Behavior of nucleation sites in pool boiling

Abstract

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...

Nucleate boiling site activation by vapor injection

“…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).…”

Formation et évolution des bulles de gaz au-dessous de l anode dans une cuve d électrolyse d'aluminium

“…Ces résidus se comportent comme un germe d'une taille finie et ainsi ils facilitent la formation des bulles (germination hétérogène). Autrement dit, des bulles se forment plutôt dans les pores de l'anode que dans le bain [47,50,51,52,53,54,55,56]. Des pores qui contribuent à la production des bulles sont souvent appelés des sites actifs ou centres de formation.…”

“…observation directe de côté arrière un bloc solide (plaque transparente dont la surface est couverte avec une couche très mince d'un substrat conducteur qui n'empêche pas la vision) [54] 2. observation de la distribution de la température sur la face arrière d'une plaque de chauffage avec une caméra infrarouge [53] 3. visualisation de la variation de température sur la face arrière d'une plaque de chauffage avec des cristaux liquides (voir appendice B) [56,57] Les techniques 2 et 3 sont basées sur le fait que durant 1'ebullition, lors de leur croissance, les bulles refroidissent la plaque dans leur voisinage dû à leur besoin de chaleur pour l'évaporation. Cependant, au milieu des larges bulles, le contact entre le gaz et le mur diminue le transfert de chaleur, ce qui peut augmenter la température locale dans le solide.…”

“…Autrement après une croissance initiale, la bulle rencontrerait le liquide froid et elle commencerait à rapetisser jusqu'à sa disparition. Ainsi la période d'attente observée est souvent longue (durant l'ébullition, parfois elle est beaucoup plus longue que la période de croissance [47,57] et elle est souvent sous-estimée par les modèles mathématiques [47,53]). Par contre, ce critère a un rôle beaucoup moins important durant l'électrolyse de l'aluminium car la solubilité du CO2 dans le bain est très faible [14], ainsi le bain doit être toujours sursaturé.…”

Heat transfer and nucleation in pool-boiling

“…Meyers and co-workers [27,28] worked with an electrically heated stainless steel sheet with a liquid crystal coating or used a high speed infrared camera, to identify the active sites on the upper surface. Barthau [29] was able to count up to 8000 sites·cm −2 on the vertical flank of a horizontal tube, using an optical method.…”