E • CONVECTIVE HEAT TRANSFER AND FRICTION 



liquid and to provide vapor for the film, and some may be transmitted 

 to the bulk fluid. No detailed investigations on the distribution of the 

 heat or on the stabihty of the film have been carried out, but some calcu- 

 lations, based on the assumptions that the film is laminar and all of the 

 heat is used to evaporate fluid, have been performed. For this case the 

 problem becomes very similar to the familiar one of the condensation on 

 a surface [77, Chap. 30]. For the heat transfer coefficient h from a 

 horizontal wire, with some additional simplification, the expression 



i 



h = 0.6 



Bpjpi — p)gL 

 nDAt 



(10-1) 



can be obtained [84]. In Eq. 10-1, p is the density of the vapor, pi the 

 density of the liquid, At the temperature difference between the wall and 

 the liquid, D the diameter of the wire; k, p., and L are the thermal con- 

 ductivity, the viscosity, and the latent heat of vaporization of the fiquid 

 respectively, and g the acceleration due to gravity. Heat transfer rates 

 predicted from Eq. 10-1 and adjusted for radiation have been compared 

 with experimental data on film boiling to both nitrogen and water. The 

 agreement between the measured and predicted values was rather 

 satisfactory. 



E,ll. Closing Remarks. In the foregoing an attempt has been made 

 to acquaint the reader with some of the problems concerning boihng heat 

 transfer. The attention of investigators has so far been directed mostly 

 towards nucleate boihng rather than toward film boiling. The principal 

 aim of studies in nucleate boiling is to arrive at a method of predicting 

 heat transfer rates — in particular at the "burnout point" — as a function 

 of fluid properties. This aim has not as yet been reached. 



In order to eventually arrive at a satisfactory method of prediction, 

 it will first be necessary to determine which of the two proposed heat 

 transfer mechanisms is essential. Then, it would seem possible, on the 

 basis of the discussion on bubble motion, to select the significant fluid 

 properties and to form the dimensionless groups on which boiling heat 

 transfer should depend. The success of this approach, however, is some- 

 what in question because the nucleation process plays a key role in boil- 

 ing and for any predictions, therefore, some information on the nuclei 

 distribution will be essential. Information of this kind is not available at 

 present. 



It is known that the number and size of nuclei in a fluid depend on 

 the previous history of this fluid and to some extent on the type and 

 treatment of the heating surface. If the nuclei distribution should be very 

 sensitive to outside influences, and if in addition various hquids should 

 react in a markedly different manner to a given treatment, it may be 

 practically impossible to predict this distribution. In that case it seems 



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