E • CONVECTIVE HEAT TRANSFER AND FRICTION 



eventually be most useful to the engineer is a correlation of the maximum 

 allowable heat transfer (burnout points) as a function of the fluid proper- 

 ties. For some special cases a correlation of the heat transfer rate in the 

 complete region of nucleate boiling may also be required, although this 

 information is believed to be of lesser importance (see Art. 8). To the 

 knowledge of the writer, satisfactory correlations of this kind are not 

 available at present. For future development of such relations it is be- 

 lieved that an understanding of the detailed processes taking place in 

 nucleate boiling will be necessary, and for this reason the present con- 

 cepts of the mechanism of nucleate boiling will be discussed below. 



The process of nucleate boiling can, for convenience, be subdivided 

 into three phases: the nucleation proper (i.e. the generation of the bub- 

 bles), the growth cycle of the bubbles, and the effect of the bubble motion 

 on heat transfer. 



Nucleation Process. As the name ''nucleate boiling" indicates, it 

 is believed that the vapor bubbles in question originate from nuclei. 

 These nuclei are imagined as consisting of small gas or vapor pockets 

 stabilized on submicroscopic solid particles of low wettability. Upon heat- 

 ing, part of the vapor (or gas) is forced away from the stabilizing particle. 

 If the resulting gas or vapor mass is sufficiently large, the inside pressure 

 overcomes the surface tension forces as well as the outside pressure, and 

 the nucleus grows into a bubble. If the detached gas mass is too small, 

 it collapses by the surface tension forces and no bubble forms. If the 

 initial cavity is spherical, the relation between the surface tension forces 

 and the pressure becomes simply 



V^-Vo = ^ (9-1) 



where p, is the pressure inside the initial cavity, y^ is the pressure of the 

 surroundings, r is the radius of the cavity, and a is the surface tension. 

 If gas is present in the cavity in addition to the vapor, p, would be the 

 sum of the vapor pressure and the partial pressure of the gas. In order to 

 create a bubble, the temperature surrounding the nucleus has to be suf- 

 ficiently high to create a pressure in the initial cavity larger than that 

 indicated in Eq. 9-1. By measuring the temperature at which a bubble is 

 observed and assuming the pressure p-, to be approximately equal to the 

 vapor pressure corresponding to this temperature, an estimate of the size 

 of the initial cavity in terms of "equivalent spherical size" can be made. 

 Nuclei of the kind described are believed to be present throughout 

 the test fluids as well as on the heating surface. The existing nuclei cover 

 a certain range of sizes, and a certain distribution curve of number vs. size 

 can be imagined in each case. Nucleate boiling first becomes noticeable 

 when the temperature near the heating surface becomes high enough to 

 cause the growth of a significant number of the largest nuclei. As the 



< 320 ) 



