Pool boiling - growth of single bubbles at a heated wall
Experimental and theoretical studies have been carried out on the growth of single bubbles at a wall in a non-uniform temperature field under conditions of zero, positive and negative gravity. Observations of bubble history were made using high speed cine photography whilst simultaneously recording transient wall temperatures at the bubble base with an array of miniature germanium resistance thermometers. The experimental results provide comprehensive growth data of bubbles in a known, non-uniform thermal environment for a wide range of conditions involving wall superheat, system pressure, thickness of the superheated layer, subcooling and gravity. A transient heating technique was developed to generate a known and controllable temperature profile near the wall. The subcooling was controlled by varying the initial temperature of the bulk liquid and the system pressure. Low gravity was achieved for about 0.25 S by allowing the apparatus to go into free fall within a drop tower. Experiments were conducted at subatmospheric pressures using controlled nucleation so that the position and the thermal environment of the subsequent bubble growth were predetermined. Results of the experiments show that the temperature distribution has a marked influence on bubble dimensions, shape change and departure, which has become clear only through making observations in low gravity at saturation conditions. The observations made using the fractional gravity facility in the drop tower demonstrated extreme shape changes and influence on bubble dynamics which occur due to gravitational field and its orientation relative to the bubble. The life of a bubble at the wall showed strong dependence on the subcooling of the bulk liquid. A theoretical model was developed to predict the bubble behaviour during the initial period of diffusion- controlled phase. This computer model offered excellent agreement with the experimental data until the shape changes were set in. The onset of shape changes were readily described by the excess pressure terms associated with the growth which were obtainable from the theoretical analysis of growth.
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