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Parabolic
Flights
50th ESA’s Parabolic
Flight Campaign (May 2009)
SCIENTIFIC OBJECTIVE Heat transfer over sub-millimeter spheroidal solids submerged in fluids is of interest in
many engineering processes such as manufacturing systems, packed beds and
many electronic components of nearly spherical shape. Apart from heat
conduction (molecular thermal diffusion) another important mechanism of heat
transfer in the above processes is natural convection which calls for the macroscopic buoyant
transport of fluid due to local density differences: hotter regions move
against gravity whereas colder regions move along gravity. The presence of
natural convection can lead to heat transfer rates many times larger than
that of pure heat conduction. Despite the huge literature devoted to
natural convection heat transfer rates over spheres (and to a smaller extent
over spheroids) there is not a generally accepted correlation especially for
small Rayleigh numbers. Existing correlations for
external (open domain) geometries predict a continuous progressively
increasing contribution of natural convection to heat transfer with respect
to gravity starting from zero gravity. This means that at the common residual
gravity level during parabolas (g-jitters) it is possible to have a
measurable effect of natural convection. Based on the experiments conducted on the 49th
PFC, evidence was provided that during parabolas
there was no natural convection in water and glycerol. Natural convection
appeared in water during normal gravity and, more distinctly, during hypergravity periods. EXPERIMENT In an effort to discriminate between the
effects of gravity level and liquid properties, we performed experiments in
the 50th ESA PFC
(May 2009) with different test liquids. Several experiments were conducted in
water, FC-72 and solid microparticles dispersed in
water, where the temperature evolution of a sub millimeter size spheroid
heater with a temperature dependent heat source was recorded at low gravity. First, we did tests with water and then, we
did tests with FC-72 (a liquid refrigerant) because of its much lower kinematic viscosity than water which allows more profound
appearance of natural convection during different g-levels. Finally, we did
tests with microparticles dispersed in water in two
forms: as a packed bed and as a dense suspension. We used two separate size
classes of particles in order to introduce different degrees of heterogeneity
in the liquid phase and therefore examine in another way whether liquid
conditions are strong enough to create natural convection currents. RESULTS Based on the data analysis of the conducted
experiments, it appears that the different thermophysical
properties of the examined liquids appeared to influence heat transfer from
the heater to an extent comparable to the effect of different gravity levels.
Moreover, the experiments in water and FC-72 showed that there was a
threshold before natural convection appears and above that threshold, the
natural convection is more profound in FC-72 than water. In addition, closely
packed particles suppress entirely natural convection but this is not so for
dense particle suspensions. Although the analysis of the data is still in
progress, it appears that heat transfer during parabolas is rather governed
by heat conduction. 49th ESA’s Parabolic
Flight Campaign (3-7 November 2008)
SCIENTIFIC OBJECTIVE Heat transfer over a sub-millimeter spheroidal solid is of interest in many engineering processes,
such as manufacturing systems, heat transfer in packed beds and for many
electronic components of nearly spherical shape. One important mechanism of
heat transfer in the above processes is the natural convection which leads to
heat transfer rates many times larger than that of pure conduction. Despite
the huge literature devoted to natural convection heat transfer rates over
spheres (and to a smaller extent over spheroids) there is not a generally
accepted correlation especially for small Rayleigh
numbers (Rayleigh number expresses the ratio of
convective to conductive transport). Existing correlations for external
geometries predict a progressively increasing contribution of natural
convection to heat transfer with respect to gravity (starting from zero
gravity). According to these correlations natural convection emerges even in
the case of g-jitters (small accelerations in reduced gravity environments
due to crew motions, mechanical vibrations, atmospheric drag, earth gravity
gradient and other sources). EXPERIMENT To test the validity of these correlations,
experiments were performed during the 49th ESA
PFC (November 2008) for the estimation of heat
transfer rates at low gravity. Heat pulses were given to a miniature thermistor with a nearly spheroidal
shape immersed in a liquid (water and glycerol) and its thermal response was
registered during heating in parabolic flights. The gravity value fluctuated
randomly (g-jitters) within ± 2.6x10-2 g during the low gravity
phases, whereas reached a peak value of about 1.6-1.8 g during the high
gravity phases. The contribution of natural convection was undoubtedly
estimated from runs in which acceleration varied from 0 g to 1.8 g. RESULTS The primary scope was to study the influence
of residual g-jitters on the heat transfer from the heater to the surrounding
liquid. The results showed that a minimum (critical) Rayleigh
number is required for the onset of natural convection (contrary to
predictions of existing theories and correlations for external geometries).
This means that there is no influence of g-jitters on heat transfer and the
only heat transfer mechanism for miniature heaters at low gravity conditions
is pure conduction. Using only conduction terms, an approximate mathematical
model was developed for the transient heat transfer problem in the
experimental set up which describes the experimental data sufficiently well.
This is an additional confirmation that the only heat transfer mechanism at
low gravity is conduction. Movie
1: Making science in 0-g (WMV 1.9 Mb) Movie
2: Spin in 0-g (WMV 3.6 Mb) 38th ESA's Parabolic Flight
Campaign (26-28 October 2004)
[Pictures Gallery]
[Movie 1:
Performing science in 0-g (2037KB)]
[Movie 2:
Performing fun in 0-g (2306KB)]
35th ESA's Parabolic Flight
Campaign (14-16 October 2003) [Pictures Gallery]
SCIENTIFIC
OBJECTIVE
This work investigates the growth of bubbles
emerging from a liquid saturated with a dissolved gas when its temperature is
locally and suddenly raised above the saturation value, yet below the boiling
temperature. This will be accomplished by fast heating of a bubble-free
saturated liquid sample by submerged heaters and registering the bubble
growth (due to gas desorption) by video recording
for later analysis. By performing runs under variable power and duration,
questions can be addressed relating to: the characteristic time of the growth
process, the effects of degree of supersaturation
and density of nucleation sites and the transient behavior of the system
during heating. Experimenting in a microgravity environment
will permit the decoupling of bubble growth from buoyancy effects and thus
facilitate understanding of the basic mechanisms governing the observed
phenomena. In addition, the low gravity conditions will permit the
investigation of large bubbles where inertia, viscosity and surface tension
are less significant and heat and mass transfer dominate the process. In this
time regime, two different phases of bubble growth will be investigated. The
earlier stages where the bubbles are large enough but still far from each
other (practically isolated) will be studied using a miniature spherical
heater. The later stages where bubbles grow so large that they interact with
their neighbors will be examined using a larger flat heater. Bubble generation and growth in liquids plays
a key role in diverse fields of technology such as polymer and glass
processing, flotation separations, pumps and hydraulic power recovery
systems. It also plays an important role in human physiology, e.g. blood
oxygenation, bubbles growing in the tissue of astronauts and divers during a
hyperbaric or hypobaric decompression. Also it is of critical value in
studying physical phenomena such as cavitation,
nucleation and boiling. TECHNICAL
DESCRIPTION OF THE EXPERIMENT
Four test fluids are to be examined in two
consecutive parabolic flight campaigns: water, blood serum, n-heptane and a refrigerant (C2F3Cl3).
The selection of the test fluids is based mainly on their potential for
applications but also on the good knowledge of their physicochemical
properties. The test liquids will be saturated with CO2 or N2,
gases chosen mainly because of their practical significance. The saturated
liquid will be contained in test cells, which will be placed inside a small
thermostat and stabilized at constant temperature. Two types of heater
geometries (Figure 1), are installed inside the test
cells to create local supersaturation: a small axisymmetrical thermistor (0.25
mm) to serve as a point heater and a flat platinum resistance layer (3x7 mm,
approx. 1mm thick) to serve as a large plane surface heater. In the first type of experiments, the
temperature of the liquid will be raised locally by the miniature point
heater, with a preset heating rate, resulting in a solution of increasing
degree of supersaturation. At fixed temperature,
the characteristic time of bubble generation and growth from a supersaturated
solution is very small, typically of the order of a few seconds. Therefore,
10 to 15 seconds of μ-gravity time is considered sufficient for one complete run. A second type of experiments calls for heat
pulses given to the plate heater resulting in a temperature gradient
perpendicular to its surface. Of particular interest then, will be the
observation of many bubbles growing in close proximity to each other over the
heater surface and their growth rate dependence on heating rate. Again, a
time span of 10 to 15 seconds allows the bubbles to grow to a sufficient size
for meaningful interactions.
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