Nucleate boiling site activation by vapor injection

Abstract

In a related paper, Myers (1985) proposed that, in the chaotic mixture of vapor and liquid in a boiling system, bubbles in the range of 10 to 100 microns in diameter (hereafter called microbubbles) could be carried by downward currents of liquid into the superheated region near the boiling plate and serve as nucleation sites. If the microbubble size and the amount of superheat were to meet certain criteria, bubble growth would occur and the location would for an instant appear to be a nucleation site. Work by Bankoff (1958) has shown that, depending on the contact angle between liquid, solid, and vapor and on the geometry of the cavities and protuberances on the heater surface, certain locations on the solid surface are capable of trapping vapor and becoming boiling sites. Should these surface conditions be sufficiently favorable, the site would continue to generate bubbles and would appear as a long-term nucleation position. If the surface conditions were less favorable, the growth and departure of a few bubbles or even a single bubble might be the end of the particular event.An extension of the hypothesis would be that by increasing the number of bubbles in a system some might be of an appropriate size to qualify as microbubbles, with the possibility that some of these would recirculate to the boiling surface and form additional nucleation sites. This work represents an attempt to give some experimental support for this hypothesis. The population of persistently active nucleation sites present in a boiling system was determined both with and without the presence of artificially introduced microbubbles. Other variables studied included the effect on the nucleation site density of heat flux, pool depth, length of run, and the location of the microbubble-injection sparger. APPARATUSThe apparatus used in all but the final part of this study consisted of a square, stainless steel vessel (15.2 cm X 15.2 cm) with a height of 25.4 cm. Boiling took place on a 0.00254 cm horizontal sheet of stainless steel that served as the bottom of the vessel. Two copper electrodes, 0.1 cm thick and 1 cm wide, were soldered to the bottom of the boiling plate for a length of 10 cm. They were 8.4 cm apart. Power was supplied from a 50 amp, 220 V outlet to an EM1 DC power supply, Model SCR 10-500. Further details of the apparatus are provided in an earlier paper by Sgheiza and Myers (1985) and in works by Witzke (1977) and Sgheiza (1981).The nucleation sites on the boiling surface were identified using an infrared scanning camera (Inframetrics, Inc., Model 209A) to display the thermal radiation pattern on the underside of the heater plate while water was boiling on its upper surface. As explained by Sgheiza and Myers, the presence of a bubble nucleating and growing on the upper surface produces a cool spot on the lower surface. This can be detected by the scanning camera and displayed as a dark spot on the screen of the special cathoderay tube attached to the camera. Actual locating of the sites was done by taping a piece of clear ac...