1,3 • RADIATION FROM FLAMES AND GASES 



significant. Assume a flame temperature of 3000°R, a fuel oil density of 1, 

 a stoichiometric air requirement of 15 lb/lb fuel, 20 per cent excess air, 

 and opaque particles which are uniform and of a diameter D equal to 

 the initial drop size. The concentration of drops can be shown to be 

 0.0000707/7rZ)3 per f^s. ^y^q projected area of a drop is tD^/4:. Then 



ef = (1 - e-i."xio-5L/o) 



For a flame of 2 ft thickness and particles of 50^ (0.000164 ft) initial 

 diameter, ej is 0.194 initially and decreases continuously as the particles 

 burn away. This is slightly more than the emissivity due to nonluminous 

 gases present, and markedly less than the emissivity due to soot in many 

 flames. Whether the residual particles themselves contribute significantly 

 to the flame radiation compared to soot luminosity or gas radiation thus 

 depends on how finely the fuel is atomized. 



Gas radiation. CO2 and H2O. Of importance in substantially all 

 evaluations of heat transmission in combustion processes, whether from 

 flames or from their cooler products, is the infrared radiation from the 

 combustion products, water vapor and carbon dioxide, which over- 

 shadows convection at combustion temperatures. This radiation has its 

 origin in simultaneous quantum changes in the energy levels of rotation 

 and of interatomic vibration of molecules and, at the temperature levels 

 reached in combustion processes, is of importance only in the case of the 

 heteropolar gases. Of the gases encountered in heat transfer equipment, 

 carbon monoxide, the hydrocarbons, water vapor, carbon dioxide, sulfur 

 dioxide, ammonia, hydrogen chloride, and the alcohols are among those 

 with emission bands of sufficient magnitude to merit consideration. Gases 

 with symmetrical molecules, Hke hydrogen, oxygen, and nitrogen, have 

 been found not to show absorption bands in those wavelength regions of 

 importance in radiant heat transmission at temperatures met in com- 

 bustion processes. Sec. H of this volume presents the molecular physical 

 background for thermal radiation from gases. 



If black body radiation passes through a gas mass containing CO2 or 

 H2O, absorption occurs at certain wavelengths. Conversely, if the gas 

 mass is heated it radiates at those same wavelengths. Consider a hemi- 

 spherical gas mass of radius L containing carbon dioxide of partial pres- 

 sure Pc, and let the problem be the evaluation of radiant heat interchange 

 between the gas at temperature Tg and a small black element of surface 

 at temperature Ti, located on the base of the hemisphere at its center. 

 Per unit of surface the emission of the gas to the surface is o-T^e^, where 

 eg denotes gas emissivity. For carbon dioxide e^ depends on Tg, the product 

 term p^L, and the total pressure Pf The emissivity of carbon dioxide for 

 Pt = 1 atm is given in Fig. I,3a, based on experimental data [22,23]. The 

 line broadening due to pressure may be estimated from Fig. I,3b, which 

 gives a correction factor C^ by which the values from Fig. I,3a are to be 



< 513 ) 



