3^8 



Table 337 



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-logh/a = 2B, 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 <f> of the heat conductivity of the gas. If W = the energy loss in watts/cm, then W = s(4>2 — <pi). 

 s may be found from the relation 



where k is the heat conductivity of the gas at temperature T in calories/cm ° C. (pi is taken at the temperature Tz 

 of the wire, (pi 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 io-«vT{(i + .oooaD/d + 777^)} 



air k = 4-6 X io^vTj(i + .0002^/(1 + i 24 7/-i)I 



mercury vapor £ = 2.4 X ierVr{i/(i 4- g 6oT- 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 338(6)) B may be taken as 0.43 cm; for H2, 3.05 cm; for Hg vapor as 0.073. Obtain 

 x from section (a) below from a/B; then from section (b) obtain (pi and (pi for the proper temperatures; the loss will 

 be s(<j>2 — (pi) in watts/cm. 



(b) Table of 4> in Watts per Cm as Function of Absolute Temp. (°K.) 



Langmuir Physical Review, 34, p. 401, 1912. 



Smithsonian Tables. 



