1,3 • RADIATION FROM FLAMES AND GASES 



surfaces which are not heat sinks or sources and for multiple reflection 

 in a system of nonblack surfaces, will be withheld until the radiating 

 characteristics of gases have been described, in order that allowance for 

 the presence of such gases may at the same time be included. 



Fig. I,2e. 



1,3. Radiation from Flames and Gases. The radiation from a 

 luminous flame is due primarily to submicroscopic soot particles, of diam- 

 eters 0.006 to 0.06m before agglomeration, in the case of pulverized coal 

 to suspended coke or ash particles of diameter 25/x and less, in the case of 

 heavy residual fuel oils to coked bitumens of initial size 200 to 50/i or 

 even less, and quite negligibly in all cases to so-called chemiluminescence, 

 the radiation characterizing the chemical reactions involved. Superim- 

 posed on this luminous radiation is the important infrared radiation from 

 combustion products, principally carbon dioxide and water vapor. 



Soot luminosity. The first of these, soot luminosity, is important 

 where combustion occurs under such conditions that hydrocarbon gases 

 in the flame are subjected to heat in the absence of prior intimate mixing 

 with sufficient air, producing thermal decomposition. This occurs, for ex- 

 ample, near the surface of a volatihzing oil droplet or in the fuel-rich 

 portion of a laminar diffusion flame. Turbulence, consequently, is of pri- 

 mary importance in minimizing luminosity due to soot; but chemical 

 differences in hydrocarbons are also important, aromatic compounds 

 being most prone to produce luminosity, and paraffins least. It is not 

 possible, in the present state of knowledge, to predict the luminosity of 

 a flame analytically; reliance must be put on experimental measurement 

 of flames similar to that of interest. The work of the International Com- 

 mittee on Flame Radiation in this field is outstanding [15,16,17]. 



The planning and use of measurements on luminous flames depends 

 on knowledge of the radiating and absorbing characteristics of a soot 

 cloud. The monochromatic emissivity ex of such a cloud equals its mono- 



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