337 research outputs found
Near-critical point phenomena in fluids (19-IML-1)
Understanding the effects of gravity is essential if the behavior of fluids is to be predicted in spacecraft and orbital stations, and, more generally, to give a better understanding of the hydrodynamics in these systems. An understanding is sought of the behavior of fluids in space. What should emerge from the International Microgravity Lab (IML-1) mission is a better understanding of the kinetics of growth in off-critical conditions, in both liquid mixtures and pure fluids. This complex phenomenon is the object of intensive study in physics and materials sciences area. It is also expected that the IML-1 flight will procure key results to provide a better understanding of how a pure fluid can be homogenized without gravity induced convections, and to what extent the 'Piston Effect' is effective in thermalizing the compressible fluids
Is CHF triggered by the vapor recoil effect?
This paper deals with the triggering mechanism of the boiling crisis, a
transition from nucleate to film boiling. We observe the boiling crisis in pool
saturated boiling experimentally at nearly critical pressure to take advantage
of the slowness of the bubble growth and of the smallness of the Critical Heat
Flux (CHF) that defines the transition point. Such experiments require the
reduced gravity conditions. Close to the CHF, the slow growth of the individual
dry spots and their subsequent fusion on the transparent heater are observed
through the latter. As discussed in the paper, these observations are
consistent with numerical results obtained with the vapor recoil model of the
boiling crisis
Quench cooling under reduced gravity
We report the quench cooling experiments performed with liquid O2 under
different levels of gravity simulated with the magnetic gravity compensation. A
copper disk is quenched from 270K to 90K. It is found that the cooling time in
microgravity is very long in comparison with any other gravity level. This
phenomenon is explained by the isolation effect of the gas surrounding the
disk. The liquid subcooling is shown to drastically improuve the heat exchange
thus reducing the cooling time (about 20 times). The effect of subcooling on
the heat transfer is analyzed at different gravity levels. It is shown that
such type of experiments cannot be used for the analysis of the critical heat
flux (CHF) of the boiling crisis. The minimum heat flux (MHF) of boiling is
analyzed instead
Radiation-cooled Dew Water Condensers Studied by Computational Fluid Dynamic (CFD)
Harvesting condensed atmospheric vapour as dew water can be an alternative or
complementary potable water resource in specific arid or insular areas. Such
radiation-cooled condensing devices use already existing flat surfaces (roofs)
or innovative structures with more complex shapes to enhance the dew yield. The
Computational Fluid Dynamic - CFD - software PHOENICS has been programmed and
applied to such radiation cooled condensers. For this purpose, the sky
radiation is previously integrated and averaged for each structure. The
radiative balance is then included in the CFD simulation tool to compare the
efficiency of the different structures under various meteorological parameters,
for complex or simple shapes and at various scales. It has been used to precise
different structures before construction. (1) a 7.32 m^2 funnel shape was
studied; a 30 degree tilted angle (60 degree cone half-angle) was computed to
be the best compromise for funnel cooling. Compared to a 1 m^2 flat condenser,
the cooling efficiency was expected to be improved by 40%. Seventeen months
measurements in outdoor tests presented a 138 % increased dew yield as compared
to the 1 m^2 flat condenser. (2) The simulation results for 5 various condenser
shapes were also compared with experimental measurement on corresponding pilots
systems: 0.16 m^2 flat planar condenser, 1 m^2 and 30 degree tilted planar
condenser, 30 m^2 and 30 degree tilted planar condenser, 255 m^2 multi ridges,
a preliminary construction of a large scale dew plant being implemented in the
Kutch area (Gujarat, India)
Dew frequency across the US from a network of in situ radiometers
Dew formation is a ubiquitous process, but its importance to energy budgets or
ecosystem health is difficult to constrain. This uncertainty arises largely
because of a lack of continuous quantitative measurements on dew across
ecosystems with varying climate states and surface characteristics. This
study analyzes dew frequency from the National Ecological Observatory Network
(NEON), which includes 11 grasslands and 19 forest sites from 2015 to 2017.
Dew formation is determined at 30 min intervals using in situ radiometric
surface temperatures from multiple heights within the canopy along with
meteorological measurements. Dew frequency in the grasslands ranges from
15 % to 95 % of the nights with a strong linear
dependency on the nighttime relative humidity (RH), while dew frequency in
the forests is less frequent and more homogeneous (25±14 %, 1
standard deviation – SD). Dew mostly forms at the top of the canopy for the
grasslands due to more effective radiative cooling and within the canopy for the
forests because of higher within the canopy RH. The high temporal resolution of
our data showed that dew duration reaches maximum values
(∼6–15 h) for RH∼96 % and for a
wind speed of ∼0.5ms-1, independent of the ecosystem type.
While dew duration can be inferred from the observations, dew yield needs to
be estimated based on the Monin–Obukhov similarity theory. We find yields of
0.14±0.12mmnight-1 (1 SD from nine grasslands) similar to
previous studies, and dew yield and duration are related by a quadratic
relationship. The latent heat flux released by dew formation is estimated to
be non-negligible (∼10Wm-2), associated with a Bowen ratio
of ∼3. The radiometers used here provide canopy-averaged surface
temperatures, which may underestimate dew frequency because of localized cold
points in the canopy that fall below the dew point. A statistical model is
used to test this effect and shows that dew frequency can increase by an
additional ∼5 % for both ecosystems by considering a reasonable
distribution around the mean canopy temperature. The mean dew duration is
almost unaffected by this sensitivity analysis. In situ radiometric surface
temperatures provide a continuous, non-invasive and robust tool for studying
dew frequency and duration on a fine temporal scale.</p
Study of fluid behaviour under gravity compensated by a magnetic field
International audienceFluids, and especially cryogenic fluids like Hydrogen H2 and Oxygen O2 , are widely used in space technology for propulsion and cooling. The knowledge of fluid behaviour during the acceleration variation and under reduced gravity is necessary for an efficient management of fluids in space. Such a management also asks fundamental questions about thermo-hydrodynamics and phase change once buoyancy forces are cancelled. For security reasons, it is nearly impossible to use the classical microgravity means to experiment with such cryofluids. However, it is possible to counterbalance gravity by using the paramagnetic (O2) or diamagnetic (H2) properties of fluids. By applying a magnetic field gradient on these materials, a volume force is created that is able to impose to the fluid a varying effective gravity, including microgravity. We have set up a magnetic levitation facility for H2 in which many experiments have been performed. A new facility for O2 is under construction that will enable fast change in the effective gravity by quenching down the magnetic field. The facilities and some particularly representative experimental results are presented
Fast heat transfer calculations in supercritical fluids versus hydrodynamic approach
International audienceThis study investigates the heat transfer in a simple pure fluid whose temperature is slightly above its critical temperature. We propose a efficient numerical method to predict the heat transfer in such fluids when the gravity can be neglected. The method, based on a simplified thermodynamic approach, is compared with direct numerical simulations of the Navier-Stokes and energy equations performed for CO2 and SF6. A realistic equation of state is used to describe both fluids. The proposed method agrees with the full hydrodynamic solution and provides a huge gain in computation time. The connection between the purely thermodynamic and hydrodynamic descriptions is also discussed
A Study of Dew Water Yields on Galvanized Iron Roof in Kothara (North-West India)
In order to determine what amount of dew water can be collected without much investment during the dry season (October –May) in north - west India, a study was performed on plain, un-insulated, corrugated galvanized iron roofs that are common in this rural region. Between October 1, 2004 and May 31, 2005, the cumulative dew yield on a 18 m2 double - sloped (30�) test roof was 113.5 L. The west side gave 35 % higher water yield than the east side. The use of thermal insulation and more IR radiative materials would have increased this yield by 40 % (160 L). An analysis of dew events is made with meteorological data. It shows that the variable relative humidity is the most important parameter, which in turn is strongly correlated with the average wind direction with respect to monsoon direction. The cumulative dew water yield (6.3 mm) remains modest when compared with the average rain fall (300 mm). But dew occurs far more frequently than rain and it forms precisely during the dry season when water is most scarce.
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