E • CONVECTIVE HEAT TRANSFER AND FRICTION 



nucleus is large, the bubble is surrounded by a wide region of greatly 

 superheated fluid. The temperature differential for the initial heat trans- 

 fer is then large and the bubble is expected to grow rapidly. A steep vapor 

 pressure-temperature curve, on the other hand, should lead to slow bub- 

 ble growth. The rate of change of vapor pressure with temperature for 

 each liquid is a function of the absolute pressure. 



Growth rate measurements over a sufficiently wide pressure range, 

 to indicate the effect of the slope of the vapor pressure-temperature curve 

 directly, have not been made. The fact, however, that the superheat re- 

 quired for boiling decreases with increasing pressure has been checked 

 experimentally [78] and the results are shown in Fig. E,9e. The curve is 



200 



400 



600 800 1000 



Time, microsec 



1200 



1400 



Fig. E,9d. Bubble radius vs. time. Distilled degassed water-aerosol solution at 1 atm 

 pressure and 90°F. Heat flux 80 per cent of burnout value. Free convection [6Ji.]- 



partly influenced, of course, by changes in surface tension and possible 

 changes in nucleus size, but these changes cannot fully explain the large 

 variation in superheat. The change in slope of the vapor pressure curve is 

 believed to be the principal factor [64]. Explosive boiUng at very low 

 pressure, a phenomenon well known to the chemical worker, is another 

 instance which may be explained by the slope of the vapor pressure curve. 

 For water at 0.1 atm pressure, e.g., the superheat necessary to produce a 

 15-lb/in.2 over-pressure would be 100°F. A nucleus, growing in such a 

 highly superheated liquid, would of course grow very rapidly, which could 

 explain the observed results. In vacuum work it is often necessary to 

 make a special effort to introduce large nuclei in order to avoid explosive 

 processes. 



Thermal diffusivity. In the brief description of bubble motion given 



( 326 ) 



