The behavior of active boiling site populations was monitored for pool boiling on an electrically heated 1-mil stainless steel sheet. The boiling sites were located using a high-speed infrared camera focused on the underside of the boiling surface. Experiments were conducted to determine factors affecting the origin, concentration, and lifetime of active sites.J. E. SGHEIZA and J. E. MYERS Department of Chemical andNuclear Engineering University of California Santa Barbara, CA 93106 SCOPEMost experimental studies of nucleation sites in pool boiling are limited either to single, artificial sites or to sparsely distributed sites found on surfaces boiling at low heat fluxes. At fluxes of even moderate value the two-phase turbulence in a boiling system prevents direct observation of the behavior of the individual sites. In the investigation reported here real-time detection of the presence of active sites on the upper surface of a heated plate was done using a high-speed infrared camera to scan the temperature field on the underside of the heater.It has been known for over two decades that the nucleation and growth of a bubble in pool boiling causes a rapid drop in the temperature of the heater surface. When the surface is an electrically heated 1-mil sheet of stainless steel, the drop in the temperature on the upper side is transmitted to the underside within a few milliseconds. Thus, the nucleation, growth, and departure of bubbles in the liquid can be followed using an infrared scanning camera to detect the temperature behavior on the underside. This noninvasive technique allows studies to be made of individual sites and whole populations of sites under the influence of such variables as heat flux and the nature of the heating cycle. CONCLUSIONS AND SIGNIFICANCETransient and permanent boiling sites were identified and counted with a high-speed infrared camera during saturated pool boiling of water and four organic liquids on a horizontal heated surface at one atmosphere, at heat fluxes between 6.0 and 30.0 W/cm2. The behavior of active boiling site populations was monitored with the infrared camera during steady and pulsed boiling tests. Infrared camera displays of boiling site activity were recorded on 16 mm film at 60 pictures/s. Frameby-frame analysis of these films produced counts of active sites and the number of movie frames showing active boiling during two-second intervals. From these counts the average lifetime per site and the fluctuation in average lifetimes was determined. Temperature profiles of several active sites were photographed from the IR camera displays at 1,000 pictures/s, and the fluctuation in bubble nucleation periods analyzed.Pulsed boiling tests with water indicated that at heat fluxes up to 17 W/cm2 only 50 to 60% of potentially active sites nucleated and that less than 30% of all active sites nucleated consistently. Site-lifetime results showed that some sites, referred to as transient sites, nucleated in a highly irregular manner with vigorously active periods alternating with periods...

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...