Pool boiling experiments in microgravity provide some insights with phenomena not encountered in Earth gravity. Pure R-113 was used as the working fluid, with a flat heater (19 x 38 mm), consisting of a semitransparent gold film sputtered on a quartz substrate, which served simultaneously as a heat source for boiling and as a resistance thermometer for the measurement of the mean heater surface temperature. The transparency of the heater permitted viewing of the boiling process through the underside of the heater simultaneously with a side view. Experiments were conducted on five space flights, with each consisting of nine different tests. In addition to the unusual nucleation and associated dynamic vapor bubble growths reported previously, photographs taken with a 16mm film camera during the experiments reveal a Marangoni-type convective bubble motion which contributes to the steady effectiveness of the boiling heat transfer process in microgravity, maintaining the heater surface temperature at an unexpectedly low level. This dominant surface tension mechanism, under high subcooling, results in behaviors such as bubble removal from the nucleation site, bubble migration along the heater surface toward larger bubbles, and bubble coalescence. The pool boiling process in microgravity is compared directly with that in Earth gravity, since the identical hardware and software were used in both circumstances.
|A Pool Boiling Apparatus constructed for shuttle experiments as part of NASA Get Away Special Program|
|A Thin Gold film Heater|
|A test run of Pool Boiling Experiments under microgravity environment.|
|A Pool Boiling Apparatus designed for a drop tower|
|Vapor bubble coalescence in microgravity pool boiling (bubbles reduced half in number due to the coalescence)|
A visual comparison of typical pool boiling with the same heater surface in both earth gravity and microgravity is presented, where the upper half presents the side view and the lower half is the bottom view through the semi-transparent gold film heater. The bright spots in the lower left are the binary time indicators. The operating conditions are almost identical for both the normal gravity and reduced gravity. Vapor bubbles in earth gravity are observed to be quite small, compared to those in microgravity, because of buoyancy forces acting to remove the vapor bubbles from the vicinity of the heater surface. The numerous relatively larger bubbles in microgravity are uniformly distributed and attached to or in the vicinity of the heater surface.
The principal mechanism which produces the steady-state pool boiling observed in microgravity is attributed to surface tension effects: A large vapor bubble hovering near the heater surface acts as a reservoir, absorbing the smaller vapor bubbles growing on the heater surface, resulting in a maximum 32 % enhancement in heat transfer. An effective enhancement of approximately 40 % was observed associated with bubble migrations, where numerous tiny bubbles nucleate and move in a consistent manner toward a large bubble in the vicinity of the heater.
Numerous very small growing bubbles on the heater surface are moving toward the large bubble with a measured velocity of approximately 2.5 cm/s, and are believed to be responsible for the enhanced heat transfer here. This motion is defined as bubble migration. In the motion picture, the amount of bubble migration appears to increase gradually with time, and is accompanied by the enhancement in the heat transfer.
A most unusual phenomenon was observed to occur under conditions of high subcooling (16.7-22.2°C) at a low heat flux of 2 W/cm². A scarcity of bubbles on the heater surface is obvious relative to that at lower levels of subcooling. A manifestation of this behavior is the approximately 40% increase in heat transfer coefficient, with the same level of heat flux and a lower level of subcooling of 11.1°C.