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Pool Boiling Curve in Microgravity

Abstract

Pool boiling experiments using R-113 were conducted in the microgravity of space on a flat heater, consisting of a semi-transparent gold film sputtered on quartz substrate, 19.05 mm x 38.1 mm (0.75" x 1.50"). Transient measurements of both the mean heater surface temperature and input heat flux are used to compute the mean heat transfer coefficient at the heater wall. Steady state pool boiling is achieved in microgravity under conditions in which a large vapor bubble somewhat removed from the heater surface is formed, which acts as a reservoir for the nucleating bubbles. The steady nucleate boiling heat transfer is enhanced materially in microgravity relative to that in earth gravity, while the heat flux at which dryout occurs is considerably less. Using quasi-steady data obtained during periods in which some significant portions of the heater surface were dried out it was possible to construct two distinct composite approximate microgravity pool boiling curves for R-113, one for the higher level of subcooling and one for the lower level of subcooling. These are compared with a Reference Curve for pool boiling at a/g=+1, constructed from all available data correlations deemed to reasonably represent the circumstances present.

Experimental Appratus

Test Vessel

(click on figure to see actual picture

The figure above is a schematic diagram of the test vessel, consisting of R-113 and nitrogen (N2) chambers. The R-113 chamber has internal dimensions of 15.2 cm Dia. by 10.2 cm High, and includes a gold film heater on a quartz substrate, a pressure transducer, thermistors, and stirrer. The stirrer functions to provide a timely uniform fluid temperature between each test. The pressure transducer measures the system pressure with an uncertainty of 0.345 kPa, while thermistors measure the liquid temperatures adjacent to the heater surface at distances of 1, 5, 10 mm, and at various other locations, with a total uncertainty of 0.06 C. The nitrogen chamber is used to maintain the desired system pressure. System subcooling is obtained by increasing the system pressure above the saturation pressure corresponding to the initial liquid temperature. In the absence of buoyancy, an initially motionless liquid remains stagnant upon heating until the onset of boiling, and the temperature distribution in the liquid at the onset of boiling can be determined from a conduction heat transfer analysis.

Heater Surface

(click on figure to see actual picture
Two heater surfaces are placed on a single flat substrate, installed so as to form one wall of the test vessel as shown in Figure 1, with one acting as a back-up. Each heater consists of a 400 thick semi-transparent gold film sputtered on a highly polished quartz substrate, seen in Figure 2, and serves simultaneously as a heater, with an uncertainty of 2 % in the measurement of the heat flux, and a resistance thermometer, with an overall uncertainty of 1.0 C. The heater is rectangular in shape, 19.05 x 38.1 mm (0.75 x 1.5 inch). Degassed commercial grade R-113 (trichlorotrifluoroethane, ) was used because of its low normal boiling point (47.6 C), which minimized problems associated with heat loss to the surroundings, and because of its electrical nonconductivity, which is compatible for direct contact with the thin gold film heater. Further experimental details may be found in Merte et al. (1995).

Conclusions

It appears that long term steady state nucleate boiling can take place on a flat heater surface in microgravity with a wetting liquid under conditions in which a large vapor bubble somewhat removed from the heater surface is formed, which acts as a reservoir to remove the bubbles from immediate vicinity of the heater surface. The steady nucleate boiling heat transfer is significantly enhanced in microgravity compared to that in earth gravity.
Surface tension has an important role in producing dryout and/or rewetting on a heated surface. The detailed circumstances describing this remain to be explored further, but the heat flux at which dryout occurs is considerably less in microgravity than in earth gravity.

Using the quasi-steady data obtained during the periods in which some significant portions of the heater surface were dried out it was possible to construct two distinct composite approximate microgravity pool boiling curves for R-113, one for the higher level of subcooling and one for the lower level of subcooling. This is compared with a Reference Curve for pool boiling at a/g=+1, constructed from all available data and correlations deemed to reasonably represent the circumstances present. The microgravity pool boiling curves bear some resemblance to the Reference Curve, although the maximum heat flux (critical heat flux) and the minimum (film boiling) heat flux is also reduced considerably.

This page was last updated: 26 April 1996 by Patrick M. Fahey, ME Senior

For questions or comments, email pfahey@engin.umich.edu