Cavitation, Tensile Strength, and the Surface Films of Gas Nuclei 



things which have been discussed in the report to ITTC, but I suppose that part 

 of the dispersion can be attributed to differences in water quality according to 

 the mechanism we have seen outlined here. However, the range of pressure 

 values is small in comparison with what we have seen reported here. 



Finally one question. What is the reason for the increase in negative pres- 

 sure with increasing water speed? Is it increased diffusion speed due to de- 

 struction of the film because of mechanical agitation ? 



DISCUSSION 



Richard L. Hummel 



University of Toronto 



Toronto, Canada 



I am interested in the results obtained using Teflon to induce cavitation 

 upon an ultrasonic transducer which contrasts with our work using spots of 

 Teflon to induce similar bubbles for the boiling process (a procedure for which 

 I hold the U.S. patent). Excess energy is similarly required to form a bubble, 

 which in boiling comes from excess temperature, while with cavitation it comes 

 from an excessive pressure reduction. For boiling, the Teflon spots are used 

 to reduce this temperature excess for bubble formation to negligible proportions, 

 resulting in easily a tenfold increase in heat transfer coefficient. An important 

 difference between cavitation and boiling is that for boiling the bubbles do not 

 collapse. 



Our boiling work and I believe that of Dr. Richard Gaertner also of the 

 General Electric Company has been under conditions which should not only ef- 

 fectively remove gas from the site, but also degas the liquid; but, with suffi- 

 ciently fresh Teflon enamel as starting material and proper processing, the 

 activity of the sites do not change with time, whether or not (a) boiling is con- 

 tinuous or (b) is periodically stopped with or without subcooling of the liquid. 

 However, at a certain age which is generally considerably short of the six- 

 month expiration date given by duPont, the Teflon enamel (which appears to go 

 on properly, and which gives initial activity before degassing is complete) will 

 become inactive for producing and maintaining bubbles for boiling within a 

 period of an hour or so. 



I wonder if the present deactivation of the Teflon surface might be due to 

 the use of Teflon after it has become relatively ineffective as occurs in boiling, 

 whether inactivation is due to the pounding of cavitation in changing the porosity 

 or surface of the Teflon then reducing the effectiveness for vapour generation 

 or retention, or whether this inactivation is in fact due to the difference between 

 cavitation and boiling. 



Ill 



