ABSTRACT
This paper reports on the authors* latest parabolic-flight experiments
(November 1996) of spray cooling. Water and FC-72 (perfluorocarbon) were
employed alternately as a test liquid sprayed from a single full-cone nozzle
onto a Cr-plated surface of an electrically heated copper block (transient
cooling experiments) or onto a transparent In2O3-coated surface of a glass
block (steady state experiments in a relatively low superheat region).
Each experimental run was accomplished during some 15 seconds through which
a reduced gravity condition (〜0.01 times the terrestrial gravity) or an
elevated gravity condition (〜2 times the terrestrial gravity) was maintained
in the laboratory used - the cabin of an MU-300 aircraft. Cooling curves
over a wide range of wall-superheat were obtained with water sprayed at
significantly different volume fluxes and with FC-72 sprayed at rather
low volume fluxes. It is demonstrated that the gravity dependency of the
spray cooling characteristics varies with the spray volume flux and the
impinging droplet velocity. Qualitative interpretations of the observed
gravity dependency are provided.
NOMENCLATURE
dm : droplet diameter (mm)
Dm: spray volume flux (m3/(m2･s))
ge: terrestrial value of gravity acceleration (m/s2)
q : heat flux (kW/m2)
qCHF: critical heat flux (kW/m2)
Vm : droplet velocity (m/s)
DTsat: superheating of heater surface above saturation temperature (K)
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INTRODUCTION
Spray cooling has been widely applied to many industrial processes
in which rapid and effective cooling is required. Because of its capability
for intensive and accurately controlled heat removal, spray cooling is
considered to be one of the key technologies for developing thermal management
systems for use in space. Despite the mass of literature on spray cooling,
the present understanding of the effect of gravity on spray cooling characteristics
is far from complete. In fact, the present authors* preceding studies(1-3)
using parabolic flights of a 15-m jet plane, Mitsubishi MU-300, are the
only published efforts directed to clarifying the gravity effect on spray
cooling. This paper aims to report on the authors* new series of experiments
which was planned to supplement the preceding studies(1-3). Before describing
those latest experiments in subsequent sections, the authors* preceding
studies are briefly reviewed below.
The authors* first attempt at evaluating the gravity effect on spray
cooling characteristics was performed with water and CFC-113 each sprayed
onto a Ni-plated surface of a copper block heater of 19 mm in diameter(1,2).
The second attempt employed water and FC-72 (perfluorocarbon), instead
of CFC-113, each of which was sprayed onto a Cr-plated surface of a copper
block heater of 50-mm in diameter and also onto a metallized surface of
a transparent glass heater for observing the behavior of liquid droplets
impinging and having impinged on it(3). The major findings obtained in
these studies may be summarized as follows. For water:
1. At spray volume fluxes below 3.0×10-4 m3/(m2.s), the heat transfer
in the film boiling region beyond the MHF (minimum heat flux) point is
degraded by about 30% with a decrease in gravity. |
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