E,9 • NUCLEATE BOILING 



mechanisms is the essential one, is difficult to obtain, but the results of 

 the following set of experiments may be pertinent. In these experiments 

 [75] a vessel containing water and a submerged heating wire was subject 

 to pressures of the order of 15,000 Ib/in.^ for approximately 10 minutes. 

 The pressure was then lowered to the atmospheric value and the wire was 

 heated electrically. It was found under these circumstances that the first 

 bubbles would form only when the wire temperature was raised consider- 

 ably above the usual nucleation temperature. Very similar results had 

 been obtained previously by several investigators [76] who observed that 

 a body of water, after being subjected to a pressure treatment as described 

 above, could be heated to temperatures much higher than the normal boil- 

 ing point before any bubbles would form. This observation was explained 

 in terms of the theory of pre-existing nuclei. In accordance with this 

 theory, the pressure treatment causes a decrease in the number and size 

 of the initial nuclei and the smaller nuclei require a higher temperature 

 before becoming capable of growing. The observed increase in nucleation 

 temperature on the other hand would be difficult to explain in terms of 

 thermal fluctuations. Since the same type of phenomenon was observed 

 in the experiments with the heated wire, it is reasonable to assume that 

 the presence of the wire did not change the nucleation mechanism and 

 that pre-existing nuclei were again responsible for the bubble formation. 

 In general, therefore, thermal fluctuations are not beUeved to play a 

 major role in boiling heat transfer. The thermal fluctuations, on the other 

 hand, are probably of essential importance in determining the maximum 

 tensile strength of a perfectly pure hquid [73], 



Growth and Collapse Process. Bubble motion has been discussed 

 in detail in [66,67,64] and the concepts given in these references will be 

 used in this section, since they appear to be the most plausible ones at 

 this time. In Fig. E,9b the typical stages of the growth and collapse of a 

 bubble are shown schematically. In Fig. E,9b (1), a nucleus is shown sur- 

 rounded by superheated hquid. The dotted line indicates the isotherm 

 which is at the temperature of the normal boihng point. The pressure in 

 the nucleus is essentially equal to the vapor pressure of the surrounding 

 hquid plus the pressure exerted by any gas present. If the nucleus is of 

 sufficient size, it begins to grow (Fig. E,9b (2), (3)). As the size increases, 

 the surface tension forces decrease rapidly and further motion depends 

 principally on the pressure inside the bubble. This pressure is a function 

 of the rate at which vapor can be supplied to the growing bubble, assum- 

 ing the fluid to be sufficiently degassed so that gas diffusion can be neg- 

 lected. The rate of this vapor flow is determined by two processes: the 

 heat transfer from the hquid to the surface of the cavity, and the evapo- 

 ration from the surface. The temperatures influencing these processes are 

 the temperature of the superheated liquid and the temperature of the 

 vapor inside the bubble. The temperature of the bulk hquid should have 



< 323 ) 



