E,9 • NUCLEATE BOILING 



temperature is increased, smaller nuclei become capable of forming bub- 

 bles, and the number of bubbles per unit area per unit time increases. 

 The temperature may be raised until the bubble population becomes so 

 high that the burnout point is reached. The number of bubbles per unit 

 area per unit time at the burnout point depends on several conditions, 

 such as the fluid temperature and velocity. As an example, for water at 

 a velocity of 10 ft/sec, at a pressure of 25 lb/in. ^ abs, and at 155°F sub- 

 cooling, a bubble frequency of 16 X 10^ bubbles/in. ^ sec was measured 

 near the burnout point [67]. According to the concept of nucleation, the 

 wall temperature at the burnout point should depend on these same con- 

 ditions. This dependence has also been observed experimentally [66]. 



The observed bubbles always occur at or in the immediate vicinity of 

 the heating surface. One should be somewhat hesitant, however, to con- 

 clude from this that the heating surface is solely responsible for the sup- 

 ply of nuclei. Nuclei existing in the fluid would also begin to grow near 

 the surface, because the temperature in this region is the highest. It 

 is of interest here to cite an experiment in which distilled degassed 

 water was heated by radiation, in such a way that the walls of the vessel 

 would be below the bulk fluid temperature [71]. In this case bubbles were 

 created inside the water at a temperature quite similar to that of the 

 heating surface in nucleate boiling. It is believed, therefore, that the nuclei 

 responsible for boiling may come from either source. Whether the sources 

 are of equal importance, or whether more nuclei come from the fluid itself 

 than from the surface probably depends on the particular fluid and the 

 particular surface. It may also be possible that the relative importance 

 depends on the rate of heat transfer. The surface, for example, may be 

 able to supply the nuclei for the rather low bubble frequencies required 

 at low rates of heat transfer but, for the high nucleation rates occurring 

 at high heat flow rates, the fluid might act as the main nucleation source. 

 The fact that the surface can influence the results has been demonstrated 

 by a set of experiments by Farber and Scorah [65]. In these experiments 

 the heat transfer rates to boiling water were measured for a set of wires 

 of different materials with results shown in Fig. E,9a. 



As mentioned previously, by measuring the wall temperature and 

 using Eq. 9-1 an estimate of the size of the original nucleus in terms of 

 "equivalent spherical diameter" can be made. From experiments with 

 distilled degassed water at 1 atm [64], this size was found to be of the 

 order of 10~^ inch. Measurements for distilled degassed carbon tetra- 

 chloride at 1 atm [64\ indicate approximately the same size. The heating 

 surface in both cases was a strip of stainless steel, type 347. Estimates of 

 equivalent nucleus size made from cavitation experiments [72] lead again 

 to the same order of magnitude. There is not, however, sufficient infor- 

 mation available to draw any general conclusions. 



As seen again from Eq. 9-1, for a given nucleus size, lower surface 



< 321 ) 



