1,3 • RADIATION FROM FLAMES AND GASES 



The formulation of radiant interchange between a gas and a black surface 

 completely enclosing it, when the gas contains CO2 and H2O, is then 





= cr(6,T^ - a,,Tf) 



(3-4) 



If the surface is gray, multiplication of the right side of the above by 

 ei(= a\) would make proper allowance for reduction in the primary beams 

 from gas to surface and surface to gas, respectively; but some of the gas 

 radiation initially reflected from the surface has further opportunity for 

 absorption at the surface because the gas is but incompletely opaque to 

 the reflected beam. Consequently, the factor to allow for surface emis- 

 sivity lies between ei and 1, nearer the latter the more transparent the gas 



1.8 

 1.6 



0.2 



0.2 0.4 0.6 0.8 



(pw + Pt)/2, atm 



1.0 



1.2 



Fig. I,3d. Correction factor Cw for converting emissivity of H2O to values 

 of Pw and Pt other than and 1 atm, respectively. 



(low yju) and the more convoluted the surface. If the surface emissivity 

 is above 0.8, use of the factor (d + l)/2 cannot be greatly in error. If 

 €1 is smaller, the more nearly rigorous method of Art. 5 may be used. 



Although ttgi approaches eg as Tx approaches Tg, 5/S of Eq. 3-4 does 

 not in consequence reduce to (ytJ^T\ — Tf) for values of Ti and Tg close 

 together. If over a restricted range of variables Cg is assumed proportional 

 to {vJ^YiT^y, then a^i is proportional to {vJ^YiT lY+^-'Tl-" where c is 

 the previously encountered exponent used in evaluating absorptivity — 

 0.65 for CO2 and 0.45 for H2O. It may be shown that, when Tg and Ti 

 are not too far apart, 





4 + a + ?> — c 



{Tt - T\) 



(3-5) 



( 517 ) 



