/'«.=-3^(^a-^)()/|^+l) 



[2] 



where 



U^ = 



2Tra 



P3' 



[3] 



In these equations and in the derivations to follow 



/ is the heating current, 



R is the average wire resistance, 



I is the wire length, 



T is the temperature at film surface, 



Tl is the temperature at wire surface, 



T is the ambient temperature of fluid, 



T is the average temperature of wire, and 



U is the flow velocity. 



The wire resistance is related to the average temperature of the wire T^ by the equation 



It will be convenient to represent the temperature elevation of the wire by an overheating 

 ratio a defined as 



R -R aR^(T -TJ 



w e 0^ w e' 



[5] 



where R is the wire resistance at the temperature T . Even though the temperature elevation 

 of the wire may be large, the temperature elevation at the surface of the film may be much 

 lower. Therefore it will be convenient to define an effective overheating ratio as 



otR^iT - T ) 



„ _ ^ a e' 



10 



