fudata using this analysis.

ISSN:0010-2202    

DOI:10.1080/00102208408923791

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Varying oxygen concentrations was found to have pronounced effects on the sooting characteristics of diffusion flames. Ethylene and propane flames were stabilized around a porous ceramic cylindrical burner mounted in a vertically oriented, low turbulence wind tunnel. Laser light scattering and extinction, laser Doppler velocimetry and thermocouple temperature measure- ments were obtained along the forward stagnation line. Measurements of soot particle size, number density and volume fraction showed that with an increasing oxygen index the soot volume fractions in both ethylene and propane flames increased at all locations along the stagnation stream- line. For ethylene flames (when the oxygen index increased from 0.18 to 0.28). the maximum soot volume fraction increased by almost an order of magnitude. Propane was found to produce about one half the volume of soot of an ethylene flame at the same oxygen index. The increase in soot volume fraction with increasing oxygen index was attributed to both an increase in the formation of particles and in the subsequent rate of surface growth. The data have yielded information on rates of surface growth. In addition, an apparent activation energy of 24 kcal/mole has been determined for the surface growth reactions in the ethylene flames.

ISSN:0010-2202    

DOI:10.1080/00102208408923792

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

An ignition criterion for lean premixed and diffusion flame burning is developed using classical empirical relations for lean premixed flammability limits and an analogy between limit premixed and diffusion flames first noted by Simmons and Wolfhard. The ignition criterion is validated by comparison with ignition experiments using a layer of products of incomplete com- bustion. The criterion, developed to allow prediction of the ignition of the upper layer in compart- ment fires, is compatible with zone fire models. Three modes of layer burning were observed. Mode one burning was a ring flame surrounding the buoyant diffusion flame feeding the layer. Mode two burning occurred when the ring flame deformed to include finger-like extensions away from the plume. Mode three was burning of layer gases at the layer/air interface. All three modes were diffusion flames. The rate of burning was controlled by the flammability of the layer such that the ignition criterion was just satisfied during layer burning.

ISSN:0010-2202    

DOI:10.1080/00102208408923793

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

This paper describes entrainment measurements made in fire plumes with a new technique. Measurements were in plumes rising from natural gas diffusion flames stabilized on 0.10, 0.19 and 0.50 m diameter burners and the heat release rates ranged from 10 to 200 kW. The heights examined ranged from elevations starting very close to the burner surface to distances about five times the average flame heights. Experiments indicate the presence of three regions: a region close to the burner surface where plume entrainment rates are independent of the fuel flow (or heat release) rates; a far field region above the flame top, where a simple point source model correlates the data reasonably well; and an intermediate region where entrainment appears to be similar to that of a turbulent plume.

ISSN:0010-2202    

DOI:10.1080/00102208408923794

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

ISSN:0010-2202    

DOI:10.1080/00102208408928008

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor