Dynamics of single and multiple bubbles and associated heat transfer in nucleate boiling under low gravity conditions.

Abstract

Experimental studies and numerical simulation of growth and lift-off processes of single bubbles formed on designed nucleation sites have been conducted under low-gravity conditions. Merging of multiple bubbles and lift-off processes during boiling of water in the parabola flights of KC-135 aircraft were also experimentally studied. The heating area of the flat heater surface was discretized and equipped with a number of small heating elements that were separately powered in the temperature-control mode. As such, the wall superheat remained nearly constant during the growth and departure of the bubbles, whereas the local heat flux varied during the boiling process. From numerical calculation it is found that peak of heat flux occurs locally at the contact line of bubble and heater surface. Dry conditions exist inside the bubble base area, which is characterized through a zero heat flux region in the numerical calculation and a lower heat flux period in the experimental results. During the merger of multiple bubbles, dry-out continues. In both the numerical calculations and experimental results, the bubble lift-off is associated with an apparent increase in heat flux. Wall heat flux variation with time and spatial distribution during the growth of a single bubble from numerical simulations are compared with experimental data.