E • CONVECTIVE HEAT TRANSFER AND FRICTION 



little influence at this stage of the growth. In Fig. E,9b (2) the bub- 

 ble is shown after some growth has taken place. The isotherm of the 

 normal boiling point has approached the bubble surface partly because of 

 the stretching of the hot film of liquid over the bubble and partly because 

 of the heat transfer from the film. The part of the film which is stretched 

 over the top of the bubble is further cooled by heat transfer to the sur- 

 rounding cold liquid. The superheat of the liquid surrounding the upper 

 part of the bubble is finally removed completely (Fig. E,9b (3)) and 

 vapor is condensed over this portion of the bubble. The rate of vapor 

 removal depends on the rate of the condensation mechanism itself, as well 

 as on the rate of heat transfer to the cold bulk liquid. The significant 

 temperatures in this case are the temperature of the bulk liquid and the 



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Fig. E,9b. Schematic representation of bubble growth. 



temperature of the vapor in the bubble. As the bubble grows, the evapo- 

 ration rate over the lower portion of the bubble also changes, because of 

 changes in the surrounding temperature and configuration. Eventually 

 the heat transfer from the upper part of the bubble may become suf- 

 ficiently large so that the condensation overcomes the evaporation. The 

 pressure inside the bubble then decreases rapidly and falls below the 

 pressure of the fluid, the resulting force reducing the momentum of the 

 surrounding fluid. If large enough, it reverses the fluid motion and brings 

 about the collapse of the bubble at the surface (Fig. E,9b (4 through 6)). 

 If the heat transfer from the bubble is low, the resulting pressure de- 

 crease in the bubble may not be sufficient to cause a reversal of the mo- 

 mentum in the fluid. In this case the bubble will be carried with the fluid, 

 away from the surface. It will then collapse in the bulk of the fluid, pro- 

 vided the fluid is subcooled. Fig. E,9b (7 through 9) illustrates this latter 



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