115 



TABLE 109.— CONVECTION AND CONDUCTION OF HEAT BY GASES AT 

 HIGH TEMPERATURES 



The loss of heat from wires at high temperatures occurs as if by conduction across a 

 thin film of stationary gas adhering to the wire (vertical and horizontal losses very 

 similar). Thickness of film is apparently independent of temperature of wire, but probably 

 increases with the temperature of the gas and varies with the diameter of the wire 

 according to the formula b log (Va) == 25, where B = constant for any gas, b = diameter 

 of film, a, of wire. The rate of convection (conduction) of heat is the product of two 

 factors, one the shape factor, s, involving only a and B, the other a function <p of the 

 heat conductivity of the gas. If W = the energy loss in watts/cm, then lV = s(<f>i — <pi), 

 s may be found from the relation 



L e--f=%; = 4.19 1 kdt, 



IT D J 



where k is the. heat conductivity of the gas at temperature T in calories/cm° C. <t>i is 

 taken at the temperature 'A of the wire, fa at that of the atmosphere. The following may 

 be taken as the conductivities of the corresponding gases at high temperatures : 



For hydrogen k = 28 X K)- a Vr{(l + .00027)/ (1 + 777'" 1 )} 



air fc = 4.6Xl0- 6 Vr{(l+.0002:r)/(l + 124r- 1 )} 



mercury vapor ... k = 2.4 X 10- 8 Vr{l/(l + 960T- 1 )}. 



To obtain the heat loss : B may be assumed proportional to the viscosity of the gas and 

 inversely proportional to the density. For air [see Table 110 part 2] B may be taken as 

 0.43 cm; for H 2 , 3.05 cm; for Hg vapor as 0.078. Obtain s from Part 1 below from a/B; 

 then from Part 2 obtain fa and fa for the proper temperatures ; the loss will be s(fa — fa) 

 in watts/cm. 



Part 1. — s as function of a/B 



SMITHSONIAN PHYSICAL TABLES 



