E • CONVECTIVE HEAT TRANSFER AND FRICTION 



sponding to section BC of the curve in Fig. E,8a is called "nucleate 

 boiling," that corresponding to section DF (and beyond) is called "film 

 boiling," and that corresponding to section CD is called "partial film 

 boiling," in reference to the observed surface phenomena. 



In Fig. E,8c, E,8d, and E,8e some actual test results of the type de- 

 scribed in the foregoing are shown. The test fluid in these cases is dis- 

 tilled degassed^ water and the pertinent test conditions are noted in the 

 figures. In Fig. E,8c data for two different temperatures of the bulk fluid 

 are given and it is seen that the burnout point has increased with de- 

 creasing fluid temperature. The behavior in the pure convection region 

 follows the usual heat transfer laws. It is interesting to note that nucleate 



50 150 950 



Temperature difference between 

 wall and boiling pointy °F 



Fig. E,8c. Effect of subcooling on boiling heat transfer. Distilled degassed water [64]- 



boiling begins only at a wall temperature of approximately 30°F above 

 the ordinary boiling temperature. 



Fig. E,8d shows the effect of velocity, which again is to increase the 

 burnout point. As before, the behavior of the curves at the lower temper- 

 atures is explained by the usual theory of forced convection. Fig. E,8e 

 illustrates the effect of fluid pressure. For better comparison, these meas- 

 urements have been made at constant liquid subcooling (normal boiling 

 point — liquid temperature) rather than at constant fluid temperature. 

 The resulting data can be plotted on practically the same curve if the 

 temperature difference between the wall and the liquid is taken as the 

 abscissa. As a consequence it has frequently been assumed that the degree 

 of subcooling rather than the absolute pressure itself was the important 



"5 The word "degassed liquid" is used to designate liquids in which the gas content 

 has been reduced to a value of less than approximately 15 per cent of saturation. 



< 316 ) 



