E • CONVECTIVE HEAT TRANSFER AND FRICTION 



transfer from the bubble to the fluid at all. The bubbles will then always 

 detach themselves from the surface. 



Velocity. The effect of the velocity of the bulk fluid on bubble mo- 

 tion is similar to that of the temperatures of the fluid. Higher velocities, 

 Hke lower temperatures, improve the heat transfer from the bubble to 

 the fluid and lead to smaller bubble sizes and faster collapse. It should 

 also be mentioned that a moving bulk fluid exerts a drag on the bubbles, 

 so that they have been observed to sHde along the heating surface at 

 velocities approximately equal to 80 per cent of that of the fluid itself [67]. 



Heat Transfer in Nucleate Boiling. 



Effect of bubble motion on heat transfer. The last phase of nucleate 

 boihng to be discussed is the effect of bubble motion on the heat transfer. 

 In early analyses it was suggested that the increased heat transfer could 

 possibly be explained by the fact that the vapor created near the heating 

 surface absorbed a large amount of heat and that this vapor was then 

 carried away with the rising bubbles. For the case in which the bubbles 

 did not detach from the surface, the increased heat transfer was explained 

 by the vapor flow from the lower to the upper regions of the bubble. 

 Numerical estimates of the amount of heat that could possibly be re- 

 moved in this way [77, Chap. 29; 79; 80], however, showed that this 

 mechanism could not account for the observed heat transfer. 



There are at present two mechanisms which have been suggested as 

 explanations for the heat transfer improvement. According to the first 

 concept, which is held by many investigators [64,66,67,80], the improved 

 heat transfer is caused by the agitation of the bubbles. The transfer proc- 

 ess is essentially one of forced convection and accordingly it should de- 

 pend on a characteristic Reynolds number and Prandtl number. The 

 typical velocity in this case should be the average fluid velocity induced 

 by the bubbles, and the maximum bubble diameter might be chosen as 

 the typical dimension. Attempts at verifying this concept have been 

 made with some success [64], although the available data do not cover a 

 sufficient range of variables to allow any definite conclusions. 



The second concept [81] is based on the following idea: After the 

 collapse of each bubble, relatively cold fluid is suddenly brought into 

 direct contact with the hot heating surface at the point of collapse. The 

 resulting large temperature gradients, momentarily and locally, cause ex- 

 treme rates of heat transfer. Initial estimates [81] indicate that these rates 

 could increase the average heat transfer sufficiently to yield the values 

 observed experimentally. 



So far it has not been possible to determine which of the two mecha- 

 nisms is predominant. From experiments, the heat transfer — and in par- 

 ticular the burnout point — seems to improve with increased bubble ac- 

 tivity. Both concepts could serve as an explanation for this observation; 

 therefore this fact alone is not sufficient to determine the actual process. 



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