E,9 • NUCLEATE BOILING 



above it was seen that the heat transfer from the hquid to the bubble 

 played a role in determining the bubble motion. The thermal diffusivity 

 of the Hquid therefore should be an important property affecting this 

 motion. A low diffusivity should favor slow bubble growth and collapse. 

 It is difficult to isolate this effect because changes in other properties are 

 usually involved when using fluids of different thermal diffusivity, but a 

 comparison of experimental data obtained for carbon tetrachloride with 

 those obtained with a water-aerosol solution is probably pertinent. The 

 surface tensions of the two hquids are quite similar and the thermal 

 diffusivity for carbon tetrachloride is only about |- that of the water 

 solution. Measurements under comparable conditions at a pressure of 



,. 100 



80 



60 



40 



20 



10 



100 100 500 1000 



Liquid pressure, Ib/in^ 



Fig. E,9e. Excess of wall temperature over saturation temperature as a function of 

 pressure for fully established boUing. Distilled degassed water. Heat transfer surface is 

 SS347 except for the point at the lowest pressure, in which case the surface is SS304 

 [78]. 



1 atm show considerably slower growth and collapse rates for the carbon 

 tetrachloride than for the water solution [6*4] . 



Temperature of bulk liquid. The temperature of the bulk hquid influ- 

 ences the heat transfer from the bubble to the bulk fluid, which in turn 

 limits the growth of the bubble and is responsible for its collapse. If the 

 temperature of the bulk fluid is low, the growth of the bubble stops early 

 and the collapse is rapid. For free convection boihng of distilled water at 

 1 atm, e.g., the maximum bubble radius decreases from 0.022 to 0.014 

 inches as the fluid temperature is changed from 170 to 60°F [64]. If the 

 fluid temperature is high, the bubble may grow to a rather large size. 

 The under-pressure eventually created in the bubble may not be suf- 

 ficient to reverse the momentum stored in the fluid during the growth 

 and the bubble may detach itself from the surface. If, in the limit, the 

 temperature of the bulk fluid is itself at the boihng point, there is no heat 



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