MOTIVATION

The mechanisms by which bubbles transfer energy from a wall are actively being investigated experimentally and numerically by numerous researchers. Many mechanisms for bubble heat transfer have been suggested, but the transient conduction model and the microlayer model are the most widely cited. Recently, a contact line model of bubble heat transfer has also been suggested. We have been obtaining direct measurements of the wall heat transfer during bubble growth and departure to investigate the validity of the above models under saturated and subcooled pool boiling conditions.

 

RESULTS

Some quicktime movies showing the heat flux underneath individual bubbles can be downloaded by clicking on the images below. Each heater in the array has been colored according to the heat transfer (dark blue=low heat transfer, red=high heat transfer). Both images and heat transfer data were obtained at a frequency of 3704 Hz. The fluid is FC-72 at 1 atm.

1). Saturated boiling, single bubble nucleation. A large increase in the heat transfer under almost the entire bubble is observed just after nucleation, consistent with evaporation from a microlayer between the bubble and the wall. The development of a low heat transfer region at the center of the bubble is observed, indicating progressive dryout of the microlayer. The dry spot size, as evidenced by the inner circle, reaches a maximum then shrinks as the bubble begins to depart. Higher heat transfer is observed on the center heaters as they are rewetted by the bulk liquid. Bubble departure is associated with a spike in heat transfer at the center heaters that decays with time.

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2). Subcooled boiling, oscillating bubble. A bubble nucleates on the heater, then grows and shrinks numerous times before departing the surface. The oscillatory bubble motion is caused by the bubble growing within the superheated layer then shrinking as condensation occurs over the bubble cap as it grows beyond the superheated layer into the colder bulk liquid. The liquid in the vicinity of the bubble becomes heated as the bubble oscillates, increasing the thickness of the superheated layer and resulting in a steady increase in the maximum bubble diameter. Higher heat transfer is observed as the bubble shrinks and liquid rewets the wall, and at the three-phase contact line.

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3). Additional results have been obtained with the heaters dissipating a constant wall heat flux and measuring the temperature variation on the surface. An inverse heat conduction technique was then used to determine the heat dissipated into the substrate–this was subtracted from the heat supplied to the heater to find the heat transfer from the heater to the liquid. These results also showed that the principal mode of heat transfer was by transient conduction.  
4). The behavior of bubble nucleating very rapidly on the surface has also been studied. Various modes of bubble behavior are observed depending on the magnitude of the heat flux impulse and the heating duration.  

Some papers describing these results are

1). Yaddanapudi, N., and Kim, J., "Single Bubble Heat Transfer in Saturated Pool Boiling of FC-72", Multiphase Science and Technology, Vol. 12, No. 3-4, pp. 47-63, 2001.

2). Demiray, F. and Kim, J., “Microscale Heat Transfer Measurements During Pool Boiling of FC-72: Effect of Subcooling”, International Journal of Heat and Mass Transfer, Vol. 47 pp. 3257-3268, 2004. (744 KB)

3). Myers, J.G., Yerrramilli, V.K., Hussey, S.W., Yee, G.F., and Kim, J., “Time and space resolved wall temperature and heat flux measurements during nucleate boiling with constant heat flux boundary conditions”, International Journal of Heat and Mass Transfer, Vol. 48, No. 12, pp. 2429-2442, 2005. (952 KB)

4). Yin, Z., Prosperetti, A., Kim, J. “Bubble Growth on an Impulsively Powered Microheater”, International Journal of Heat and Mass Transfer, Vol. 47, No. 5, pp. 1053-1067, 2004.

This work was sponsored by NASA 's Office of Biological and Physical Resarch.