Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

The research carried out in the Heat Transfer Laboratory of the Johns Hopkins University was motivated by previous studies
indicating that in terrestrial applications nucleate boiling heat transfer can be increased by a factor of 50 when compared to values
obtained for the same system without electric fields. Imposing an external electric field holds the promise to improve pool boiling
heat transfer in low gravity, since a phase separation force other than gravity is introduced. The influence of electric fields on
bubble formation has been investigated both experimentally and theoretically.
Author (revised)
Electric Fields; Heat Transfer; Microgravity; Nucleate Boiling
20010024895 Maryland Univ., Dept. of Mechanical Engineering, College Park, MD USA
Saturated Pool Boiling Heat Transfer Mechanisms
Kim, J., Maryland Univ., USA; Yaddanapudi, N., Maryland Univ., USA; Proceedings of the Fifth Microgravity Fluid Physics and
Transport Phenomena Conference; December 2000, pp. 128-159; In English; See also 20010024890; No Copyright; Avail: CASI;
A03, Hardcopy; A10, Microfiche
In the present work, saturated pool boiling was studied in a reduced gravity environment provided by a KC-135 aircraft. The
objective of this work was to identify the boiling processes associated with nucleate boiling, critical heat flux (CHF), and transition
boiling. Saturated pool boiling of FC-72 at 1 atm on an array of 96 heaters, each 0.27 mm x 0.27 mm in size was studied. Each
of the heaters was maintained at a constant temperature by means of electronic feedback circuits, and the time-resolved heat flux
from each heater was calculated from the instantaneous voltage across that heater. At each temperature, the voltage across the
individual heaters were sampled at a rate of 1250 Hz for 6.4 seconds. Boiling curves for microgravity and earth gravity are shown.
to avoid hysteresis associated with boiling incipience, the measurements were started off at a high wall superheat. The earth gravity
data are seen to be very repeatable. The boiling curves for microgravity agreed with each other for superheats up to about 30 K,
but differed at higher superheats. The discrepancy between these two boiling curves is probably due to g-jitter, which tends to
remove the larger vapor bubbles from the heater surface, allowing liquid to rewet the surface. At lower wall superheats, nucleate
boiling was observed wherein multiple bubbles grew, often merged, then departed with relatively small departure diameters. At
35 K and 40 K, a single large bubble was seen to cover the entire heater. In the low heat flux nucleate boiling regime (from 15
K to 25 K), microgravity heat fluxes were slightly larger than the corresponding values in earth gravity due to more nucleation
sites and larger bubbles in microgravity. Similar trends were observed by other researchers. A critical heat flux (CHF) of about
7.5 W/sq cm was observed in microgravity at a wall superheat between 30 K and 35 K. CHF was not reached in earth gravity over
the superheats studied. However, a CHF of 22 W/sq cm was observed in a previous study of saturated pool boiling in earth gravity
using a similar heater. The CHF values in microgravity were about 35% of those in earth gravity, similar to that observed by other
researchers. The visual observations in microgravity seemed to indicate that bubble coalescence governs CHF. Images of boiling
in microgravity were correlated with time resolved heat flux maps. It was observed that high heat flux was associated with areas
covered with small, rapidly nucleating bubbles, while low heat flux was associated with the large bubbles. The array heat transfer
in microgravity is governed by the extent of the surface covered by high heat flux, small bubble boiling. The time resolved heat
flux data were conditionally sampled according to whether or not boiling occurred on the surface, providing an average heat flux
during boiling. to do this, a boiling function, B(t), was generated from the time-resolved heat transfer signal whose value is 1 when
boiling occurs on the surface and 0 otherwise. The boiling function was used to obtain the boiling heat flux (the heat flux that occurs
only when boiling is present on the surface) over the entire heater array. The time and array averaged boiling heat flux is plotted
vs. wall superheat for the earth gravity and microgravity runs. Boiling heat flux for both gravity levels increase monotonically
over the range of superheats studied, and collapses onto a single curve. This result is not unexpected since inertia forces dominate
bubble growth when the bubbles are small. In microgravity, large heat transfer rates were observed where numerous small bubbles
(on the order of the individual heater size) nucleated, with very little heat transfer associated with the large bubble. Since large
heat transfer rates are associated with small bubbles, and since small bubbles are not affected significantly by gravity, it is not
surprising that the boiling heat flux is relatively insensitive to gravity. In summary, heat transfer during boiling in microgravity
seems to be governed by two parameters: 1) the size of the large primary bubble on the surface, and 2) the heat flux associated
with the small scale boiling at a given superheat. The first parameter appears to be governed by bubble coalescence, and determines
the extent of the surface covered by small bubble boiling. The second parameter can be obtained from the earth gravity boiling
heat transfer data. Future work should concentrate on determining the effect of gravity on coalescence phenomenon.
Author (revised)
Gravitational Effects; Heat Flux; Heat Transfer; Microgravity; Nucleate Boiling
20010024896 Rensselaer Polytechnic Inst., Isermann Dept. of Chemical Engineering, Troy, NY USA
Wetting and Partially Wetting Fluid Profiles in a Constrained Vapor Bubble Heat Exchanger
Wang, Y. -X., Rensselaer Polytechnic Inst., USA; Plawsky, J., Rensselaer Polytechnic Inst., USA; Wayner, P., Jr., Rensselaer Poly-
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