E,9 • NUCLEATE BOILING 



process. For a particular combination of properties and temperature, 

 bubbles may also become stationary on the heating surface. The tendency 

 of the bubble to leave the surface, as well as its shape during the growth 

 cycle, probably also depends on the contact angle between the fluid, the 

 vapor, and the surface [77]. 



In the following, effects of some of the more important variables on 

 bubble motion will be discussed in the light of the foregoing description. 

 Comparisons with experimental data will be made whenever possible. 



Effect of Variables on Bubble Motion. 



Nucleus size and surface tension. As mentioned previously, the nu- 

 cleus size determines the temperature at which boiling begins. Since this 

 temperature also influences the initial growth rate (see Fig. E,9b), nucleus 



0.020 



400 600 800 



Time, microsec 



1000 



1200 



Fig. E,9c. Bubble radius vs. time. Distilled degassed water at 1 atm pressure and 

 78°F. Heat flux 50 per cent of burnout value. Free convection [6Jf\. 



size and initial growth should be interdependent. Smaller nuclei should 

 correspond to higher initial growth rates. In a very similar way, a reduc- 

 tion in surface tension should lead to a decrease in the initial growth rate. 

 This latter effect is believed to be principally responsible for the differ- 

 ence in initial bubble growth rates in water with and without dissolved 

 aerosol (see Fig. E,9c and E,9d). 



Shape of vapor pressure curve. The size of the nucleus and the surface 

 tension actually determine the pressure which is required to initiate boil- 

 ing; they only indirectly control the required temperature of the surround- 

 ing liquid. The temperature, however, is an important factor in the growth 

 of the bubble, and the relation between vapor pressure and temperature 

 of the liquid therefore becomes a property affecting this growth. If the 

 amount of superheat required to produce the necessary pressure in the 



( 325 > 



