E • CONVECTIVE HEAT TRANSFER AND FRICTION 



When the pressure reaches the critical value, the entire boiling phe- 

 nomenon disappears, of course, and heat transfer is by pure convection. 

 As the critical pressure is approached, the differences between the liquid 

 and the vapor phase diminish. The vapor then becomes almost as good a 

 heat transfer agent as the liquid and the decrease in the heat transfer rate 

 with the appearance of film boiling should vanish. The curve of "heat 

 transfer rate" vs. "wall temperature" will then show a continuous in- 

 crease, and there will be no more temperature jump when the vapor film 

 forms. 



The effect of large pressure changes has been illustrated by a set of 

 experiments with Freon (Fig, E,8f). In these experiments the burnout 





1.2 



1.0 



0.8 



0.6 



D 



^ 0.4 



0.2 



X 



o 



---. 4r 



q, by forced convection on 



0.2 0.4 0.6 0.8 1.0 



Reduced pressure p/pcr 



1.2 



Fig. E,8f. Maximum heat transfer to Freon as a function of reduced pressure. The 

 fluid is Freon No. 114. Mass velocity 6.2 Ib/in.^-sec. Annular cooling passage [68]. 



point was measured as a function of pressure at constant subcooling and 

 constant fluid velocity, and the pressure was increased above the critical 

 value. The resulting curve shows a continuous decrease until finally the 

 value corresponding to simple forced convection is reached, a result which 

 should be expected from the foregoing discussion. The disappearance of 

 the temperature jump in the graphs of the heat transfer rate vs. temper- 

 ature is well illustrated by a set of curves in Fig. E,8g. The data in Fig. 

 E,8g were obtained for a "jet fuel" consisting of a mixture of hydrocarbons. 

 The critical pressure in this case was approximately 600 lb/in. ^ abs. An 

 extensive set of experiments showing the effect of pressure on the maxi- 

 mum heat transfer rates of hydrocarbons is reported in [70]. 



In the following pages the information presently available on nucleate 



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