E • CONVECTIVE HEAT TRANSFER AND FRICTION 



Reynolds numbers with the properties evaluated at the bulk temper- 

 ature, or at some temperature between the wall and bulk temperatures, 

 can be calculated by using the relation 



S^ = 1 - m 



(5-4) 



Fig. E,5b and E,5c show fully developed experimental and predicted 

 Nusselt numbers for air (Pr = 0.73) plotted against Reynolds number 

 for various values of ^. In Fig. E,5b the physical properties including 

 density, in the Reynolds and Nusselt number, are evaluated at the fluid 



1000 



Q 



100 



10 

 1000 



1 0,000 



100,000 



1,000,000 



Ret 



PbUbD 

 lib 



Fig. E,5b. Variation of Nusselt number with Reynolds number for flow of air 

 with heat addition and properties evaluated at bulk temperature. Prandtl num- 

 ber = 0.73. 



bulk temperature. Both the experimental and predicted Nusselt num- 

 bers at a given Reynolds number show a decrease with increasing values 

 of iS or of ratio of wall to bulk temperature. If the properties are evalu- 

 ated at the wall, rather than the bulk temperature, the Nusselt numbers 

 increase as (3 increases [14]- The same trends with (8 were obtained in the 

 data for average heat transfer coefficients given in [60]. 



In Fig. E,5c the properties in the Nusselt and Reynolds numbers are 

 evaluated at a reference temperature To a, which is slightly closer to the 

 bulk temperature than the average of the wall and bulk temperatures. 

 It is observed that the effect of ^ or of ratio of wall to bulk temperature 

 on both the experimental and predicted Nusselt numbers is practically 

 eliminated when the properties are evaluated at this temperature. The 



< 306 ) 



