E • CONVECTIVE HEAT TRANSFER AND FRICTION 



tension should lead to boiling at a smaller temperature excess above the 

 normal saturation temperature. Experiments with distilled degassed water 

 and a degassed water-aerosol solution [64] tended to verify this result. In 

 the first case a wall temperature of approximately 30°F above the normal 

 boiling point was required to initiate nucleate boiling, whereas only about 

 15°F was required for the aerosol water solution, the surface tension of 

 which was considerably below that of pure water. The presence of gas in 

 the initial nucleus according to Eq. 9-1, should have the same effect as 

 decreasing the surface tension. This fact has also been shown experi- 

 mentally [64]- 



The concept that nuclei are responsible for the boiling as discussed in 

 this section is the most widely accepted theory at present. The precise 



c 



'UUL 



cor; 



o 



X 



100,000 



10,000 



1000 



100 



10 



10 



Wall temperature 



100 1000 



fluid temperature, °F 



Fig. E,9a. Heat transfer coefficient for four different surface materials. Bulk boiling, 

 1 atm pressure; wire diameter 0.040 inches; wire length 6 inches. DistUled water, free 

 convection, horizontal wire \6S\. 



role of surface and liquid in supplying nuclei, as well as the size dis- 

 tribution of nuclei in each case, is still not known. 



There is one other mechanism of bubble generation which has been 

 considered. In any liquid, cavities are continuously formed due to the 

 random fluctuations of the molecules. These cavities, if large enough, 

 could grow into bubbles. The probability that cavities of sufficient size 

 would form [75] to cause boiling at the observed temperatures, however, 

 is practically zero. It has further been suggested that these cavities might 

 form at the surface of " nonwettable " solids [7^], in which case the re- 

 quired size would be much smaller. The probability that the necessary 

 fluctuations would occur at such specific points is, however, again ex- 

 pected to be very small. 



Experimental evidence, which would clearly indicate which of the two 



( 322 ) 



