14 



motion of the bubbles. The actual tests were not begun until all the small bubbles reached 

 the surface of the liquid. 



The rate of bubble flow was then regulated by the needle valve so as to release bubbles 

 with a minimum spacing of 24 in. This reduced the effect of the wake created by the passage 

 of a bubble on the motion of a bubble following. At higher bubble rates, the velocity of the 

 individual bubble is increased. 



The same precaution was observed for the larger bubbles that were formed by dumping. 

 An additional precaution was to rotate the dumping cup with steady speed in order to avoid 

 splitting of the bubble or the formation of satellites upon release. The slow passage of the 

 air through the brass tube inside the tank allowed the air to reach the temperature of the li- 

 quid. Contact of the air at the nozzle tip or inside the dumping cup with the liquid allowed 

 saturation of the air with liquid vapor, so that the air bubble can, in each instance, be as- 

 sumed to be saturated with the vapor of the liquid in which it rises. 



MOTION PICTURES AND THEIR EVALUATION 



The velocity, path, and shape of the bubbles were obtained from motion pictures made 

 with a Mitchell 35mm camera using a special lens attachment to permit close-ups. Film speeds 

 of 25 to 35 frames per second and back lighting from a white reflector were used. For the first 

 few tests, the film speed was obtained by photographing a rotating clock dial; subsequently a 

 neon timing light with a 60-cycle voltage source was utilized. The film speed was determined 

 from the marks of the timing light on the film. The field of the camera varied from 1.4 x 1.8 

 to 1.75 x 2.3 in. depending upon the refractive index and horizontal depth of liquid. A trans- 

 parent scale photographed in the plane of the bubble provided the distance scale factor for 

 the evaluation of displacement and size. The camera lens was placed at approximately the 

 midpoint between the liquid level and the bottom of the tank for all tests. The camera loca- 

 tion was in each instance sufficiently above the nozzle tip so that the bubbles reached their 

 terminal velocity before passing in front of the camera. A summary of camera location and 

 depth of liquids in the tanks is given in Figure 2. 



Changes in bubble volume due to differences in liquid depth were minimized by making 

 velocity measurements over a very short vertical displacement (less than 2 1/2 in.). The 

 rate of rise of bubbles was determined by measuring the displacement of a bubble from a 

 reference point on successive frames of the film by means of a Bausch and Lomb contour- 

 measuring projector using a magnification of twenty-five (Figure 3). These displacements 

 were then plotted against the frame number. The straight-line plot indicates that the veloc- 

 ity of the bubble remained constant during the time it passed the field of the camera. From 

 the slope of the line, the frame speed, and the scale factor, the velocity of the bubble is 

 computed. 



*Napier showed the absence of proximity effect for air bubbles in water, ranging in equivalent radius from 

 0. 14 to 0.38 cm, if the frequency was below 30 bubbles per minute. 



