The mechanism of heat transfer in nucleate pool boiling—Part I: Bubble initiaton, growth and departure
Abstract
A criterion is developed for bubble initiation from a gas filled cavity on a surface in contact with a superheated layer of liquid. It is found that the temperature of bubble initiation on a given surface is a function of the temperature conditions in the liquid surrounding the cavity as well as the surface properties themselves. It is also found that the delay time between bubbles is a function of the bulk liquid temperature and the wall superheat, and is not constant for a given surface.
By consideration of the transient conduction into a layer of liquid on the surface, a thermal layer thickness is obtained. With this thickness and a critical wall superheat relation for the cavity, a bubble growth rate is obtained.
Bubble departure is considered and it is found that the Jakob and Fritz relation works as long as the true (non-equilibrium) bubble contact angle is used. At one gravity the primary effect of bubble growth velocity on bubble departure size is found to be due to contact angle changes.
Résumé
On expose un critère pour la formation de bulles à partir d'une cavité remplie de gaz sur une surface en contact avec une couche surchauffée de liquide. On trouve que la température de formation de bulles sur une surface donnée est une fonction des conditions de température dans le liquide entourant la cavité aussi bien que les propriétés de surface elles-mêmes. On trouve aussi que la durée entre les bulles successives est une fonction de la température globale du liquide et de la surchauffe de la paroi et n'est pas constante pour une surface donnée.
En considérant la conduction transitoire dans une couche de liquide sur la surface, on obtient une épaisseur de couche thermique. Avec cette épaisseur et une relation critique de surchauffe de paroi pour la cavité, on obtient une vitesse de croissance de bulle.
On considère le détachement de la bulle et on trouve que la relation de Jakob et Fritz est valable aussi longtemps qu'on emploie le vrai angle de contact (en non-equilibre). Pour une gravité unité, on trouve que le principal effet de la vitesse de croissance des bulles sur la taille des bulles au détachement est due aux changements de l'angle de contact.
Zusammenfassung
Für das Entstehen von Blasen aus einer mit Gas gefüllten Vertiefung an einer Oberfläche in Berührung mit einer überhitzten Flüssigkeitsschicht wird ein Kriterium entwickelt. Es ergab sich, dass die Temperatur beim Blasenentstehen an einer gegebenen Oberfläche eine Funktion der Temperaturbedingungen in der Flüssigkeit ist, die die Vertiefung ebenso wie die Oberflächenbeschaffenheiten selbst umgibt. Es zeigte sich ebenfalls, dass die Verzögerungszeit zwischen den Blasen eine Funktion der Flüssigkeitstemperatur und der Wandüberhitzung und keine Konstante für eine gegebene Oberfläche ist.
Unter Berücksichtigung der Ableitung in eine Flüssigkeitsschicht an der Oberfläche erhält man eine thermische Schichtdicke. Mit dieser Dicke und einer Beziehung der kritischen Wandüberhitzung für die Vertiefung erhält man eine Blasenwachstumsgeschwindigkeit. Das Ablösen der Blasen wird beobachtet und dabei gefunden, dass die Beziehung von Jakob und Fritz solange gilt, wie der wahre (nicht im Gleichgewicht befindliche) Randwinkel der Blase eingesetzt wird. Bei Normalschwere findet man, dass der primäre Einfluss der Blasenwachstumsgeschwindigkeit auf die Ablösegrösse der Blasen von Veränderungen des Randwinkels hervorgerufen wird.
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