dp 

 dx 



E,10 • FILM BOILING 



where 



q = heat transfer rate per unit area 



= local pressure gradient 



p = fluid density 



V = average fluid velocity (adjusted for bubble obstruction) 

 Cp — specific heat of fluid 



d = effective diameter of unobstructed flow passage 

 T^ — wall temperature 

 Tb = bulk temperature of fluid. 



It may be pointed out that the average velocity V required in the 

 above equations is not equal to the ratio of the volume flow rate to the 

 unobstructed cross-sectional area of the pipe. The vapor bubbles on the 

 wall may represent a significant reduction of the available flow area and 

 the velocity V may differ substantially from the above-mentioned ratio, 

 particularly where the diameter of the tube is small. 



The results of the experiments [83] indicated that the relation between 

 the two coefficients was approximately given by the simple equation 



This, however, is the relationship obtained from Reynolds' analogy, 

 either for the case where a laminar boundary layer at the heating surface 

 is nonexisting or for the case where the Prandtl number of the fluid is 

 equal to unity. For the temperatures of the water in the above experi- 

 ments, the Prandtl number of the water was relatively close to unity, 

 and any conclusion will have to be limited to that case. For this limited 

 case, however, the experimental results indicate that the postulate which 

 forms the basis of Reynolds' analogy for forced convection still applies 

 in the nucleate boiling region. This would mean that the mechanisms of 

 heat transfer and momentum transfer are ''similar," within the meaning 

 of Reynolds' analogy. These results, if verified over a wider range of 

 variables, would incidentally tend to confirm the point of view that the 

 improved heat transfer in nucleate boiling is obtained by increased agi- 

 tation rather than by the creation of periodic steep temperature gradients. 



E,10. Film Boiling. In addition to the nucleate boiling region, the 

 film boiling region is of engineering importance, although considerably 

 less experimental work has been published concerning results in this region 

 than in the nucleate region. In film boiling, the heating surface is sepa- 

 rated from the fluid by a continuous, stable vapor film. The film is in 

 motion due to free or forced convection, and the flow of the film may be 

 either laminar or turbulent. The heat from the surface is largely trans- 

 mitted through the film to the liquid. Some of the heat serves to evaporate 



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